Seasonal variations in microbial abundance and transparent exopolymer particle concentration in the sea surface microlayer of temperate coastal waters

The sea surface microlayer (SML) is defined as the less than 1 mm uppermost layer of the ocean water column, and it plays critical roles in global biogeochemical cycles and climate change. In the SML, organic matter and microorganisms are usually enriched compared to the subsurface water (SSW). To describe seasonality in the enrichment of microorganisms and transparent exopolymer particles (TEPs; gelatinous particles of acidic polysaccharides) in the SML of temperate coastal waters, annual monthly variations in microbial abundance and TEP concentration in the SML and SSW (0.5 m depth) were investigated in Sagami Bay, Japan, from September 2013 to September 2015. The abundance of microorganisms, such as bacteria and heterotrophic nanoflagellates, and TEP concentrations were significantly higher in the SML than in the SSW during the study period. No significant relationship between the enrichment of TEPs and microorganisms in the SML indicated that the enrichment of microorganisms in the SML is not enhanced by that of TEPs. By collecting monthly samples over a period of 2 consecutive years, seasonality in the behavior of microorganisms and TEPs in the SML was observed. Specifically, the enrichment of both microorganisms and TEPs in the SML was particularly high in spring (April 2014 and May 2015). The present study indicates that the formation of the unique SML of temperate coastal waters during spring phytoplankton blooms is possibly due to particular accumulation of TEPs.