Relationships between biological and enrichment indicators were explored to demarcate hallmarks of enrichment that can be used in managing eutrophication in large rivers. Biological indicators were sensitive to the measured range of enrichment and generally showed linear responses. However, fish biotic integrity scores, 24-h ranges in dissolved oxygen, and biological oxygen demand all showed strong nonlinear responses coinciding with sestonic chlorophyll concentrations in the range of 20-50 μg/l. Hallmarks of overt overenrichment were defined as sestonic chlorophyll ≥ 100 μg/l, 5-day biological oxygen demand ≥ 6.0 mg/l, and 24-h range in dissolved oxygen ≥ 9.0 mg/l. Acceptable levels were defined as chlorophyll < 30 μg/l, 5-day biological oxygen demand < 2.5 mg/l, and 24-h dissolved oxygen range < 7.0 mg/l. Relationships between nutrient concentrations and sestonic chlorophyll were weak, mainly due to high ambient levels of nutrients. However, total phosphorus concentrations averaged lower at sites showing less overt signs of enrichment, suggesting 130 μg/l total phosphorus as a management target for presently overenriched waters.
Keywords: Biological indicators; Enrichment; Eutrophication; Nutrients; Phosphorus; Rivers.