•
, ..
lCES 1988
PAPER
C.M. 1988/L: 9 Ref. E
RESPONSE OF GONYAULAX TAMARENSIS TO THE PRESENCE OF A PYCNOCLINE IN AN ARTIFICIAL WATER COLUMN.
by
Jeff Rasmussen and Katherine Richardson
Danish Institute for Fisheries and Marine Research
Charlottenlund Castle
DK-2920 Charlottenlund, Denmark
ABSTRACT
In nature, large concentrations of the toxic bloom forming
dinoflagellate, Gonyaulax tamarensis, are frequently observed in the vicinity of the pycnocline. In the absence of a
pycnocline, the organism is usually recorded near the surface where light levels are more advantageous for photosynthesis. In this paper, we examine the swimming behaviour of G.
tamarensis when exposed to varying degrees of stratificatIOn
in order to address the question of whether the maintenance
of a subsurface (pycnocline) population is the result of retention of the algae by a physical barrier or active accumu~
lation of the organisms at a density interface. The study
indicates that G. tamarensis cells presented with a halocline of ~S < ca.6=7 0/00 (occuring over a few centimeters)
cross this salinity barrier and accumulate a~2t~~ highest
available photon flux density (ca.lOO pmol m s
). Cells
exposed to a gradient of ~Z ~lca. 7 0/00 remain ~t the halocline (pfd = ca.40 pmol m s
). However, when l1ght above
the pycnocline is attenuated by the addition of foodcolour
to the medium, the cells cross a halocline of ~s = 10 0/00
and accumulate at-the highest available photon flux density.
In the absence of added nutrients (inorganic N and P), the
organism fails to exhibit a phototatic response. Thus, the
presence of a strong halocline does not represent an inpenetrable physical barrier for G. tamarensis and the development of pycnocline populations of this organism is a function of density, light and nutrient climate.
- 1 -
Introduction
In re cent years, there ~as developed a widespread understanding that marine community structure and biological production are closely related to physical interfaces in the marine environment. Legendre et ale (1986) postulated that the
occurrence of high production in aquatic ecosystems and,
thus, the temporal blooming of phytoplankton and the occurrence of subsurface chlorophyll maxima can be interpreted
as a reflection of non-equilibrium responses to physical variability in the environment. These "ergoclines" (Legendre
and Demers 1984) or perturbations in the environment have
characteristics in common which lead to the hypothesis that
community structure is a function of spatial and/or temporal
scales in the spectrum of physical processes. These scales
can produce structures which result in increased primary
production and, thereby, available food for higher trophic
levels (prieur and Legendre 1988). More generally, subsurface chlorophyll maxima are most often observed in a stratified water column near the pycnocline. Peaks in phytoplankton abundance can also often be observed at horisontal density interfaces, such as fronts and upwelling areas (Simpson
et ale 1982, Seliger et ale 1981, Pingree et al 1982).
•
A vertical density discontinuity, which involves a steep
gradient in temperature or salinity will, by itself, act as
a barrier for sinking particles. Variation in density and
viscosity will have an influence on any particles with a
tendency to sink and, thereby, plant cells can accumulate at
distinct vertical interfaces within the water column (Yamamoto 1984). This process may account for.the development of
some subsurface chlorophyll maxima. However, subsurface
- 2 -
chlorophyll maximum can also be the result of in situ growth
by phytoplankton and/or be related to an increase in chlorophyll content per cell resulting from adaption to low
light at the pycnocline (Richardson et ale 1983).
It has been argued that growing phytoplankton populations
form at the pycnocline in order to exploit the optimal combination of. nutrient and light conditions available in a
stable water column (where surface waters may be nutrient
depleted and water below the pycnocline light limited). However, it remains to be demonstrated what or which factor
(s) trigger the development of subsurface chlorophyll maxima
occurring near the pycnocline. The purpose of this study was
to examine the response of the toxic dinoflagellate Gonyaulax tamarensis Lebour to the presence of a pycnocline under
controlled laboratory conditions. This organism is frequently found in subsurface (pycnocline) maxima during naturally
occuring blooms. Thus, understanding the mechanism of the
formation of these subsurface maxima is crucial to an understanding of ~ tamarensis bloom formation.
Materials and methods
•
I. culturing of Gonyaulax tamarensis .
A unialgal culture of Gonyaulax tamrensis (clone designation
"22") was supplied by the Institute for Sporeplants (University of Copenhagen). The culture derives from Cjaldavik,
Suduoy, in the Faroe Islands. The algae were maintained in
batch-culture (18 +/- 2°C, no aeration) in "Bl"-medium
(Klein Breteler pers. com.) and under continuous i1lumina-
-
3 -
tion (100 pmol m- 2 s- 1 ), measured with a spherical quantum
sensor (Li-cor, Li 185 B). Cultures were maintained in both
19 0/00 and 29 0/00 salinity. The flasks were shaken once or
twice a week, to maintain the algae in suspension and to mix
the culture prior to sampling or the set up of experiments.
For experiments in which Q. tamarensis cells started at the
same salinity in which they had already been growing, a large (ca. 10 1) exponentially growing culture was established. This was diluted (50 %) with fresh medium and used
directly as inoculum in the large tank. In cases where cells
were given a salinity shock at the onset of an experiment, a
given volume «1 1) containing cells was added to 20 1 of
the new medium immediately prior to inoculation of the large
tank. The same procedure, with volume scaled appropriately,
was used in the small water columns.
•
Counting of cells was done in a sedimentation chamber (1
ml) following the addition of Lugols Iodine. This method
proved more reliable than using either a Coulter Counter (TA
11) or haemocytometer (Fuchs Rosenthal). The highest cell
concentration of G. tamarensis observed in the culture was
-1
about 10.000 cell ml
and the maximum growth rate was measured to be 0.32 div./day. These results agree well with results obtained by other workers (White 1978, Watras et al.
1982) for this species .
The effect of the addition of foodcolour (Brilliant blue
FCF, Acetic acid, 12 pl 1- 1 ) on the growth of G. tamarensis
was tested by comparing growth of the organism at 19 0/00
both with and without the addition of foodcolour. No significant difference was noted between growth rates with and
without foodcolour or between 19 and 29 0/00.
- 4 -
II.Experimental set up
4t
In the first series of investigations (1-7), 6 2liters columns were used. These were surrounded by black plastic in
order to eliminate interference from daylight. Photon flux
density at the surface of the water columns was 140 pmol m2 s -1 from fluorescent tubes. A ventilator circulated air
near the column and between the light source and the water
surface so that temperature was kept relatively constant
throughout the water column ( ~t < 0.5°C). In the second series of investigations (A-E), a 40 liter column (h = 60cm.,
diam. = 30cm.), made of transparent PVC, was used. It was
closed at both ends and equipped with an opening where media
could be added and samples removed. Fluorescent tubes were
placed above the column.
The column's design is shown in Figure 1. Photon flux density at the water surface was ca. 100 pmol m-2 s -1 • As with the
smaller system, air was circulated around the column to prevent temperature increases in the tank. Both set ups were
placed at constant temperature (19 +/_ 1°C) with a 12:12
(light/dark) period. The light period was from 0800 am to
2000 pm.
•
111. Sampling programmes •
In experiments 1-7, a pycnocline (halocline) was introduced
into the column and the response, in terms of vertical accumulation, of ~ tamarensis was determinated. Different
combinations (strength of halocline, preconditioning of
cells, etc.) were tested in the water column. In order to
produce the halocline, low salinity (19-22 0/00) medium was
- 5 -
placed in the column. Following stabilisation of this water
mass, high salinity water (ca. 29 0/00) was added through a
tube at the bottom of the column. This results in the lifting of the "lighter" low salinity water by the "heavy" high
salinity water,due to density differences. This procedure is
carried out slowly and carefully to avoid mixing between the
two water masses. G. tamarensis could then be introduced
either above or below the
pycnocline.
Prior to conducting the experiments, the stability of this
pycnocline was investigated over aperiod of days to determine how quickly molecular and turbulent diffusion mixed the
two water masses in the 40 1 tank. The results of this study
are shown in Figure 2. The pycnocline can be seen to be
slowly eroding. However, for the time frame of the experi.ments carried out here, the pycnocline can be considered to
be stable.
The conditions for experiments 1-7 and A-F are described in
Table I. The results from the numbered experiments aided in
selection of experiments to be conducted in the (more time
and labour consuming) 40 1 tank. In experiment C,
(40 1
tank) a continuous salinity gradient was established through
the water column. The purpose of this study was to examine
•
the possible accumulation of ~ tamarensis at a specific salinity. To create this gradient, two separate bottles, with
respectively 19
0/00
and 29
0/00
salinity, were serially
coupled to a peristaltic pump which delivered medium to the
experimental tank. The bottle containing 29
tained exponentially growing
Q.
0/00
also con-
tamarensis. A magnetic stir-
rer ensured a homogeneous salinity in the second bottle in
the series. DUe to mixing of the 19 0/00 with the 29 0/00
water, the salinity in the medium in the experimental tank·
-
6 -
decreased gradually from the bottom (ca.29 0/00) to the surface (ca.19 0/00).
For all experiments, sterile filtered seawater (0,22 pm) enriched with "B1"-medium was used. Where necessary, salinity
was lowered by the addition of distilled water, filtered
through a (0.22 pm) Millipore filter. Media and media containing cells were always added to the experimental tank
using a peristaltic pump. Microscopic investigations indicated no cell damage following passage through the pump. In
all studies, samples for cell enumeration were removed from
chosen depths and preserved. Counting took place within 1
week of sampling. Salinity was determinated by registration
of conductivity and temperature with a conductivity cell
(Radiometer, CDM 83) and calculated from supplied tables.
In experiments 1-7, sampling was carried out exactly 24
hours following experimental set up while, in the experiments using the 40 1 tank, sampling took place from 2 to 3
•
days after set up. This extra time was necessary due to the
larger volume to inoculum ratio in the 40 1 than in the 2 1
tanks.
In all experiments, samples were taken with greatest intensity within and around the pycnocline - if one
existed - and at the surface. To avoid mixing and contamination of samples by cells transported under sampling, sampling was carried out from the surface to the bot tom of the
water column. A theoretical analysis of the energy requirement of swimming activity by Q. tamarensis through homogeneous and a stratified water column was also carried out.
- 7 -
Results
Figure 3 shows vertical profiles of cell distributions in
experiments 1-3 and 7 presented as apercent of the total
number of G. tamarensis cells recovered through sampling.
The results from Experiment 1 are illustrated in Figure
3a. In this early experiment, a vertical salinity profile
was not determined. However, the surface to bottom salinity
difference was calculated to be (left to right in the figure) from 10 0/00 to 0 0/00 in intervals of 2 0/00. In the
two first columns, where ~s was calculated to be > 8 0/00,
the cells remained below the halocline. Where ~s < ca. 6
0/00, cells crossed the halocline and, in most cases, significant accumulations were noted at the surface. In this study, cell accumulations were also noted near the bottom of
the water column (but above the actual bottom). We cannot
explain this observation and it was never repeated in subsequent studies in which algae were added below the pycnoclineo
•
In Experiment 2 (Figure 3b.), similar salinity gradient were
established in the tank as in Experiment 1 and, again, the
cells used had been preincubated at 29 0/00. Here, however,
cells were added to the lower salinity water above the halocline. In all cases where a halocline was present, cells accumulated at the surface and the pycnocline. In the homogeneous water column (29 0/00) almost 80 % of the cells sampled were recovered at the surface.
In order to determine if there was an effect of osmotic
shock to the cells on the observed swimming patterns, cells
were preconditiored to 19 0/00 and added below the pycnocline (experiment 3; Figure 3c). In this case,cells remained
- 8 -
in high salinity water and concentrated at the pycnocline in
all columns where ~s 0/00 > 6. In columns with a weaker or
no salinity gradient, cells ente red the surface and significant cell accumulations (up to 80 % of total) were observed
at the surface itself. Thus, a salinity shock at the time of
experimental set up does not appear to affect the swimmming
pattern of the cells.
It was assumed that the swimming of cells toward the surface
is a function of phototaxis. In order to test this assumption, Experiment 7 (Figure 3d.) was carried out in which the
light source was moved to beneath the tank and cells (preconditioned to 29 0/00) were added above the pycnocline. In
this case, cells swam down towards the light. When ~ S 0/00
was> ca. 7 0/00, cells (up to 80 % of total sampled) were
recovered at the pycnocline. When presented with weaker salinity gradients cells penetrated the pycnocline and significant accumulations were observed near the bottom at the
highest photon flux density. (ca. 140 pmol photons m-2 s -1 )
•
The effect of the absence of inorganic nutrients (N0 and
3
P0 ) on the observed swimming pattern of ~ tamarensis was
4
examined in Experiments 4,5 and 6 (Figure 4). Experiment 4
is similar to Experiment 1 where cells are added below the
halocline. However, inorganic N and P were only added to the
medium above the salinity barrier. In Figure 4a, it can be
seen that most of the cells confronted with the low nutrient
medium, sedimented out of the water column and accumulated
at the bottom. Microscopic examine of these cells revealed
no swimming activity.
- 9 -
In Experiment 5 (Figure 4b), cells were added below the halocline in nutrient replete medium. Relatively weak salinity gradients were used ( ~S = 5 0/00 and 2 0/00) and no
inorganic nutrients were added to the surface water. Although cells had penetrated these salinity gradients in all
studies where nutrients were available on both sides of the
halocline, in this study, they remained at the halocline
ie.,in the upper layer of the nutrient replete water.
The physical set up in Experiment 6 (Figure 4c.) was the
same as in 5 (no nutrients above the pycnocline). However,
cells were added in the surface waters. In all cases, significant cell accumulations were observed at the pycnocline
and, in only one case, were significant numbers of cel1s
seen at the surface as would have been expected from the results of Experiments 2 and 7.
When no nutrients were added to either the surface (S = 19
0/00) or bottom (8 = 29 0/00) water and actively growing
cells were added above the halocline, non-motile cells accumulated at the bottom of the tank (results not shown).
To ensure that the observed results were not artifacts of
sampling time, diel studies of cell distribution patterns
were conducted in both a homogeneous (Figure 5) and a stratified water column (Figure 6). Major differences in the distribution patterns throughout the day were not observed.
However, the most pronounced accumulations at both the surface and the halocline occurred during the light period. During the dark period, cells were more homogenously distributed throughout the mixed water column or beneath the pycnocline in the stratified water column.
- 10 -
In Figure 7a, the response of G. tamarensis cultured at 29
0/00 and presented with an even salinity gradient from 29 19 0/00 is illustrated. In the absence of a sharp density
gradient, most cells accumulated near the surface, in the
spite of the fact that this surface water was considerably
lower in salinity than that in which the cells had been precultured.
Figure 7b shows the response of
Q.
tamarensis grown at 29
and added below a sharp halocline ( ~s = ca. 10 0/00)
in the 40 1 tank. As in the smaller tanks, the cells concen-2 -1 ).
trated at the density gradient (pfd = ca.37 pmol m s
However, when the experiment is repeated in all respects except that blue foodcolour (12 ~l 1-1) is added to the surface water so that the photon flux density at the halocline
-2 -1 , the cells penetrate the density
is only ca.10 pmol m s
gradient and accumulate at the surface (Figure 7c).
0/00
Discussion
Many dinoflagellates have been demonstrated to be positively
phototactic (Eppley et ale 1968, Hasle 1950) and in this
study, Q. tamarensis showed a phototatic response by swimming towards the light source. However, under nutrient replete conditions, when confronted with a sharp salinity gradient of > ca. 7 0/00, the organism interrupted its movement
towards light and accumulated at the salinity (density) gradient.
- 11 -
All published light response curves for this dinoflagellate
as weIl as our own unpublished data indicate that maximum
growth and photosynthetic rates are achieved at higher photon flux densities than those found near the pycnocline in
-2 -1
this study (37 pmol m s
). Glibert et al. (1988), for
example, report P
for this organism to be at > 400 pmol
-2 -1
max.
m s
. Thus, it seems unl~kely that the observed accumulation of Q. tamarensis at the pycnocline here is a response
to the light enviroment. That the cells, in the absence of a
halocline accumulate at the highest available photon flux
density also argues against light being the controlling factor in the pycnocline accumulation.
It also seems unlikely that this accumulation is a response
to salinity as Q. tamarensis presented with a range of salinities (from ca. 19 to 29 0/00) but where no sharp salinity
discontinuity was present, responded as though they were in
a homogeneous water column and concentrated at the highest
available photon flux density (Figure 7a).
We speculated that the osmotic shock involved in an abrubt
transition from 29 0/00 to 19 0/00 (or vice versa) might
hinder the organisms in migration through the pycnocline.
However, cells given a salinity shock of ca. 10 0/00 at
the onset of an experiment responded identically to those
preconditioned to the experimental medium (Figure 3c).
The possibility that a strong halocline presents a inpenetrable physical barrier for ~ tamarensis was also considered. However, when the light climate at the salinity barrier
was'sufficiently reduced, the organism showed itself to be
quite capable of traversing the pycnocline (Figure 7c). This
conclusion is supported by theoretical consideration of
the energy requirements for a model dinoflagellate (spheri-
- 12 -
-----
- - - - - - - -
cal cell with a radius of 40 pm) to swim through a homogeneous and a stratified water column.
In nature, typical diel vertical migrations are 5-10 m (Eppley et al. 1968, Blasco 1978) and dinoflagellate swimming
velocit;-appears to be from 200-500 pm s-1 (HasIe 1950,1954,
Hand et al.1965, Eppley et al.1968). If we assume that a
dinoflagellate moves vertically in the water column (either
up or down) for 12 out of 24 hours and swims at a rate of
500 pm s-l, then the organism swims a total of 10.8 m in the
vertical (Raven and Richardson 1984). We will use this value
of 10.8 m as the vertical distance migrated in 24 h by our
model dinoflagellate in the following analysis and assume
that, as in this study, vertical swimming is always towards
light (upwards). To carry out the comparison of the energetic requirements for the model dinoflagellate to swim 10,8
(upwards) in a homogeneneous and a stratified water column (
~s = 10 0/00), we make the following assumptions:
As the density of water increases nearly linearly with salinity (Rutther 1963), a 10 % difference in salinity will
cause a density difference of approximately .001. Viscoscity
is only minimally affected by salinity (Wetzel 1983) and
disregarded in our analysis.
The volume of our model dinoflagellate is 3.4*10- 14 m3 . If
we assume that its density is the same as the water in which
migration begins (30 0/00) , then the mass of our organism
is 3.5*10-11 kg . Using stokes Law, the viscosity of water (at
-2
-1
-1
.
20°C = 1.005*10 cm
9 s
: Alonso and Flnn 1980) and a
-1
swimming speed of the organism of 500 pm s
,we calculate
that the force of friction on our organism is 1.89*10- 9 N.
-
13 -
In both a homogeneous and a stratified water column, an organism must overcome the force of friction and buoyancy
(gravity) in order to swim vertically through the water.
Friction exerts essentially the same force on an organism in
both cases. However, buoyancy changes as a function of salinity. Here we compare the energy required to overcome friction and gravity in order to climb 10.8 m in a homogeneous
water column of 30 0/00 and in a heterogeous water column
•
where the cells must move through a water column where they
start at 30 0/00, immediately cross the halocline and swim
10.8 m in 20 0/00. The work required to swim is the sum of
the forces which act in opposite directions. We assurne in
this calculation that:
Work
sum of force of friction and gravity multiplied by
distance.
Thus, when looking at swimming in a homogeneous water column, we can ignore the effect of the force of gravity. (We
assurne that the density of our model organism is identical
to that of the surrounding enviroment). Thus, we get:
Work = friction * 10.8 m = 2.1*10- 8 J
and the energy required for swimming upwards for 12 hours in
a homogeneous water column is:
4.72*10- 14 W
•
We now assurne that the cell starts just below the pycnocline
in high salinity water (30 0/00) and moves up through a water column of low salinity water (20 0/00) without changing its density. That cell, then, must use more energy to
remain buoyant above the pycnocline, as the cell is now heavier with respect to the medium than the cell moving at all
times through a medium to which it is adapted. We calcula-
- 14 -
te, therefore, the energy demand to overcome the effect of
the force of gravity. We estimate the change in the effect
of gravity (g') according to Archimedes principle:
g'
.1g
where
.1
P-Pref
P
The "extra" work required, then, for a cell adapted to 30
to climb through a water column of 20 0/00 is:
11
Work = 3.45*10J
•
0/00
and, the increased energy demand will be:
P
= 7.98*10- 16 W
ave
Thus, the "extra"
energy required for model cell to climb 10.8
m through a stratified water column ( .15 = 10 0/00) is only
about 1 % of that required to climb through a homogeneous
water mass and it seems unlikely that this small increase in
energy demand would hinder migration through a salinity
barrier.
We are left,then, with the conclusion that Gonyaulax tamarensis cells are able to sense and respond to a sudden
•
change in density. Field studies on other dinoflagellate
species (Prorocentrum micans and Ceratium furca) suggest
that this ability to respond to sharp density diffences may
be widespread (Edler and GIlson 1985). However, field studies can be difficult to interpret as nutrient concentrations as weIl as other chemical parameters are seldom constant above and below the pycnocline.
- 15 -
The presence of a subsurface phytoplankton peak occurring
at the pycnocline has often been interpreted as an attempt on
the part of phytoplankton to exploit that region of the water column where the optimal combination of light intensity
and nutrient concentrations are available. No attempt has
been made, however, to elucidate whether it is actually nutrient concentration that triggers the concentration of
cells in the region where surface and bottom water meet. We
.show here that, even where both surface and bottom water are
nutrient replete, G. tamarensis will accumulate at the pycnocline at a less than optimal photon flux density.
•
•
In the total absence of added N and P, this dinoflagellate
ceases all migratory activity. Cells placed in nutrient
poor surface water do not seek high photon flux densities as
their counterparts in nutrient replete medium (s~e Figure
4c and 3b) but, instead, sink out and accumulate at the pycnocline. Cells introduced to nutrient replete bottom water do not cross a weak pycnocline into nutrient poor surface water although under identical physical conditions but
when surface water is nutrient replete, the organism will
seek the high surface photon flux density (Figure 4b and
3a). This failure of ~. tamarensis to migrate upwards in the
absence of nutrients is also supported by field studies from
Great Salt pond, ~lassachusetts, USA (Anderson and Stolzenbach 1985) .
In this study, when presented with nutrient replete medium
both above and below the pycnocline, ~. tamarensis did not
cross a salinity barrier of ~s > ca.? 0/00 when the incident
2 1
light level at this barrier was ca. 40 pmol m- s- . However, when the incident photon flux density was reduced to
.
.
b arca. 10 pmol m-2 s -1 ,~. tamarens~s
crosse d t h e d enslty
- 16 -
rier (~S = 10 0/00) and accumulated at the highest available
photon flux density (Figure 7c).
•
•
We were not able to demonstrate a marked diel migration pattern in our experimental water column. This is in contrast
to a number of field observations of dinoflagellate migration (i.e. Blasco 1978). Dur diel studies were, however,
carried out under nutrient replete conditions and with limited light attenuation through the water column (from 100 to
ca.10 pmol m-2 s -1 ). It may be possible to stimulate more,
marked migration through the entire water column by increasing light attenuation and decreasing surface nutrient concentrations. However, as the purpose of diel observations in
the context· of this study was to ensure that the observed
results were not,a function of sampling time, no effort was
made to stimulate a more marked migration pattern.
We conclude that the occurrence of peaks in the concentration of Q. tamarensis in relation to a pycnocline is a complicated function of nutrient, light and density conditions.
It may weIl be that, in nature, the formation of a pycnocline population allows the organism to utilise the high nutrient concentrations contained in bottom water while still
receiving enough light to maintain photosynthesis. However,
the results presented here indicate that it is not to light
and nutrient concentrations alone that the organism is responding when forming a pycnocline population. The density
of the enviroment (or change therein) also plays an important role in the occurrence of surface populations and must
also be considered as an underlying factor in phytoplankton
patchiness.
- 17 -
Acknowledgement
We would like to thank Carsten Jurgensen, (Institute of Hydrodynamics and Hydraulic Eginering, Danish Technical University) for help in computing the energy required for dinoflagellate migration .
•
•
- 18 -
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•
•
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- 19 -
•
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- 20 -
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•
•
-
21 -
Figure 1. Experimental set up. The dimensions of the tank
are diameter = 30 cm, height = 60 cm and material (PVC)
thickness = 0,5 cm.
Figure 2. Long-term breakdown of the halocline in the 40 1
tank. The graph presents the vertical conductivity throughout the water column. (D) = day 1, (e) = day 7, (~) = day
11 and (0) = day 14. No correction for vertical variation in
temperature has been made but controls showed only a 0,3°C
variation through the column.
•
Figure 3. G. tamarensis distributions and salinity depth
profiles for experiments 1-3 and 7. See Table I for description of experiments. G. tamarensis distributions are shown
as concentration of cells expressed in percentages of total
recovered through sampling in this and all other experiments. Only cells in the water column were sampled, i.e .
inactive cells lying on the bottom of the tank are not included. Salinity depth profiles are measured from samples
taken at the surface, just above the halocline, beneath the
halocline and near the bottom. When the concentration of G.
tamarensis was zero or indistingnishable from zero on the-graph, the percent distribution is labelled with (.).
Figure 4. G. tamarensis distributions and salinity depth
profiles for experiments 4-6. See Table I for description of
experiments. Scales and labels as for Fig. 3.
•
Figure 5. G. tamarensis depth profiles made through a 24hour perioo-in a homogeneous water column. Distributions
shown as concentration of cells at varying sampling depths.
a: (. ) = 1330 pm, (0) = 1 730 pm, b: ( .) = 2130 pm, ( 0) =
0130 am, c: (.) = 0600 am, (0) = 1000 am, (~) = 1200 am •
Figure 6. Vertical distributions of G. tamarensis through a
24h made in a stratified water column: Concentration of
cells a: (.) = 0830 am, (0) = 1300 pm, (~) shows the depth
profile of salinity, b: (.) = 1700 pm, (0) = 2130 pm, c: (.
) = 0130 am, (0) = 0700 am.
Figure 7. Salinity and G. tamarensis depth distribution profiles for; a: a water column with gradually increasing salinity from about 19 0/00 near surface to 29 0/00 at the bottom. b: a heterogeneous water column with a salinity difference across the halocline of ca. 10 0/00. c: the same water
column as in b but with the addition of foodcolour above the
pycnocline. Areas shown above the stippled line represent
the region where foodcolour reduced the incident light. Concentration of cells labelled (.) and salinity shown by (0)
in all graphs .
•
•
Table I. EXperimental program. The numbered experiment were
carried out in 2 liter water columns. The lettered experiments in a 40 liter column. G.tam. = Gonyaulax tamarensis
and indicates to which part of the water column the organism
was added. The number in parenthesis following indicates the
salinity of the pre-experiment culturing medium. (-NP) = nutrients are omitted. In all experiments where salinity is
presented as an interval, the step between the experimental
columns is ~s = 2 0/00 •
•
•
Table I.
Number and volume of column
Surface
water
Bottom
water
1
6x2 liters
19-29 0/00
29 0/00
G.tam.(29)
2
6x2 liters
19-29 0/00
G.tam.(29)
29 0/00
3
6x2 liters
19-29 0/00
4
6x2 liters
19-29 0/00
29 0/00, (-NP)
G. tam. ( 29 )
5
3x2 liters
24-29 0/00
(-NP)
29 0/00
G. tam. (29)
6
3x2 liters
19-24 0/00
(-NP)G.tam.
( 29 )
29 0/00
7
6x2 liters
19-29 0/00
29 0/00
G.tam.(29)
(light moved to bottom)
A
40 liters
Oiel study in a homogeneous
water column (29 0/00).
G.tam.(29) mixed in water
column at onset of experiment.
B
40 liters
Oiel study in a heteroge
neous water column (~S = 10
0/00). G.tam.(29) added be
low the halocline.
c
40 liters
Gradually increasing salini
ty from 19 to 29 0/00
throughout the water co
lumn.G. tam.(29) added
throughout the water column
(see text).
o
40 liters
Heterogeneous water column
with stratification (AS =
10 0/00),G.tam.(29) added
below the halocline.
E
40liters
Heterogeneous water column,
stratification ÄS = 10 0/00
foodcolour added above
the halocline G. tam added
below the halocli~
F
40liters
Timecourse of breakdown of
Experiment
•
29 0/00
G. tam. ( 19 )
stratification. Initial AS
=
10 0/00.
Figure 1.
tlI'"
@o
0000
I~
pump
j
+
©
uuuu
t
I~ sarnple
U
•
t
incubation
Figure 2 .
•
28
34
uctivity (mS/cm)
con d31
40
Figure 3.
o
cxpl. 1
(a)
10
20
0
I
0
10
E
u
\
\
0
\
\
0
20
o~
0 __0
0
\
\
expt. 2
(b)
'0
\
0
0
0
.......
\
\
0'0
0
0
0
0
0
0
0
..c
'-'
0.
QJ
'0
0
0
0
I
\
0
10
0
0
\
20
0
\
0
0
10
0
0
I
0
I
\
0
\
0
0
0
I
0 ..........
\
0
I
expt. 7
(d)
0
\
0
0
0
0 -0
-0
\
20
(c)
'0
0
0
expt. 3
0
0
0
0
I
0
40
80
0
40
I
I
I
I
I
20
24
28
20
24
80
J
28
0
40
j
I
20
24
e
80
I
28
0
I
20
40
I
24
80
0
40
80
0
40
80
I
I
I
I
(
I
I
28
20
24
28
20
24
28
e
%cells m1
-1
I
%0 salini ty
Figure 5.
.---~ - - 0,
a'
/.
0
•
/
/
,. ~O
• 0
\ I
10--
f
1
/
/
b
I.01'0:::----...........
~
0
~
\
~
40"~
dJ
i
-
•
\
•
-
1 \
o •
~j
\/
o
o
-
\•
Ir
.0
I
•
.0
50"- ~
c
-
r
>-I
30
0-
\/r
........
i
'"',
-0
ee.O
~
[) 20
6.~
I
I
. I
200
400
600
r.\~
I
o
100
200
cells/ml
300
o
200
I
400
I
600
Figure 6.
salinity ( %0)
0
20
I
e 0
24
28
/
v
20
S
.........
~
~
oe
.j.J
\\
tP.~Ä~
.
---Ä- e _
~
(J)
'0
30
e
==.
==Ä-O
0--
o~
~o~
o~
;0
e
s
odo
.0
/1
~o
I
•
Ä
200
o
~.O
•
6
oV
.Y
0
\• \
0
~
100
0
"·~O
r---
...,.
Ad
Y
100
e
./.
-::.---o-~
•
50
\\
\
0_
\ l
40
'\/
~.-- .=::
Ä
0
1?
•
1\
c
v
r~
0
~
•
\i
.0
u
b
11
C»
10
.\
a
l
•
0
200
cells/ml
•
300
o
100
200
Figure
19
7.
salini ty (%0)
19
29
24
24
29
19
24
a
c
•
10
reduced light
•
20
30
e
40
I
•
e_
•
50 ~
o
o
100
200
300
o
e
100
200
cellsjml
300
o
e
500
1000
1500