Trophic status of Missouri River floodplain lakes in relation to basin type and connectivity

Limnological data were collected over a two-year period from 12 lakes in the Missouri River floodplain in order to evaluate lake trophic status and the influence of basin type and connectivity on nutrient, seston, and phytoplankton dynamics. The lakes were located in west-central Missouri and included eight scour basins formed by levee breaks during a 1993 flood and four older oxbow lakes. The Missouri River was also sampled. Five of the scour basins (connected scours) were inundated by the Missouri River for varying periods during the study. The other lakes (protected lakes) were hydrologically isolated from the river by levees. On the basis of mean total phosphorus (TP-61-282 mu g/L), mean total nitrogen (TN-0.7-2.1 mg/L), mean chlorophyll (CHL-22-67 mu g/L), and mean suspended solids (TSS-16-93 mg/L), all 12 sites were highly eutrophic compared to other lakes in Missouri. Nutrient fractions and seston composition indicated dominance of inorganic nutrients and abiotic seston in the riverine lakes. Maximum concentrations of nutrients and TSS occurred in oxbow lakes during periods of sediment resuspension and in connected scours during floods in spring 1995 and 1996 when connected sites were inundated by the Missouri River. Algal blooms with peak CHL >200 mu g/L occurred sporadically in the oxbow lakes. Similarly large blooms (peak CHL 81-689 mu g/L) occurred in connected scours when low river levels reduced exchange with the river. Reduced connectivity was accompanied by rapid loss of dissolved N and P fractions, especially nitrate. Protected scours had lower average TP, TN, and TSS than the other sites and were less temporally variable.