Nutrient Distributions, Transports, and Budgets on the Inner Margin of a River-Dominated Continental Shelf
NAVAL RESEARCH LAB STENNIS DETACHMENT STENNIS SPACE CENTER MS
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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.
- Physical and Dynamic Oceanography