Temperature and phosphorus interacts in controlling the picoplankton carbon flux in the Adriatic Sea: an experimental versus field study.

Temperature and phosphorus positively interacted in controlling picoplankton biomass production and its transfer towards higher trophic levels. Two complementary approaches (experimental and field study) indicated several coherent patterns: (1) the impact of temperature on heterotrophic bacteria was high at temperatures lower than 16°C and levelled off at higher temperatures, whereas this impact on autotrophic picoplankton was linear along the entire range of the investigated temperatures; (2) the addition of phosphorus increased the values of picoplankton production and grazing, but did not change the nature of their relationships with temperature substantially; (3) the picoplankton carbon flux towards higher trophic levels was larger during the warmer months (grazing by HNF dominated during the warmer period and by ciliates during the colder period) and also strengthened in conditions without phosphorus limitation; (4) the hypothesis that the available phosphorus can be better utilized at higher temperatures was confirmed for both autotrophic and heterotrophic picoplankton; (5) the hypothesis that the rise in temperature stimulates growth only in conditions of sufficient phosphorus was confirmed only for heterotrophic bacteria. Therefore, in the global warming scenario, an increase of the picoplankton carbon flux towards higher trophic levels can be expected in the Adriatic Sea, particularly under unlimited phosphorus conditions.

Relations between mercury fractions and microbial community components in seawater under the presence and absence of probable phosphorus limitation conditions.

Microbial transformations of toxic monomethylmercury (MMHg) and dissolved gaseous mercury (DGM) at the lower levels of the marine food web are not well understood, especially in oligotrophic and phosphorus-limited seas. To examine the effects of probable phosphorus limitation (PP-limitation) on relations between mercury (Hg) fractions and microorganisms, we determined the total mercury (THg), total methylated mercury (MeHg), DGM, and microbiological and chemical parameters in the Central Adriatic Sea. Using statistical analysis, we assessed the potential microbial effects on Hg transformations and bioaccumulation. Only in the absence of PP-limitation conditions (NO-PP-limitation) is MeHg significantly related to most chemical and microbial parameters, indicating metabolism-dependent Hg transformations. The heterotrophic activity of low nucleic acid bacteria (abundant in oligotrophic regions) seems responsible for most of Hg methylation under NO-PP-limitation. Under these conditions, DGM is strongly related to microbial fractions and chlorophyll a, indicating biological DGM production, which is probably not metabolically induced, as most of these relations are also observed under PP-limitation. MMHg biomagnification was observed through an increased bioaccumulation factor from microseston to mesozooplankton. Our results indicate that Hg transformations and uptake might be enhanced under NO-PP-limitation conditions, emphasizing their impact on the transfer of Hg to higher trophic levels.