Shelf sea subsurface chlorophyll maximum thin layers have a distinct phytoplankton community structure

The Western English Channel is a seasonally stratified temperate coastal sea where a subsurface chlorophyll maximum (SCM) is typically detectable within the seasonal thermocline. The SCM often develops as a thin layer(<5m) that may contain elevated concentrations of phytoplankton (subsurface chlorophyll maximum thin layer;SCMTL). During summer 2013 a study was conducted offshore of Falmouth, UK to assess spatial and short-termtemporal variability in SCM thickness in relation to water column structure and physical conditions and to evaluateany associated changes in phytoplankton community structure. SCMTL were observed in 18 of 52 verticalprofiles, typically characterised by higher chlorophyll concentrations than broader SCM. SCMTL were generallyassociated with a ‘stepped’ thermocline, likely representing the presence of one or more shallow mixed layersforming above/within the seasonal thermocline, and related to increased stratification and stability compared tobroader SCM. Pseudo-nitzschia was almost exclusively the dominant diatom taxon in SCM, yet statistically distinctdifferences in community structure existed between SCMTL and broader SCM. Within the phytoplankton,the distinction was largely due to a greater biomass of Proboscia alata and other rhizosolenid diatoms, and thedinoflagellate Ceratium lineatum in SCMTL, and a smaller population of the diatom Chaetoceros spp. There wasalso a distinction amongst heterotrophic dinoflagellates, with enhanced biomass of Gyrodinium spp. in SCMTLand a reduction in Diplopsalis lenticula. We propose that this observed difference resulted from promotion of phytoplanktonbetter adapted to environmental conditions more specific to SCMTL compared to broader SCM. Withmore intense and prolonged stratification projected for the NW European shelf, there may be increased prevalenceof SCMTL and the associated larger-sized specialised taxa, with implications for increased carbon export.This study adds to a growing body of evidence of the importance of SCMTL in coastal and shelf seas, and highlightsthe requirement for improved understanding of physical forcing, and the ecology and physiology of keytaxa, particularly as predicted changes in stratification could alter the role of SCM phytoplankton in a future influencedby climate change