Establishing Relationships Among Nutrient Concentrations, Phytoplankton Abundance, and Biochemical Oxygen Demand in Minnesota, USA, Rivers

In this study, we demonstrated significant and predictable relationships among nutrients, algae, and biochemical oxygen demand (BOD) in five medium to large Minnesota, USA, rivers. These rivers were distributed across three ecoregions ranging from the Northern Lakes & Forests (NLF) (low nutrients), to the North Central Hardwood forests (NCHF) (intermediate nutrients), to the Western Corn Belt Plains (WCBP) (high nutrients). River order varied, allowing comparisons of periphyton-dominated rivers to phytoplankton-dominated rivers. Summer 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 as evidenced by paired comparisons between sites and years. Seston composition varied not only in terms of origin of the seston: benthic vs. planktonic, but also along a gradient of nutrient enrichment. Benthic diatoms were a significant proportion in clear, low nutrient rivers in the NLF but declined in significance in more nutrient rich NCHF rivers where planktonic green and blue-green algae became more prominent. In the more turbid and high nutrient WCBP rivers, highly tolerant blue-greens were common. Regardless of origin of the seston, rivers with high nutrients exhibited high chlorophyll and high BOD while those with low nutrients exhibited the inverse. The linkages established here will contribute to nutrient criteria development and nutrient or dissolved oxygen (DO)-based Total Maximum Daily Loading (TMDL) studies.