Cross-shelf Distribution of Dimethylsulfide in the South Atlantic Bight

The cross-shelf distribution of the climatically-important dimethylsulfide (DMS) and its precursor dimethylsulfoniopropionate (DMSP) were examined along a transect starting near Gray’s Reef (coastal Georgia) during three expeditions in 2015. Gulf Stream meanders and eddies coupled with shelf-break upwelling of nutrient-rich water can have a profound effect on the productivity and taxonomy of the phytoplankton community in the South Atlantic Bight. In 2015, Gulf Stream waters were observed more than 20 km inshore with cold, upwelled waters reaching the 40 m isobath. Yet, despite the presence of nutrient-rich water inshore of the shelf break, Chl a was low at all depths (less than 4 mg L^‑1) during the expeditions. In the neritic zone, pigment analyses revealed the existence of cyanobacteria in the surface layer and a mixture of prymnesiophytes and cryptophytes associated with upwelled water in the bottom layer. In the oligotrophic waters of the western Sargasso Sea, cyanobacteria were prevalent with prochorophytes dominating at depth. The pigment signatures for prymnesiophytes and cryptophytes were also noted at depth in the oceanic zone. Modest levels of both dissolved and particulate DMSP were measured across the shelf (2-40 nmol L^‑1) with lower levels observed in oligotrophic waters. However, DMS levels were uniformly low across the entire transect (typically less than 2 nmol L^‑1) and are thought to result from phytoplankton cells being entirely ingested by tunicates rather than broken open during grazing by copepods. As a consequence, the South Atlantic Bight does not appear to be a significant contributor to global DMS sea-to-air fluxes.