Hydrology and aquatic habitat characteristics of a riverine swamp: II. Hydrology and the occurrence of chronic hypoxia

We examined the temporal and spatial occurrence of hypoxia (dissolved oxygen concentrations<2.0 mg L−1) in a 254-km2 portion of the Atchafalaya Basin floodway for 25 months (September 1993–September 1995) and analysed its relationship to local and basin-wide hydrology. Hypoxia occurred in channels with low turbulence whenever a flood pulse traveled through the system and mean water temperature exceeded 15°C. The number of hypoxic stations was linearly related to both river stage and air temperature. Tidal fluxes and weather-related variables (i.e. wind speed and direction, accumulated rainfall) did not significantly influence when, or the number of stations at which, hypoxia occurred. Additionally, hypoxia occurred at different times within different regions of the swamp. Total organic-carbon concentrations were relatively low throughout the study area when hypoxia occurred, and (although we acknowledge limitations of our data) we suspect that regional differences in organic-matter concentrations (leading to differences in metabolic consumption of dissolved oxygen) had little effect on when or where hypoxia occurred. Instead, we believe that regional paths of water flow affected how water was transported from turbulent-flowing channels into and through channels containing low- or non-turbulent flows. Water movement out of channels with low turbulence was impeded in each region during different stages of the flood pulse, and this event coincided with the most extensive occurrence of hypoxia within each region. The magnitude and duration of the flood pulse as well as the interior paths of water flow significantly influenced the severity and duration of hypoxia in this riverine swamp. Copyright © 1999 John Wiley & Sons, Ltd.