Influences of watershed land cover pattern on water quality and biotic integrity of coastal plain streams in Mississippi, USA.

摘要

This study examined the role of spatial distribution of land cover on water quality and stream fish assemblages in watersheds of low-order streams in the Mississippi coastal plain. I found that the growing proliferation of urbanized land into landscapes with dominant rural or forest character decreases water quality and diversity of aquatic biota.;A reconstruction of local land use history identified the contemporary landscape mosaic as legacy of the Southern Lumber Boom and management decisions based on individual land-ownership. Such decisions transformed fire-dominated longleaf pine savanna into a landscape characterized by active fire suppression and second-growth Southern mixed deciduous forest, non-industrial pine silviculture, and an expanding urban core. Commercial development is concentrated in floodplains and along major transportation routes, while diffuse parcel-size residential development across the study area increases fragmentation of the forest-dominated landscape matrix.;Contemporary land cover distribution was evaluated using a new hybrid classification method combining panchromatic aerial photographs, high-resolution multispectral remote sensing data, and Landsat5 TM images. A spatially explicit modeling approach using GIS quantified watershed land cover based on distance to streams and relative upstream distance from sampling sites.;Water chemistry, stream geomorphology and fish assemblage metrics identified direct and indirect linkages between land cover, landscape features, and stream ecology. In the Mississippi coastal plain land cover influence exceeded geomorphological effects on stream conditions. Fish assemblages varied among sites in composition and diversity, and differed between watersheds with contrasting dominant land cover, suggesting integration of watershed-scale and local-scale influences.;Fish assemblage metrics identified species richness, assemblage dominance, trophic guild membership, and perturbation tolerance as best descriptors of dissimilarities between sites. The amount of Southern Mixed Forest in the total watershed and within 300 meter upstream from sampling sites correlated with degree of impairment of streamwater quality and composition of fish assemblages. Negative influences of land cover on stream ecology were greatest from urbanized land cover within 60 meter from streams along the entire upstream network. Strongest effects from impervious surface and managed green area on fish assemblages were through habitat degradation due to increases in the natural variability of stream hydrology, absence of large woody debris, and amplified fine sediment delivery, changes in pH, and electric conductivity during low-flow and pulse-flow.;Future land cover change will likely arise from diffuse low-density residential development in rural watersheds to the west of the study area, and increases in impervious surface from commercial land use concentrated in low-lying areas along major highways. The greatest future threats to existing water quality and stream ecology include runoff from transient land cover with site preparation or active construction, and from urbanized land cover, with increased sediment delivery to streams and further alteration of natural stream hydrology.;The involvement of private landowners in watershed stewardship and the establishment of stream management zones with introduction of best management practices for private forest ownership may ameliorate future development pressure on streams and aid in preservation of water quality and conservation of existing diversity in aquatic biota.