Lack-of P-limitation of phytoplankton and heterotrophic prokaryotes in surface waters of three anticyclonic eddies in the stratified Mediterranean Sea

By Tsuneo Tanaka, T. Thingstad, U. Christaki, J. Colombet, Véronique Cornet‐Barthaux, C. Courties, Jean-David Grattepanche, A. Lagaria, J. Nedoma, L. Oriol and 3 more. 2011

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We investigated the identity of the limiting nu- trient of the pelagic microbial food web in the Mediter- ranean Sea using nutrient manipulated microcosms during summer 2008. Experiments were carried out with surface waters at the center of anticyclonic eddies in the Western Basin, the Ionian Basin, and the Levantine Basin. In situ, the ratio of N to P was always higher in both dissolved and particulate organic fractions compared to the Redfield ra- tio, suggesting a relative P-starvation. In each experiment, four different treatments in triplicates (addition of ammo- nium, phosphate, a combination of both, and the unamended control) were employed and chemical and biological param- eters monitored throughout a 3-4 day incubation. Temporal changes of turnover time of phosphate and ATP, and alkaline phosphatase activity during the incubation suggested that the phytoplankton and heterotrophic prokaryotes (Hprok) communities were not P-limited at the sites. Furthermore, statistical comparison among treatments at the end of the in- cubation did not support a hypothesis of P-limitation at the three study sites. In contrast, primary production was con- sistently limited by N, and Hprok growth was not limited by N nor P in the Western Basin, but N-limited in the Ionian Basin, and N and P co-limited in the Levantine Basin. Our re- sults demonstrated the gap between biogeochemical features (an apparent P-starved status) and biological responses (no apparent P-limitation). We question the general notion that Mediterranean surface waters are limited by P alone during the stratified period.

Subfields: Ecology, Oceanography

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