Biological thresholds of nitrogen and phosphorus in a typical urban river system of the Yangtz delta, China

Abstract

River health and associated risks are fundamentally dependent on the levels of the primary productivities, i.e., sestonic and benthic chlorophyll-a. We selected a typical urban river system of the Yangtz delta to investigate nutrient and non-nutrient responses of chlorophyll-a contents and to determine biological thresholds of N and P. Results showed the mean contents of sestonic and benthic chlorophyll-a across all sampling points reached 10.2 μg L(-1) and 149.3 mg m(-2). The self-organized mapping analysis suggested both chlorophyll-a contents clearly responded to measurements of N, P, and water temperature. Based on the chlorophyll-a criteria for fresh water and measured variables, we recommend the biological thresholds of N and P for our river system be set at 2.4 mg N L(-1) and 0.2 mg P L(-1), and these be used as initial nutrient reference values for local river managers to implement appropriate strategies to alleviate nutrient loads and trophic status.

Keywords: Biological threshold; Chlorophyll-a; Nitrogen; Phosphorus; SOM; Urban river.