The Response of the Hypoxic Area to the Ratio of

One of the most important conclusions from the river diversion experiments is the two sources of fresh river water, the Mississippi and the Atchafalaya, have great impacts on where hypoxia occurs on the Louisiana shelf. The Atchafalaya River plays an important role in delivering the fresh river water to the west Louisiana shelf. Although the effect was not apparent when the Atchafalaya had 33.3% of the total flow, it was clear when the Atchafalaya had 66.7% of the total flow. Large amount of the Atchafalaya river water brought large amount of nutrients to the west shelf, intensifying the stratification at the same time. As a result, both the area of hypoxia and its frequency on the west Louisiana shelf increased. The western extension of the hypoxic zone for the 1/3 MR case reached the western boundary of our current model domain setting at Texas border. The hypoxic area could extend to Texas coast if the model domain were larger. In contrast, as the river flow of the Mississippi was reduced, both the area of hypoxia and its frequency decreased on the east Louisiana shelf.

The consequences of shifting flow from the Mississippi River to the Atchafalaya River showed that although the total nitrogen loading and freshwater discharge were the same, the case with 1/3 of river flow from the Mississippi River had a larger hypoxic area than either the base case with 2/3 flow or the case when 3/3 river flows come from the Mississippi River. The hypoxic area in the 2/3 river flow from the Mississippi River case was the smallest. A possible reason for the relatively small hypoxic area in the 2/3 MR case was the change of bathymetry along the Louisiana coast. There is a large shallow area next to the Atchafalaya Bay. As a result, the bottom stress on the flow is strong. A small flow diversion to the Atchafalaya could

not overcome the strong bottom stress. Therefore, the freshwater stayed attached to the bottom and confined to a narrow nearshore band west of the discharge. Because the nearshore freshwater contacted the bottom, the stratification in the nearshore band was weak. As a result, the hypoxic area in the base case was the smallest. As the diversion of flow to the Atchafalaya increased, the freshwater overcame the bottom stress and floated on top of the seawater, resulting in stratification on the west shelf being more intense, creating the condition that favors the formation of hypoxia.

Another interesting feather on the frequency of hypoxia on the Louisiana shelf in the all MR case was that the frequency of hypoxia was distributed patchily rather than continuously. In addition to the area next to the Mississippi River mouth on the east Louisiana shelf, there was a frequent hypoxic region (frequency > 75%) on the west shelf. The patchy pattern was created by the combined river and wind effects. The river discharge was from high to low during our selected period (July 14th – August 02). The fresh river water from the Mississippi was able to flush to the west Louisiana shelf during the high discharge period. As the river flow decreases through time, the freshwater layer on the west shelf become thinner. The west shelf was then easier disrupt with the wind, making the frequency of hypoxia more variable there. In contrast, the Mississippi River discharge was on the east Louisiana shelf. A thick freshwater layer could be maintained even when the river discharge was low in the all MR case. As a result, the region next to the Mississippi River on the east Louisiana shelf had a continuous frequent hypoxic zone in the all MR case.

Over the past 200 years, the relative discharge of the Atchafalaya River has increased (Fisk , 1952; Day et al., 1995). Krug (2007) examined the implications of the river diversion. He argued that the large wetland loss during the diversion process released large amounts of nutrients to the Louisiana shelf, and thus increased

the hypoxic area. These results show that the circulation over the hypoxic region of the Louisiana shelf is highly influenced by shifting the river discharge. Although more nutrients are released to the Louisiana shelf from the Atchafalaya, how far west and south they reach depended on the strength and direction of the currents. The shift in the stratification also changed because the flow changed the freshwater distribution on the shelf. The alteration of the physical oceanic conditions intensified the effect of nutrient addition from wetland loss.

Krug’s studies recommended that an integrated assessment on the effect of the Atchafalaya’s partial capture of the Mississippi River should be conducted to improve the current Gulf hypoxia management strategy. The present study suggests that in addition to the nutrient addition effect caused shifting discharge, the change in the physical oceanic conditions, including the stratification and circulation, needs to be included in any evaluation. Such an evaluation could be conducted through the present three dimensional, coupled-biogeochemical model.

4. The Response of the Hypoxic Area to the Different Nitrogen Form from the