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 À3.14 mmol N m À2 d À1 and a net sink for total phosphorus in the amount of À0.28 mmol P m À2 d À1 . These sinks were approximately 33% and 59% of the total N and P sources, respectively, to the inner LCS.

Carbon Dynamics on the Louisiana Continental Shelf and Cross-Shelf Feeding of Hypoxia

“…Samples were collected as part of annual 5-8-day shelfwide cruises in late July (exact dates were 22-26 July 2006, 21-28 July 2007, 21-28 July 2008, 20-24 July 2009, and 25-31 July 2010. Both the Atchafalaya River and the mainstem Mississippi River influence the central shelf, carrying about 70-80 % of the total river flow (Dinnel and Wiseman 1986;Roberts 1998;Etter et al 2004;Hetland and DiMarco 2008;Lehrter et al 2013), with largest outflows at Southwest (SW) Pass (Fig. 1).…”

“…The calculations of Table 1 and Lehrter et al (2013) indicate an overwhelming dominance of the river nutrient load as the underlying fuel for hypoxia. This would mean that substantial nutrient reductions are needed before the true significance of other factors contributing to hypoxia can be evaluated in a meaningful way.…”

Application of a one-dimensional model to explore the drivers and lability of carbon in the northern Gulf of Mexico

“…Nutrient and chlorophyll-a samples were collected every 3-6 h over a 33-36 h time period at four representative depths: near-surface, two middle depths, and the bottom of the water column. (For analysis methods see Lehrter et al, 2009Lehrter et al, , 2013. These observations were used to estimate the model initial conditions and to compare model output to field data.…”

“…We describe the development of the model and calibration and corroboration using field observations collected during 2006 as part of a multi-year research project studying carbon, oxygen, and nutrient dynamics on the LCS (Murrell and Lehrter, 2011;Murrell et al, 2013;Lehrter et al, 2009Lehrter et al, , 2013. The model is then applied to investigate the carbon cycle including external sources of organic matter, primary production, and respiration.…”

Estimating the Cross‐Shelf Export of Riverine Materials: Part 1. General Relationships From an Idealized Numerical Model

“…The alongshore transport is calculated analogously. F FW is the "freshwater fraction" (analogous to a concentration of fresh water; following Lehrter et al, 2013), a measure of the proportion of river water (salinity of 0) within a given parcel or grid cell, calculated as follows:…”