Temporal and Spatial Variability of Sediment Fluxes of Carbon and Nutrients on a River Dominated Shelf

Temporal and spatial patterns of sediment-water fluxes are poorly understood for most systems because of under sampling. In this study, sediment fluxes of carbon and nutrients were measured and modeled across the inner shelf of the northern Gulf of Mexico to quantify temporal and spatial patterns in relation to Mississippi River loads. Temporal patterns were evaluated by measuring fluxes at three stations during four cruises in April, July, October, and January of 2017-2018. Spatial patterns were determined by measuring fluxes at 12 stations during one cruise in October 2010. For temporal patterns, fluxes of DIC, alkalinity, and pH showed a seasonal pattern with maximum effluxes of DIC occurring during April and July (max 24.9 mmol m^-2d^-1) and minimum during October and January (min 2.41 mmol m^-2d^-1). Some of the nutrient fluxes exhibited seasonal patterns with NO₃^-, NH₄⁺, and Si fluxes being generally largest in the summer. In contrast, PO₄^3-fluxes were uniform across all seasons. For spatial patterns, for all fluxes there was a general trend of decreasing fluxes with increasing depth across the inner shelf. This pattern held for transects that were close to the inputs from the Mississippi River as well as those located further away. The effects of a hurricane were captured in our October 2017 sampling when Hurricane Nate transited through our study area during the cruise. Fluxes at one station measured before and after Hurricane Nate indicated that fluxes increased by approximately 30% following the hurricane. This difference is on the same order of magnitude as some of the seasonal changes observed. Finally, we made use of a sediment diagenesis model to examine how seasonal changes in temperature, organic matter deposition to the sediments, and seasonal hypoxia affect flux rates. In sum, these data provide a more complete picture of flux dynamics on a river-dominated shelf that will be useful for constraining the role of the sediments in driving bottom water carbon and nutrient cycling processes.