PATTERNS OF NUTRIENT DISTRIBUTIONS IN THE ST. CROIX AND UPPER MISSISSIPPI RIVERS: PRELIMINARY EVALUATION OF VARIATION AMONG CHANNELS, FLOWING BACKWATERS AND ISOLATED BACKWATERS

William Richardson, USGS Upper Midwest Environmental Sciences Center, La Crosse, Wisconsin
Lynn Bartsch, USGS Upper Midwest Environmental Sciences Center, La Crosse, Wisconsin
Michelle Bartsch, USGS Upper Midwest Environmental Sciences Center, La Crosse, Wisconsin
Richard Kiesling, USGS Minnesota Water Science Center, Moundsview, Minnesota
Brenda Moraska Lafrancois, National Park Service, Midwest Region, St. Croix Watershed Research Station
Kathy Lee, USGS Minnesota Water Science Center, Moundsview, Minnesota

Connectivity among channels and backwaters can exert considerable control over biogeochemical, water quality, and biologic processes in floodplain river ecosystems. Effects of connectivity can vary tremendously among rivers with variable geomorphology, hydrologic regimes and watershed land use. A collaborative research project among USGS and National Park Service scientists was initiated in the St. Croix (SCR) and Upper Mississippi Rivers (UMR) during the summer of 2007 to characterize: 1) Spatial differences in nutrient concentrations and cycling in main channel versus backwater habitats; 2) Effects of flood versus base river flow on nutrient concentrations and biogeochemical processes. 3) Biogeochemical processes affecting nitrogen cycling among rivers and habitats; 4) Nutrient enrichment on key indicator biota and ecological processes (growth/survival of juvenile unionid mussels, production of benthic algae, and ecosystem metabolism). This paper will focus on nutrient distributions; biogeochemical processes may be described if data becomes available.

The study included 2 sites each, in the main channel (MC), flowing backwater (FBW) and isolated backwaters (IBW) of the SCR (near Close Slough at Osceola) and UMR near the 9th Street landing and in River Lake. Water column nutrient concentrations and water quality variables measured included total nitrogen (TN), nitrate-N (NO3), ammonium-N (NH4), total phosphorus (TP), soluble reactive P (SRP), chlorophyll a, pH, DO, conductivity, temperature.

As expected NO3 and TN concentrations in the flowing habitats of the UMR (~1.5 and 3 mg-N L^-1, respectively) were 2 - 4x higher than in than SCR. Nitrate in isolated backwaters, in both rivers, was extremely low, suggesting high rates of assimilation or denitrification were occurring. In both rivers concentrations of NH4 were higher in FBW than MC or IBW, particularly in August. Observed springs and seeps along these slow flowing bluff-side channels suggests groundwater inputs likely explained this pattern. Elevated NH4 concentrations (0.2 mg-N L^-1) in the IBW of UMR in August, suggests high rates of mineralization of organic nitrogen were occurring. Total P concentrations were 4x higher in UMR (~220 µg L^-1) than SCR, with little difference among habitats in either river. SRP concentrations in the UMR (~70 µg L^-1) were ~10x greater in channels and 4x greater in IBW than those in SCR.

From this and other work on flood plain river nutrient dynamics, connectivity and isolation appear to effect nitrogen and phosphorus differently. Isolation (long hydraulic residence) of backwaters results in depletion of nitrate (and reduction of TN) due to heterotrophic and autotrophic processes. Ammonium can be generated and released from sediments but at low rates. Connection to main channels replenishes depleted water column nitrogen. Processes regulating phosphorus concentrations are dominated by redox conditions and sedimentation; hence, habitat isolation will not necessarily result in depletion of soluble P, particularly where anoxia is common in highly organic sediments.