Seasonal and Interannual Variability of Areal Extent of the Gulf of Mexico Hypoxia from a Coupled Physical-Biogeochemical Model: A New Implication for Management Practice

摘要

The extent of hypoxia on the Louisiana shelf has been measured during July since 1985. The measured area was assumed to represent the seasonal maximum each year and was related to the Mississippi-Atchafalaya riverine May NO_2+3 loading and May-June total nitrogen loading, for planning management strategies. In this study, we analyze 25 years of simulations from a coupled physical-biogeochemical model. An empirical orthogonal function analysis of the hypoxia frequency reveals that the dominant pattern is east-west, with the seasonal maximum occurring in June, July, or August. This indicates that the July hypoxic area may or may not always be the largest of the year. A simple linear regression model was constructed to examine the explained variance of hypoxia attributable to nutrients. Results reveal that the May NO_2+3 (May-June total nitrogen) loading can explain 35% ± 7% (54% ± 7%) of the maximum cumulative hypoxic area in contrast to 22% ± 9% (48% ± 12%) of the July cumulative hypoxic area. Our results suggest that the current nitrogen loading reduction suggested by the Hypoxia Task Force 2013 is not sufficient to reduce the 5-year moving average Gulf's hypoxic zone to less than 5,000 km~2. A reduction of at least 66% (48%) of May NO_2+3 (May-June total N) loading is needed when using July hypoxic area as the criterion, while a reduction of 77% (60%) of May NO_2+3 (May-June total N) loading is needed when using maximum hypoxic area.