HELGOLANDER MEERESUNTERSUCHUNGEN
Helgol/inder Meeresunters. 42, 435-467 {1988)
Phytoplankton and nutrients in the Helgoland region*
Max Gillbricht
Biologische Anstatt Hetgotand (Zentrate); Notkestr. 31, D-2000 Hamburg 52,
Federal Republic of Germany
ABSTRACT: During recent decades, phytoplankton stock on the one h a n d a n d inorganic nutrients
(P a n d b o u n d N) on the other have increased considerably in the southern North Sea, as demonstxated at a p e r m a n e n t station (since 1962) near the island Helgoland. This correlation b e t w e e n
phytoplankton and inorganic P and N n e e d not have anything to do with causality; exceptional algal
blooms have b e e n observed and reported in the literature since in the 1 9 th century. Furthermore,
these increases (four-fold for phytoplankton a n d two-fold for nutrients) are in the same r a n g e as the
fluctuations from year to year u n d e r different hydrographical conditions. A detailed investigation
carried out in 1981 demonstrated the presence of a slowly growing phytoplankton population.
Starting with a considerable stock of flagellates in spring, it r e a c h e d a peak in cell n u m b e r s over a
long reproduction period which contrasted with the normal duration of a spring bloom of diatoms.
These processes were not related to a limited production by P or N. A considerable concentration of
these nutrients was p e r m a n e n t l y available in the form of inorganic compounds. The total amount of
nutrients surpassed by far the portion incorporated in the phytoplankton. This is a consequence of
the fact that small organisms have a high metabolic rate. Therefore, the relation b e t w e e n stock and
production (daily production=stock) is completely different from that k n o w n e.g. in agriculture. The
nutrients exist during the vegetation period mainly in the form of dissolved organic matter that is
accessible to plankton. The great dynamics of this system, including a phase shifting during the year
b e t w e e n inorganic P, N, Si, and production, indicates the significance of p e r m a n e n t a n d fast
reminerahzation. Calculations demonstrate that the n a t u r a 1 nutrient content of seawater normally satisfies the d e m a n d s of phytoplankton present in the North Sea area u n d e r study. Only in the
more productive coastal region (sahnity < 30 associated with fresh water run-offs of low nutrient
content - an unrealistic assumption in the G e r m a n Bight) m i g h t some hmitation b e observed. For
diatoms, silicate may represent a critical component, but a high dynamic force exists in the presence
of small Si concentrations. Therefore, a lack of sihcon must not represent any limitation; however,
knowledge on the sihcon system is insufficient up to now.
INTRODUCTION
A t t h e b e g i n n i n g of t h i s c e n t u r y , s o m e y e a r s a f t e r t h e first p l a n k t o l o g i c a l p u b h c a t i o n
b y H e n s e n (1887), B r a n d t (1899, 1902) s t a r t e d t h e d i s c u s s i o n o n t h e c o n n e c t i o n b e t w e e n
n u t r i e n t s a n d p h y t o p l a n k t o n . H e w a s h i g h l y i n f l u e n c e d b y L i e b i g ' s " l a w of t h e
m i n i m u m " a n d e s p e c i a l l y i m p r e s s e d b y t h e r e c e n t l y d e t e c t e d n i t r o g r e n cycle. O n a
geological time scale, the easily soluble nitrogen salts should have reached a critical
concentration in the sea by now, but this state has not yet been observed. Brandt, on the
c o n t r a r y , a s s u m e d N to b e g r o w t h - l i m i t i n g as a c o n s e q u e n c e of d e n i t r i f y i n g b a c t e r i a , as
f o u n d b y B a u r (1902) i n t h e Kiel Fjord. M a n y s u b s t a n c e s s h o u l d h a v e b e e n e n r i c h e d i n
* Dedicated to Dr. Dr. h. c. Peter Kornmann on the occasion of his eightieth birthday.
9 Biologische Anstalt Helgoland, H a m b u r g
436
M a x Gillbricht
the sea over the time r e a c h i n g concentrations d a n g e r o u s for (normal) life. This situation
cannot b e observed. Therefore, p r o c e s s e s must exist that h i n d e r s u c h i n c r e a s e s b y
s e d i m e n t a t i o n (flocculation, precipitation, adsorption), b y d e g a s s i n g o r b y c h e m i c a l
transformation, or m a y b e b y m e a n s of s p e c i a h z e d organisms. The s e a is n o t in a state of
p e r m a n e n c y but, at the most, in one of d y n a m i c equihbrium or e v e n in o n e of continuous
imbalance.
The c o n s e q u e n c e of t h e s e r e m a r k s b y Brandt has not b e e n c o n s i d e r e d in full up to
now, a n d his i d e a s about the p r i m a c y of nutrients in p h y t o p l a n k t o n d e v e l o p m e n t are still
b e i n g d i s c u s s e d a n d are in g o o d a g r e e m e n t with the findings b y h m n o l o g i s t s that a m a n m a d e i n p u t of P a n d / o r N m a y a d d to the organic production e n o r m o u s l y . T h e r e is,
however, a f u n d a m e n t a l difference in this r e s p e c t b e t w e e n the situation i n the s e a a n d in
freshwater. The p h o s p h a t e content in the d e p t h s of the Atlantic O c e a n (50 ~ N) is in the
r a n g e of 1.3 ~g a t o m . d m -3 (Redfield et al., 1963) as the c o n s e q u e n c e of a l o n g lasting
input, whilst the n a t u r a l content of total phosphorus in l a k e s m a y b e m u c h smaller,
possibly only 0.1 ~g a t o m - d m -3 (Chapra & Robertson, 1977). However, t h e w a r m w a t e r
sphere n e a r the surface of the oceans located in l o w e r latitudes is e x t r e m e l y
i m p o v e r i s h e d with r e s p e c t to nutrients. In the u p w e l l i n g region off N W Africa the
p h y t o p l a n k t o n d e v e l o p m e n t m a y be initiated therefore b y nutrient i n p u t as stated b y
N a t h a n s o n (1906), a n d is afterwards r e g u l a t e d by a n o t h e r c o m p o n e n t , p o s s i b l y b y
vertical w a t e r m o v e m e n t ( r e d u c e d sinking velocity) (Gillbricht, 1977).
One of the first p r o b l e m s m e t with w h e n carrying out investigations o n the subject of
p h y t o p l a n k t o n stock r e g u l a t i o n b y nutrients is that usually only the i n o r g a n i c nutrients
are d e t e r m i n e d or, to b e quite exact, the substances that react with t h e r e s p e c t i v e
reagents. Since the twenties of this century, it is k n o w n that o r g a n i c a 11 y b o u n d
nitrogen a n d p h o s p h o r u s exist in solution. T h e s e substances w e r e first d e t e c t e d in fresh
w a t e r (Birge & Juday, !926; J u d a y & Birge, 1931) and later on in the s e a (cf. s u m m a r y b y
Armstrong & Harvey, 1950, for P). T h e s e authors c o n c l u d e d that most of the inorganic
nutrients that d i s a p p e a r in s p r i n g t i m e are not incorporated into organisms or detritus b u t
are t r a n s f o r m e d into soluble organic substances, as d e m o n s t r a t e d b y B u t l e r et al. (1979)
for nitrogen. Accordingly, t h e r e is a g r e a t nutrient depot in the sea that is not n o r m a l l y
detected. This situation, o b s e r v e d world-wide, has also b e e n o b s e r v e d i n the southern
North Sea with r e s p e c t to P (Kalle, 1937) a n d to N (Eberiein et al., 1985).
W h a t is the origin of t h e s e g r e a t quantities of organic substances? S t e i n e r (1938)
d e m o n s t r a t e d that up to 90 % of the p h o s p h o r u s in d e a d limnic p l a n k t o n w a s t r a n s f o r m e d
into p h o s p h a t e b y e n z y m a t i c processes within three days, associated with the i m p o r t a n t
effect that p h o s p h a t a s e is r e l e a s e d into the w a t e r reacting t h e r e with s o l u b l e organic
p h o s p h o r u s compounds. Hoffmann (1956) i n v e s t i g a t e d m a r i n e p l a n k t o n (diatoms, peridinians, copepods). After b l o c k i n g the enzymes, 13 % of the p h o s p h o r u s w a s o b s e r v e d in
the organisms as (inorganic) p h o s p h a t e , whilst 40 % of the organically b o u n d p h o s p h o r u s
was in a soluble form. W h e n the e n z y m e s w e r e not blocked, 29 % of the o r g a n i c P was
r e m i n e r a h z e d after 24 h, a n d 77 % of the rest of the organic P from t h e c o p e p o d s w a s
dissolved in the water. T h e s e results d e m o n s t r a t e that d e a d o r g a n i s m s m a y not only b e a
continuous source of r e c e n t l y r e l e a s e d i n o r g a n i c nutrients b u t also of g r e a t e r quantities of
dissolved organic compounds.
The other possibihty is that all this m a t e r i a l is mainly a b y - p r o d u c t of assimilation, as
d e m o n s t r a t e d b y Braarud & F o y n (1931) for the flagellates Carteria a n d Chlamydomonas,
Phytoplankton a n d n u t ri en t s n e a r H e l g o l a n d
437
and m e a s u r e d directly by F o g g (1958) in lake waters with laC for b l u e - g r e e n algae. T h e
m e a n v al u e of the r e l e a s e d substances is in the r a n g e of 5 % of t h e production u n d e r
optimal conditions (exponential growth phase) (Williams, 1975). This 5 % is p r o b a b l y the
main source of the dissolved organic material, which has its m a x i m u m P an d N content
during spring a n d s u m m e r (Table 1). All particulate nutrient c o m p o u n d s are only a small
Table 1. Distribution of nutrients in the English Channel and the Dutch coastal water. S -- summer
(April to September); W = winter (October to March)
Phosphorus
total Ps ~---80 % total Pw
S: organically bound P = 50 % total P and
W: organically bound P = 25 % total P (a), (c)
Bound
Nitrogen
total Ns = 80 % total Nw
S: organically bound N = 80 % total bound N and
W: organically bound N-~ 55 % total bound N (b), (c)
(a) Armstrong & Harvey (1950), measurements: 1947-1949 in the Plymouth area (English
Channel); phosphorus, total P = dissolved + particulate.
(b) Postma (1966), measurements: 1960-1962 from the light vessel 'Texel' (Dutch coast);
nitrogen, total organic N = dissolved or dissolved + particulate; both calculations give a
similar result because more than 80 % of organic N is dissolved in contrast with a near
coastal region with more detritus (Taft et al., 1975); one surface sample omitted (completely different water with respect to salinity etc.).
(c) Butler et al. {1979), measurements: 1969-1977 (corrected with respect to NH4: measurements 1976-1978) in the Plymouth area (English Channel); phosphorus and bound
nitrogen, total P and N = dissolved.
fraction of the total stock, which has nearly the s a m e quantity in s u m m e r as in winter.
This does not contradict the assumption that the total short-cut nutrient cycle is mainly
within the wat e r column with an e x c h a n g e b e t w e e n inorganically a n d organically b o u n d
dissolved P and N (p. 463). With r e g a r d to stratification according to density, nutrients
may be distributed relatively h o m o g e n e o u s l y but with many organic c o m p o u n d s in the
surface layer whilst P and N are r e m i n e r a l i z e d near the bottom; by m e a s u n n g the
inorganic components, a situation is simulated that in fact does not exist (Armstrong &
Harvey, 1950; Butler et al., 1979). A c o m p l e t e l y different p r o b l e m is w h e t h e r the r e l e a s e
of assimilation products d e p e n d s on distinct species (GiUbricht. 1952, 1983) as well as on
the en v i r o n m en t a l conditions (Fogg, 1958).
All these facts indicate that there is a lot of different organic P an d N c o m p o u n d s an d
a d ep o t of inorganic nutrients. It is therefore possible to describe these i n o r g an i c
c o m p o n e n t s as m e r e i n t e r m e d i a t e products b e t w e e n the destruction of (mainly dissolved)
organic substances by physico-chemical (UV), biological (bacteria, zooplankton), or any
other processes on the one hand, and the assimilation of nutrients b y p h y t o p l a n k t o n (and
also by bacteria a n d other organisms) on the other (Tarapchak & N a l e w a j k o , 1986).
Therefore, the dynamics of the system cannot be s e e n directly, but only t h e relatively
Max
438
Gillbricht
Table 2. M e a n composition of marine organisms (atomic relations)
P
:
N-
S.i
l
:
1
1
1
l
1
:
:
:
:
:
19
16
15
16
16
14
t
:
16
:
:
C
14
:
107
-
:
-
-
:
-
(c)
-
:
106
(d)
16
17
:
:
-
(e)
(f)
16
:
106
(g)
(a)
(b)
(a) Brandt & Raben (1920), phytoplankton by net catches, Si: in the respective samples
certainly not exclusively diatoms, therefore true relation possibly somewhat higher than
14.
(b) Schreiber (1927), misprints corrected, nutrient uptake by Cartefia in culture.
(c) Braarud & Foyn (1931), same method as Schreiber (1927) with Chlamydomonas.
(d) Fleming (1939), commonly used standard in good agreement with observations in sea
(GiUbricht, 1977).
(e) Richards (1958), Si content of diatoms is difficult to determine (frustules as function of cell
dimensions, mostly investigated bottom forms with thicker walls), plankton samples, no
information about the obviously dominant diatom part, value for Si maybe shghtly too
low.
(f) Grill & Richards (1964), remineralization during a tank experiment obviously influenced
by bacteria; value for Si may be somewhat too high; calculated from nutrient changes
between day 51 and day 93.
(g) Used in this investigation.
small fluctuations in the different compartments, a n d the inorganic release of P a n d also
N takes place before the complete destruction of organic material occurs (Gillbricht,
1977). Therefore, a n essential question is w h e t h e r or not p h y t o p l a n k t o n c a n use organically b o u n d nutrients directly. This possibility was first described b y Schreiber (1927)
(uptake of glycin by Carteria), who established, however, that organically b o u n d P
cannot be assimilated directly. Both findings are still valid in the most cases discussed for
urea (Verlencar, 1985) a n d amino acids (Plynn & Butler, 1986). It is not k n o w n what
proportion of this organic N source can b e used directly b y phytoplankton, possibly with
different u p t a k e velocities (Schell, 1974) a n d efficiencies for inorganic a n d organic
c o m p o n e n t s (Schreiber, 1927; Braarud & Foyn, 1931). Organically b o u n d P can be
resorbed after hydrolysis by cell-surface alkaline phosphatase or other dissolved enzymes p r o d u c e d by phytoplankton, especially in the case of phosphate deficiency, a n d by
micro-organisms (Stewart & Wetzel, 1982; T a r a p c h a k & Nalewajko, 1986).
M i n u t e n u t r i e n t concentrations allow a n o p t i m u m growth rate of phytoplankton.
Natural populations have a n a d v a n t a g e i n this respect over monocultures i n laboratories
(Tarapchak & Nalewajko, 1986). Nutrient-deficient cells should not normally b e observed
in the sea. The results given by S a k s h a u g et al. (1983) m a y h a v e b e e n i n f l u e n c e d by the
statistics u s e d (Gillbricht, 1974).
Discussions about n u t r i e n t s a n d organisms n e e d some information with respect to the
composition of the latter. Examples given in literature for the atomic relations, i n c l u d i n g
Phytoplankton a n d nutrients n e a r Helgoland
1927
I
Jan.
I
2.5.
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439
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Nov.
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Fig. 1. Observations near Helgoland: after Raben in 1911/12 (N), ~fter Schreiber in 1926/27 and the
permanent station in 1981. P and INhave the same scale with respect to the composition of organisms
Si in the case of diatoms, are s u m m a r i z e d in Table 2. This outline indicates that the m e a n
relations b e t w e e n the different elements allow rough estimates about the nutrient
r e q u i r e m e n t s of phytoplankton. The practical c o n s e q u e n c e of this fact ts impressive in
that it m e a n s that 1 btg atom P is e q u i v a l e n t to 16 btg atom N or Si {in diatoms) a n d to 1.3
m g C. In this way, the potential phytoplankton stocks can be calculated from observed
n u t r i e n t concentrations in winter. The inorganic n u t r i e n t content in the waters near
H e l g o l a n d (Fig. 1) is therefore m u c h higher than is n e e d e d to build up the p h y t o p l a n k t o n
population during s u m m e r (Fig. 4).
A method described b y Schreiber (1927) deserves special attention. The m a x i m a l cell
n u m b e r developing i n a bacteria-free seawater sample with a monoculture of algae ceils
is d e t e r m i n e d directly a n d after a d d i n g a surplus of P or N to find out the concentrations
a n d the "limiting" nutrient. C o m p a r e d with chemical methods, this m e t h o d is a d v a n t a g e ous in that it m e a s u r e s exactly the quantities accessible for phytoplankton, or as
Schreiber writes with respect to the nitrogen problem: "Die o b e n angeffihrte Lficke, dab
die fiir das Plankton assimilierbaren N - Q u e l l e n bisher noch u n b e k a n n t sind, wird bei der
n e u e n Methode
d a d u r c h (iberbrfickt, dab bei der
'physiologischen'
Stickstoffanalyse tatsfichhch n u t diejenigen N - V e r b i n d u n g e n des Meerwassers
angezeigt werden, die auch wirklich v a n der Algenzelle direkt assimihert w e r d e n
kSnnen."
440
M a x Gillbricht
This, for a n o r m a l (chemical)laboratory, complicated m e t h o d h a s n e v e r b e e n u s e d to
a g r e a t e r extent, b u t w e m u s t b e a r in m i n d that a monoculture cannot p r o d u c e the species
succession n e c e s s a r y for different a n d continually c h a n g i n g situations, a n d that the
p o p u l a t i o n i n v e s t i g a t e d cannot p e r m a n e n t l y achieve an optimal a d a p t a t i o n . Additionally, observations m a d e in a small vessel cannot or can with restrictions o n l y b e a p p h e d to
the sea. Nevertheless, it w o u l d b e of some interest to r e p r o d u c e a n n u a l cycles (1926/27)
using this m e t h o d (single observations) a n d r e c e n t m e a s u r e m e n t s (1981), a n d some N
v a l u e s b y R a b e n (1914) from this region (Fig. 1). A comparison of the different results is
difficult w h e n one considers the different m e t h o d s of d e t e r m i n a t i o n a n d w h e n the
different h y d r o g r a p h i c a l conditions are unknown. P a n d N h a v e the s a m e scale with
r e s p e c t to the (mean) composition of organisms. The N concentration m a y h a v e i n c r e a s e d
d u r i n g the Iast d e c a d e s , whilst the situation is not so clear with r e s p e c t to P (in this case in
contrast to Fig. 2 a n d Weichart, 1986).
It is n e c e s s a r y to gain k n o w l e d g e of the p h y t o p l a n k t o n production n o t m e a s u r e d
directly. The optimal daily i n c r e a s e of a diatom stock d u r i n g a spring b l o o m is ~ 15 %
(decrease ~ 2 5 %), o b s e r v e d in a h a r b o u r basin (Gillbricht, 1955) a n d in t h e North Sea
d u r i n g FLEX '76. This is small c o m p a r e d with the production b y n a t u r a l m a r i n e populations, c a l c u l a t e d as 140 % from the d a t a given b y T h o m a s et al. (1978) a n d as m o r e than
200 % in culture e x p e r i m e n t s (SchSne, 1977; Baars, 1981). T h e s e v a l u e s d e m o n s t r a t e the
well k n o w n fact that small organisms have relatively high metabolic rates. In this way, a
m i n u t e p o p u l a t i o n with a m i n u t e nutrient content is the basis for h i g h p r o d u c t i o n in
contrast to the situation with large plants i n agriculture or forestry. In the r a n g e of 1/z of
the gross production is r e s p i r e d over 24 h, as can b e d e d u c e d from the information given
b y S t e e m a n n N i e l s e n & H a n s e n (1959). T h e s e findings indicate that a p h y t o p l a n k t o n
p o p u l a t i o n has high p e r m a n e n t losses, a n d g r e a t d e c r e a s e s in production c a u s e the stock
to practically d i s a p p e a r from the w a t e r column within a short time. If t h e r e is a n y
p l a n k t o n in the sea, t h e n its multiplication rate must b e relatively constant.
This fact is u s e d here for r o u g h calculations a s s u m i n g a daily p r o d u c t i o n of diatoms
of 100 %, in a g r e e m e n t with Krey (1953), as an i n t e g r a t e d m e a n v a l u e of the euphotic
zone.
The p r o d u c t i o n of other p h y t o p l a n k t o n species (Thomas et al., 1978) - in the first
r a n g e flagellates, p r e f e r a b l y p e r i d i n i a n s (Nordli, 1957; Elbr~chter, 1977) - is a b o u t 50 %.
This value is u s e d considering that these organisms optimize their vertical distribution
with r e s p e c t to light (Gillbricht, 1983).
MATERIAL A N D M E T H O D S
Surface s a m p l e s are t a k e n on every w o r k i n g d a y a r o u n d 9 a. m. on H e l g o l a n d Roads
( G e r m a n Bight; 54 ~ 11.3 'N; 7 ~ 54.0 'E) in a n a r r o w p a s s a g e b e t w e e n the i s l a n d a n d the
Dune. In the water, the following m e a s u r e m e n t s a r e t a k e n : t e m p e r a t u r e (thermometer),
sahnity (sahnometer), p h o s p h a t e , nitrate, nitrite, a m m o n i u m , silicate (Grasshoff, 1976),
a n d p h y t o p l a n k t o n b y counting with the Uterm6hl (inverted) microscope a n d b y transformation into c a r b o n b y calculation (Hagrneier, 1961). This information is correct for the
a n n u a l cycle 1981. During the long p e r i o d of observation, some c h a n g e s h a v e b e e n m a d e
with r e s p e c t to the f r e q u e n c y of the m e a s u r e m e n t s a n d m e t h o d s used. T h e s e c h a n g e s
h a v e h a d no practical c o n s e q u e n c e s with r e s p e c t to the p r o b l e m s d i s c u s s e d here.
The m e a s u r e m e n t s w e r e t a k e n in w a t e r b o d i e s that are p e r m a n e n t l y c h a n g i n g due to
Phytoplankton and nutrients near Helgoland
441
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./
P~ytoplankton
10-
/
9..-'~E N w
I
s
:2.
o
30
s
,./j"
I
/
/
9
,~I 20E
"~
...~
/
.~,.....~_.'J
._.
3-
3
.
2
1.5-
15
e,
s
0.~
s
0.7
."
r , ' 19' 6 5 '
'
'
' 1970
' '
' '
' 1975
' ....
' '
1980
'
'1985
' '
Pig. 2. Helgoland Roads 1962-1985: smoothed curves calculated from half-year means, S = summer
(April to September), W = winter (October to March). Phosphate tPw) and inorganic nitrogen (~:Nw
= NH 4 + NO2 + NO3} near the surface are given in [~tg at. dm-3], phytoplankton (C) as [~tg. dm-3].
The curve Diatomss is not significant; the ordinate has a logarithmic scale
Max Gillbricht
442
-120
1200
100-
/~
80
80
I
I
I
1980- 85
I
I
/
60"
60
~..1~
I
o
\ 9 //\\
/2;,~.
~.\
II
40'
I
1962-67
I
, /
I
20"
100
Diotom s
,~_o/
z,0
\\
o\ . \
I /
!
E
13
6~
\
~|
0"
o_ ..~__=~
JQn. Feb. IMorch Ap~
. . MQy
. .June July
0
O ~
i
100"
Flclgellotes
A~ . Sept,
' c~t.
Nov. Dec.
/~
100
I
\
I
80
/
60 84
/
80
I
1980- 85
\
/
I
\
60
o
\
/
z.0
jO
I
O~
20-
~oO ~
Jan
t.0
\
/
.-..o
20
1962-67
~O'"
/
•/O_____O /
I
\
~0
///
0
20
i
e
i
l
0"-- 9
i
i
" 0 ~ 0
i
,
Feb Morch Apf: MQy June July Aug
l
i
Sept Oct
I
i
Nov Dec
Fig. 3. Helgoland Roads: monthly m e a n s over six years (1962-67 and 1980-85) of diatoms a n d
flagellates
Phytoplankton and nutrients near Helgoland
9
443
O
200-
\
/1",
.
,"
-1.2
-1.0
150
Phytoplonkton s
A
-08 "13
~3
"%/
~I00~
_o
/..o-4
&
-0.6 B,
-0.4
50
o/
~
-0.2
Phytoplc~nkton - Ps
/\
0""
0-
"0#
0 ~
--O,.,.~ O
~
0
1976'
'1978 '
' 1980'
'1982'
'198&'
'1986'
Fig. 4. Helgoland Roads 1976-1986: yearly means of phosphate (P) in summer -- S (April to
September) and in winter = W (November to February), and of phytoplankton in summer, also given
as its P content (Phytoplankton-Ps)
p e r i o d i c (tides), a p e r i o d i c (meteorology), a n d r e s i d u a l currents. It is t h e r e f o r e difficult to
find out w h a t b i o l o g i c a l e v e n t s t a k e p l a c e in a distinct w a t e r m a s s (Gillbricht, 1983). To
o v e r c o m e this p r o b l e m as best o n e can, t h e results of t h e m e a s u r e m e n t s are s m o o t h e d as
f u n c t i o n s of t i m e a n d salinity. T h i s m e t h o d e l i m i n a t e s r e a s o n a b I y t h e e f f e c t of f r e s h w a t e r
i n p u t b y rivers, b u t it is not a g o o d i n d i c a t o r of t h e m e a n coastal d i s t a n c e (different w a t e r
d e p t h a n d v e r t i c a l t u r b u l e n c e ) , b e c a u s e s a h n i t y h a s an a n n u a l c y c l e that differs f r o m y e a r
to year. T h e r e s i d u a l current p a r a l l e l to t h e c o a s t f r o m W to E n e e d s the a s s u m p t i o n of t h e
s a m e c o n d i t i o n s o v e r a l o n g d i s t a n c e to i n t e r p r e t our o b s e r v a t i o n s as t r u e d e v e l o p m e n t s
in t i m e (Gillbricht, 1983). T h e f o l l o w i n g is a n o p t i m i z a t i o n of t h e e q u a t i o n ( T -- time, S =
salinity):
y = a+b.TC.S d +e.Tf.Sg+...
444
Max Gillbricht
t rc]
a
9
14-
t
32
12.
/
10-
31
864-
30
~176
Jan.'
'March
'May'
'July'
'Sept'.
'Nov.'
o
3000-
40
~
30-
-
}o,:
? 2000
"13
-]
.1.5
o
cJ
J
J
o"
.1.0 ~:
1000
0
9
, "-~-.
, .......
Jan.
March
' Moy'
9
' July'
'Sept'.
"
, " ,'"
Nov
, 0
0
Fig. 5. H e i g o l a n d Roads 1981: m o n t h l y m e a n values, a: t e m p e r a t u r e (t) a n d safinitu (S), b: p h o s p h a t e
(P); inorganic nitrogen (N) and flagellates. Scales are equal with respect to the mean composition of
organisms
It is a polynomial function with (practically} error-free values on the right h a n d side as
required by theory (Gillbricht, 1974). The solution of this e q u a t i o n in the case of k n o w n
exponents b y m i n i m i z i n g the sum of squares {observation - calculation) is w e l l - k n o w n
a n d not problematic. The optimization of the exponents must be done by approximation,
a n d this is a difficult procedure. This formalistic m e t h o d evades to a great e x t e n t pressing
nature into a preconception. So as m a n y terms (up to six) with T a n d S on the right h a n d
side are used as are "significant" in a statistical sense (0~< 0.05}, b e a r i n g in m i n d that this
value is dubious u n d e r these conditions (no i n d e p e n d e n c e of the terms etc.}.
Phytoplankton a n d nutrients n e a r Helgoland
33-
445
.~9 :."
-33
S
:
32-
".
.
"..
9
.l.
.I
.
9
9
S
:.
#~'r
.#
9
9
~
-32
9
4,
.31
9
..
..'..'\
9
30- , "."
29 ~
"'..
".:
~
." " :
,.
~
: ,
.
"
9
~
.
,
.30
9
"
" "\
29
-28
28
9
e,
27
-27
'"Jan. '
'March'
' May ' 1981
i
July
i
i
Sept.'
J
No~
J
Fig. 6. H e l g o l a n d Roads 1981: s a l i n i t y o b s e r v a t i o n s a n d c a l c u l a t e d m e a n s a l i n i t y as a f u n c t i o n o f
time. N = n u m b e r of o b s e r v a t i o n s = 244, R = m u l l i p l e c o r r e l a t i o n c o e f f i c i e n t = 0.410, ~ = p r o b a b l e
error< 0,001
In this respect, there is a special problem with the p h y t o p l a n k t o n - diatoms a n d the
rest (mainly flagellates [peridinians]). The quantifies of these organisms vary e x p o n e n tiaUy in time a n d space (salinity). They can only be described r e a s o n a b l y in a logarithmic
system which systematically gives smaller results t h a n a n u m e r i c a l one, a n d they must be
corrected before b e i n g compared with the n u t r i e n t situations. This is done by c o m p u t i n g
the logarithmic s t a n d a r d deviations of the observations from the calculated values in
order to find the differences b e t w e e n the logarithmical a n d the respective numerical
means. For this purpose, the two logarithmic plus a n d m i n u s deviations are transformed
into numerical values, a n d their m e a n may be something like the difference b e t w e e n the
logarithmic a n d the numerical m e a n s that are to be found out. For diatoms and flagellates, a factor of about two has b e e n determined, which has b e e n used for the quantitative considerations. This method presents problems, but it m a y b e better t h a n directly
comparing results found out b y numerical a n d logarithmical smoothing. This fact is not
exclusively disadvantageous. It indicates that the exponential gradients built up by
biological processes are not destroyed to a n y extent b y mixing, the e l e m e n t a r y prerequisite for calculating true courses in a water body. Furthermore, the n u t r i e n t contents are
not disturbed too much by the year cycles of the concentrations in the inflowing river
water (ARGE Elba, 1982).
RESULTS
The p h y t o p l a n k t o n stock (especially the flagellates) a n d t h e concentrations of inorganic nutrients in winter time (P and N ) i n c r e a s e d continuously n e a r H e l g o l a n d from
Max Gillbricht
446
50"
I
/
I
/
%%
i I
I
%%
-3
iI
l
40"
I 9
I
, ,
I s
I I
E 30"
"%
I
". ~
\
%,
\
~
~
%
%
~
32
I
Si
~
~n
%%
,| \ iI - ~ ~
i/
%
%
\
/
P
28
x
, ."
/
l
%%
%,,
~\
,,#,
I Il
N
I
,
I
I
I
II
~
,,
iI
I
9
%%
,,
%'%
Il
,,
,
-2.5
9
/
I
ii
-2
,
,' /
iI
iI
,'
,,
-1.5
b5 20z"
~'~28
10-
3
-. .-
.f30-._~.
/*
~32-'~.~'~.'~
x, \
"--,2_,,J~
""
"" "<':-~
"'~'.-~..
/ " " - "
--'--
0=
Jan. '
,'
,""
"~"~
'March'
' May
,
i
1981
I
,
i
,
Sept.
. / " !~-,Jz
...<'~2/
. .-~...~._~:..
-- ....
July
"~o/
.co30 -0.5
_~T--~- ~-'"
,
,
Nov.
i
Fig. 7. Helgoland Roads 1981: smoothed annual cycles of the inorganically bound components of the
nutrients P, N, and Si for the salinities 28, 30, and 32. Phosphate: N = 244, R = 0.866, ~<<0.001;
nitrogen: N = 243, R = 0.909, 0c<<0.001; silicate: N = 244, R = 0.935, o~<<0.001
1962 to 1985. The respective smoothed curves are given in Figure 2. They are calculated
from half-year m e a n s a n d presented (not calculated) in a logarithmic scale to find out
w h e t h e r or not c h a n g e s in the quantities of different c o m p o n e n t s are more or less
proportional to one another. We must b e a r in mind, however, that good correlations m a y
be purely formalistic a n d may have n o t h i n g to do with causality. This is especially the fact
in the case of time series b e c a u s e a lot of c o m p o n e n t s m a y c h a n g e their q u a n t i t i e s in one
direction only, i n d e p e n d e n t of each other over a long time span. This is a difficult
problem from a theoretical point of view; in the main, n e g a t i v e results are rehable. We
must keep this fact in m i n d w h e n investigating excIusively the interrelations b e t w e e n
nutrients a n d phytoplankton.
The m e a s u r e m e n t s prove that phosphate in winter time (Pw), the sum of the inorganic
nitrogen c o m p o u n d s in winter time (ENw), the p h y t o p l a n k t o n (C) a n d its c o m p o n e n t s
without diatomsw demonstrate something like parallelism (the p h y t o p l a n k t o n stock
increases four times a n d the concentrations of P a n d N twice) in so far as not to b e
completely contradictory to the assumption of a causahty. But why does the flagellate
(peridinian) stock, not fastidious with respect to nutrients (Gillbricht, 1983), increase not
only in s u m m e r time b u t also in winter?
To gain better insight into the p h y t o p l a n k t o n dynamics over the last two decades,
monthly m e a n s t a k e n over periods of six years are calculated. The four a n n u a l cycles
Phytoplankton and nutrients near H e l g o l a n d
i~-/
/
\
447
-1.2
\
\
28
12.
\
/~\
~o, /
j
/
/
/
Si
\
N
E 8
"8
k
30
32
\
\
O
Diatoms
/28
x \\ \
//
\
/
\ \ \
/
J
32
~2-
/
/
~\
130
i
\\\
\ \ \\ x x
\xx\\~.,
/
-,
//32
3
.0.6 c~
3.
B
-0.4
/ / / // /
\
"x
~.~
i
-0.8
/
/
//
//
-0.2
0
0
' Jan. '
'March'
' May '
' July
'
' Sept.'
' Nov. '
'
1981
Fig. 8. H e l g o l a n d Roads 1981: s m o o t h e d a n n u a ! cycles of silicate an(] d i a t o m s f o r the sa[init/es 28, 30,
and 32
obtained in this w a y d e m o n s t r a t e a clear d e v e l o p m e n t during this time, as can b e s e e n for
the first an d the last cycles in Figure 3. Whilst flagellates s h o w a continuous increase
during all seasons with a mass d e v e l o p m e n t of Ceratium in s u m m e r 1981, the situation is
m u c h more c o m p l i c a t e d with respect to diatoms. It is a w e l l - k n o w n fact that the latter
organisms p r o d u c e a p r o n o u n c e d spring bloom. This was not o b s e r v e d during most of the
investigation period. A m a x i m u m c o u l d be s e e n only in s u m m e r time, an d the usual
situation was (re-?)established during the last years with a spring bloom (Coscinodiscus)
in 1985. T h e s e observations indicate that natural fluctuations in the sea m a y h a v e v e r y
different time scales that cannot b e completely d e t e c t e d during the life span of m a n or the
duration of a p e r m a n e n t station. To find out m o r e on this subject w e turn to the literature
w h e r e w e find information from the 19 th century on p h y t o p l a n k t o n blooms that w e r e just
as impressive as those o b s e r v e d today (cited by Steuer, 1910, and v an B e n n e k o m et al,,
1975).
The fluctuations of p h o s p h a t e in s u m m e r and winter, a n d of p h y t o p l a n k t o n in
s u m m e r , also g i v e n as incorporated P, d e m o n s t r a t e some interesting characteristics
(Fig. 4). All the values scatter in a r a n g e like the i n c r e a s e s during the l o n g - t e r m
investigation (Fig. 2). Only a small proportion of t h e p h o s p h a t e m e a s u r e d in w i n t e r can be
o b s e r v e d in s u m m e r time in th e plankton. Nev er t h el ess, a r e a s o n a b l e quantity disappears from the w a t e r as usual, b u t the r e m a i n i n g amount indicates that t h er e is always
e n o u g h i n o rg an ic P available as not to limit p h y t o p l an k t o n d e v e l o p m e n t , a s already
Max Gillbricht
448
-300
300,
-100
100t,,.
_ / ,/~ / - /
30-
iI ~ 2 8 ~
10E
/,"
3
(D
1
\N'\
\\
30
\x. ~
"--
\
,,'/.
l\
\
,,\ x
/
\ i
/
Flagellates
/
"~ \ \ ,
Diatoms
\
0.3
/
J/,.//
/
Jan.'
~3
3)
\\ \
,z7
\ ~//
\\
\\ \\
/~z /
0.3
0.03
,~._\
~---,.~
///
I\
0.1-
/ /
~
10 o
!'
I/
"I3
32
'0.1
' March'
' May
July
' Sept. '
No'z '
0.03
1981
Fig. 9. Helgoland Roads 198 t: smoothed annual cycles of diatoms and flagellates for the salinities 28,
30, and 32. Values as C (corrected) in a log scale; log (diatoms): N = 243, R = 0.773, ~<<0.001; log
(flagellates): N = 243, R = 0.815, o~<<0.001
proved for P a n d N in 1981 (Gillbricht, 1983). A shght parallehsm b e t w e e n n u t r i e n t
content a n d following p h y t o p l a n k t o n stock may be seen, b u t we must b e a r i n m i n d that P
is also a n indicator of the hydrographic situation (salinity).
To investigate problems of this sort in detail it would b e better to a b a n d o n long-term
observations a n d to work with one single a n n u a l cycle only. For this purpose, the
interesting year 1981 was selected with a strong Ceratium bloom that has a l r e a d y b e e n
e x a m i n e d exhaustively (Gillbricht, 1983) (Fig. 5). The scales in Figure 5b are e q u a l with
respect to the (mean) composition of organisms, a n d the monthly m e a n s indicate the
already k n o w n fact that the n u t r i e n t decrease in spring is high c o m p a r e d with the
increase of the m a i n p h y t o p l a n k t o n c o m p o n e n t (flagellates) in that year. No n u t r i e n t
limitation can be seen, a n d the r e g e n e r a t i o n of phosphate, difficult to u n d e r s t a n d
r e g a r d i n g the short period of bloom in s u m m e r (Gillbricht, 1983) a n d practically not
h i n d e r e d by the Ceratium bloom, starts earlier in the year t h a n the r e l e a s i n g of n i t r o g e n
components. We must a s s u m e therefore that the turnover of nutrients is e n o r m o u s (shortcut r e g e n e r a t i o n a n d consumption) a n d cannot be seen by m e a n s of our m e a s u r e m e n t s .
The better r e g e n e r a t i o n of P compared with N m a y be the m a i n reason for the smaller
amplitude of P over the year.
All these reflections are a little doubtful for the present, as can be s e e n i n Figure 5a.
Whilst the t e m p e r a t u r e curve is normal, the salinity demonstrates irregularities e v e n
using m o n t h l y means. This indicates considerable water e x c h a n g e processes, which may
Phytoplankton a n d nutrients n e a r H e l g o l a n d
449
/ \
400-
/
~-~1/
\
~T2
\\
[9 C'm4"d"]
301
-F-"
E
200
O
-1
,o9e,,oes
//
oio,o
/
///
iJ
\
t
\
.3o'"-\
\
,o
100
Jan. '
'March'
'Moy
'
' July ~
\
", 4 k
' Sept.'
'Nov.
.o,
'
1981
Fig. 10. Helgoland Roads 1981: smoothed annual cycles of diatoms and flagellates (stock and daily
production of an euphotic zone of 5 m depth [g C.m-2.d-1]) for the salinities 28, 30, and 32.
Numerical scales
m a s k the p u r e l y biological run, the object of this investigation. Before starting with
smoothing p r o c e d u r e s as discussed a l r e a d y (p. 443), it is of interest to k n o w s o m e t h i n g
about the a b n o r m a l l y low salinity (Gillbricht, 1983) and its fluctuations during the year
(Fig. 6). The single observations scatter in a w i d e range, a n d the impression is therefore
that of an a p p r o x i m a t e and v e r y abstract m e a n s curve with a dubious course in the
b o u n d a r y regions. It can b e s e e n that a salinity r a n g e of 28 to 32 can b e o b s e r v e d
throughout the year. It is therefore possible to describe different a n n u a l cycles of
nutrients a n d p h y t o p l a n k t o n within this interval of salinity without e x t r a p o l a t i n g the
observations. The respective curves are given in the following figures.
U n d e r these conditions, the nutrients P, especially N, and, with limitations, Si show
almost the s a m e a m p h t u d e s i n d e p e n d e n t of salinity in the o b s e r v e d r a n g e (Fig. 7). T h e s e
findings are contradictory to the w o r l d - w i d e e x p e r i e n c e of l a r g e r p l a n k t o n stocks in
coastal regions (e.g. Gillbricht 1959) u n d e r the a s s u m p t i o n that there is a proportionality
b e t w e e n nutrients a n d phytoplankton. In this investigation, the scales are e q u a l with
r e s p e c t to the m e a n composition of organisms, thus indicating that there is n o r m a l l y a
surplus of N b u t with a p h a s e difference in r e m i n e r a h z a t i o n c h a n g e d into a P surplus at
least in the case of h i g h e r sahnities. For diatoms, the low silicate concentrations at
sahnities b e l o w 30 may b e smaller than Si i n c o r p o r a t e d in frustules (Fig. 8), in this w a y
indicating the possibility of growth limitation b y this element.
The s m o o t h e d curves of diatoms a n d flagellates are c a l c u l a t e d in a log s y s t e m a n d
M a x Gillbricht
450
',k
-300
140"
1981
120"
\\
\\
,
100"
\
\
\
200
,
E 80-
\
\
\
" \ Flagellates
ProdUction
\ \ \
U3
\
0
Diatoms
60"
3.,4,
\
\
\
,\
.-<
,._f
\\
\,,
100
\\
z,O-
20
i
28 s
i
2'9
'
3'0
i
3'1
32
Fig. 11. Helgoland Roads 1981: annual means of diatoms and flagellates and the resulting production for an euphotic zone of a depth of 5 m [g C .m-2.y -1] as functions of salinity and corrected with
respect to the disturbances at the beginning of the year
afterwards corrected (Fig. 9; p. 445). T h e y d e m o n st r at e only a slight c o n n e c t i o n with the
nutrients (Fig. 7). It can be s e e n that the curves in the b o u n d a r y regions, e s p e c i a l l y at the
b e g i n n i n g of the year, are s o m e h o w disturbed. Th e log scale elucidates the relative
fluctuations in time indicating that the p r o n o u n c e d bloom of peridinians has n o t h i n g to
do with a high multiplication rate as the true indicator of good growth conditions, whilst
the diatoms are m u c h more active in this r e s p ect mainly at the b e g i n n i n g of t h e year. T h e
absolute differences in the stocks d u r i n g the annual cycle with respect to salinities and
the differences b e t w e e n diatoms a n d flagellates are d e m o n s t r a t e d b e t t e r in a n u m e r i c a l
scale (Fig. 10). T h e r e are no major differences b e t w e e n the m a x i m a of diatoms an d
flagellates, whilst in both diatoms a n d flagellates the d e c r e a s e s of the m a x i m a of t h e
Phytoplankton a n d nutrients near Helgoland
45I
8 -05
600-
N P
Si -0.4
6-
500-
/*00"
,A-300"
~200-
-Q3 "C
--
i
i
i
i
to
0.2
I
.<..
..-..,
-,
100-
i
.-L
.-
,\.
.....
\
Jan. '
' March'
\
--x..
9
'July
0.1
-_
"
' Moy '
2-
\
~.=._~.-r'...~
0"
o
5.
B
'
' S~-nt '
....
' Nov.'
....
,0-
"0
1981
Fig. 12. Helgoland Roads 1981: smoothed annual cycles of the sums of (diatoms + flagellates) and of
the quantities of P, N, and Si (diatoms only) incorporated in the organisms for the sahnities 28, 30,
and 32 and for the mean salinity curve given in Figure 6
stocks with increasing sahnity are impressive. Daily productions u n d e r the a s s u m p t i o n of
a depth of the euphotic zone of five metres are given; they indicate reasonable values for
such a region. These results can be used to calculate the m e a n a n n u a l quantities of
diatoms a n d flagellates as functions of sahnity (after correction with respect to disturbances at the b e g i n n i n g of the year) (Fig. 11). Prom these values, the a n n u a l production can
b e deduced. This method does not improve our k n o w l e d g e on the m e a n yield in this
region because it is not exact enough. The year investigated was an atypical one with
respect to p l a n k t o n stock, a n d the depth r a n g e suited for assimilation should increase
with increasing salinity (reduced turbidity), thus simulating an excessive production
gradient but giving a good impression of the connection b e t w e e n p h y t o p l a n k t o n a n d
coastal distance. Nevertheless, the results are comparable with other observations
(Steele, 1958; Postma, 1973; CadSe, 1986).
Superposing the curves of diatoms a n d peridinians gives b e l l s h a p e d structures with
a special description of the p l a n k t o n "development" n e a r H e l g o l a n d observed b y m e a n s
of the salinity fluctuations (S) (Fig. 12), The scales of C and the incorporated quantifies of
P, N, a n d Si (diatoms only) may b e c o m p a r e d with the respective i n o r g a n i c concentrations in Figures 7 a n d 8.
As found out already, the relative increases and decreases of p h y t o p l a n k t o n stock
per day, normally more p r o n o u n c e d in the case of lower salinities (first derivative}, are
small compared with observations d u r i n g true spring blooms (p. 440), especially with
452
Max Gillbricht
f
_
~ v
-8
%
%
6-
-6
30
4-
-4
/
_
-2
0-
-0
-2-
--2
Diatoms
-A-
-6-
.-4
III
'Jan. '
.-5
'March'
' M a y ~1981 ' J u l y '
'Sept.'
' Nov.'
Fig. 13. Helgoland Roads 1981: smoothed annual cycles of the daily increases or decreases (-) in %
of the stocks of diatoms and flagellates for the salinities 28, 30, and 32
respect to flagellates (Fig. 13). This m e a n s that the bloom has n o t h i n g to do with a n
explosion of the population but is the c o n s e q u e n c e of a n e x t e n d e d growth period
(diatoms) a n d a relatively large initial stock (flagellates) with a slow-speed d e c h n e
afterwards. The respective absolute alterations (Fig. 14) are highest a short time before
a n d after the m a x i m a of the stocks, with highly differing values for different salinities,
a n d with similar findings for diatoms a n d flagellates with a time lag of two months in
between.
It is of some interest to investigate the turnover rates of the system in more detail b y
comparing the c h a n g e s per day (P, N, phytoplankton), a n d the calculated production, as
carried out for S = 28 in Figure 15a. The p h y t o p l a n k t o n values are g i v e n inversely
b e c a u s e production or a n increase in p h y t o p l a n k t o n stock m e a n s a decrease in nutrients
in water. It can be seen that the fluctuations in the inorganic nutrient stocks have almost
n o t h i n g to do with the level a n d time of production. This fact indicates the existence of a
highly active remineralization system besides a direct u p t a k e of n u t r i e n t c o m p o u n d s not
d e t e r m i n e d b y our methods. This comphcated situation is the reason w h y the n u t r i e n t
fluctuations observed (Fig. 7} are fairly i n d e p e n d e n t of p h y t o p l a n k t o n activity (Fig. 10},
a n d are, as a consequence, relatively higher in the case of small p r o d u c t i o n (S = 32;
Fig. 15b). It can b e seen that a higher u p t a k e rate of nutrients by higher p h y t o p l a n k t o n
Phytoplankton a n d nutrients n e a r Helgoland
453
+6-
-+5
+4_
-§
///I \\,
+2-
-+2
"T
"O
013
C~
',-
OI--
-2
"ol-o -2-
//
Jan.
'
' March
' May
' July
' SepL'
' Nov.'
.-4
'
1981
Fig. 14. Helgoland Roads 1981: smoothed annual cycles of the absolute daily increase (+) or
decrease (-) of the stocks of diatoms and flagellates for the sahnities 28, 30, and 32
production is c o m p e n s a t e d for by better r e m i n e r a h z a t i o n after an initial period, especially of P, the concentration of which increases a g a i n at the time of maximal p l a n k t o n
production. The same t e n d e n c y is shown by nitrogen, glvmg a phase difference b e t w e e n
the different curves in such a way that P a n d N are always a h e a d of organic production
The d e v e l o p m e n t in this respect may be a function of the absolute concentration of
organic nutrients, especially for the time span n e e d e d to produce optimal r e m i n e r a h z a tion.
These facts r e g a r d i n g P a n d N can be explained. However, it is not so easy to
u n d e r s t a n d the situation with respect to inorganic Si (organic Si c o m p o u n d s are rare in
nature; Jorgensen, 1955b), which is incorporated practically only in the frustules of
diatoms. In Figure 16 this is demonstrated, a n d even a more p r o n o u n c e d a n d perman e n t l y effective remineralization for S = 28 a n d S = 32 than d e m o n s t r a t e d earher for
other nutrients i n c l u d i n g phase shifting etc. (p. 462).
Pigure 17 summarizes the behaviour of diatoms a n d flagellates during the v e g e t a t i o n
period in 1981. The log scale of p h y t o p l a n k t o n quantities provides a good opportunity to
compare the relative increases of stocks, given in differences b e t w e e n lowest values i n
winter for diatoms (DL) a n d flagellates {FL), a n d the respective m a x i m a l values m s u m m e r
(DH resp. PH). These differences are m u c h smaller for the latter than for the former a n d
Max Gillbricht
454
-§
dN
--0.2
-+2 I
§
-§
O-
"(3
,?,"
i
.-0.1
~k
dC
d"T-
d._EP
dT
-o - ~---__.~.~.
~ ._-=- .~__ _-__-..-_~-~--~. ~- -':-~ ---:--- :---- -~ ~ - ~ _
.-1
N
~
0
,-,a.
--41c~
/
+0.1
E
-o _ 0.1-
r~
r
O
5
En
:3_
.§
ff
C)
%1~ o.2-
9
.*0.3
&
&
-0.3-
+0.4 ~
0.5
-0.4
' Jan. '
'March'
' May
July '
' Sept.'
' Nov.'
1981
d__P dN
dT
*0.02-
aTI I
.§
,\
-+o21
+0.01-
0 o
,
/t
\
9.
I
.+o.1 ;
,
\
E
"?. -0.01
dP
,dT
,
\
.--'--2"...
/
",,
/
'
"
-10
'\
,,
\
0
.o
,
"O
9"
-2O
/'\
-o.1 t
;
,
_
,,
', \,
",\
/
,"
,
,
."
~j
t~
",,-/~,
,
/
C)
r~
",|
,10
5
(m
--1
-0.0 2
--o3" I;/
\\'---J'~
~
3
.
&
/t
*20
-"
- 0.03-
I
Production
/
as'.
!b
+30
--0.6
' Jan. '
'March'
' May '
~ July '
' Sept.'
' Nov.'
'
1981
Fig. 15. Helgoland Roads 1981: a: S = 28, b: S = 32. Daily changes of p h o s p h a t e (dP/dT),
inorganically b o u n d N (dN/dT), and phytoplankton stock (dC/dT) as well as daily production of
phytoplankton with the carbon scale inverse to the others
Phytoplankton and nutrients near Helgoland
455
dC
-0
\
dS~i
T
X
/
\
/
\
-I
/
\
/
\
,'7
/
\
\
\
-o[-o_3
/
\
\
-4
a
\
'March'
S
17
' May"'
/
C-
1
5
/
I
200
o
o_
3
/
Production
of ~,diatoms
'
"0
/
\
E -2
"o
.L.
O
c~
100 -41o
/
\
Jan.
/
f
/
' July
/
o_
i
300
' Sept.'
' Nov. '
1981
§
-10
r~o_
--~lc>
\
f J
\
_'2-"
J
8
r~
f
dC
I
0
o
O
Z..
0
C~
o
-o.1+10
~%
--
"C
Production
&
3
z.
+20
-o.4
Jan.
'N•rch'
'Mc W
' July
'Sept_'
' Nov.
1981
F i g 1 6 H e l g o l a n d R o a d s 198t: a: S = 28, b: S = 3 2 O b s e r v e d daily c h a n g e s of Silicate (dSi/dT) a n d
d i a t o m s t o c k (dC/dT) as well a s daily p r o d u c t i o n of d i a t o m s with t h e c a r b o n s c a l e i n v e r s e to t h e o t h e r
456
Max Gillbricht
days
%
-'~--DH.~..'~_'"
",,..
190
"'-
"-...
4
"~....~...._.
~'"
FL
-
X
""0.%
ISO
"\
"'-.
"Ddays..~
170
10~
"'-
1O:
"-
..""
/
9
-"~ 10~
-
"5"104
\
............
F ~/~...
"-...
..J
~8 s
a
\
.---" --...
1 .150
-540~
\
"'-
. I0 t
.
2'9
3'0
~'1
5.10'
.
5.10"2
3'~
Fig, 17. Helgoland Roads 1981: phytop]ankton development as a function of salinity; D = diatoms; F
= flagellates; L = lowest calculated value; H =-highest calculated value (log scale}; days = number
of days from minimum to maximum; % = mean increase of phytoplankton stock (D or F} from day to
day during this time
decrease with increasing salinity varying b e t w e e n the following extreme values: for
diatoms and S = 28 the mulfiphcation rate during s u m m e r is 10 000, a n d for flagellates
a n d S = 32 less than two. Between 155 a n d 200 days or roughly half a year were
necessary to produce these highly different results, which m e a n s a long p e r m a n e n t
growth season for phytoplankton. This period is generally slightly longer for flagellates
than for diatoms (Fa~ys resp. Dd~ys), a n d the resulting m e a n rate of increase per day r a n g e d
b e t w e e n 0.5 a n d 5.5 % (F % resp. D %).
The dynamics of p h y t o p l a n k t o n m a y be discussed in more detail t a k i n g the diatoms
as subject, which are better k n o w n in this respect than flagellates, a n d starting with the
following definitions (Table 3):
I = factor for daily increase of stock = 1 in the case of a stable p o p u l a t i o n (an
i m p r o b a b l e situation in reahty); increase in % = ( I - 1}. 100.
Fp = factor for daily gross production; respiration over 24 h = 1/3.Fp; therefore net
production = 2/3.Fp; Fp = 1 m e a n s dally production = quantity of stock (assumed
to be the fact in the case of I = 1.
Fl = factor for daffy losses {except respiration) = 0.6 {or the loss itself = 40 %) in the
case of I = 1 and Fp = 1; the simple calculated dally increase (D = diatom stock):
D . I = D-(I +~/a'Fv)'F:.
Results obtained in this way c a n n o t b e very correct for two reasons. Firstly, a single
year is only one single year a n d does not give a description of a m e a n situation, which is
Phytoplankton and nutrients near Helgoland
457
Table 3. Dynamics of diatoms
Daffy
increase
Days
Daffy gross"
prod. 1
Daily gross
prod. 2
Daily
loss 1
Daffy
loss 2
(a)
(b)
(c)
(d)
(e)
(f)
25
20
15
10
5
1
31
38
49
72
142
694
162
150
t38
125
112
102
117
114
110
107
104
101
25
28
3t
34
37
39
30
32
34
36
37
40
0
~
100
100
40
40
1
688
135
98
88
99
96
41
43
40
42
66
43
31
24
75
62
50
38
93
89
85
81
46
49
52
55
44
47
49
51
(%)
+
+
+
+
+
+
-
5
- 10
- 15
- 20
- 25
(%)
(%)
(%)
(%)
(a) Daffy increase (+) or decrease (-) of diatom stock given in %.
(b) N u m b e r of days n e e d e d to get a 103-fold stock increase or a 10-~-fold stock decrease.
(c) Daffy gross production of stock (Fp) given in % u n d e r the assumption that daily loss is the
same as for increase 0 % {40 % or factor = FI = 0,6); respiration = 'A gross production or net
production = 2/3 gross production,
(d) Daily gross production of stock given in % u n d e r the assumption that the relation FI/Fp is
constantly 0.6.
(e) Daily loss of (stock + net production) in % u n d e r the assumption that daily production is
the same as for increase 0 % (100% or Fp = 1).
(f) Daffy loss of stock given in % u n d e r the assumption in (d).
a n a b s t r a c t i o n l o o s e l y b o u n d to r e a l o b s e r v a t i o n s i n n a t u r e , b e c a u s e t h e d e v e l o p m e n t is
d i v e r s e i n e v e r y a n n u a l cycle (Fig. 4). S e c o n d l y , m e a s u r e m e n t s a r e n o t t a k e n o f t e n
e n o u g h a n d a r e n o t c o m p l e x e n o u g h to s a t i s f y all t h e s t a t i s t i c a l d e m a n d s a n d all t h e
q u e s t i o n s a s k e d to clarify s u c h a m u l t i d i m e n s i o n a l s y s t e m . Is it r e a l l y p o s s i b l e to s o l v e
s u c h a p r o b l e m to a s a t i s f a c t o r y e x t e n t e v e n u n d e r f a v o u r a b l e c o n d i t i o n s ? T h e c o m p l i c a t e d p r o c e d u r e s u s e d i n t h i s i n v e s t i g a t i o n a r e t h e r e f o r e s u p p l e m e n t e d , for p r a c t i c a l
r e a s o n s , b y a s s u m p t i o n s a n d b y d a t a f r o m l i t e r a t u r e . T h e r e s u l t s o b t a i n e d i n t h i s w a y will
b e s a t i s f a c t o r y w i t h r e s p e c t to t r e n d s , r a n g e s , r e l a t i o n s , a n d so on, b u t t h e a b s o l u t e v a l u e s
s h o u l d b e d i s c u s s e d w i t h c a u t i o n . V a r i a t i o n s i n t h e d a i l y i n c r e a s e s of p h y t o p l a n k t o n
s t o c k s ( T a b l e 3, C o l u m n a) h a v e b e e n i n v e s t i g a t e d o n a l a r g e r s c a l e t h a n o b s e r v e d h e r e ,
but our observations do not contrast completely with observations made elsewhere
( + 1 5 % to - 2 5 %) d u r i n g s p r i n g b l o o m s . T h e t i m e s p a n n e e d e d to g e t a r e a s o n a b l e
i n c r e a s e or d e c r e a s e (factor: 1000) of a p l a n k t o n p o p u l a t i o n is i n t h e r a n g e of m o n t h s
under the conditions discussed here (Table 3, Column b). This means that a changed
growth situation cannot be seen within a very short time, as is sometimes assumed
(Gillbricht. 1983). But what does happen in the case of increase or decrease of a plankton
458
Max Gillbricht
stock by production a n d loss? A s s u m i n g that only one of these two factors c h a n g e s whilst
the other retains its value in the case of a zero increase (Fp -- 1 or F1 = 0.6, as g i v e n in
T a b l e 3, C o l u m n c a n d e), the increase (I) is m u c h more sensitive to c h a n g e s in losses t h a n
to c h a n g e s in production, which c a n be seen b y the greater relative differences of the
latter compared with the former to get the same effect u p o n I. The practical c o n s e q u e n c e
of this conclusion is that it is more realistic to state that phytoplankton is i n f l u e n c e d to a
greater extent b y different losses (turbulence, s i n k i n g velocity, zooplankton) t h a n b y
c h a n g i n g production. This result can b e affirmed b y a calculation u n d e r the a s s u m p t i o n
that F1/Fp = constant -- 0.6- this m e a n s variations of both components i n the same sense
(Table 3, C o l u m n d a n d f). Whilst the c h a n g e s in C o l u m n (d) are small c o m p a r e d with
C o l u m n (c), no such great differences are seen b e t w e e n Column (f) a n d (e). In this way,
the higher controlling influence of loss is indicated. The w e a k n e s s of this conclusion is
that it is d e p e n d e n t on the formula given above. This reduces the accuracy of our
findings. It is, for instance, not correct to calculate with a respiration r a t e of "zero" for
non-assimilating pliytoplankton as a s s u m e d before (p. 440). But i t is not only the
incomplete information of m a n that hinders the a c q u i r e m e n t of more precise results
a b o u t life in the sea. It is the sea itself, which c h a n g e s its state p e r m a n e n t l y i n m a n y
d y n a m i c processes thus continually i n f l u e n c i n g m a r i n e organisms. This i n t e n s e fluctuation of a n u n k n o w n n u m b e r of variable factors essential to life masks the significance of
every single factor. This situation hinders or impedes, at least, the discovery of the true
biological dynamics a n d its regulations in the sea. This fact has considerable theoretical
a n d practical consequences.
DISCUSSION
The sea is a multiple system with n u m e r o u s interPelations b e t w e e n different compon e n t s where the proportionality b e t w e e n a n y two c o m p o n e n t s may not only b e l i n e a r b u t
also e.g. logarithmic. The complex influence of m a n y components m a y p r o d u c e completely different effects; sometimes p l u s / m i n u s reactions may b e f o u n d (survival or
death), a n d so on. Biological processes are especially sensitive in this respect. Small
variations in the (exponential) growth rate of a population produce impressive differences
in stock; a n d experimental results in small vessels cannot be easily extrapolated to
describe the situation in the sea (Gillbricht, 1969}; b u t they can be used to g r a d u a l l y gain
n e w insight, w h e n the practical value of the results obtained are w e i g h e d u p realistically.
Simple formalistic statistics used in the investigations on nutrients a n d p h y t o p l a n k ton can give only a primitive picture of the situation, whereas n e g a t i v e findings are
m a i n l y valid for theoretical reasons. First of all, it is clear that neither P nor N could have
limited the p h y t o p l a n k t o n growth in 1981, b e c a u s e both of them were always available to
a sufficient extent (p. 447}. This is not the effect of a great nutrient stock, b u t of a highly
d y n a m i c a l process (Ketchum, 1947), as can be seen in Table 4. The m i n i m a l t u r n o v e r time
of P a n d N (inorganic nutrient c o n c e n t r a t i o n / p h y t o p l a n k t o n production} g i v e n in days is
long compared with observations in limnic systems {Taft et al., 1975}, a n d is observed
a b o u t the time of the p h y t o p l a n k t o n maximum.
Higher values give the m i n i m a l quotients linorganic n u t r i e n t concentration/first
derivative) observed earlier in the year:
Phytoplankton a n d nutrients n e a r H e l g o l a n d
459
Table 4. Helgoland Roads 1981: Minimal turnover time of nutrients
Salinity
Date
Phospha.te
P' = dP/dT
P/Production
Days
28
30
32
July
June
June
8
28
18
1.7
3.1
6.7
28
30
32
E (Inorganically
bound
N' = dN/dT
N/Production
July
3t
3.2
August
6
5.9
August
11
8.1
28
30
32
Silicate
Si' = dSi/dT
Si/Production
July
30
.44
August
7
1.2
August
11
5.6
- (P/P')
Date
May
May
May
Days
3
5
9
78
60
45
N)
June
June
July
- (N/N')
22
29
15
101
77
40
July
July
July
- (Si/Si')
4
13
15
59
63
68
(T = time, P' = dP/dT, N' = dN/dT, Si" = dSi/dT)
- ( P / P ' ) resp. - ( N / N ' ) resp. - ( S i / S i ' )
thus d em o n s t ra t in g the significance of p e r m a n e n t remineralization.
T h es e findings do not indicate any nutrient limitation in this region, with better
growth conditions for p h y t o p la n k to n n e a r e r the coast in the salinity r a n g e observed. This
latter result cannot be extrapolated to the shore; Elbrfichter (pers. comm.) could not
detect a Ceratium bloom close to t h e island Sylt. This comes as no surprise b e c a u s e great
turbidity of the w a t e r r e d u c e s the light intensity, thus hindering p h y t o p l a n k t o n growth,
especially in flat regions with strong (tidal) currents (Postma, 1973; Cadge, 1986). In
contrast, turbidity in the open sea is p r o d u c e d first of all by the plankton itself and - to a
h i g h e r d e g r e e - by its decomposition products (detritus) (Gillbricht. 1959). O n the other
hand, a mass d e v e l o p m e n t of Phaeocystis was o b s e r v e d at the s a m e time (Eberlein et al.,
1985) and in other years (B/itje & Michaelis, 1986) n e a r the coast a c c o m p a n i e d by
impressive sea foam caused by the production of substances that r e d u c e the surface
tension; such substances w e r e d e t e c t e d in smaller quantities by N a n s e n (1902) in Arctic
waters, and they coincide with phytoplankton n e a r H e l g o l a n d t h r o u g h o u t the y e a r
(Goedecke, 1956; G a s s m a n n & Gillbricht, 1982). Th e fact that this species exists only n e a r
the coast is not typical, as could be seen in the G r e e n l a n d area (Gillbricht, 1959), an d
spectacular p h y t o p l a n k t o n blooms n e a r the shore m ay occur from time to time anywhere.
Steuer (1910) r e p o r t e d on a mass d e v e l o p m e n t of b l u e - g r e e n al g ae in the Baltic in 1855
p r o d u c i n g walls of d e c a y i n g remains of these organisms on the b e a c h of the island
460
M a x Gfllbricht
Gofland. Life is too c o m p l e x t o - b e e x p l a i n e d b y simple formula; our k n o w l e d g e is
incomplete; not e v e r y situation has b e e n investigated. H o w e v e r , calculations do give the
i m p r e s s i o n t h a t e x t r e m e mass developments, n e a r the coast (low salinity) n e e d more
nutrients t h a n are available m e r e l y b y a d m i x t u r e of seawater.
F r e s h w a t e r is often poor in natural nutrients c o m p a r e d with s e a w a t e r (p. 436), with
the c o n s e q u e n c e that in mixing zones the possible m a x i m u m of p h y t o p l a n k t o n stock m a y
b e limited b y l a c k of nutrients (Sakshaug & Olsen, 1986). T h e situation is different off
highly p o p u l a t e d coasts (Kalle, 1953), as discussed h e r e with no exact k n o w l e d g e of the
"natural" n u t r i e n t content of freshwater. T h e r e is, however, sufficient information on
nutrients in the inflowing seawater, which can b e c a l c u l a t e d with = 0.75 ~g a t o m P.dm -3
resp. = 11 ~g a t o m N . d m -3 from the d a t a given b y Postma (1973), i n a g r e e m e n t with
J o h n s t o n & J o n e s (1965). T h e s e quantities in w a t e r of S = 35 are diluted in p r o p o r t i o n to
the a d m i x t u r e of freshwater. To a certain d e g r e e , however, t h e y are t r a n s f o r m e d into
organic c o m p o u n d s , to b e u s e d partly at least b y p h y t o p l a n k t o n a n d p a r t l y lost b y
s e d i m e n t a t i o n in the freshly i m p o r t e d s e a w a t e r n e a r the coast (counter-clockwise circulation). T h e s e losses in the old w a t e r of the C e n t r a l North S e a m a y b e g r e a t e r over a l o n g e r
time span. T h e y do not hinder, however, the b u i l d i n g - u p of r e a s o n a b l e b l o o m s of
coccolithophorids in this region, as o b s e r v e d e. g. b y the author in 1975. T h e o r g a n i c a l l y
b o u n d nutrients m a y exist in a form that cannot b e u s e d or can b e u s e d only with
difficulty b y p h o t o p l a n k t o n . T h e r e are certain loopholes in our k n o w l e d g e w i t h r e g a r d to
this; w e do not know, therefore, w h a t quantity of nutrients c o m i n g from the s e a to this
r e g i o n is r e a s o n a b l y available for p h y t o p l a n k t o n . The problem, nutrient a n d freshwater,
is also c o m p l i c a t e d to a certain d e g r e e . Total nutrient is not a conservative c o m p o n e n t in
a strict sense, a n d t h e r e are special complications: freshwater does not mix directly with
seawater, b u t m a y b e chemically transformed during its p a s s a g e t h r o u g h t h e turbidity
zones at the m o u t h s of the rivers (Postma & Kalle, 1955) a n d d u r i n g the crossing of the
i n t e r c o n n e c t e d W a d d e n s e a w a t e r b e t w e e n the chain of islands a n d the m a i n l a n d (Gillbricht, 1956).
The situation with r e s p e c t to p h o s p h o r u s a n d p h y t o p l a n k t o n is d e s c r i b e d in T a b l e 5.
For different salinities (Table 5, Column a) the n e c e s s a r y P content of f r e s h w a t e r is
calculated to p r o d u c e the m a x i m u m in w i n t e r (PO4 + p h y t o p l a n k t o n ) b y m i x i n g with a
nutrient-free s e a w a t e r (Column b). The results are in r e a s o n a b l e a g r e e m e n t with the
values c a l c u l a t e d from the d a t a given b y Postma (1973) of 18 ~g a t o m . d m -3, a n d b y ARGE
Elbe (1982) for 1981 of i 0 ~g a t o m . d m -3. The m a x i m a l P content of p h y t o p l a n k t o n (Table
5, Column c) is relatively small a n d can therefore b e c o m p a r e d with the P c o n c e n t r a t i o n
g a i n e d b y m i x i n g s e a w a t e r with a nutrient-free freshwater (as an unrealistic theoretical
supposition). C o l u m n (d) a s s u m e s for this mixture the P v a l u e of inflowing s e a w a t e r ; this
concentration m a y b e too high, b e c a u s e certain losses m u s t b e e x p e c t e d , a n d a small
surplus of P c o m p a r e d with the quantity i n c o r p o r a t e d in p h y t o p l a n k t o n m a y b e necessary. Therefore calculations of 2/3 (Column e) a n d I/2 (Column f) of m a x i m a l P concentration are given, thus, probably, including realistic quantities. It can b e s e e n that only
w a t e r of low salinity has P concentrations in t h e s e calculations that are too s m a l l for such
an e x t r e m e p h y t o p l a n k t o n bloom. This effect is s t r e n g t h e n e d b y the e x p o n e n t i a l i n c r e a s e
of p h y t o p l a n k t o n stock t o w a r d s the coast. This situation can b e h i n d e r e d n e a r e r the shore
b y l a c k of nutrients d u e to the nutrient-poor freshwater input. It m a y b e r e g u l a t e d in this
region b y h i g h turbidity n e a r t h e shore (p. 459), m a y b e with different reactions of
Phytoplankton and nutrients near Helgoland
461
Table 5. Halgoland Roads 1981: phosphorus [~g at 9
-3] in water and phytoplankton, calculated
mixing conditions under different assumptions, P, = phosphorus in seawater (S = 35); PF =
phosphorus in freshwater (S = 0); P = phosphorus in mixed water or phytoplankton
Salinity
S
Winter
Ps = 0
PF
Phytoplankton
P
PF = 0
Ps = 0.75
PF = 0
Ps = 0.5
PF = 0
Ps = 0.375
(a)
(b)
(c}
(d)
(e)
(f)
28
30
32
9
I1
16
0.53
0.20
0.05
0.60
0.64
0.69
0.40
0.30
0.43
0.46
0.32
0.34
Salinity
S
P in
Phytopl.
PF = 0
Ps
P in
Phytopl.
Ps = 0
PF
P in
Phytopl.
Ps = 0.375
PF
P in
Phytopl.
Ps = 0.5
PF
(a)
(g)
(h)
(i)
(k)
28
30
32
0.66
0.23
0.05
2.6
1.4
0,64
0.76
-
0.13
-
(b) PF necessary to produce the P concentration observed (phytoplankton + PO4, m e a n from
winter and autumn without the disturbance at the beginning of the year, Fig. 7) u n d e r the
assumption Ps = 0.
(c) Maximal phosphorus content of phytoplankton.
(d) Ps = 0.75, PF = 0, calculated P contents at different salinities by mixing.
(e) Ps = 0.5, PF = 0, calculated P contents at different salinitie&by mixing; concentration smaller
than maximal P content of phytoplankton given in bold print.
(f) Ps = 0.375, PF = 0, calculated P contents at different salinities by mixing: concentration
smaller than maximal P content of phytoplankton given in bold print.
(g) PF = 0, Ps = P content in seawater necessary to produce a concentration equal to P content in
maximal phytoplankton stock in mixed water.
(h) Ps = 0, PF = P content in freshwater necessary to produce a concentration equal to P content
in maximal phytoplankton stock in mixed water.
(i) Ps = 0.375, PF = P content in freshwater necessary to produce a concentration equal to P
content in maximal phytoplankton stock in mixed water.
(k) Ps = 0.5, PF = P content in freshwater necessary to produce a concentration equal to P
content in maximal phytoplankton stock in mixed water.
d i f f e r e n t s p e c i e s i n this r e s p e c t . T h e P c o n t e n t n e c e s s a r y to p r o d u c e m i x e d w a t e r w i t h a
c o n c e n t r a t i o n e q u a l to t h e m a x i m a l p h y t o p l a n k t o n stock, u n d e r t h e c o n d i t i o n t h a t
s e a w a t e r is m i x e d w i t h n u t r i e n t - f r e e f r e s h w a t e r or v i c e v e r s a , is g i v e n i n T a b l e 5 ( C o l u m n
g a n d h). A s m a l l n u t r i e n t d e m a n d also in a r e g i o n n e a r t h e c o a s t is i n d i c a t e d . A s s u m i n g a
c e r t a i n q u a n t i t y of P i n s e a w a t e r r e d u c e s t h e P c o n t e n t of f r e s h w a t e r n e c e s s a r y for t h e
g r o w t h of a n e x t r e m e q u a n t i t y of p h y t o p l a n k t o n as s e e n in T a b l e 5 ( C o l u m n i a n d k).
Similarly, if w e s e t a s i d e t h e fact t h a t s o m e P r e s e r v e [s n e c e s s a r y b e s i d e s t h e q u a n t i t y
i n c o r p o r a t e d i n t o p h y t o p l a n k t o n , t h e a p p r o x i m a t e d c o n c e n t r a t i o n of P n e c e s s a r y for g o o d
d e v e l o p m e n t of p l a n k t o n c a n b e d e d u c e d b y t h i s m e t h o d . It is of s p e c i a l i n t e r e s t w h e t h e r
or n o t g r o w t h c o n d i t i o n s c a n b e c o m e so o p t i m a l o n e d a y t h a t a p l a n k t o n s t o c k c a n b e
462
M a x Gillbricht
p r o d u c e d n e e d i n g m o r e nutrients than discussed. Such an event i s outside of our
experience. It m a y h a p p e n sometime, with the hmitation that g r e a t e r p o p u l a t i o n s n e a r
the coast m u s t hmit further d e v e l o p m e n t by. self-made, direct or i n d i r e c t increase of
turbidity. The time s p a n n e e d e d to build up a g r e a t p l a n k t o n stock cannot, however, b e
g r e a t l y prolonged in our latitudes.
Silicate p r e s e n t s a special p r o b l e m in this r e s p e c t (Fig. 16; T a b l e 4). Similar to the
situation with P or N, the consumption of Si is much h i g h e r t h a n the d e c r e a s e in nutrient
stock; there are, however, other reasons for this (no incorporation into o r g a n i c substances). Only a p e r m a n e n t restitution of losses on a large scale is, however, a b l e to explain
the situation. A n e a r l y constant, small Si concentration m u s t not n e c e s s a r i l y b e the
m i n i m u m factor in such a d y n a m i c system. The only portion of p h y t o p l a n k t o n that n e e d s
Si (practically) is the diatom population. It could b e r e g u l a t e d in this w a y s e p a r a t e l y as
discussed since the b e g i n n i n g of p l a n k t o n r e s e a r c h (Brandt, 1902). The concentration of
Si is small c o m p a r e d with that of other nutrients, a n d with d e m a n d (Figs 7 a n d 8). The
solution n e c e s s a r y for the dynamics o b s e r v e d (Fig. 16) - e s p e c i a l l y with r e s p e c t to the
large spring b l o o m in 1985 (p. 447) - is not found in e x p e r i m e n t s : d e a d d i a t o m s release a
daffy a m o u n t of only 0.5 to 1% of Si b o u n d in the frustules (J~rgensen, 1955b), which is
not e n o u g h to m a i n t a i n a long lasting bloom. The silicon of l i v i n g cells is practically
insoluble, with solubility i n c r e a s i n g b y m e a n s of heating, EDTA, etc., o p t i m i z e d b y
treating with nitric acid, thus obtaining a fast solution of Si (Lewin, 1961). It could b e
d e m o n s t r a t e d that frustules are p r o t e c t e d b y metal c o m p o u n d s . A n o r g a n i c coating, as
discussed b y C o o p e r (1952), w a s not detected.
T h e s e results e x p l a i n the findings in this investigation to a certain e x t e n t only.
Moreover, the Si content of diatoms was s e e n to b e r e d u c e d u n d e r critical conditions, e. g.
in culture (Paasche, 1973); cell n u m b e r s continue to i n c r e a s e e v e n after r e m o v a l of Si
(Jorgensen, 1955a). This indicates a significant d y n a m i c force in the s y s t e m that is not
seen directly. If t h e s e findings are correct, they m a y i n d i c a t e that the c a l c u l a t e d Si
consumption b y diatoms is too high (Fig. 16) but correct in principle. To i n v e s t i g a t e this
p r o b l e m once more, the following points w e r e considered:
(a) Phosphorus, nitrogen, a n d silicon are n e e d e d b y p h y t o p l a n k t o n o v e r a short time
during the v e g e t a t i o n p e r i o d in g r e a t e r quantifies t h a n those o b s e r v e d in inorganic
form in the water.
(b) The quantities of nutrients i n c o r p o r a t e d in p h y t o p l a n k t o n are m u c h smaller than
the quantities o b s e r v e d in w a t e r in winter time.
(c) N e v e r t h e l e s s the sum (inorganic nutrients o b s e r v e d + nutrients in plankton) is
smaller in s u m m e r t h a n in winter. With r e s p e c t to P a n d N, t h e difference is
c o m p e n s a t e d l a r g e l y b y dissolved (and particulate) organic c o m p o u n d s .
Such a system n e e d s fast short-cut r e g e n e r a t i o n processes. T h e s e a r e not directly
d e m o n s t r a b l e b y nutrient m e a s u r e m e n t s or are d e m o n s t r a b l e to a m i n o r d e g r e e only
during the biologically active time of the year. With r e s p e c t to P a n d N, no p r o b l e m exists
in principle, b u t w h a t h a p p e n s with silicon? W h e r e does it d i s a p p e a r to a n d from w h e r e
does it come afterwards if n e e d e d ? Two simple a s s u m p t i o n s for the discussion of this
p r o b l e m a r e as follows:
(a) Remineralization is controlled b y the v o l u m e of a - m a y b e u n k n o w n - depot. This
effect has s o m e t h i n g to do with the difference b e t w e e n i n o r g a n i c nutrient, winter
m a x i m u m a n d s u m m e r minimum.
Phytoplankton a n d nutrients near H e l g o l a n d
463
(b) T h e r e is a constant nutrient concentration always buffered by a depot.
As can b e s e e n clearly in T a b l e 6, the situation of N can be described as (too) g o o d by
assumption a. T h e difference is i n d e p e n d e n t .of the different plankton activities at
different sahnities (Q), whilst the m i n i m u m v a l u e is not constant. Th e situation with P is
Table 6. Helgoland Roads 1981: L o w e s t nutrient concentrations in summer (L) and nutrient
d e c r e a s e from winter maximum to summer minimum (D) of silicate (Si), phosphate (P), and
inorganically bound nitrogen (N) given in [~g at 9 -3] for different salinities (S). Q -- the quotient
(highest value/lowest value) of every column
S
SiL
SiD
PL
PD
NL
28
30
32
1.39
1.20
1.13
13.4
9.97
7.55
0.631
0.398
0.235
1.30
1.12
1.01
18.8
12.5
4.24
Q
1.24
1.78
2.69
1.29
4.43
ND
32.5
33.2
33.3
1.02
not so clear, but the d e c r e a s e s are more similar than the m i n i m al values. This finding is in
a g r e e m e n t with a b e t t e r r e m i n e r a h z a t i o n in the case of poor P concentrations (p. 438). In
contrast to these observations, the b e h a v i o u r of Si can be e x p l a i n e d best by a c c e p t a n c e of
a buffering system (assumption b) not d e t e c t e d by our methods.
T h e s e considerations can only d e m o n s tr a te the principle: the situation is too comphcated. Th es e findings, however, are in a g r e e m e n t with observations by C o o p e r (1933),
who d e t e c t e d a nearly e q u a l d e c r e a s e of P and N w i t h , r e s p e c t to the composition of
organisms, whilst the Si stock c h a n g e d to a smaller degree. This is the result, probably, of
a m o r e successful regeneration. No doubt, the fluctuations of inorganic nutrient co n cen trafions o b s e r v e d are the tops of i c e b e r g s only, in c o m p a n s o n with the d y n a m i c processes
going on b e h i n d these small differences. But w h a t sort of d e p o t p e r m a n e n t l y restores Si
losses to a stable m i n i m u m concentration of 1.2 ~tg atom Si. d m -3 over a relatively long
period of time (Fig. 8)? More clarity in this respect may b e g i v e n by the reflections of
C o o p e r (1952) on the different modifications of Si c o m p o u n d s in the w at er in e q u i l i b r i u m
with one other a n d therefore counteracting the silicate co n su m p t i o n of diatoms (buffered
system). On the other hand, Hart (1934 1942) m a d e some observations in the northern
part of the Antarctic seas indicating certain growth limitations, especially a relative
increase of spineless Corethron cells, with d e c r e a s i n g Si concentration.
The question is w h e t h e r the small quantity of Si o b s e r v e d can b e u s e d effectively by
diatoms. Such an assumption should be in r e a s o n a b l e a g r e e m e n t with the results of some
culture e x p e r i m e n t s (Goering et al.. 1973), b e a r i n g m m i n d the problems of such a
comparison. This finding does not exclude the possibility of an influence of Si concentration upon spemes composition of the populations b e c a u s e of different d e m a n d s in this
r es p ect (Paasche, 1973).
In coastal regions, the influence of the bottom upon the nutrient system must be
considered. S h g h t fluctuations of the total nutrient content are o b s e r v e d o v e r the y e a r
(Table 1) - at least in the waters of the Southern North Sea an d the English C h a n n e l . This
finding m a y d e n y the essential influence of s e d i m e n t a t i o n processes since d e c a y i n g
464
Max Gillbricht
organic particles release P a n d N in the form of dissolved i n o r g a n i c a n d organic
components that reach the bottom in a highly modified chemical form (p. 438). As a
consequence, nutrients in spring are gener.ally not lost from the system b u t are u s e d
several times b y p h y t o p l a n k t o n before settling on the sediment after a m u c h longer time
span. Here the greatest portion is remineralized a n d redistributed into the water especially u n d e r unstratified winter conditions. This simple explanation m a y b e modified by a
r e a s o n a b l e e x c h a n g e b e t w e e n s e d i m e n t a n d water as s u m m a r i z e d by Zeitzschel (1980)
especially i n flat regions with strong tidal currents, b u t with the p r o b l e m of the relatively
stable total n u t r i e n t concentration in water. The situation is similar with respect to Si,
b e c a u s e the greater quantity of this e l e m e n t exists in the bottom i n a n insoluble form
{quartz} in contrast to the easily soluble silicate (Cooper, 1952).
There are more problems c o n c e r n i n g r e m i n e r a h z a t i o n t h a n discussed up to now. The
parallehsm of nitrogen curves for different salinities (Pig. 7) compared with very different
quantities of p h y t o p l a n k t o n a n d production (Fig. 11) indicates r e a s o n a b l e differences in
the n u t r i e n t dynamics. The d e v e l o p m e n t in time of the destroying system (bacteria)
regulated by the quantity of (dissolved} organic matter m a y have p r o d u c e d this result, b u t
one must b e a r in m i n d that the statistics used m a y also have i n f l u e n c e d this f i n d i n g to a
certain degree. In contrast to this observation, phosphate demonstrates a t e n d e n c y to
decrease in a m p h t u d e during the a n n u a l cycle with regard to smaller total P concentrations (Table 6). This looks hke a regulation i n d u c e d by a certain shortage. We must not
accord too m u c h weight to this observation, as the P system easily acts in this way. A true
hmitation of P a n d N should produce cycles like Si. No doubt, the quantities of P a n d N
are m u c h higher than necessary to build up the p h y t o p l a n k t o n stock. The only question
is, which portion of organically b o u n d nutrients is n e e d e d to secure reminerahzation, thus
increasing the inorganic P a n d N concentrations observed parallel with great p l a n k t o n
stocks. These findings indicate that this i n d i s p e n s a b l e n u t r i e n t surplus (P, N) is small
compared with the actual situation in this area.
In any discussion on p h y t o p l a n k t o n a n d nutrients one must b e a r i n m i n d that these
substances present a comphcated p r o b l e m due to complex d y n a m i c processes that are
difficult to study, highly u n k n o w n , a n d not to be explained by simple comparisons of
static situations.
Acknowledgements. This investigation of tong duration was carried out with the collaboration of
numerous people including the crews of the research vessels and many colleagues of the
"Biologische Anstalt Helgoland" on the island of Helgoland. P. Mangelsdoff and K. Treutner,
technical assistants of long standing experience, produced the highly qualified data set necessary for
all further calculations. Thanks are due to B. Freier for the assistance needed daily in compiling this
paper, from data handling to updating the collection of literature; also to Dipl. Bibl. I. Schritt and
Dipl. Bibl. B. Sysoew for their painstaking accuracy in correcting the "Literature cited". C. Berger
improved the English text, and J. Marschall produced the drawings. Now that I am at the end of my
official service I should like to thank them all cordially for their patient cooperation.
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