Structure of Submesoscale Fronts of the Mississippi River Plume

Submesoscale currents (SMCs), in the forms of fronts, filaments, and vortices, are studied using a high-resolution (~150 m) Regional Oceanic Modeling System (ROMS) simulation in the Mississippi River plume system. Fronts and filaments are identified by large horizontal velocity and buoyancy gradients, surface convergence, and cyclonic vertical vorticity with along-coast fronts and along-plume-edge filaments notably evident. Frontogenesis and arrest/destruction are two fundamental phases in the life cycle of fronts and filaments. In the Mississippi River plume region, the horizontal advective tendency induced by confluence and convergence plays a primary role in frontogenesis. Confluent currents sharpen preexisting horizontal buoyancy gradients and initiate frontogenesis. Once the fronts and filaments are formed and the Rossby number reaches O(1), they further evolve frontogenetically mainly by convergent secondary circulations, which can be maintained by different cross-front momentum balance regimes. Confluent motions and preexisting horizontal buoyancy gradients depend on the interaction between wind-induced Ekman transport and the spreading plume water. Consequently, the direction of wind has a significant effect on the temporal variability of SMCs, with more active SMCs generated during a coastally downwelling-favorable wind and fewer SMCs during an upwelling-favorable wind. Submesoscale instabilities (~1–3 km) play a primary role in the arrest and fragmentation of most fronts and filaments. These instabilities propagate along the fronts and filaments, and their energy conversion is a mixed barotropic–baroclinic type with horizontal-shear instabilities dominating.

Influence of the Mississippi River plume and non-bioavailable DMSP on dissolved DMSP turnover in the northern Gulf of Mexico

Environmental contextDimethylsulfoniopropionate (DMSP) comprises an important fraction of the organic carbon produced by phytoplankton, and is a major source of carbon and sulfur for heterotrophic bacteria. Here, we show that a non-bioavailable fraction of DMSP recently discovered in coastal waters also exists in oligotrophic open-ocean waters. Taking account of the non-bioavailable pool improved estimates of cycling rates of DMSP and its contribution to bacterial nutrition. AbstractMicrobial cycling of dissolved dimethylsulfoniopropionate (DMSPd) and the fate of DMSP-sulfur were measured in the northern Gulf of Mexico off the Louisiana coast in September 2011 using the tracer 35S-DMSPd. Salinity ranged from 31.2ppt in the Mississippi River plume to 36.5ppt offshore. Total DMSP concentrations were significantly higher at the river-influenced stations (12–27nM) than offshore (6–14nM). From 8.7 to 27% of the measured DMSPd, equivalent to 0.1 to 0.23nM, was refractory (i.e. non-bioavailable). We subtracted refractory DMSPd from the measured DMSPd concentrations when calculating DMSPd consumption rates with the tracer 35S-DMSPd. DMSPd consumption and bacterial production were respectively 8 and 7 times higher in the river plume compared with offshore. Incorporation of DMSP-sulfur into biomass was almost three times higher in river-influenced water, whereas the dimethylsulfide (DMS) yield was 3.5 times lower in the river plume compared with offshore. Accordingly, DMSP contributed over 3 times more to the bacterial demand for both carbon and sulfur in the river-influenced water. Despite lower DMS yields in the river plume, the fast DMSP turnover resulted in a 2.7-times higher DMS production compared with offshore waters. Mississippi River inputs and the resulting high productivity led to a rapid turnover of DMSP and high DMS production in the river plume compared with oligotrophic Gulf of Mexico waters. Failure to account for refractory DMSPd when using the 35S-DMSP method will lead to significant overestimation of the DMSPd turnover flux and its contributions to C and S cycling.