Large floodplain rivers consist of a diverse array of aquatic areas that differ in their connectivity with the main channel. Connectivity with the main channel affects the rate of nutrient delivery to backwaters and their water residence time. We investigated the effect of connectivity to the main channel on nutrient concentrations and rates of ecosystem metabolism in backwaters of the Upper Mississippi River (UMR) near La Crosse, Wisconsin. We measured nutrient concentrations, water column primary production (using light/dark bottles) and ecosystem metabolism (using continuous in situ dissolved oxygen measurements) in three contrasting aquatic areas of the UMR: backwaters with a single connection to the main channel (BW-SC), backwaters with multiple connections to the main channel (BW-MC), and the main channel (MC).

Results/Conclusions

June total nitrogen (TN) concentrations were consistently highest in MC (mean: 2.4 mg L^-1), lowest in BW-SC (mean: 1.2 mg L^-1) and intermediate in BW-MC (mean: 1.6 mg L^-1) indicating the importance of rates of nutrient input (from the main channel) and removal (plant uptake and denitrification) in determining nitrogen concentrations in these backwaters. In contrast, total phosphorus (TP) concentrations were generally higher in the backwaters than in the MC and similar between BW-SC and BW-MC. The water column net production was higher in MC (mean: 4.9 g O₂ m^-2 d^-1) than in backwaters (means: 2.8 to 3.9 g O₂ m^-2 d^-1). However, net ecosystem production was lower in the main channel (about 0, indicating near equality of rates of gross primary production and respiration) than in the backwaters (means: 2.3 to 3.2 g O₂ m^-2 d^-1). Differences in connectivity among backwaters were not clearly associated with differences in water column or ecosystem production. Our results indicate that backwater connectivity does affect nutrient concentrations, but does not have a clear effect on rates of net ecosystem production.