P-limited bacteria but N and P co-limited phytoplankton in the Eastern Mediterranean - A microcosm experiment

An on-board microcosm experiment was set up to test the hypothesis that the observed lack of phytoplankton biomass increase response to a mesoscale in situ P-enrichment experiment in the P-limited Eastern Mediterranean (Krom et al., 2005a) was a consequence of co-limitation by P and N availability in this ultraoligotrophic environment. Six microcosms were filled with subsurface seawater (ambient DIN: 90-100 nM) taken from inside a P-enriched patch (IN), which in the absence of biological activity would have had ca. 22 nM of PO ₄^-3. Another six microcosms were filled with unfertilized (<2 nM PO₄^-3) subsurface seawater from outside the patch (OUT). The bottles were either supplemented with 1600 nM NH ₄⁺ or not, incubated on-deck and subsampled daily, or at the first and last (fourth) day of the experiment, for a suite of biological parameters. The addition of N to OUT water did not induce cell abundance increases in either the phototrophic or heterotrophic sides of the food chain, in line with previous assessments that the Eastern Mediterranean is not purely N-limited. The IN and OUT treatments, to which no NH₄⁺ was added, mimicked the behavior of the in situ experiment, with an order of magnitude higher bacterial production of IN vs. OUT water, but no noticeable phytoplankton response. The addition of N to IN water, previously exposed to P, led to substantial responses of the entire microbial community, including 4 to 80-fold increases in chlorophyll, other pigments, bacterial activity, and the abundance of ciliates - relative to IN water to which N was not added. The ca. 10-fold increase in chlorophyll within 4 days was mostly due to a major increase in both abundance (×4) and fluorescence per cell (×17) of Synechococcus, whereas Prochlorococcus disappeared. These changes were accompanied by removal from the water of 570 nM of the added NH₄ ⁺, equivalent to 570/22 or N: P ratio of 26: 1, similar to the ratio measured for POM in the area. Possibly, non-Redfield ratios were maintained, still leaving by day 4 some 1100 nM of N that could not be used due to the lack of P. These results support our hypothesis that the lack of response of phytoplankton to the mesoscale P-enrichment was due to their concurrent N-starvation, i.e. N and P co-limitation. In contrast, bacteria could grow when only P was added, implying pure P-limitation. Thus, the heterotrophic and autotrophic components of the same aquatic community experienced different limitations.