Linking aquatic metabolism, gas exchange, and hypoxia to impacts along the 300-km Grand River, Canada

The Grand River is a 7th-order 300-km river draining the largest watershed (6800 km(2)) in southern Ontario, Canada. The watershed has experienced large landuse changes during a period > 100 y, resulting in increasing agricultural and urban nutrient inputs to the river. As a result, the Grand River is highly degraded from its source to mouth. We studied longitudinal and temporal changes in aquatic community metabolism (photosynthesis and respiration) and O2 gas exchange over 3 seasons using O-2 and delta O-18-O-2. Diel changes in O-2 saturation were > 50 percentage points along the river. In some parts, the diel O-2 change was > 10 mg/L. Strong daily variation in delta O-18-O-2, up to 22%, was observed at all 23 sampling sites in the river. Despite consistently high nutrient levels and high productivity from headwaters to mouth, wastewater treatment plant (WWTP) effluents strongly affected O-2 saturation in all seasons. Modifications to river flow or nutrient inputs that affect the O-2 gas-exchange coefficient or O-2 demand could exacerbate current nighttime hypoxia problems. Strong diel variability in O-2 may not directly indicate changes in metabolic rates because changes in gas-exchange coefficients alone are enough to mask changes in metabolic rates. Photosynthetic rates immediately downstream of WWTPs did not increase because nutrients were already high because of agricultural nutrient loading upstream of WWTPs. As a result, WWTP nutrients were exported downstream rather than used immediately below the WWTPs and the zone of impact from WWTP nutrients extended farther downstream than would otherwise be expected. O-2 is the measure used by ecosystem managers, but metabolism and gas exchange must be managed to achieve the desired O-2 outcomes.