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Nutrient and Chlorophyll Relations in Selected Streams of the New England Coastal Basins in Massachusetts and New Hampshire, June-September 2001

APPLICATION OF NUTRIENT AND CHLOROPHYLL a DATA TO IN-STREAM CRITERIA DEVELOPMENT BY WATER-RESOURCES MANAGERS

The nutrient and chlorophyll data collected for this study can provide information useful for nutrient-criteria development. One approach is to use linear regression to estimate the total nitrogen and total phosphorus concentrations that corresponded to various chlorophyll a concentrations at open-canopy sites (figs. 7 and 8). The periphyton chlorophyll a concentration at which a stream is considered mesotrophic can be identified on the regression line, and the corresponding nutrient concentrations may be applicable as estimates for setting nutrient criteria. The mesotrophic concentration should be used rather than the eutrophic concentration because this criterion is intended to prevent the adverse effects of nutrient enrichment. Biomass data from multiple studies including Lock (1981), Horner and others (1983), Bott and others (1985), and Biggs and Price (1987) were summarized by Biggs (1996) to determine typical algal biomass concentrations for different site types according to levels of nutrient enrichment. On the basis of these data, the median concentration of periphyton chlorophyll a from moderately enriched streams (mesotrophic) was determined to be 21 mg/m2, which is similar to the 50th-percentile concentration of periphyton chlorophyll a (17 mg/m2) calculated with the data from all open-canopy sites in this study. An estimated concentration of 1.3 mg/L for total nitrogen and 0.12 mg/L for total phosphorus corresponded to the periphyton chlorophyll a concentration (21 mg/m2) for moderately enriched streams (figs. 7 and 8 ).

fig7
Figure 7.  Graph showing Relation between total nitrogen and periphyton chlorophyll a concentrations for the six open-canopy sites sampled five times from June through September 2001 in the New England Coastal Basins study area in Massachusetts and New Hampshire.

 

fig8
Figure 8.  Graph showing Relation between total phosphorus and periphyton chlorophyll a concentrations for the six open-canopy sites sampled five times from June through September 2001 in the New England Coastal Basins study area in Massachusetts and New Hampshire.

 

The allowable or desired concentrations of nutrients and chlorophyll a may depend on the designated use or existing quality of a water body. Lines representing the 25th percentile of periphyton chlorophyll a from all 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 on figures 7 and 8 to provide additional total nitrogen and total phosphorus concentrations. An estimated concentration of 0.45 mg/L for total nitrogen and 0.027 mg/L for total phosphorus corresponded to the periphyton chlorophyll a concentration of 5.2 mg/m2 (25th percentile from the open-canopy sites) and an estimated concentration of 0.90 mg/L for total nitrogen and 0.075 mg/L for total phosphorus corresponded to the periphyton chlorophyll a concentration of 16 mg/m2 (75th percentile value for open-canopy reference sites). It is reasonable to assume that the data for impaired open-canopy sites would represent conditions most favorable for the growth of nuisance algae. If nutrient criteria were established from data collected under these conditions, it could also be applied to other conditions (closed-canopy sites) that are less favorable for the growth of nuisance algae because of light limitation, and could be considered a conservative limit.

The concentrations of total nitrogen and total phosphorus obtained using several potential methods for nutrient criteria development using data in this study are summarized in table 4. These methods are some of the possible techniques that can be used by water-resources managers; depending on the designated use of a specific area, other calculated percentiles or estimates might be more appropriate.

 

To print out table 4, please use this pdf (265KB).

 

Table 4.   Summary of total nitrogen and total phosphorus concentrations for potential methods of nutrient criteria development in the Northeastern Coastal Zone in Massachusetts and New Hampshire

[mg/L, milligrams per liter]

Criteria method Total
nitrogen
(mg/L)
Total
phosphorus
(mg/L)

Subecoregion 59 preliminary criteria. 0.57 0.024
25th percentile-all sites in this study. .64 .030
10th percentile-impaired sites in this study. .73 .036
Estimated concentrations of total nitrogen and total phosphorus1
Estimated concentration at the 25th percentile of chlorophyll a from all open-canopy sites. .45 .027
Estimated concentration at the 75th percentile of chlorophyll a from all open-canopy reference sites. .90 .075
Estimated concentration at the median literature concentration for chlorophyll a for moderately enriched streams. 1.3 .120

1 Concentrations of total nitrogen and total phosphorus are estimated from chlorophyll a concentrations in this study
(figs. 7 and 8).

 

The resulting total nitrogen and total phosphorus concentrations are generally similar for the various approaches. The lowest total nitrogen and total phosphorus concentrations were the subecoregion 59 preliminary criteria developed by the USEPA (0.57 mg/L for total nitrogen and 0.024 mg/L for total phosphorus) and the estimated total nitrogen and total phosphorus concentrations that corresponded to the 25th percentile of chlorophyll a from all open-canopy sites in this study (0.45 mg/L for total nitrogen and 0.027 mg/L for total phosphorus). The highest total nitrogen (1.3 mg/L) and total phosphorus (0.120 mg/L) concentrations corresponded to the estimated concentration from the median literature concentration of chlorophyll a from moderately enriched streams (21 mg/m2). A combination of these methods may aid in the selection of an appropriate range of total nitrogen and total phosphorus concentrations to be used for nutrient criteria development in wadeable New England Coastal streams.

 


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