Nitrate-enrichment structures phytoplankton communities in the shallow eastern Mediterranean coastal waters. Phytoplankton communities in Eastern Mediterranean coastal waters

Phytoplankton biomass and activity is limited primarily by nitrate (NO3-) availability in many coastal environments. Here, we followed autotrophic microbial biomass, activity and community composition at a site strongly influenced by groundwater discharge (SGD) and a nearby nutrients-poor reference site at the oligotrophic Israeli shallow rocky coast (SE Mediterranean Sea) between 2011-2019. The surface water at the SGD-affected area had significantly higher NO3+NO2 (~10-fold) levels compared to the reference site, while no significant differences were observed for PO4 or NH4. This resulted in a significant increase in autotrophic algae biomass (~3.5-fold), which was mostly attributed to elevated Synechococcus (~3.5-fold) and picoeukaryotes (~2-fold) abundances at the SGD-affected site, as well as in elevated primary production rates (~2.5-fold). DNA sequencing of the 16S and 18S rDNA supported these findings. These results highlight the influence of SGD on the diversity, abundance and function of shallow-water microbial populations. Our observations are consistent with the results of recent studies showing that phytoplankton along the Israeli coast are likely nitrogen+silica limited, and may have important ecological and regulatory implications for environmental policy and management of coastal aquifers.