Ecosystem metabolism in the Upper Mississippi River: The role of light, nutrients, and connectivity to the main channel

Large floodplain rivers consist of a diverse array of aquatic areas. Hydraulic connectivity of these areas with the main channel (MC) may affect important drivers of primary production. We investigated the effect of connectivity to the main channel on nutrient concentrations, water clarity, and rates of ecosystem metabolism. We measured nutrient concentrations, water column primary production (light/dark bottles), ecosystem metabolism (continuous in situ dissolved oxygen measurements), and water clarity at six sites across a gradient of connectivity with the MC in the Upper Mississippi River near La Crosse, Wisconsin. June total nitrogen concentrations increased with connectivity to the MC, whereas total phosphorus concentrations were inversely related to this connectivity. Water column net primary production rates were higher in the 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), but among backwater sites, production rates were not related to connectivity to the MC. In contrast, net ecosystem production was lower in the MC than in the backwaters. Our results suggest that connectivity to the MC more strongly affects nutrient concentrations than rates of ecosystem metabolism.