Nearshore shallow waters (i.e. < 2 km from the coast and < 20 m depth) represent a transitional area between land and open ocean where key marine ecological processes occur. During summer, when oligotrophic conditions prevail offshore in the Mediterranean Sea, an alongshore zone with enhanced phytoplankton biomass is often evidenced in these waters. Here, using continuous underway measurements as well as discrete samples, we describe the cross-shore environmental typology and the associated microbial planktonic communities of these waters. Cross-shore temperature, salinity and fluorescence transects obtained from coast to offshore reveal widespread presence of modified water bands generated by nearshore temperature warming and diffuse groundwater discharges in the Mediterranean Sea. Intense phytoplankton biomass buildup was associated with these modified waters (up to 50 fold offshore chlorophyll). We show that the physical and chemical characteristics of this nearshore stripe dramatically vary at scales of hours following a diurnal cycle that is regulated by heating and wind forcing. Plankton communities characterized by increased microplankton and bacterioplankton populations are able to actively exploit these nearshore conditions, constituting an independent and distinct assemblage from that prevailing further offshore. Our study argues the relevance of the formation of a nearshore niche and its temporal stability for the development of a distinct microbial community which could eventually favor the emergence of harmful algal blooms

Association for the Sciences of Limnology and Oceanography (ASLO) Aquatic Sciences Meeting, Mountains of the Sea, 26 February - 3 March 2017, Honolulu, Hawai`i

Peer Reviewed