Establishing Relationships Among In-stream Nutrient Concentrations, Phytoplankton and Periphyton Abundance and Composition, Fish and Macroinvertebrate Indices, and Biochemical Oxygen Demand in Minnesota USA Rivers

Significant and predictable relationships were demonstrated among summer nutrient, chlorophyll a, and algal concentrations and biochemical oxygen demand (BOD) in five medium to large Minnesota, USA, rivers. Summer (June – September) flows in 1999 were significantly higher than 2000, and as a result, the “age” of the water (residence time) was greater in 2000. The lower and more stable flows generally resulted in higher algal concentrations in 2000 than in 1999. Algal composition varied not only in terms of origin: benthic vs. planktonic, but also along a gradient of nutrient enrichment. Benthic diatoms comprised a significant proportion of the algal community in clear low nutrient rivers but declined in significance in more nutrient rich rivers where planktonic green and blue-green algae became more prominent. In more turbid and high nutrient rivers highly tolerant blue-greens were dominant. Subsequent studies provide further insights into the linkages and relations among total phosphorus (TP) concentrations, chlorophyll-a and BOD. Water quality data from independent sites indicated that the previously-defined relationships were valid for most sites; however, these relationships did break down in the highly turbid Red River of the North because of extremely high inorganic turbidity that limited the growth of algae and subsequent nutrient assimilation. Diurnal dissolved oxygen (DO) flux (based on submersible data recorders over a period of three to six days) was found to be strongly positively correlated to summer mean TP and chlorophyll-a concentrations at 12 stream sites tested in 2000. Likewise, fish index of biotic integrity (IBI) scores were found to be inversely correlated with summer-mean TP. Linkages established here will contribute to nutrient criteria development and nutrient or DO-based TMDLs. Examples of approaches for setting river nutrient goals are offered.