HELGOL,~NDER MEERESUNTERSUCHUNGEN
Helgol~inder Meeresunters. 43, 29-43 (1989)
M e i o f a u n a l e m e r g e n c e from intertidal s e d i m e n t
m e a s u r e d in the field: significant contribution
to nocturnal planktonic b i o m a s s in s h a l l o w w a t e r s
W. Arrnonies
Biologische Anstalt Helgoland (Wadden Sea Institute Silt); D-2282 IAst,
Federal Republic of Germany
ABSTRACT: Field studies on the occurrence of meiobenthos in the water column above intertidal
sandflats have been performed near the Island of Sylt in the northern Wadden Sea. Swimming
meiobenthos was strongly dominated by harpacticoid copepods. Many of them have a semiplanktonic fife-style. They rest in superficial sediment layers at low tide and swim in the water column at
high tide. Swimming activity correlated negatively with light. The abundance in the water column
was one order of magnitude higher during the night. Strong currents caused by storm tides
significantly decreased meiobenthic abundance in the water column. Light and flow being constant,
no significant changes of meiobenthic abundance per unit area occurred over a tidal cycle. Since
holoplankton and meroplankton abundances correlated positively with the height of the water
column, semiplanktonic meiobenthos may dominate the mesozooplankton in shallow waters. On an
average, emergence of meiobenthos increased the mesozooplanktonic biomass by about 2 % during
diurnal high tides over the entire tidal cycle, and by about 50 % during nocturnal high tides. Because
of seasonal cycles of the dominant harpacticoids, this high contribution to planktonic biomass may
be a summer phenomenon.
INTRODUCTION
Traditionally, m e i o b e n t h o s was t h o u g h t to be confined to either living in the
s e d i m e n t or its interstices, or p e r f o r m i n g an e p i b e n t h i c hfe-style with i n t e r m i t t e n t hiding
in the sediment. H o w e v e r , r e c e n t investigations r e v e a l e d that at least some m e i o f a u n a l
taxa m ay actively l e a v e the s e d i m e n t and swim in the w at er column (Alldredge & King,
1977; Hobson & Chess, 1979; H a m m e r , 1981; Sibert, 1981; Ohlhorst, 1982; Hicks, 1986;
Walters & Bell, 1986; recently r e v i e w e d by Palmer, 1988). Most of these studies deal with
tropical to w a r m t e m p e r a t e subtidal habitats. Laboratory ex p er i m en t s on the e m e r g e n c e
b e h a v i o u r of boreal intertidal m e i o b e n t h o s (Armonies, 1988a, b, c} y i e l d e d the following
predictions: (1) C o p e p o d a and Plathelminthes m ay actively enter the w a t e r column;
Ostracoda perform an e p i b e n t h i c life-style, and N em at o d a, Gastrotricha and O l i g o c h a e t a
s e e m to live endobenthically. (2) T h e c o p e p o d and plathelminth s w i m m i n g activity
correlates n e g a t i v e l y with light, i.e. their a b u n d a n c e in the w a t e r column should be
higher during nocturnal than during diurnal high tides. (3) Currents affect the a b u n d a n c e
of active swimmers in the w a t e r column. A b u n d a n c e should be h i g h est at w e a k currents,
decrease as currents b e c o m e stronger, a n d increase again w h e n the s e d i m e n t b e c o m e s
eroded. In the latter case, N e m a t o d a , Gastrotricha an d O h g o c h a e t a should also b e found
9 Biologische Anstalt Helgoland, Hamburg
30
W. Armonies
in the water column. (4) The s w i m m i n g activity of harpacticoid c o p e p o d s a n d plathelminths is also affected by physical factors such as temperature, salinity, a n d oxygen.
However, the effects of these factors seem to b e species-specific a n d in m o s t cases there
is no g e n e r a l pattern. The potential benefits a n d risks of active s w i m m i n g in the water
c o l u m n are discussed b y Alldredge & King {1980, 1985) a n d Armonies (1988a, c}.
The present study combines field s a m p h n g in the water column a n d s e d i m e n t to test
for (1) the presence of active swimmers in the water column above b o r e a l intertidal
sandflats, (2} a hght effect, a n d (3) a decrease in s w i m m i n g activity d u r i n g periods with
stronger currents. (4) A n o t h e r aim of this study is to quantify the c o n t r i b u t i o n of
s w i m m i n g meiobenthos to the biomass of mesozooplankton.
MATERIAL AND METHODS
S t u d y sites a n d s a m p l e c o l l e c t i o n
Meiobenthic e m e r g e n c e into the water column was studied at the e x t e n s i v e tidal
flats of the K5nigshafen w a d d e n area n e a r the Island of Sylt in the N o r t h Sea. For a
g e n e r a l description of the area see Reise (1985). Three sites were s a m p l e d b e t w e e n July
a n d S e p t e m b e r 1988 (Fig. 1): (1) Site I was a 20 x 2 0 m square of a s h e l t e r e d s a n d y tidal
flat NE of the village of List. The s e d i m e n t was mainly composed of fine s a n d (median
diameter 150~tm, sorting coefficient 1.5}. During an average tidal cycle, this site was
s u b m e r g e d for 4 h {approx. 33 %) a n d the average height of the water c o l u m n at high tide
was 0.6m. (2) Site II was a 40 • 10m plot of m e d i u m s a n d (median d i a m e t e r 324~tm,
sorting coefficient 2.46} north of the village of List. The average s u b m e r g e n c e was 8 h per
tidal cycle {approx. 66 %) a n d the average high water level was 1.1 m. (3} Site IfI marks a
series of 10 locations e x t e n d i n g from site I to site II (distance approx. 300 m of shorehne)
a n d further west to the Lister Haken. The total distance of shorehne c o v e r e d by the 10
locations was about 600m, a n d individual locations were at least 5 0 r n apart. The
granulometric composition of the s e d i m e n t varied from fine sand (site I) to coarse sand
n e a r the Lister Haken. The height of all locations was as at site I {i.e. a b o u t 33 % of tidal
submergence}.
S e d i m e n t samples were collected at low tide u s i n g glass tubes of 1 cm 2 i n s e r t e d into
the s e d i m e n t to a d e p t h of 2 cm, thus only covering the brownish superficial s e d i m e n t
layers. Each set of both s e d i m e n t a n d water samples consisted of at least 10 replicates.
Three methods were applied to collect seawater samples above the i n t e r t i d a l sandfiat. (1)
Pipe samples were collected b y vertically inserting plexiglas pipes of 10 cm 2 a n d variable
l e n g t h (0.3, 0.5, 1.0, 1.5 m d e p e n d i n g on the water depth) into the water c o l u m n d o w n to a
few centimetres above the sediment. The pipe was closed with a r u b b e r s t o p p e r at the
u p p e r e n d a n d w i t h d r a w n until the lower e n d was just below the w a t e r surface. The
lower e n d was closed a n d t h e n the pipe r e m o v e d from the water. Such s a m p l i n g yields a
water column of 10 cm 2 surface area r e a c h i n g from the water surface to s o m e cm above
the sediment. Samples c o n t a i n i n g sand grains (by contact with the s e d i m e n t surface)
were rejected. To avoid artificial s u s p e n s i o n of s e d i m e n t a n d associated m e i o f a u n a , pipe
samples were collected from a r u b b e r d i n g h y which glided along a cable a n c h o r e d at the
sample site a n d the shore. The d i n g h y was p u s h e d by h a n d along the cable, thus no oars
or paddles disturbed the water layer or s e d i m e n t surface. The cable also h e l p e d our
M e i o f a u n a l e m e r g e n c e from i n t e r t i d a l s e d i m e n t m e a s u r e d in t h e field
31
N
t
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y
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Fig. 1, Position of sampling sites (I, II, III) off the Island of Sylt in the northern Wadden Sea. htl = high
tide level, lfl = low tide level
r e t u r n to the s a m e locations in t h e c a s e of r e p e a t e d s a m p h n g d u r i n g t h e tidal c y c l e (see
below). (2) S a m p l e s of s u r f a c e w a t e r w e r e c o l l e c t e d b y s c o o p i n g w i t h a 12 d m 3 b u c k e t
w i t h a r e c t a n g u l a r o p e n i n g (top 30 c m of t h e w a t e r column). (3) A 64 ~m p l a n k t o n n e t w i t h
t h e o p e n i n g f i x e d to a r e c t a n g u l a r f r a m e of 20 x 40 c m w a s m o u n t e d o n t o a s l e d g e to
s a m p l e the w a t e r c o l u m n h o r i z o n t a l l y b e t w e e n 10 c m (position of t h e l o w e r o p e n i n g of
the f r a m e a b o v e t h e s e d i m e n t ) a n a 50 c m a b o v e t h e s e d i m e n t . A 12.5 m t o w of this n e t
32
W. Armonies
filters 1 m 3 of seawater. Such a sample from site II was used to estimate the individual
biomass values of various taxa (see below).
Water samples were sieved through 63 ~m m e s h e s a n d the r e m a i n d e r r e t u r n e d to the
laboratory. The s e d i m e n t a n d n e t samples were i m m e d i a t e l y preserved i n 5 % formalin.
From the other water samples, animals were sorted alive using a dissecting microscope,
a n d hard bodied m e i o f a u n a was s u b s e q u e n t l y preserved in 5 % f o r m a h n for later
c o u n t i n g a n d identification. All m e t a z o a n taxa except copepod nauplii (harpacticoid as
well as calanoid) were counted b u t s p e c i m e n s that were retained on a 500 btm sieve (e.g.
shrimps, b r a c h y u r a n megalopae) were excluded from later analysis.
T e s t i n g for d i u r n a l d i f f e r e n c e s of m e i o b e n t h i c e m e r g e n c e
At site III, a 10 dm 3 sample of surface seawater was scooped from each of the 10
locations, once during daytime at high tide (August 27th), a n d once d u r i n g the succeeding n o c t u r n a l high tide (August 28th). A b u n d a n c e of the various taxa in the water
samples is compared b y Wilcoxon's m a t c h e d pair signed r a n k statistic a p p h e d to the
diurnal a n d nocturnal samples in each of the 10 locations. During the d a y t i m e s a m p h n g ,
the wave height was 5 crn in the most sheltered a n d up to 15 cm in the most exposed
locations. There was less w i n d d u r i n g the n i g h t a n d the wave height was mostly below
5 cm with a m a x i m u m of 10 crn at one location.
I n f l u e n c e of s t r o n g c u r r e n t s
Laboratory experiments suggest d e c r e a s e d a b u n d a n c e of s e m i p l a n k t o n i c species in
the water column as flow increases above a current speed of approx. 1 cm- s -1 (Armonies,
1988b; current speed m e a s u r e d 1 cm above the s e d i m e n t surface). At the I s l a n d of Sylt,
westerly gales greatly increase the current s p e e d i n the W a d d e n area a n d increase the
high tide level. Water samples of 1 dm 3 v o l u m e (pipe samples from the top I m of the
water column) were collected d u r i n g two nights with westerly storms ( S e p t e m b e r 11th,
n - - 2 4 , a n d September 12th, n = 20) at site II. A b u n d a n c e of the various taxa was
compared with the daytime water samples of S e p t e m b e r 8th, collected at the same site
with the same method (n '-- 24) but d u r i n g periods w h e r e there were only slight winds. In
all cases, sample collection started I h after highest water level (falling tide). O n
S e p t e m b e r 8th (daytime), the water level did not significantly differ from the average
(1.0 rn, I h after high tide). The surface current s p e e d was about 10 cm. s -1 (measured by
drifting objects). During the stormy nights, the tidal height was i n c r e a s e d to 1.3 a n d
1.5~m, a n d the surface current velocity was about 0 . 5 m . s -1 a n d 0 . 8 m . s -1 on Sept e m b e r 11th a n d 12th, respectively.
Q u a n t i f i c a t i o n of m e i o b e n t h o s i n t h e w a t e r c o l u m n
This was done using pipe samples of the entire w a t e r column from a r u b b e r dinghy.
Each of the seawater samples at each of the s a m p l i n g dates consisted of 10 replicate
water columns collected at least 2 m apart over a total distance of 20 m (site I) or 30 m
(site II). In the July samples, each of the rephcates was separately transferred into plastic
bottles while the 10 replicates of the A u g u s t a n d S e p t e m b e r samples w e r e pooled.
Meiofaunal emergence from intertidal sediment measured in the field
33
Depending on the actual height of the water column, the volume of collected seawater
varied b e t w e e n 0.7 and 12 dm 3 per 10 rephcates. There were 5 sampling occasions: (1)
July 10th, 2 h before nocturnal high tide (-- ht - 2 h), at site II; (2) July 12th, daytime, site I,
ht-2h, ht-lh,
h t + l h , h t + 2 h ; (3) July 13th, nighttime, site I, h t - 2 h , h t - l h ,
ht + 1 h; (4) August 8th, nighttime, and {5) September 9th, daytime, site II. Nineteen
sampling dates distributed over the entire tidal cycle: at first, 6 samplings were carried
out at 15 min intervals, starting 15 min after the sample site had b e e n r e a c h e d by the
incoming tide; then, at 30min intervals until the tide left the site again (see Pigs 1, 2, 3).
Light, air and seawater temperature, tidal height, and wave height were monitored at the
same intervals.
Biomass estimation
A 64 ~m plankton net sample from site II (August llth) was used to determine the
average individual dry weights of the dominant zooplankters and meiobenthic taxa,
following the procedure described by Widbom (1984) for formahn-preserved specimens.
These determinations for the more abundant taxa, and literature data from Widbom
{1984), Hickel (1975), and Faubet {1982) were fitted according to the actual percentages
of size classes in the above net sample and thus yielded the following average individual
dry weights (all in ~tg) used for biomass calculation: holoplanktonic Appendicularia and
Chaetognatha 1.0, calanoid copepods 1.5 (including copepodites but excluding nauplii),
planktonic PlatheIminthes 1.0, Rotatoria 0.2 (estimate}; meroplanktonic taxa, nectochaeta
larvae and barnacle nauphi 0.3, barnacle cypris larvae 3.0, spionid larvae 5.0, juvenile (5
to 6-setiger) Arenicola marina L. and trochophore larvae 1.0; semiplanktonic harpacticoids 0.8 (including copepodites but excluding nauphi), cyclopoids 0.3, plathelminths
1.0; benthic Ostracoda 2.0, Nematoda 0.5, and Plathelminthes 1.0 (all ~tg DW).
Statistical a n a l y s i s ,
A b u n d a n c e data of both meiobenthic and planktonic taxa frequently deviated from a
normal distribution (significant deviation of the variance-to-mean ratio from the appropriate reference of a chi-square table, p = 0.05, G a g e & Geekie, 1973). In some cases, the
10 replicates forming a sample were pooled and thus no information is available on the
between-replicate variation of abundances. Therefore, only non-parametrical methods
are used: Wilcoxon's matched pair signed rank statistics, U-test (Wilcoxon et al., in Sachs,
1984), and Spearman's rank correlation coefficient (abbreviated by "rs"). Significant
results are indicated by * (p < 0.05), ** (p < 0.01) and *** (p < 0.001), respectively.
RESULTS
D i u r n a l d i f f e r e n c e s in t h e c o m p o s i t i o n of m e s o z o o p l a n k t o n
The daytime zooplankton above intertidal sandflats of the W a d d e n Sea is strongly
dominated by holoplanktonic and meroplanktonic taxa which together comprise > 90 %
of metazoans. Abundances of these taxa are not significantly different in the nocturnal
samples but swimming harpacticoids increased by an order of magnitude (Table 1).
Therefore, holo- and meroplanktonic taxa each comprised less than 30 % of the individu-
34
W. A r m o n i e s
Table i. Abundances-10dm -3 of seawater ( _ sd) and percentage composition of the metazoan fauna
in diurnal and noctumal plankton samples collected above an intertidal sandflat near the Island of
Sylt (site Ill, August 27th and 28th, 1988). Copepod nauplii were excluded from this study. Minor
taxa are not included in this table. Arenicola mar/na are all juveniles < I mm length. * * denotes a
significant difference between daytime and nighttime samples (Wilcoxon matched pair signed rank
test, p < 0.01)
Daytime
Abundance
Taxon
Holoplankton, total
- Calanoidea
Rotatoria
- Appendicularia
Chaetognatha
Plathelminthes
Meroplankton, total
Barnacle nauplii
- Barnacle cypris
-
-
-
-
- Arenicola marina
- Spionid larvae
Nectochaeta larvae
Semiplankton, total
Harpacticoids
-
-
-
H. flexus
- Ectinosomatidae
Benthos, total
Total metazoans
39.3
26.1
5.9
5.6
1.0
0.5
33.8
17.9
0.3
6.4
2.5
4.1
4.8
4.5
0.9
0.3
2.4
80.3
(18.1)
(11.4)
(5.2)
(4.4)
(0.9)
(1.0)
(26.9)
(15.5)
(0.5)
(5.9)
(2.2)
(5.0)
(2.6)
(2.4)
(0.9)
(1.0)
(3.0)
(34.7)
%
49
42
6
3
Nighttime
Abundance
28.2
19.8
2.9
3.9
0.7
0.4
27.3
9.2
0.5
8.5
5.0
3.2
40.4
39.5
30.7
5.3
2.0
97.7
(13.7)
(12.1)
(2.7)
(2.2)
(0.7)
(0.7)
(9.6)
(5.6)
(0.7)
(7.3)
(3.6)
(3. I)
(9.0)*"
(8.8)* *
(5.8) * '
(4.3) * ~
(1.1)
(21.5)
%
29
28
41
2
als in t h e n o c t u r n a l w a t e r c o l u m n w h i l e h a r p a c t i c o i d s w e r e t h e m o s t a b u n d a n t t a x o n w i t h
the s p e c i e s H a r p a c t i c u s f l e x u s B r a d y & R o b e r t s o n .dominating. T h e p e r c e n t a g e a n d
a b u n d a n c e of s u s p e n d e d b e n t h o s ( m a i n l y N e m a t o d e , T a r d i g r a d a a n d O s t r a c o d a ) a r e
s h g h t l y h i g h e r in t h e d a y t i m e s e a w a t e r s a m p l e s , c o r r e l a t i n g w i t h t h e h i g h e r w a v e s . As H.
f l e x u s as w e l l as s o m e o t h e r s p e c i e s a r e v e r y a c t i v e s w i m m e r s w h i c h s p e n d a s u b s t a n t i a l
p a r t of t h e i r life in t h e w a t e r c o l u m n , t h e y a r e h e n c e f o r t h d e n o t e d as " s e m i p l a n k t o n " a n d
thus separated from both entirely benthic species and species performing occasional
m i g r a t i o n s in the w a t e r column.
Low abundance
of s e m i p l a n k t o n
during
stormy nights
I n c r e a s e d c u r r e n t v e l o c i t y d u r i n g t h e s t o r m y n i g h t s of S e p t e m b e r 11th a n d 12th w a s
a c c o m p a n i e d b y a n e x t r a o r d i n a r i l y l o w a b u n d a n c e of s e m i p l a n k t o n i c s p e c i e s in t h e
w a t e r c o l u m n . C o m p a r e d to t h e d a y t i m e s a m p l e s of S e p t e m b e r 8th, t h e r e w a s n o
s i g n i f i c a n t i n c r e a s e in h a r p a c t i c o i d a b u n d a n c e d u r i n g t h e s t o r m y n i g h t s (0.33 •
0.56 - d m -3 v e r s u s 0.51 • 0.66 a n d 0.45 • 0.76 9 I d m -3 of s e a w a t e r , U-test: p > 0.1) w h i l e
all o t h e r n i g h t t i m e s a m p l e s (n = 24 x 10 r e p h c a t e s ) y i e l d e d s i g n i f i c a n t l y h i g h e r a b u n d a n c e s of s e m i p l a n k t o n t h a n t h e r e s p e c t i v e d a y t i m e s a m p l e s (see b e l o w ) .
M e i o f a u n a l e m e r g e n c e from intertidal s e d i m e n t m e a s u r e d in the field
N o c h a n g e s of s e m i p l a n k t o n i c a b u n d a n c e
35
with tidal height
A b u n d a n c e of h o l o p l a n k t o n and m e r o p l a n k t o n in the w a t e r column a b o v e 100 cm 2 of
s e d i m e n t surface correlates positively with the h e i g h t of the w a t e r column (rs* * * ; Fig. 2).
In contrast, a b u n d a n c e of s e m i p l a n k t o n p e r s e d i m e n t surface unit r e m a i n e d fairly
constant over the dieI cycle b u t s h o w e d irregular variations at night (Fig. 2). B a s e d on
v o l u m e units of seawater, a b u n d a n c e of holo- a n d m e r o p l a n k t o n r e m a i n e d fairly constant
while the nocturnal a b u n d a n c e of b o t h s e m i p l a n k t o n a n d passively s u s p e n d e d b e n t h o s
correlate n e g a t i v e l y with tidal h e i g h t (rs* * * ; Fig. 3). In the case of s u s p e n d e d benthos, the
correlation is p r e s u m a b l y caused b y stronger w a v e erosion in shallow t h a n in d e e p water.
During the first 6 s a m p l i n g dates, a b u n d a n c e of s e m i p l a n k t o n shows a d e c r e a s e w h i c h
correlates significantly with i n c r e a s i n g light during sunrise (rs*),
Nocturnal dominance
of s e m i p l a n k t o n
In nocturnal shallow water, s e m i p l a n k t o n can numerically d o m i n a t e the z o o p l a n k t o n
a s s e m b l a g e (Fig. 4). S e m i p l a n k t o n e m e r g e s from the s e d i m e n t surface a n d the n u m b e r of
s w i m m e r s p e r surface unit r e m a i n s r a t h e r constant over the tidal cycle. But i n c r e a s i n g
tidal h e i g h t r e d u c e s their a b u n d a n c e p e r volume unit (rs* * *). In contrast, h o l o p l a n k t o n
a n d m e r o p l a n k t o n a b u n d a n c e p e r v o l u m e unit is rather constant, a n d i n c r e a s i n g tidal
h e i g h t causes a h i g h e r a b u n d a n c e p e r surface unit (rs* * *). On an a v e r a g e of the entire
tidal cycles, s e m i p l a n k t o n contributed 2.5 % of m e t a z o a n a b u n d a n c e in the w a t e r column
during daytime, a n d 47.8 % at night ( S e p t e m b e r a n d A u g u s t samples, site II). In terms of
biomass, the contribution is 2 % a n d 32 %, respectively. The m e t a z o a n b i o m a s s m a y thus
b e a b o u t 50 % h i g h e r at night. In the shallower w a t e r of site I, positioned h i g h e r up in the
tidal gradient, the contribution of s e m i p l a n k t o n to total m e s o z o o p l a n k t o n a b u n d a n c e
m a y even e x c e e d 60 % (Fig. 5).
P e r c e n t a g e s of s w i m m i n g h a r p a c t i c o i d s
During d a y t i m e high tide of S e p t e m b e r 7th at site II, less than 1% of the h a r p a c ticoids p r e s e n t in the superficial s e d i m e n t layer s w a m in the overlying w a t e r column
(Table 2). In the nighttime samples of A u g u s t 8th at the same site, an a v e r a g e of 66 % of
harpacticoids w e r e simultaneously s w i m m i n g in the w a t e r column. In the nighttime
s a m p l e s at site I (July 13th) as well as in some single s a m p h n g dates of A u g u s t 8th (site II),
the p e r surface a r e a a b u n d a n c e of m e i o b e n t h i c harpacticoids in the w a t e r column
e x c e e d e d the a b u n d a n c e calculated from s e d i m e n t sampling. This indicates a significant
horizontal transport of harpacticoids with the tidal currents.
S p e c i f i c c o m p o s i t i o n of h a r p a c t i c o i d s
During the entire p e r i o d of investigation, the harpacticoid fauna of all s a m p l e d sites
was strongly d o m i n a t e d b y only two species, Harpacticus flexus a n d Tachidius discipes
Giesbrecht. T o g e t h e r they c o m p r i s e d over 95 % of the harpacticoids, in the w a t e r column
as well as in the u p p e r (top 2cm) s e d i m e n t layer. A m o n g the r e m a i n i n g species,
Longipedia helgolandica, Canuefla perplexa, IntermedopsylIus intermedius, Mesochra
36
W. A r m o n i e s
_=_--
-100
DAY
~
cm
~=
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---0
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.
1 ~o ,o
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M e i o f a u n a l e m e r g e n c e from intertidal s e d i m e n t m e a s u r e d in the field
37
inconspicua, Evansula pygmaea, Paraleptastacus spinicauda and P. holsaticus, F.nhydrosoma propinquum, Asellopsis hispida, a n d A. intermedia could b e identified
(authorities of all species can b e referred to in Mielke, 1975). Ectinosomatids a n d some
c o p e p o d i t e s r e m a i n e d unidentified. All species occurred both in the w a t e r column a n d in
superficial sediment.
DISCUSSION
S p e c i f i c c o m p o s i t i o n of h a r p a c t i c o i d s w i m m e r s
T h e joint occurrence of all identified h a r p a c t i c o i d species in both the w a t e r column
a b o v e a n d in the superficial s e d i m e n t layer of the intertidal sandflats m a y in p a r t b e an
artefact of (1) the stronger s a m p l i n g effort in the w a t e r column ( > 5 0 0 s a m p l e s of
s e a w a t e r b u t only 50 s e d i m e n t samples), a n d (2) m a y result from p a s s i v e s u s p e n s i o n of
some of the rarer species b y w a v e action a n d tidal currents or s e d i m e n t rafting (Hicks,
1988). In a n y case, the species list is subject to seasonal change. For instance, pilot
s a m p h n g p e r f o r m e d since April 1988 often r e v e a l e d high a b u n d a n c e of the diosaccid
Amphiascoides debflis (Giesbrecht) in both the s e d i m e n t a n d the w a t e r column. At the
s a m e time, the dominants of July to S e p t e m b e r , Harpacticus flexus a n d Tachidius
discipes, w e r e significantly less a b u n d a n t . T h e s e findings correspond to t h e a n n u a l
a b u n d a n c e cycles of the latter two species in a n e a r b y s e m i - e x p o s e d sandflat (Mielke,
1976). Their high a b u n d a n c e in the w a t e r column therefore m a y b e restricted to the
s u m m e r months. It is not known, up to now, if there are other species that t a k e over their
role in the water column during winter.
14. flexus a n d T. discipes are b o t h w i d e l y distributed in the W a d d e n Sea a n d the
adjoining brackish waters (Noodt, 1957). This w i d e distribution m a y relate to high
tolerance to salinity a n d s e d i m e n t t y p e (especially in T. discipes, Noodt, 1957) a n d
coincide with high nocturnal s w i m m i n g activity. Up to 75 % of an H. flexus p r e s e n t in the
s e d i m e n t at low tide w e r e s i m u l t a n e o u s l y active in the w a t e r column during nocturnal
high tide (Table 2). Therefore, the classification of this species as e p i b e n t h i c s e e m s no
l o n g e r a d e q u a t e , a n d a term such as s e m i p l a n k t o n i c or s e m i b e n t h i c should b e considered.
T i d a l t r a n s p o r t of s e m i p l a n k t o n
In some cases, more harpacticoids w e r e found in the w a t e r column t h a n h a d b e e n
p r e s e n t in the s e d i m e n t during the p r e c e d i n g low tide (Table 2). This d e m o n s t r a t e s that
high n u m b e r s of s e m i p l a n k t o n i c species can b e t r a n s p o r t e d with the tidal currents a n d
m a y explain some of the s u d d e n c h a n g e s of a b u n d a n c e in the s e d i m e n t o b s e r v e d in
previous studies (e.g. Palmer & Brandt, 1981; WiUems et al., 1984). The significant
i n c r e a s e in s e m i p l a n k t o n i c a b u n d a n c e in the w a t e r column d u r i n g nighttime w h e n
c o m p a r e d to d a y t i m e s a m p h n g with similar h y d r o g r a p h i c conditions, a n d the l a c k of such
Fig. 2. Abundance of holoplankton, meroplankton, and semiplankton (left ordinate) in the water
column over 100 cm 2 of sediment surface during a diel (Sept 7th) and a nocturnal (Aug 8th) high tide
(site II). Abscissa indicates time (h) since samphng site was reached by the incoming tide
W. A r m o n i e s
38
NIGHT
i
w- "''-+
plankton
9
]i!!ii :i i ij iiiiiil
I I I | I I
....~
.9
I
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I
I
I
I
I
I
I
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I
9149
9 .9
9
-I
~
.-
"-
. ' " "'" "%,
meroplankton
~
.."'
"....
!~ii~i~i~i~ii~!i~i!iii!~i~iii!i~iiiii~i~ii~i~i~i~i!iii!~i~!~iii!i!!~ii~iiiii~i~i~i~i~iiii!~!i!ii~i~!iii~i~i~i~iii~ii!i~!i~i!~!i!i~i
i!i!iii!i~iiiii~iii~iii!iiiiiiiiiii~iiiiiiiiiiiiiiiiiiiiiiiiiiiii~i~iiiii~i~i~i!ii~i~i~!iiiiiiii!iiii!iiiiiiiiiiiiiiiiiii~iiii~iii~iiiiiii~iii~iiiiiiiiii!iiiiiiiiiiiiiiiiiiii~i~i
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period
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4
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(h) o f w a t e r
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7
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cover
Pig. 3. Abundance [log(x + 1) transformed], per I d m a of seawater, of holo-, mero- a n d semiplankton
and s u s p e n d e d benthos above an intertidal sandflat (site II) during daytime high tide of September
7th and nocturnal high tide of August 8th. The dotted line indicates the height of t h e water cover
Meiofaunal emergence from intertidal s e d i m e n t measured in the field
39
%
9O
crn
E
-6
50
o
t-
"5
t..f::
0
0.15
~]
1
2
holoplankton
3
~
4
0
5
meroplankton
6
7
8h
semiplankton
100
90
~-
0
~
Cm
o
~
6O
r-
.o 5O
e3
o
o.
E
0
o
30
c
~ 10
a
0
0.15
1
2
3
4
5
6
7
8h
period of water cover
Fig. 4. Percentage composition of zooplankton in the water column above an intertidal sandflat
(site II} during daytime high tide of S e p t e m b e r 7th and the nocturnal h i g h tide of August 8th. The
diffuse hne marks the actual height of the water column (right scale}
40
W. Armonies
Table 2. Average abundance of meiobenthic harpacticoids per m 2 of sediment surface area and in
the overlying water column. In July site I, sediment samples were only once collected (during the
intermittent low tide). At the other dates, sediment samples were collected during the low tide
preceding the high tide of water sample coUection
Site and Date
Taxon
(II) Sept. 7, daytime
Total harpacticoids
H. flexus
(II) Aug. 8, nighttime
Total harpacticoids
H. flexus
T. cfiscipes
(I) July 12, daytime
(I) July 13, nighttime
Total harpacticoids
Total harpacticoids
Abundance in the
Water column
Sediment
370
305
15,730
14,000
1,320
2,030
11,600
48,300
31,700
23,700
18,700
4,200
4,200
4,200
a n i n c r e a s e d u r i n g stormy nights b o t h corroborate the thesis that e m e r g e n c e is primarily
a n active process in this intertidal a s s e m b l a g e . Contrasting results from o t h e r g e o g r a p h i c
a r e a s (i.e. p r e v a l e n c e of passive s u s p e n s i o n of organisms, e.g. Palmer, 1984; Palmer &
Gust, 1985) m a y in p a r t be e x p l a i n e d b y the n o n - l i n e a r r e s p o n s e of h a r p a c t i c o i d s to
current s p e e d (Armonies, 1988b). However, since e m e r g e n c e t u r n e d out to b e a speciesspecific p r o c e s s (e.g. Waiters, 1988) a n d most species h a v e clear p r e f e r e n c e s for certain
s e d i m e n t p r o p e r t i e s correlating with the h y d r o g r a p h i c conditions (e.g. F e n c h e l , 1978),
variations in the d e g r e e of active s w i m m i n g in the w a t e r column over s e d i m e n t types are
to b e expected.
Semiplankton
worldwide significant in shallow waters
The records of s e m i p l a n k t o n i c species from other g e o g r a p h i c a r e a s (e.g. H a u s p i e &
Polk, 1973; Porter & Porter, 1977; H a m m e r , 1981; Sibert, 1981; Fulton, 1984; Decho, 1986;
Hicks, 1986; J a c o b y & G r e e n w o o d , 1988; Waiters, i988) indicate that e m e r g e n c e of
s e d i m e n t - a s s o c i a t e d m e i o f a u n a m a y occur on a w o r l d w i d e scale. H o w e v e r , t h e high
i n c r e a s e of p l a n k t o n i c biomass b y s w i m m i n g m e i o f a u n a is certainly r e s t r i c t e d to shallow
inshore waters. Their a b u n d a n c e in the w a t e r column relates to units of s e d i m e n t surface
a r e a a n d their a b u n d a n c e p e r v o l u m e unit d e c r e a s e s with i n c r e a s i n g w a t e r depth.
Therefore, their contribution to p l a n k t o n i c b i o m a s s will strongly d e c r e a s e in d e e p e r
waters (Fig. 5). Thus, s e d i m e n t - a s s o c i a t e d m e i o f a u n a is only an insignificant c o m p o n e n t
of traditional s h i p - o p e r a t e d p l a n k t o n s a m p l e s (e.g. Hickel, 1975; M a r t e n s , 1980). H o w ever, s e m i p l a n k t o n i c s w i m m e r s m a y a g g r e g a t e close to the s e d i m e n t surface, as has b e e n
d e m o n s t r a t e d b y D ' A m o u r s (1988). In d e e p e r w a t e r s t h e y m i g h t therefore p l a y a role in
the s u p r a b e n t h o s .
Semiplankton
resembles macrofaunal
suprabenthos
M e i o f a u n a l e m e r g e n c e from the s e d i m e n t r e s e m b l e s the life-style of s u p r a b e n t h i c
a r n p h i p o d s a n d cumaceans. T h e y also s p e n d the d a y t i m e in superficial s e d i m e n t Iayers
a n d e n t e r the w a t e r column at n i g h t (e.g. H e s t h a g e n , 1973; S a i n t e - M a r i e & Brunel, 1985),
Meiofaunal emergence from intertidal sediment measured in the field
41
100
I
I
_
Holo- and
-. Meroplankton
c"-,
,
~
O
N
c~
E
II Semiplankton . . . . . . . .
0
~
I
site I
"B"
-
----A
J
site II
htl
inte
rt
i dal
!
subt
id
al
Fig. 5. Percentage composition of metazoans in the nocturnal water column at high tide above an
intertidal sandflat (sites I, II) off the Island of Sylt, a n d the expected tendency along the tidal
gradient. A: numerical abundance, B: biomass, htl, ltl: high and low tide level, respectively. For
further explanations see text
a l t h o u g h t h e r e w e r e n o s i g n i f i c a n t d a w n or d u s k e f f e c t s o n
s t u d i e d i n t e r t i d a l a s s e m b l a g e of h a r p a c t i c o i d s (cf. A n g e r &
t h e s e s i m i l a r i t i e s i n life-style r e f l e c t c o n v e r g e n t l y e v o l v e d
e n d o b e n t h i c p r e d a t i o n , e f f e c t i v e i n all size c l a s s e s of b e n t h i c
swimming activity in the
V a l e n t i n , 1976). Possibly,
r e a c t i o n s to f a c t o r s h k e
organisms.
Acknowledgements. Thanks are due to Prof. Dr. K. Reise and an u n k n o w n reviewer for improving
the manuscript by critical comments, and to Mr. B. Ipsen for his expertise in constructing the
plankton sledge. This study was supported by a grant of the Biologische Anstalt Helgoland.
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42
W. A r m o n i e s
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43
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