Seasonal microgel and microbial dynamics in the sea surface microlayer and sub-surface waters, L4 coastal time series station, Western English Channel

Results from a time-series study of coastal sea surface microlayer (SML) and sub-surface water marine gel and biological dynamics (L4 station, Western English Channel) during the transition from low to high productivity indicate the SML is an interfacial film with variable potential to suppress ocean-atmosphere gas exchanges dependent on bulk water processes. Phytoneuston communities were consistently enriched from sub-surface phytoplankton by a mean enrichment factor (EF) of 3.3 showing indication of reduced photosynthetic rates and increased production of extracellular polysaccharides. Use of a new method to determine the potential for coagulation of carbon rich marine gel aggregates, termed transparent exopolymer particles (TEP) from dissolved polysaccharide precursors (P-TEP), suggests there is a greater and more dynamic total pool of organic carbon in the SML than previously reported. P-TEP was enriched (mean EF = 1.3) as were concentrations of in-situ TEP (mean EF = 1.1), similar to values from mesotrophic/eutrophic waters. In-situ TEP aggregates were at the highest concentration 4 weeks prior to the chlorophyll-a maximum (5.47 mg/m³). This large bloom marked a reduction of enrichments of both gel phases and a change in the ratio of gels in particulate and dissolved states in the following weeks. A significant negative correlation with increased high nucleic acid (HNA) content bacteria was also found. Bacterial hydrolysis of aggregates, together with potential downwards particle flux at wind speeds > 3.7 m s-1 and sea states ranging from choppy to rough may have caused lowered in-situ TEP enrichments within a complex and dynamic system progressed by seasonal environmental forcing (e.g. > temperature, light, pCO2). The findings support previous conclusions that although the SML is generally enriched, changed trophic states initiated by spring/summer phytoplankton blooms can prevent enhanced formation of the gelatinous biogenic film