Effect of P and N addition to oligotrophic Eastern Mediterranean waters influenced by near-shore waters: A microcosm experiment

Phosphate (P), nitrate (N) or P+N added in a microcosm experiment to oligotrophic waters of the Eastern Mediterranean influenced by near-shore waters triggered a range of responses in the autotrophic and heterotrophic compartments of the system. Chlorophyll a increased in all treatments, including the no-addition control, implying that nutrients became available also from internal sources (recycling). Larger and faster biomass increase as well as a larger P utilization took place in the P+N treatments. Diatoms bloomed in the P+N treatments whereas coccolithophores bloomed following the addition of P ultimately reaching N-limitation. Bacterial activity responded with a transient peak to both low P-alone and N-alone additions (0.01 and 1 μM, respectively). For reasons not well understood, no such response was observed at higher P-alone additions (0.05 and 0.5 μM), whereas at the two highest P+N additions the positive response was delayed. We therefore were unable to conclude conclusively on bacterial limitation. In most cases, the increase in bacterial activity was not matched by an increase in abundance, suggesting a tight top-down control of the biomass. Instead, heterotrophic nanoflagellate and ciliate abundances increased in all treatments. A slightly elevated orthophosphate turnover-time (T_t) (32 h) in the initial waters did not give a clear indication of P-limitation, although the system could absorb the lowest P-addition (0.01 μM) without increase in T_t. N alone lead to a reduction in T_t as would be expected in an N-limited system consuming existing surplus P after N-addition. The response of the near-shore influenced system used in this study was in accord with the 'classical' response to nutrient introduction - increase in chlorophyll a and in large size phytoplankton. In contrast, in the ultraoligotrophic Cyprus Eddy [Krom, Thingstad, Carbo, Drakopoulos, Fileman, Flaten, Groom, Herut, Kitides, Kress, Law, Liddicoact, Mantoura, Pasternak, Pitta, Polychronaki, Psarra, Rassoulzadegan, Skjoldal, Spyres, Tanaka, Tselepides, Wassmann, Wexels-Riser, Woodward, Zodiatis, Zohary, 2005. Overview of the CYCLOPS P addition lagrangian experiment in the Eastern Mediterranean. Deep-Sea Research II, this volume.], the short T_t (<4 h) indicated P-limitation, the combined addition of P and N (as ammonium) induced a bloom of picocyanobacteria [Zohary, Herut, Krom, Mantoura, Pitta, Psarra, Rasssoulzadegan, Stambler, Tanaka, Thingstad, Woodward, 2005. P-limited bacteria but N&P co-limited phytoplankton in the Eastern Mediterranean - a microcosm experiment. Deep-Sea Research II, this volume.] and the in situ P alone addition led to a decrease in chlorophyll.