Use of diatoms for developing nutrient criteria for rivers and streams: A Biological Condition Gradient approach

Nutrient enrichment is one of the most important causes of ecological impairment in surface waters. More effective management of this problem is constrained by a lack of nutrient criteria that link phosphorus and nitrogen concentrations to levels of biological impairment as defined in management agency regulations. We used a new approach for identifying potential nutrient criteria using diatom assemblages and the Biological Condition Gradient (BCG). We first used diatom count data to assign study sites to BCG impairment categories (levels), then determined P and N concentrations corresponding with the boundaries between those levels. We studied diatom counts and environmental data from 95 river and stream sites throughout the state of New Jersey (NJ), USA. The sites represent five ecoregions, primarily forested uplands and coastal plains, and a wide range of nutrient concentrations (total P range of 15 to >200 µg L−1 and total N 0.5 to >4 mg L−1). Advantages of NJ as a study region are the ready availability of streams with a large gradient in nutrient concentrations and similarity to nutrient-stressed systems in many other geographic regions. BCG levels represent a range of ecological conditions from natural to highly impaired. A panel of experts on use of diatoms as ecological indicators assigned 57 study sites to BCG levels based on diatom assemblage composition. Potential nutrient criteria were subsequently derived by determining the measured TP and TN concentrations associated with the boundary between impaired sites and non-impaired sites (BCG levels greater than 4). Based on this approach, statewide criteria of no greater than 50 µg L−1 TP and 1.0 mg L−1 TN are indicated for maintaining or restoring sites to unimpaired condition. In some least-disturbed ecoregions, lower concentrations (e.g., 25 µg L−1 of TP) would be more appropriate to maintain current ecological conditions. Existing diatom TP and TN metric values for the study sites correlate well with BCG level assignments and can be used to determine if nutrient criteria are met. Overall, results demonstrate the usefulness of data derived from diatom assemblages in management of nutrient issues, and the potential widespread applicability of the BCG approach to develop nutrient criteria and to monitor compliance.