摘要：It is generally believed that interannual variability in the areal extent of the Gulf of
Mexico hypoxia is driven primarily by the magnitude of the Mississippi River
freshwater and nutrient fluxes. It has recently been proposed that outwelling of
carbon from deteriorating coastal wetlands into the surrounding Gulf of Mexico
could be an important mechanism promoting the development of hypoxia. We
used a coupled hydrology–hydrodynamics model of the Barataria estuary, a site
of massive wetland loss, to calculate the fluxes of nitrogen, chlorophyll a and
carbon at the estuary–ocean interface. The hydrology model calculates runoff
from rainfall and evaporation data, and then feeds it into the high-resolution (100 m × 100 m grid, 1.3 million elements), two-dimensional depth-integrated hydrodynamic
model. Model results show substantial outwelling of total organic carbon (TOC,
110 × 106 kg yr − 1), dissolved organic carbon
(DOC, 94.3 × 106 kg yr − 1), particulate
organic carbon (POC, 15.7 × 106 kg yr − 1)
and chlorophyll a (Chl a, 0.3 × 106 kg yr − 1) from the estuary to the coastal waters and an import of nitrate (N–NO3,
6.9 × 106 kg yr − 1) from
the nutrient-rich coastal waters into the estuary. Estuarine fluxes of TOC, DOC, POC, Chl a and N–NO3, account for 2.8%, 2.7%, 3.4%, 7.5% and 1%, respectively, of the annual fluxes carried
by the lower Mississippi River. The flux of total nitrogen was not statistically
significant. Overall, this study supports the conclusion of the previous modeling
study (Das et al 2010 Ecol. Modeling221 978–85), suggesting that the Barataria
estuary supplies a relatively small amount of the carbon consumed in the Gulf's
hypoxic zone. Importantly, our results indicate that import of nitrate from the
coastal waters and its assimilation within the estuary could account for 38% and
208%, respectively, of the calculated TOC and Chl a exports, demonstrating
the pervasive control of the Mississippi River on the productivity of this shelf.
