The role of zooplankton in the nitrogen cycle of a Mediterranean brackish lagoon
R. Gaudy

The water and zooplankton of a semi-closed brackish lagoon near Marseilles (Etang de Berre) were regularly sampled throughout an annual cycle in surface and bottom (-8 m) layers. The variation of the environmental factors and of the nitrogen biomass of seston and zooplankton were analyzed. During the same period, 12 sets of experiments were carried out to investigate the grazing and the excretion (mineral and total part) of zooplankton; the animals were fed with natural particulate material under prevailing temperature and salinity conditions. The copepod Acartia tonsa and the rotifer Brachionus plicatilis constituted the bulk of the zooplankton population which showed several seasonal peaks more or less related to the variation of the seston biomass. The daily food intake (in particulate volumes or in nitrogen equivalents) depended on temperature and seston-nitrogen biomass. The coarse fraction of zooplankton (size over 200 mm, mainly adult copepods) showed higher grazing rates (up to 91 % of the body nitrogen content) than the fine fraction (size under 200 mm, mainly copepods young stages or rotifers): maximum 34 % of body nitrogen weight. The whole zooplankton population used less than 1.7 % of seston nitrogen per day and, on an annual basis, about 8 % of the "new" primary production. The excretion rate of N-NH4 varied seasonnaly between 2.1 and 50.6 % of zooplankton "body-N" (coarse fraction) and 1.3 and 80 % for the fine fraction. Ammonia represented a highly variable proportion of total N excreted (10 to 100 %), depending on the season and on the quality of zooplankton, copepods excreting more ammonia than rotifers. Assuming an assimilation rate of 80 % and adopting the experimental values of food intake and excretion, a nitrogen budget was built in order to assess the production capacities of the population of coarse zooplankters (essentially composed of Acartia tonsa). In several cases, this assessment was negative or just sufficient to allow a small part of the reproduction needs. This discrepancy was supposed to result from an underestimation of the actual food intake in nature: small size seston from the water column must be used, but also other sources of food, particularly seston accummulated in the density interface with its bacteria, or animal prey.

Keywords: zooplankton, Berre lagoon, nitrogen cycle, secondary production.
Contents of this volume Sci. Mar. 53(2-3) : 609-616 Back PDF
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