Response of Bacterial Communities to Rainwater Additions in the Oligotrophic SE Mediterranean Coast; Experimental and In-Situ Observations

Wet deposition may serve as a source of external nutrients for phytoplankton and heterotrophic bacteria in low nutrients low chlorophyll (LNLC) marine systems. Here, we examined the effects of rainwater deposition on phytoplankton and bacterial abundance and activity in the LNLC SE Mediterranean coast. In-situ observations and experimentally controlled incubations show enhanced abundance and productivity of the bacterioplankton communities attributed to the addition of rainwater-born nutrients. While rainwater relieved N limitation to phytoplankton resulting in enhanced abundance and primary production of autotrophic microbial communities (mostly pico-eukaryotes), no single nutrient (N or P) could be identified as a sole limiting factor of the stimulated heterotrophic bacteria. In both the experimental and in-situ observations, the changes in phytoplankton biomass following rainwater deposition/addition was observed only after 24–48 h. Contrary, an increase in primary and bacterial productions was visible within a few hours, suggesting high nutrients turnover rates by bacterioplankton communities following rain deposition. These results emphasize the importance of short-term wet deposition events to new N production and phytoplankton temporal variability in the LNLC SE Mediterranean coast.