A modeling study examining the impact of nutrient boundaries on primary production on the Louisiana Continental Shelf
Citation:
Pauer, J., T. Feist, A. Anstead, P. DePetro, W. Melendez, J. Lehrter, M. Murrell, X. Zhang, AND D. Ko. A modeling study examining the impact of nutrient boundaries on primary production on the Louisiana Continental Shelf. ECOLOGICAL MODELLING. Elsevier Science BV, Amsterdam, Netherlands, 328:136-147, (2016).
Impact/Purpose:
A mass balance model, Gulf of Mexico Dissolved Oxygen Model has been developed and applied to describe nitrogen, phosphorus and primary production in the Louisiana shelf of the Gulf of Mexico. This work supports the Mississippi River/Gulf of Mexico Watershed Nutrient (Hypoxia) Task Force and the Gulf Hypoxia Action Plan. Model sensitivity analysis demonstrated that nitrogen boundary concentrations had a bigger impact on the shelf area nitrogen and primary production than the river loads at most locations (impact from atmospheric loads was low). However, areas closer to the rivers were strongly affected by river loads, especially at the surface. The sensitivity analysis also showed that primary production in general was not very sensitive to changes in either the nitrogen boundary conditions or loads. This and other model analysis suggested that production on the shelf area can be limited by either nitrogen or phosphorus depending on season (e.g. high versus low river flows) and location on the shelf and that this limitation can rapidly shift between the two nutrients. Understanding nutrient and production dynamics is critical to understanding hypoxia on the shelf.
Description:
A mass balance eutrophication model, Gulf of Mexico Dissolved Oxygen Model (GoMDOM), has been developed and applied to describe nitrogen, phosphorus and primary production in the Louisiana shelf of the Gulf of Mexico. Features of this model include bi-directional boundary exchanges, an empirical site-specific light attenuation equation, estimates of 56 river loads and atmospheric loads. The model was calibrated for 2006 by comparing model output to observations in zones that represent different locations in the Gulf. The model exhibited reasonable skill in simulating the phosphorus and nitrogen field data and primary production observations. The model was applied to generate a nitrogen mass balance estimate, to perform sensitivity analysis to compare the importance of the nutrient boundary concentrations versus the river loads on nutrient concentrations and primary production within the shelf, and to provide insight into the relative importance of different limitation factors on primary production. The mass budget showed the importance of the rivers as the major external nitrogen source while the atmospheric load contributed approximately 2% of the total external load. Sensitivity analysis showed the importance of accurate estimates of boundary nitrogen concentrations on the nitrogen levels on the shelf, especially at regions further away from the river influences. The boundary nitrogen concentrations impacted primary production less than nitrogen concentration on the shelf due to co-limitation from phosphorus and light. The regions close to the rivers, due to high nitrogen loadings, are limited by both nitrogen and phosphorus, and, in some cases are exclusively phosphorus-limited.
