MEPS 158:41-50 (1997)  -  doi:10.3354/meps158041

Experimental examination of the effects of rainwater on microbial communities in the surface layer of the NW Mediterranean Sea

C. Klein*, J. R. Dolan, F. Rassoulzadegan

Marine Microbial Ecology Research Group, CNRS URA 2077, Station Zoologique, BP 28, F-06230 Villefranche-sur-Mer, France

The effect of rain-borne nutrients in spring on the marine microbial community was studied in Villefranche Bay (France), a system in which previous studies have suggested that competition exists within the microbial food web for nutrient salts. In 5 incubation experiments, conducted from May 1994 to March 1996, surface seawater was amended with 2 to 8% (v/v) rainwater additions. The additions yielded final nutrient concentrations from 0.98 to 5.93 µM of dissolved inorganic nitrogen (NH4+ + NO3- forms) and 0 to 0.05 µM of inorganic phosphorus (PO43-). In 4 out of 5 experiments, significant increases in chlorophyll a (chl a) were detected. The ratio of added N (µg l-1) to apparent increase in chl a (µg l-1) averaged about 100. The increases in chl a were correlated with increases in abundances of phototrophic nanoflagellates (PNAN). In contrast to chl a and PNAN stocks, no effect of rainwater addition was evident among the populations of autotrophic bacteria (Synechococcus sp.), heterotrophic bacteria, or the predators of bacteria, heterotrophic nanoflagellates. In one experiment, there was no significant effect of rainwater addition; however, nutrient concentrations of unamended seawater samples were high (1.0 µM DIN, 0.06 µm PO43-). Our data indicate that in late spring, when the NW Mediterranean is most likely nitrogen limited, rain-borne 'new nutrients' appear to benefit small eucaryotic phytoplankton, PNAN, rather than autotrophic or heterotrophic bacterial populations or larger phytoplankton.

Nitrogen · Rain · Bacterioplankton · Phytoplankton · Synechococcus

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