A modeling study examining the impact of nutrient boundaries on primary production on the Louisiana continental shelf

Nutrient inputs to the Louisiana continental shelf (LCS) from lateral ocean boundaries can be significant, but the effect of these nutrients on LCS primary production has not been examined. Herein, we apply a three-dimensional physical-biogeochemical model to calculate nitrogen and phosphorus mass balances on the LCS and quantify the contributions of riverine and offshore nutrient inputs to primary production. A model sensitivity analysis to different offshore nutrient concentrations indicated that modeled primary production was most sensitive to boundary nitrogen concentrations, whereas changing boundary phosphorus concentrations had little effect. The primary production response also varied spatially and temporally, with its greatest response being to changing boundary nitrogen concentrations in areas furthest from the river plume, and during the late summer for all regions of the shelf when Mississippi River discharge approaches its annual minimum. These results indicate that even for river-dominated shelves like the LCS, uncertain boundary condition nutrient concentrations are likely to contribute significantly to uncertainty in modeled primary production. The modeling study highlights the need for further observational studies to understand the sources and variability of nutrients at LCS offshore boundaries and the impacts to LCS primary production.