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. Plain Language Summary Rainwater contain high concentrations of nutrients required for life such as nitrogen and phosphorus compounds. Therefore, rainwater deposition over the oceans may be an important source of external nutrients to phytoplankton (contrary to nutrients regeneration within the water column), especially in nutrients‐poor marine areas. In this study, we examined the effects of rainwater deposition on “bottom of the food‐web” microorganisms, namely phytoplankton and heterotrophic bacteria, in the SE Mediterranean coastal water. We used two approaches; follow in‐situ phytoplankton dynamics post rain events (between 2014 and 2019), and laboratory‐controlled incubation experiments where rainwater and nutrients were added to seawater and the temporal variability of the microorganisms was followed for several days. We show that rainwater relieved nitrogen limitation to phytoplankton, resulting in their increased primary production (i.e., photosynthesis) within a few hours and abundance after 24–48 h post addition/deposition. Contrary, rainwater‐born nitrogen or phosphorus could not be identified as the sole limiting factors for heterotrophic bacteria. These dif