The contribution of heterotrophic plankton to nitrogen (N) regeneration in the water column, and its significance for the requirements of phytoplankton, were studied at the seasonal scale in the coastal upwelling ecosystem of A Corun˜a (Galicia, NW Spain). During 1995-1997, monthly measurements were taken of hydrographic conditions, dissolved nutrients, and abundance and biomass of microplanktonic heterotrophs (bacteria, flagellates and ciliates), phytoplankton and mesozooplankton (>200 _m). Additionally, series of experiments were conducted to quantify N fluxes, including primary production (14C method), phytoplankton uptake of nitrate, ammonium and urea (15Nlabelling techniques), microheterotrophic regeneration of ammonium, mesozooplankton grazing (chlorophyll gut-content method) and excretion of ammonium by mesozooplankton. Two N budgets were built for the average situations of high (>100 mg C m_2 h_1) and low (<100 mg C m_2 h_1) primary production. The results revealed that phytoplankton relied strongly on regenerated ammonium all year round (33 and 43% of total N uptake in high and low production situations, respectively). This demand for ammonium was closely matched by regeneration rates of microplankton (0.14-0.25 mmol N m_2 h_1), whereas zooplankton contributed on average <10% to N regeneration. Likewise, zooplankton grazing had little direct control on phytoplanktonic biomass. The results obtained indicate that in the A Corun˜a upwelling system, N biomass of heterotrophic plankton is generally higher than phytoplankton N biomass. The high rates of N regeneration measured also suggest that a large proportion of the organic matter produced after an upwelling pulse is recycled in the water column through the microbial food web. |
