Longitudinal trends and discontinuities in nutrients, chlorophyll, and suspended solids in the Upper Mississippi River: implications for transport, processing, and export by large rivers
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Across the distances spanned by large rivers, there are important differences in catchment characteristics, tributary inputs, and river morphology that may cause longitudinal changes in nutrient, chlorophyll, and suspended solids concentrations. We investigated longitudinal and seasonal patterns in the Upper Mississippi River (UMR) using long-term data (1994–2005) from five study reaches that spanned 1300 km of the UMR. Lake Pepin, a natural lake in the most upstream study reach, had a clear effect on suspended material in the river. Suspended solids and total phosphorus (TP) concentrations were substantially lower downstream of the lake and percent organic material (OM%) in suspension was higher. Below L. Pepin, mean total and organic suspended solids (TSS, OSS) and TP increased downriver and exhibited approximately log-linear relationships with catchment area, whereas OM% declined substantially downriver. Despite the downriver increase in TSS and OSS, concentrations similar to those above L. Pepin do not occur until ~370 km downriver indicating the extent of the influence of L. Pepin on the UMR. Chlorophyll concentrations were lower in the most downstream study reach, likely reflecting the shorter residence time and poor light climate, but there was not a consistent longitudinal decline in chlorophyll across the study reaches. Dissolved silica (DSi), DSi:TN, and DSi:TP declined downriver suggesting that DSi uptake and sedimentation by river phytoplankton may be reducing DSi transport in the river, and indicating that the eutrophication of the river may contribute to a reduction of DSi export to the Gulf of Mexico.
KeywordsNitrogen Phosphorus Silica Stoichiometry Mississippi River
We thank the LTRMP water quality sampling crews from the Minnesota Department of Natural Resources, Wisconsin Department of Natural Resources, Iowa Department of Natural Resources, Illinois Natural History Survey, and Missouri Department of Conservation, who collected the data reported here. This manuscript benefitted from discussions with staff from these field stations. Barry Johnson, Eric Strauss, Paul Bukaveckas, and anonymous reviewers provided valuable comments on earlier versions of this manuscript. The LTRMP funded this study. The LTRMP is funded by the U.S. Army Corps of Engineers and administered by the U.S. Geological Survey, Upper Midwest Environmental Sciences Center. Shirley Yuan and the USGS Upper Midwest Environmental Sciences Center water quality laboratory conducted all chemical analyses. Brian Gray provided statistical consultation. The land cover analysis was performed by J. C. Nelson and Cassi Bauch and funded in part by the Great Rivers Environmental Monitoring and Assessment Program of the U.S. Environmental Protection Agency. Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government.
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