Biological response to P addition in the Eastern Mediterranean Sea. The microbial race against time

The response of the microbial food web to P-addition was studied during a 10-day Lagrangian experiment in the Eastern Mediterranean during which orthophosphate was added to the surface water of the Cyprus anticyclonic eddy. Very low levels of all microbial populations (heterotrophic bacteria, Synechococcus, Prochlorococcus, autotrophic and heterotrophic nanoflagellates, ciliates) and bacterial production were detected, verifying the extreme oligotrophic character of this area. The microbial biomass was dominated by heterotrophs and the heterotroph/autotroph ratio increased from 1.05 before to 2.8 after the P-addition, on day 4. Bacteria took advantage of the supply of the putative limiting factor (P) increasing their production but not their abundance. The heterotrophic nanoflagellates remained stable in numbers. By contrast, Synechococcus and autotrophic nanoflagellates decreased after the P addition. This is an indication of consumption by ciliates which were the only organisms that showed a significant increase in abundance during the first 4 days after the P addition, relative to the period before the addition as well as to the second phase of the experiment (days 5–9). In particular, the mixotrophic ciliate biomass increased by 50% after the P release. In environmental conditions of general resource scarcity as is the case of the Eastern Mediterranean, the addition of the putative limiting factor (P) was reflected in the increased abundance of microzooplankton shortly after the enrichment as a result of a quick transfer of energy (“heterotrophic by-pass”) through bacteria and heterotrophic nanoflagellates. A “mixotrophic by-pass” of phytoplankton primary producers also occurred, transferring the P-addition driven primary production to higher trophic levels through mixotrophic ciliates.