Water quality of pocosins and associated wetlands of the Carolina Coastal Plain

Abstract

We reviewed literature data on pocosins and associated wetlands of the Carolina coastal plain to 1) characterize the surface and subsurface water quality of undisturbed pocosins, 2) identify the processes controlling solution C, N, and P concentrations, and 3) examine the effects of disturbance on water quality and nutrient export. We hypothesized that the key to predicting nutrient losses from disturbed pocosins lies in understanding how disturbance affects both hydrologic export and the processes controlling solution C, N, and P concentrations. Since the development of pocosins for commercial forestry and agriculture involves a complex series of disturbances (drainage, vegetation removal, fire, and fertilization), we analyzed the specific effects of these component disturbances as well as the composite effects of agricultural and silvicultural development.

Surface and subsurface waters of undisturbed pocosins are dominated by H⁺ cations (100–1000 μeq/L) and SO ²⁻₄ anions (475 μeq/L), with Ca²⁺ concentrations below 50 μeq/L, and are chemically similar to more northem ombrotrophic bogs. However, pocosin waters are more acid and contain higher concentrations of Na⁺, SO ²⁻₄ , and Cl⁻, probably due to their coastal environment. Solution export of N from undisturbed pocosins is primarily as organic N (74–88%), but even though organic P comprises 73–93% of total dissolved P in subsurface pore waters, 57–87% of P exported is inorganic PO ³⁻₄ . C:P ratios increase during the growing season in subsurface waters from 2355∶1 in June to 9563∶1 in August, as soluble organic P and PO ³⁻₄ supply toin situ anion-exchange resins decline.

Biological processes control C, N, and P transformations in pocosin soils and are strongly influenced by hydrology and P availability. Seasonal changes in water table depth suggest seasonal fluctuations in soil redox potential that could affect a variety of processes controlling water quality and nutrient export and may cause changes in the source of pocosin drainage waters (surface, subsurface, or mineral soil), thus affecting water quality. Phosphorus availability limits plant growth and may play an important role in controlling nutrient export due to its potential influence on rates of C, N, and P cycling.

Drainage of otherwise undisturbed pocosins does not affect total runoff volume but may affect nutrient export due to changes in the source of drainage waters. Drainage coupled with vegetation removal increases total runoff, increasing freshwater inputs downstream. Fire increases soil nutrient availability and may increase nutrient export when coupled with vegetation removal. Fertilization effects on nutrient export will be determined by fertilization frequency and by the degree of saturation of the immobilization capacity of soil microorganisms, the uptake capacity of plant roots, and the sorption capacity of soil minerals. Agricultural development increases C, N, and P concentrations in drainage waters and N and P export. Silviculture may have less severe effects on nutrient export, since harvesting and fertilization occur less frequently, but may reduce runoff, reducing freshwater inputs downstream.