Phytoplankton response to a Lagrangian phosphate addition in the Levantine Sea (Eastern Mediterranean)

In order to test the hypothesis of P-limitation on primary production and microbial biomass, a mesoscale Lagrangian phosphate-enrichment experiment was performed in the warm core of the Cyprus Eddy (Eastern Mediterranean Sea) in May 2002. This study reports the effects of the phosphate addition on the phytoplankton community, by measuring induced changes in biomass, primary production, and community structure. Prior to the addition, primary production and chlorophyll a were very low ( 0.107 μ gC l - 1 h - 1 and 18 ng l - 1 , respectively), typical of the ultra-oligotrophic conditions prevailing in the area. The autotrophic community was dominated in terms of both abundance and biomass (60% of total chl- a ) by picoplankton (mostly Synechococcus). More than 90% of the autotrophic biomass was confined to particles < 10 μ m , while larger phytoplankton (diatoms, dinoflagellates, coccolithophores) were very scarce. Unexpectedly, the addition of P resulted in a decrease in phytoplankton biomass. Total HPLC-chlorophyll slightly declined to 11 ng l - 1 in the P-enriched patch, five days after the P-addition. As the patch was diluted away (7–9 days), chlorophyll concentrations returned to background levels. Similar trends were observed in chlorophyll determined fluorometrically and in primary production. Picophytoplankton and smaller nanophytoplankton abundance declined (by 49 and 65%, respectively) within the first four days of the experiment. At the same time, there was a small increase of larger nanophytoplankton (10– 20 μ m ) and microphytoplankton ( > 20 μ m ) species. Small nanophytoplankton (2– 10 μ m ) were the group most affected by the addition. These results together with an onboard microcosm experiment (reported elsewhere in this issue), in which ammonia was added to the P-enriched water, indicate that the system was N and P co-limited for phytoplankton. The decrease in pico and smaller nanophytoplankton was probably caused by increased predation by micrograzers that became more active because of the increased heterotrophic bacterial activity and/or the increased P content of their prey. The immediate decrease in Synechococcus numbers, as the system became temporarily N-limited implies that N-fixation caused by this particular cyanobacteria was unlikely to occur.