Nutrient distributions, transports, and budgets on the inner margin of a river-dominated continental shelf

Abstract

[1] Physical and biogeochemical processes determining the distribution, transport, and fate of nutrients delivered by the Mississippi and Atchafalaya river basin (MARB) to the inner Louisiana continental shelf (LCS) were examined using a three-dimensional hydrodynamic model and observations of hydrography, nutrients, and organic carbon collected during 12 cruises. Two aspects of nutrient transport and fate on the inner LCS (<50 m depth) were evaluated: (1) along-shelf and cross-shelf transports were calculated and (2) nutrient sinks and sources were inferred. On average, 47% of the lower Mississippi River freshwater traveled westward on the LCS, but this percentage was reduced during summer when currents reversed to a predominately upcoast direction. Changes from mainly inorganic to organic nutrients were observed at salinity between 20 and 30, and above 30, organic nutrients were the dominant forms. Westward transport of dissolved inorganic nitrogen (DIN) was about 25% of the combined DIN load from the MARB, whereas westward transport of dissolved organic nitrogen (DON) was 2.8-fold larger than the MARB DON load. Different from dissolved inorganic nutrients, for which the rivers were the primary source, the dominant source of organic nutrients was advection from offshore. Overall, the inner LCS was estimated to be a net sink for total nitrogen in the amount of −3.14 mmol N m−2 d−1 and a net sink for total phosphorus in the amount of −0.28 mmol P m−2 d−1. These sinks were approximately 33% and 59% of the total N and P sources, respectively, to the inner LCS.