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Nutrient distributions, transports, and budgets on the inner margin of a river-dominated continental shelf
Citation:
Lehrter, J., D. Ko, M. Murrell, Rick Greene, Jim Hagy, B. Schaeffer, R. Gould, AND B. Penta. Nutrient distributions, transports, and budgets on the inner margin of a river-dominated continental shelf. JOURNAL OF GEOPHYSICAL RESEARCH: OCEANS. American Geophysical Union, Washington, DC, 118:4822-4838, (2013).
Impact/Purpose:
This manuscript describes the transport and fate of Mississippi River nutrients delivered to the Louisiana continental shelf. The results set the upper bound for the potential river contribution to the enhanced primary production on this shelf that has been linked to the development of hypoxia.
Description:
Physical and biogeochemical processes determining the distribution and fate of nutrients delivered by the Mississippi and Atchafalaya rivers to the inner (<50 m depth) Louisiana continental shelf (LCS) were examined using a three-dimensional hydrodynamic model of the LCS and observations of hydrography, nutrients, and organic carbon collected from the shelf. Two hypotheses of nutrient transport and fate on the inner LCS were evaluated: 1) Along shelf transport is greater than across shelf transport and 2) The inner shelf is a net a sink for nutrients and a source of organic matter. With respect to hypothesis 1, nutrients and organic carbon transport pathways were predominately downcoast, i.e. westward along the shelf towards Texas, supporting this hypothesis. Generally, hypothesis 2 was also supported. The downcoast transport of dissolved inorganic nitrogen (DIN) was calculated to be about one-third of the flux of DIN from the Mississippi River, whereas the downcoast transport of dissolved organic nitrogen (DON) was 3-fold larger than the Mississippi River load. These differences could be accounted for by primary production on the inner shelf adjacent to the river discharge, where a uptake rate of -7.3 mmol N m-2 d-1 and an approximately equal DON production rate of 7.2 mmol N m-2 d-1 were estimated. DIP and DOP dynamics were similar to those of nitrogen. Overall for the inner LCS, there was a net sink of total nitrogen and phosphorus, -2.7 mmol N m-2 d-1 and -0.19 mmol P m-2 d-1, respectively. These inner shelf N and P sinks represented approximately 32% of the N and 6% of P exported from the Mississippi-Atchafalya River Basin.
