Micro
and Mesozooplankton
Methods
used
to study the zooplankton
The
zooplankton biomass was monitored on Tikehau for 15 days in
April 1985, and then once a week from April 1985 to April 1986.
The biomass of the micro- and meso-zooplankton was monitored for
a period of two years on the atoll of Takapoto.
Studies
on the metabolism were carried out on Tikehau in April 1985 and
April 1986 and on Takapoto in November 1993 and February 1994.
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The
zooplankton were sampled on Tikehau and Takapoto using
vertical drag-nets, with a
WP2 net to catch meso-zooplankton (mesh size: 200 µm)
and a net with a 35-µm mesh to catch micro-zooplankton.
(see
figure)
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The
biomass was expressed in dry weight (DW) and ash-free dry weight
(AFDW). The animals were collected in the vertical drag-nets at
9:00 a.m. every day. After being screened with a mesh size of
200 µm (micro-zooplankton) and 2 mm (meso-zooplankton), they
were placed on pre-weighed 35- and 100- µm Nylon filters,
respectively. They were then rinsed with 100 ml of soft water to
remove the salt and left to dry at a temperature of 60° C for
24 h. The dry weight was determined to within 1/10 mg using
Metler scales. The samples were then placed on pre-weighed
aluminium cover slips previously baked at 450° C for 3 h in
order to prevent any organic contamination. The ash weight was
determined to within 1/10 mg by weighing the samples after
baking them at 550° C for 1 h 30.
The
ash-free dry weight is the difference between the dry weight and
the ash weight. It was decided to use this parameter rather than
the dry weight since it corresponds to the organic matter, the
fraction of the biomass which can be assimilated by the higher
organisms.
The
biomass is also sometimes expressed in DW m-3, the
mean concentration between the bottom and the surface, and in DW
m-2, the total biomass in the whole water column.
The
meso-zooplankton were collected using the same method as that
used to determine the biomasses and fixed with 10% formaldehyde
upon landing. The animals were sorted and counted with
binocular microscope.
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The
zooplanktonic production, assimilation and ingestion rates
were assessed on the basis of the C/N/P ratios. This
method gives the production rate (PR) based on the
excretion rate (ER) and the net production yield (K2)
(Le Borgne 1978).
K2
was calculated for phosphorus (K2P) and
nitrogen (K2N) based on the atomic N/P ratio
between the preys (a1), the excretion (a2),
the zooplankton produced (a3) and the
ratio (a4) between the assimilation
coefficients (D) as follows:
PR=ER
x K2/(1-K2)
Rate
of assimilation TA = PR+ER
Rate
of ingestion TI = (PR+ER)/D
Coefficient
of assimilation D = (f'-e')/f' x (100-e')
where
: e' = organic content of the faeces (%)
and f' = organic content of the particles (%)
(Conover
1966). |
After
screening the plankton using filters with mesh sizes of 200 µm
and 35 µm, it was crushed. One hundred µl of the diluted pulp
thus obtained was then transferred with a pipette to a
pre-weighed aluminium
crucible which was previously baked at a temperature of 450 °C
for 3 h in order to prevent any organic contamination. The
crucibles were then dried in a drier at 60° C before being
weighed to within one µg, and the C, N and P contents were then
determined.
One
litre of sea water screened with a 35-µm mesh filter was re-filtered
using a 25-mm GF/F filter, rinsed with HCl
(0.1 N), and baked at a temperature of
450° C for 3 h and pre-weighed. These filters were
deep-frozen and kept for analysis using the same method as that
described above.
The
meso-zooplankton collected in the vertical drag net was left in
a beaker for 1 h 30 in the shade. The pellets deposited by
copepods (which were the most easily visible) were transferred
with a pipette to baked crucibles prepared in the same way as
for determining the elementary composition. By comparing the C,
N and P contents of the particles and those of the fecal
pellets, it was possible to calculate the assimilation
coefficient ( assimilated food /
ingested food) using the method described by Conover
(1966).
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Control
test-tubes were prepared by filling them with sea water
filtered with a 35-µm mesh filter. The experimental tubes
were those containing plankton collected with a vertical
drag net at 17 h30 p.m. and filtered with mesh sizes of 35
et 200 µm and aspirated at random. The latter tubes were
then placed in a crate which was immersed 50 cm below the
surface for approximately 12 h (the time required for the
excretion rate to stabilize) (Le Borgne et al., 1989). The
water was then analyzed to determine the dissolved oxygen,
PO4, NH4, nitrogen and total
phosphorus contents and the incubated
organisms were collected on 47-mm GF/F filters
which had been pre-weighed and baked. The incubation of
the total zooplankton carried out under these conditions
can be said to faithfully reproduce what occurs in the
animals' natural habitat. |
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The
differences in the O2, NH4, PO4,
NT and PT contents in comparison
with the control samples were calculated per mg of dry
weight in order to obtain the respiratory and excretion
rates.
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|
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The
atomic ratios were calculated between the respiratory
rates(O), the mineral nitrogen and phosphorus excretion
rates (NH4 and PO4), and the total
nitrogen and phosphorus excretion rates (NT
and PT). These data provide information
about the nature of the oxidated substrate and the rate of
assimilation by the zooplankton. |
Results
Table
1 :
Respective
contributions of the various size classes to the total
zooplankton
| Size
(size class) |
04/1985 |
04/1986
|
| Micro-zooplankton
35-200 µm |
11
%
|
49
% |
| Meso-zooplankton
200-500 µm |
16
% |
28
%
|
| Meso-zooplankton
500-2000 µm |
42
% |
24
% |
| Macro-zooplankton
>2000 µm |
31
% |
0
% |
|
Total |
46
mg m-3 |
31
mg m-3 |
43 %
of the organisms constituting the micro-zooplankton in the
Tikehau lagoon were found to measure <100 µm, and 73 %
of them were protozoa (tintinides, ciliata). There were very few
foraminifers or radiolarians present in these waters. Metazoans accounted for 27 % of the population, however. The
micro-zooplankton were found to form a living biomass with a dry
weight of 3.3 mg C m-3
.
Two
peaks in the meso-zooplankton populations were found to occur in
October. Microscopic observations showed the occurrence of
periodic blooms of copepods, larvae, pteropoda and salps.
The mean biomass was 5 times larger in the lagoon (dry weight:
35 mg m-3) than in the surrounding ocean waters.
In
the atoll of Takapoto, the zooplankton are characterized by the
proliferation of little jelly-fish, Linuche unguiculata,
which the Paumotu call "kea
keas". The "kea keas" proliferated conspicuously
between October 1991 and March 1992, as well as during the two
campaigns carried out in November 1993 and February 1994,
whereas no such proliferation was observed in 1992.
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|
Mean
±
SE micro-zooplankton
biomass values ;
k = presence of " kea keas"
(Linuche unguiculata)
|
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Two
peaks in the micro-zooplankton populations were observed:
in August 1991 and in May 1992. The mean annual figure is
17.3 + 10.8 mg DW
m-3. |
|
Mean
±
SE meso-zooplankton
biomass values ;
k = presence of " kea keas"
(Linuche unguiculata) |
|
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The most abundant quantities of
meso-zooplankton were observed in July, after the peak
in the abundance of the micro-zooplankton. The lowest
quantitative levels were recorded in November 1993, and
these were attributed to predation by the jelly-fish.
the mean annual figure is 8.4 + 5.5 mg DW m-3,
which is only half the level recorded in the case of the
micro-zooplankton. |
A
marked increase in the biomasses of both the micro-plankton and
the meso-plankton occurred at night. No significant variations
occurred, however, during the daytime.
Mean ±
SE (n) micro-zooplankton
DW values, depending on the time of collection
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Mean ±
SE (n) meso-zooplankton
DW values, depending on the time of collection
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Among
all the size classes combined, copepods were found to be by far
the most abundantly occurring taxon, since they accounted for
87% of all the animals collected. 85% of them were Paracalanus
parvus, and these were followed by Calanopia minor.
The other taxa found to be present were mainly
gasteropod larvae living among the mero-plankton: the
former accounted for 70 % of the total number of individuals
constituting the mero-plankton. The only carnivores observed were
chaetognathes, including two species of Sagitta. One
appendicular species, Oikopleura fusiformis, was observed,
which accounted for only 0.6 % total number of individuals. (See
the tables giving the taxonomic patterns on Takapoto).
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Mean
percentages corresponding to the main taxa observed in
November 1993 and February 1994
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On
Tikehau :
The
ingestion of zooplankton of all kinds measuring >35 µm
amounts to 31 % to 65 %
of the total
estimated
phytoplankton production. The non-organic excretion of
zooplankton provides 32 - 18 % of the N requirements and 17 %
of the P requirements of the phytoplankton. The zooplankton
production rate amounts to 15 to 19 % of the total
phytoplankton production rate.
On
Takapoto:
the rates of metabolism of the micro- and meso-zooplankton were
found to be very similar.
The
micro-zooplankton production and ingestion rates were both
distinctly higher on Takapoto than on Tikehau, whereas those
recorded in the case of the meso-zooplankton were of the same
order of magnitude on both atolls.
Table
2 :
rates of metabolism (mg C m-3 j-1)
recorded in the zooplankton on Tikehau and Takapoto
| Atoll |
Size |
Production |
Ingestion |
| Tikehau |
micro-zooplankton |
0.2
à 1.2 |
0.6
à 2.6 |
| |
méso-zooplankton |
4.1
à 4.2 |
10
à 18 |
| Takapoto |
micro-zooplankton |
5.24 |
13.7 |
| |
méso-zooplankton |
3 |
14 |
Conclusions
-
The
meso- and micro-zooplankton inhabiting the atoll lagons
consist mainly of copepods. In some lagoons, the jelly-fish
propagate regularly.
-
Episodic
blooms occur all year round, and a peak in the biomass was
observed in October.
-
The
zooplankton measuring >35 µm consume 31 % to
65 % of the total phytoplankton produced, and the
zooplankton production accounts for 15 to 19 % of the
total phytoplankton production.
This
page was based on :
Charpy
L., Langy S., Le Borgne R., Lo L., Rochette J.-P. (1994) Etude de
la perte de Matière Organique Particulaire pour la nacre par les
compétiteurs planctoniques. Rapport définitif de la Fiche 14 du
PGRN (EVAAM)
Le
Borgne R.P., Blanchot J., Charpy L. (1989). Zooplankton
of the atoll of Tikehau (Tuamotu Archipelago) and its relationship
to particulate matter. Marine
Biology 102 : 341-353.
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