Influence of wind and river discharge on the hypoxia in a shallow bay

Abstract

Perdido Bay is a shallow bay with narrow inlets connecting with Gulf of Mexico. It is affected by severe hypoxia problem due to its narrow tidal range of around 0.4 m. An existing calibrated eutrophication model, as described in Xia et al. (J Coastal Res 719: 73–86, 2011a), was used to simulate the bottom hypoxia of Perdido Bay in response to the changing local wind and river discharge. In addition, the response of nutrient dynamics and algae distribution to these physical forces was also discussed, in order to understand the hypoxia dynamics. Compared to a no wind case, we found that southerly (onshore) and westerly favorable winds with speed of 3 m/s or less inhibited the nutrient and algae transport, while easterly favorable winds and northerly (offshore) winds favored nutrient and algae transport. Onshore and westerly winds were most effective at inducing significantly broader and thicker hypoxic/anoxic conditions. Increasing wind speed could limit the nutrient-rich freshwater to the upper bay. On the other hand, a 5-m/s southerly wind or above was sufficient to make vertical distribution of nutrients uniform, significantly reducing areal coverage of hypoxia/anoxia. We also discovered that increasing river discharge drove the nutrient-rich, high algae waters down to the bay mouth. Under a high-volume river discharge (100 m3/s, henceforth denoted as cms), the freshwater could reach the bottom in shallow areas (e.g., depth < 1 m), whereby the severity of anoxia and hypoxia was reduced, while increasing river discharge had little effect on the bottom hypoxia and nutrient variation in deepwater.