Confirming the “Rapid phosphorus transfer from microorganisms to mesozooplankton in the Eastern Mediterranean Sea” scenario through a mesocosm experiment

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P-transport through the microbial food web was investigated in a mesocosm experiment in which orthophosphate was added to oligotrophic Cretan Sea water. As non-exclusive alternatives to traditional phytoplankton–zooplankton succession, two mechanisms for P-transport have previously been proposed: a stoichiometric shift in food quality (Tunnelling); and a transport through a predatory food chain from heterotrophic bacteria, bypassing phytoplankton (Bypass). Following P-addition, particulate C:P-ratio dropped from 436 to 44 (molar) within 1 day, and egg production increased after 2 days. This confirms the hypothesized stoichiometric shift and rapid copepod response of a “Tunnelling” scenario. Bacterial abundance responded positively to P-addition on Day 1, ciliates increased after Day 5 and new egg production peaks occurred on Days 5 and 9; a succession suggesting additional Bypass transport although the response expected in heterotrophic flagellate abundance was not confirmed. A small, but statistically not significant, increase in Chl a in the 0.6- to 2-µm size fraction also suggests possible additional P-transport through a phytoplankton–zooplankton succession. On the basis of the magnitude of the stoichiometric change and the short delay in egg production response, we consider the Tunnelling mechanism to be the most likely signal initiating egg production, but a Bypass, and possibly a traditional succession, may have contributed to maintain the elevated egg production.

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