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 oxyge n concentrations < 2.0 mg L-1) in a 254-km(2) portion of the Atchafalaya Ba sin floodway for 25 months (September 1993-September 1995) and analysed its relationship to local and basin-wide hydrology. Hypoxia occurred in channe ls with low turbulence whenever a flood pulse traveled through the system a nd mean water temperature exceeded 15 degrees C. The number of hypoxic stat ions was linearly related to both river stage and air temperature. Tidal fl uxes and weather-related variables (i.e. wind speed and direction, accumula ted rainfall) did not significantly influence when, or the number of statio ns at which, hypoxia occurred. Additionally, hypoxia occurred at different times within different regions of the swamp. Total organic-carbon concentra tions were relatively low throughout the study area when hypoxia occurred, and (although we acknowledge limitations of our data) we suspect that regio nal differences in organic-matter concentrations (leading to differences in metabolic consumption of dissolved oxygen) had little effect on when or wh ere hypoxia occurred. Instead, we believe that regional paths of water flow affected how water was transported from turbulent-flowing channels into an d through channels containing low- or non-turbulent flows. Water movement o ut of channels with low turbulence was impeded in each region during differ ent stages of the flood pulse, and this event coincided with the most exten sive occurrence of hypoxia within each region. The magnitude and duration o f the flood pulse as well as the interior paths of water flow significantly influenced the severity and duration of hypoxia in this riverine swamp. Co pyright (C) 1999 John Wiley & Sons, Ltd.