Factors influencing chlorophyll in an agriculturally-impacted river.
Hatch, Lorin K.
The Minnesota River (USA) drains nearly 40,000 km² of land in southern and western Minnesota, 92 % of which is under intensive agricultural production. The river is a significant source of point and non-point source pollutant loads, including sediment, nutrients, pesticides, and pathogens. This study presents the analysis of 18 years of water quality monitoring data (approximately 50 samples per year) at a site located upstream of the major metropolitan area of Minneapolis/St. Paul. Mean monthly soluble reactive phosphorus and dissolved inorganic nitrogen values ranged from 57 to 150 mug/l and 2.82 to 7.09 mg/l, suggesting that algal nutrient limitation is not relevant in the lower Minnesota River. There is a strong negative relationship between discharge and chlorophyll a concentration during the April-to-October period (r²= 0.70), but this relationship did not hold during the 1988-1990 drought period. Mean monthly chlorophyll values showed a small peak in late spring, lowered values during June/July, and the highest values during the August-October period. Elevated sediment levels in June/July (> 150 mg/l) may be partially responsible for the lowered chlorophyll levels at that time, possibly due to shading and/or abrasive mechanisms. Although algae appear to be a small component (1 %) of suspended matter in the Minnesota River, algal respiration and decomposition may be contributing significantly to biochemical oxygen demand.