Total nitrogen trends for 1980-90 (Harned and others, 1995) indicate increases in the tributaries at the upper end of Kerr Lake and decreases in the Neuse River Basin. The increases in nitrogen concentrations upstream from Kerr Lake reflect changes in atmospheric inputs of nitrogen, possibly caused by construction of powerplants near the lake. Decreases in nitrogen concentrations in the Neuse River Basin are dissimilar, suggesting differing causes for the decreases. Trends observed for total ammonia and organic nitrogen were similar to those observed for total nitrogen and nitrate nitrogen. Upward trends in nitrate also were detected for two sites in the Tar River Basin and probably are due to changes in agricultural practices. A gradual increase in nitrate concentrations in the Chowan River near Riddicksville was detected.
Total phosphorus concentrations generally showed upward trends for streams in Virginia and downward trends for streams in North Carolina. The effect of the 1988 phosphate-detergent ban in both States is evident at several streams, including the Nottoway River, Neuse River, and Contentnea Creek. Total phosphorus is strongly associated with suspended sediment; therefore, locations having high suspended-sediment concentrations, such as those in the upper Roanoke and Dan River Basins, generally have high phosphorus concentrations. Although decreases in nitrogen and phosphorus have been observed, concentrations are still high enough in the Tar and Neuse River Basins to cause nuisance algal growth.
Suspended-sediment concentrations for the major rivers in the Albemarle-Pamlico Drainage Basin from 1980 to 1990 generally were less than 50 mg/L with greater concentrations occurring in Piedmont streams than in Coastal Plain streams because of steeper slopes and more readily erodible soils in the Piedmont (Harned and others, 1995). Kerr and Gaston Lakes are effective traps for sediment in the upper Roanoke River Basin, as is Falls Lake in the Neuse River Basin. Substantial decreases in suspended-sediment concentrations are evident in the Neuse River as it flows from the Piedmont to the Coastal Plain. Agricultural activities, particularly highly erosive corn and tobacco farming, combined with the steep slopes of the Piedmont produce high sediment yields.
Median sediment concentrations for data collected from 1993 to 1995 during the NAWQA study were highest in the Neuse River (25 mg/L), Devil's Cradle Creek (15.5 mg/L), and the Tar River (16.5 mg/L), probably due to the greater slopes and higher degree of erosion of the Piedmont drainage areas compared to those of the Coastal Plain. However, high median concentrations in Coastal Plain streams--Albemarle Canal (15 mg/L) and Chicod Creek (14 mg/L)--indicate that agricultural sources can have a substantial effect on sediment concentrations in streams even in areas of low slopes in the Coastal Plain.
Suspended- and dissolved-solids concentrations have decreased throughout the Albemarle-Pamlico Drainage Basin over the past 15 years (1980-95). The decrease is probably a result of (1) construction of new lakes and ponds which trap solids, (2) improved agricultural soil management, and (3) improved wastewater treatment. Decreasing concentrations of suspended solids in the sounds and estuaries may result in clearer water and deeper light penetration which, in turn, enhance conditions for algal blooms in nutrient-enriched areas.