Trophic role and carbon budget of metazoan microplankton in northwest Mediterranean coastal waters

Abstract

We determined the feeding rates, trophic effect, and growth efficiencies of natural assemblages of metazoan microplankton from a coastal site in the northwest (NW) Mediterranean over a seasonal cycle in laboratory incubations. Micrometazoans, i.e., multicellular heterotrophic plankters between 20 and 200 µm, were mainly constituted by invertebrate larval stages. Copepod nauplii and copepodites dominated the community, except in April, when polychaete larvae dominated. We analyzed the grazing pressure of micrometazoans on chlorophyll a (Chl a; total and > 10 µm), nanoflagellates, phototrophic nanoflagellates, dinoflagellates, diatoms, and ciliates. Micrometazoans grazed on all the prey groups, with carbon‐specific ingestion rates ranging from 0.31 to 1.24 d−1. The gross growth efficiencies for the entire metazoan microplankton community, calculated as the slope of the linear regression relating specific growth rates vs. specific ingestion rates, varied between 0.27 and 0.39. The respiratory carbon losses of micrometazoans depended on temperature and ranged from 0.16 to 0.36 d−1, with a Q10 = 2. The average net growth efficiency, 0.41, was independent of temperature and food availability. Overall micrometazoans have higher specific growth rates than, but similar food conversion efficiencies to, mesozooplankton. The grazing effect on the standing stock of the different prey was < 1% d−1 for Chl a (total and > 10 µm) and < 2.5% d−1 for the other studied prey, which seems insufficient to exert relevant control on phytoplankton and protozoan dynamics. The inclusion of micrometazoans did not change appreciably our current view of the role of metazooplankton in marine trophic webs of NW Mediterranean coastal waters.