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Spatial and temporal water quality variability in aquatic habitats of a cultivated floodplain

Shields Jr., F. Douglas, Lizotte Jr., Richard E., Knight, Scott S.
River research and applications 2013 v.29 no.3 pp. 313
agricultural land, agricultural runoff, aquatic habitat, diurnal variation, dry season, ecological restoration, ecosystem services, fish, floodplains, habitat destruction, hydraulic structures, hydrochemistry, hypoxia, intensive farming, nitrogen content, phosphorus, rivers, seasonal variation, spatial variation, species diversity, spring, summer, temperature, total solids, water quality, water supply, wet season, wetlands, winter, Mississippi
The floodplains of lowland rivers contain diverse aquatic habitats that provide valuable ecosystem services but are perturbed when intensively cultivated. Hydrologic, water chemistry and biological (fish) conditions in five aquatic habitats along the Coldwater River, Mississippi, were measured for more than 4 years: the river, two severed meanders that functioned as backwaters, a managed wetland and an ephemeral channel draining cultivated fields. Off-channel habitats were connected to downstream regions 0.10% to 32% of the dry season and 24% to 67% of the wet season. The median temperatures for the five monitored sites ranged from 18°C to 23°C, the median total solids concentration for all sites was 135 mg L-1, the median total phosphorus was 0.29 mg L-1 and the median total nitrogen was 1.56 mg L-1. Chemical and physical water quality displayed strong seasonal differences between the wet winter/spring and the dry summer/fall periods so that temporal variation consisted of gradual seasonal trends superimposed on strong diurnal variations. All off-channel habitats exhibited periods of hypoxia and temperatures >30°C during the dry season. Between-site gradients of water and habitat quality were strongly coupled to water depth and runoff loading. The rehabilitation of one backwater by increasing water depth and diverting agricultural runoff was associated with improved water quality and fish species richness relative to an adjacent untreated backwater. The diversion of polluted runoff and the use of water control structures to maintain greater water depth were observed to be effective management tools, but the former reduces the water supply to habitats that tend to dry up and the latter reduces connectivity.