SECONDARY PRODUCTION OF THE BRACKISH COPEPOD COMMUNITIES AND THEIR CONTRIBUTION TO THE CARBON FLUXES IN THE WESTERSCHELDE ESTUARY (THE NETHERLANDS)

The zooplankton community of the brackish part of the Westerschelde estuary (November 1989-0ctober 1990) was dominated by two calanoid copepods, Eurytemora affinis and Acartia tonsa. Eurytemora was present during a longer period of the year and was much more important in terms of total abundances and biomasses than Acartia. The secondary production of these species was estimated by means of the growth rate method, using weight-specific growth rates obtained from laboratory cultures (Eurytemora) or from the literature (Acartia). Due to the substantially higher growth rates of Acartia compared to Eurytemora, total yearly productions of both communities were comparable, notwithstanding the large discrepancies in biomass. They amounted to about 5 and 6 g C m(-2) y(-1) by Acartia and Eurytemora respectively. The food needed to realise this production was estimated to be about 14 and 17 g C m(-2) y(-1) by Acartia and Eurytemora respectively. Provided that the copepods are able to selectively ingest the phytoplankton, in situ net production provides sufficient carbon for zooplankton demands for a short period of the year only. As phytoplankton standing stock is very low and net phytoplankton productivity is negative from late fall to early spring, nutritional demands of the copepods have to be fulfilled by other than algal food at least during this period of the year. Although the copepods in the brackish part can have an important impact on some food items, their contribution to total carbon fluxes in the brackish zone is negligible: each year some 6% of all consumed carbon in the brackish part of the estuary passes through the copepod food web.