The
trophic food web in the lagoon
We
have drawn up a simplified model for the trophic food webs in
the lagoons of Tikehau and Takapoto, based on all the data
obtained in the ATOLL, CYEL, PGNR1 and PGNR2 programmes.
The
biomasses of the various compartments are expressed in mg C m-2
(these values are integrated over depths of 25 m) and the
fluxes, in mg C m-2 day-1. It is thus
possible to model the planktonic ecosystem side by side with the
benthic ecosystem characteristic of the loose sediments. The
measurements on the biomass and the metabolism of the protozoa
were carried out by 2 different groups on Tikehau and Takapoto.
This explains why the heteroflagellata and ciliata on Tikehau
were classified in 2 different compartments , whereas the
protozoa on Takapoto were subdivided into
2 size classes, one smaller and one larger than 35 µm.
The measurements carried out on the food ingested by the
zooplankton were carried out using the method based on the
ratios between the C: N: P contents of the particles, and the
origin of the prey was therefore not taken into account.
|
Schemes
of the Tikehau and Takapoto atolls' trophic food
webs. |
|
Tikehau |
Takapoto |
|
P
= production, I = ingestion, Ex.= Exportation out of the
atoll, S = sedimentation, HNF=heteroflagellates. All
the fluxes (arrows) are expressed in
mg C m-2 day-1 and the
stocks (rectangles), in mg C m-2. |
A
large proportion of the particulate organic matter was found to
consist of debris. The biomasses of the phytoplankton,
heterotrophic bacteria and protozoa were all of a similar order
of magnitude. The highest level of production was that of the
small coccoid cyanobacteria (0.5 g C m-2 day-1).
The protozoa feed on pico-cyanobacteria and heterotrophic
bacteria. The highest grazing rate was that of the meso-zooplankton
on Takapoto. The fact that this rate was higher than the rate of
phytoplankton production indicates that the zooplankton must
feed on organic particles originating from sources other than
the water column.
|
There
is a "microbial loop" in the atoll lagoons:
this means that the phytoplankton is produced by very
small cells called pico-cyanobacteria. These are
ingested by protozoa measuring less than 35 µm, on
which other protozoa of a larger size subsequently feed,
and these are then ingested in turn by the zooplankton.
The further requirements of the zooplankton are supplied
by particles of debris produced at the benthic level.
|
Bibliography :
Charpy-Roubaud
C.J., Charpy L., Lemasson L. (1988). Benthic and Planktonic
primary production of an open atoll lagoon (Tikehau, French
Polynesia). Proc. 6th Int. Coral Reef Symposium, Australia, 2:
551-556
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
Charpy
L., Charpy-Roubaud C.J. (1990). Trophic Structure and
Productivity of the lagoonal communities of Tikehau atoll
(Tuamotu Archipelago, French Polynesia). Hydrobiologia,
207 : 43-52
Charpy
L., Charpy-Roubaud C.J. (1991). Particulate Organic Matter
fluxes in a Tuamotu Atoll Lagoon (French Polynesia). Mar.
Ecol. Progr. Ser, 71 : 53-63
Charpy
L. (1996) Phytoplankton biomass and production in two Tuamotu
atoll lagoons (French Polynesia). Mar Ecol Progr Ser 145 :
133-142
Niquil
N, Pouvreau S, Sakka A, Legendre L, Addessi L, Le Borgne R,
Charpy L, Delesalle B (2001) Trophic web and carrying capacity
in a pearl oyster farming lagoon (Takapoto, French Polynesia).
Aquatic living resources 14: 165-175
|
Atoll_site_