Coastal land loss and hypoxia: the ‘outwelling’ hypothesis revisited

Abstract

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 × 10 6 kg yr −1 ), dissolved organic carbon (DOC, 94.3 × 10 6 kg yr −1 ), particulate organic carbon (POC, 15.7 × 10 6 kg yr −1 ) and chlorophyll a (Chl a, 0.3 × 10 6 kg yr −1 ) from the estuary to the coastal waters and an import of nitrate (N-NO 3 , 6.9 × 10 6 kg yr −1 ) from the nutrient-rich coastal waters into the estuary. Estuarine fluxes of TOC, DOC, POC, Chl a and N-NO 3 , 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. Modeling 221 978-85), 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.

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

“…First, we used the decomposition results for estuarine and river particulates to update the outwelling calculations presented in Das et al (2010Das et al ( , 2011, using the C/N=106/16 Redfield ratio to convert inorganic nitrogen inputs to carbon units. Most terms in these calculations (Table 1) were directly taken from Das et al (2010Das et al ( , 2011, with the only changes being the conversion from nitrogen to carbon units and the inclusion of the percent lability estimates from this study.…”

“…Most terms in these calculations (Table 1) were directly taken from Das et al (2010Das et al ( , 2011, with the only changes being the conversion from nitrogen to carbon units and the inclusion of the percent lability estimates from this study. The results given in Table 1 indicate that the nutrient and carbon sources for the shelf region are dominated by the Mississippi River nitrate load (95 %), with a small contribution from Mississippi River particulates (4.8 %), and a very small contribution (0.2 %) for estuarine outwelling from one of the five major estuaries along the southern Louisiana coast.…”

“…In this case, extra carbon is required to fuel the observed extensive hypoxia, and carbon export or outwelling from local estuaries has been hypothesized as possibly important in this regard (Dagg et al 2007;Bianchi et al 2011). However, two recent modeling studies reporting long-term measurements for one important deltaic estuary of the region, Barataria Bay, do not support the carbon outwelling hypothesis (Das et al 2010(Das et al , 2011. Because confidence generally is lower when models do not agree in their major outcomes (Scavia et al 2004;Justić et al 2007), we undertook repeated field studies during annual summer shelfwide cruises from 2006 to 2010 to test how riversupported carbon productivity is coupled to hypoxia.…”

“…To complement these field efforts, a preliminary biogeochemical budget is presented linking riverine nitrogen loading with carbon productivity important for summer hypoxia. This budget is largely synthesized from existing literature, with an exception that estimates of DOC and POC carbon loading from the Mississippi River and adjacent estuaries are updated from Das et al (2010Das et al ( , 2011. These updates are made with new data on carbon lability from 28-day decomposition experiments reported here.…”

Rescaling stable isotope data for standardized evaluations of food webs and species niches

“…1 & 2). A more phytoplankton-based diet with lower N isotope values may be indicated at GT, an area with consistently elevated chlorophyll levels (Das et al 2011). More zooplanktivory with higher N isotope values may be indicated for menhaden at the 2 other more freshwater stations, MV and FP.…”