Impact of nutrient enrichment on productivity of coastal water along the SE Mediterranean shore of Israel - A bioassay approach

The coastal waters of the southeastern Mediterranean-Sea (SEMS) are routinely enriched with naturally-occurring and anthropogenic land-based nutrient loads. These external inputs may affect autotrophic and heterotrophic microbial biomass and activity. Here, we conducted 13 microcosm bioassays with different additions of inorganic NO₃-(N), PO₄-(P) and Si(OH)₄-(Si) in different seasons along the Mediterranean coast of Israel. Our results indicate that cyanobacteria are mainly N-limited, whereas N or Si (or both) limit pico-eukaryotes. Furthermore, the degree to which N affects phytoplankton depends on the ambient seawater's inorganic N and N:P characteristics. Heterotrophic bacteria displayed no response in all treatments, except when all nutrients were added simultaneously, suggesting a possible co-limitation by nutrients. These results contrast the N + P co-limitation of phytoplankton and the P-limitation of bacteria in the open waters of the SEMS. These observations enable the application for a better science-based environmental monitoring and policy implementation along the SEMS coast of Israel.