Local differences in phytoplankton-bacterioplankton coupling in the coastal upwelling off Galicia (NW Spain)

Abstract

We simultaneously studied microbial-mediated carbon fluxes at 2 contrasting sites within the coastal upwelling system off Galicia, near A Coruna and Vigo, Spain, over an annual cycle in order to compare the fraction of primary production released as dissolved organic carbon (DOC) and the degree of coupling between bacteria and phytoplankton. A significant fraction of primary production was released as DOC at both sites, averaging ~30%. DOC release ac counted for, on average, 30% of the total bacterial carbon demand, which is indicative of a moderate trophic dependence of bacteria on phytoplankton in these coastal ecosystems. Nevertheless, dif- ferences in hydrographic conditions associated with stronger upwelling pulses off Vigo, and deeper upper mixed layers during the downwelling period off A Coruna, led to significant differ- ences in phytoplankton dynamics and the subsequent direct coupling with heterotrophic bacteria. Strong direct coupling between phytoplankton extracellular release and bacterial production (BP) was found off Vigo, which could be related to the quality of the DOC produced by actively grow- ing phytoplankton. By contrast, DOC release and BP rates were decoupled off A Coruna, likely due to unaccounted DOC associated with indirect trophic processes or to the low availability of freshly produced exudates associated with overflow or photoinhibition mechanisms.