Assessing aquatic productivity in the housatonic river using the Algal assay:bottle test

Dense algal growth and large diurnal oxygen variations in both free flowing and impounded reaches of the Housatonic River in Connecticut and Massachusetts prompted a concentrated study to assess the need for a comprehensive basin wide phosphorus abatement program and the contribution of point source discharges to eutrophication of the river. Inorganic nutrient levels and bioassay data indicated a predominance of nitrogen limitation below Pittsfield, MA resulting from discharges of phosphorus-rich, municipal and industrial wastewater into the river. N:P ratios suggested that the nutrient status of the river varied, to some extent, even over as short an interval as 2 months and was probably attributable to flow variations in the river. Algal assays of water samples confirmed the nutrient limitation predicted by N:P ratios in 10 out of 13 cases. Additions of varying percentages of untreated, and alum-treated Pittsfield domestic wastewater treatment plant effluent to Housatonic River water, resulted in a linear increase in algal yield that was within 20% of values predicted by inorganic nutrient content. No significant reduction in algal growth response was observed, however, in the alum-treated sewage since its N:P ratio was not sufficiently shifted to phosphorus limitation after 66% phosphorus removal (Al:P equal to 1.3:1). Phosphorus removal at Best Practicable Technology levels would therefore probably not change the nutrient status of the Pittsfield wastewater to phosphorus limitation.