Chemical limnology, algal growth potential, and nutrient limitation factors of the Upper New River, Virginia, and predictions concerning trophic status for the proposed Blue Ridge reservoirs

Abstract

Waters in the upper New River basin are well oxygenated, soft, bicarbonate poor, and relatively low in dissolved solids content, Water quality appears to be generally good, although concentrations of heavy metals present are often in excess of recommended water quality standards.

Algal growth potential tests performed on water from mainstream and tributary stations identified the waters in Peach Bottom Creek as having the greatest growth potential and indicated that P availability might control the potential productivity of water in the basin.

Nutrient limitation studies were conducted utilizing indigenous periphyton, indigenous plankton, and a selected, cultured organism (Selenastrum capricornutum). Uptake of ¹⁴C by indigenous periphyton and phytoplankton was greatest in river water cultures enriched with phosphorus. Regardless of whether or not in situ primary productivity is nutrient limited in the upper New River, these experimental enrichment results suggest that P would become limiting if physical factors were optimized or if these waters became impounded. Studies conducted on water from the 6 major tributaries to the proposed impoundments indicated that P was the primary nutrient factor limiting productivity in all tributaries except Peach Bottom Creek, where N was limiting. All waters in the basin containing a NO₃-N to orthophosphate-P ratio of 8.5 or greater appear to be P limited. A multiple regression model was calculated to predict algal growth potential in these waters.

Impoundment of lotic waters often leads to a reduction in color, turbidity, BOD, and numbers of coliform bacteria. On the other hand, objectionable odors, higher levels of soluble Fe and Mn and loss of oxygen resources may result. Impoundments are often characterized by vertical stratification of O₂, temperature, alkalinity, free CO₂, pH, Fe, Mn, N,and P during different seasons of the year. As a result, the location of the penstocks can greatly influence downstream water quality during periods of stratification. Floral composition is drastically altered by impoundment, and often productivity is greatly increased. Algal blooms are common initially, and depending upon nutrient loading, may continue to occur or may subside within a few years.

If the proposed Blue Ridge Reservoirs are constructed, tributary streams inundated soils, surface runoff, and precipitation will contribute significant amounts of nitrogen and phosphorus. Based on projected inputs from these sources N and P budgets were calculated using 4 different models. Ten different criteria were subsequently utilized to predict the trophic status of the proposed reservoirs. Indications are that the reservoirs will be mesotrophic or eutrophic, immediately following inundation and will remain in this state until N and P inputs are reduced.