Nutrient and Chlorophyll Relations in Selected Streams of the New England Coastal Basins in Massachusetts and New Hampshire, June-September 2001
The U.S. Environmental Protection Agency is currently developing guidance to assist state water-resources managers with establishing nutrient and algal criteria to prevent streams and rivers from becoming impaired and to preserve them for designated water uses (such as the protection of wildlife). Nutrient levels differ among regions because of variations in geology, climate, soil, and other physical factors. Therefore, for the criteria to be most effective, ecoregional variations need to be considered.
Thirteen stream sites, primarily in eastern Massachusetts, were selected for a nutrient-chlorophyll assessment study. The sites represent a range of water-quality impairment conditions (reference, moderately impaired, impaired) based on state regulatory agency assessments and previously assessed nitrogen, phosphorus, and dissolved-oxygen data. In addition, a combination of open- and closed-canopy sections were sampled at six of the sites to investigate the effect of sunlight on algal growth. Among the sites sampled, median nutrient concentrations were lowest at the two reference sites and highest at the six impaired sites. When sites were grouped by site type, there were statistically significant differences (p < 0.05) among all site types for total nitrogen and total phosphorus.
The chlorophyll a concentrations from periphyton samples were positively correlated with total nitrogen and total phosphorus concentrations at the open- and closed-canopy sites. Correlations were higher at the open-canopy sites than at the closed-canopy sites because light stimulates the production of algal biomass and enhances algal-nutrient relations. Median concentrations of chlorophyll a from periphyton were highest at impaired sites and were significantly higher at open-canopy sites than at sites with closed canopies. Results of analysis of periphyton chlorophyll a concentrations from the six sites that had open- and closed-canopy sampling locations indicated that open-canopy sites had higher concentrations of chlorophyll a among all three site types. Nutrient concentrations, types of substrate, and stream velocities were similar between the closed- and open-canopy locations at the same site, indicating that light was affecting algal biomass as measured by chlorophyll a.
Linear regression was used to estimate the total nitrogen and total phosphorus concentrations that corresponded to various chlorophyll a thresholds. Nutrient concentrations were plotted as a function of chlorophyll a concentrations. A regression line was drawn and used to estimate concentrations of total nitrogen and total phosphorus from concentrations of chlorophyll a. On the basis of a median value in the literature for periphyton chlorophyll a of 21 mg/m2 for moderately enriched streams, total nitrogen and total phosphorus for this ecoregion (subecoregion 59) were estimated from a regression line using the literature value to yield concentration thresholds of 1.3 mg/L for total nitrogen and 0.12 mg/L for total phosphorus. Lines representing the 25th percentile of periphyton chlorophyll a concentrations for all of the open-canopy sites in this study (5.2 mg/m2) and the 75th-percentile value for open-canopy reference sites (16 mg/m2) also were plotted to provide additional nutrient limits based on allowable levels of algae. The 25th-percentile total nitrogen and total phosphorus concentrations were calculated with all of the study data as a possible additional threshold. From these data, concentrations of 0.64 mg/L for total nitrogen and 0.030 mg/L for total phosphorus were obtained. As another possible method of threshold development, the 10th-percentile concentrations of total nitrogen and total phosphorus were calculated using all of the impaired sites in this study. Concentration thresholds of 0.73 mg/L for total nitrogen and 0.036 mg/L for total phosphorus were obtained. These nutrient concentrations derived from current data (2001) may contribute to development of a potential set of regional nutrient criteria. The combination of these methods may aid in the selection of an appropriate range of total nitrogen and total phosphorus concentrations to be used by water-resources managers for nutrient criteria development.
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