The effects of urbanization on wetland ecosystem structure and function: A case study of freshwater riparian wetlands in Fairfax County, Virginia

Urbanization alters watershed hydrology and degrades water quality with profound effects on the structure and function of riparian wetlands (RWs) and aquatic ecosystems downstream. The objective of this study was to use freshwater RWs in Fairfax County, Virginia, as a model system to examine the effects of increasing urbanization (as indexed by % impervious surface cover (ISC) in the surrounding watershed) on: (1) N and P concentrations in plant tissues and surface soils; (2) soil P saturation; (3) soil Fe chemistry as an index of altered hydrology; and to examine the relationship between: (4) RW plant and soil nutrient chemistries with the physical and biotic integrity of adjacent streams; (5) recent land use history and current plant and soil nutrient and Fe chemistries; and to compare (6) P retention in RWs and stormwater detention facilities (SW). Soil total P, and two soil P fractions representing P bound to Al and Fe hydrous oxides (NaOH and oxalate-extractable P), varied significantly, but non-linearly with % ISC (r2 = 0.69, r2 = 0.57, and r2 = 0.41, respectively). This relationship was best described by a 2 nd order polynomial; RWs appear to receive increasing nutrient loads between low and moderate % ISC (1.0--6.1% to 8.6--13.3%), than at higher % ISC (25.1--29.1%). These observations are consistent with alterations in watershed hydrology that occur with increasing urbanization, that direct water and nutrient flows away from natural RWs. Significant increases in total and crystalline Fe (r2 = 0.57 and r2 = 0.53, respectively), and decreases in relative soil Fe crystallinity, with increasing % ISC, suggest the mobilization and deposition of terrestrial sediments in RWs is likely associated with construction rather than hydrologic modification. Increases in plant and soil P concentrations in RWs were negatively correlated with the physical and biotic integrity of adjacent streams. Finally, SWs designed to provide both flood mitigation and water quality benefits, had significantly greater surface soil P concentrations than both natural RWs and SWs designed solely for flood control (831.9 +/- 32.5 kg/ha, 643.3 +/- 19.1 kg/ha, and 652.1 +/- 18.8 kg/ha, respectively). Data suggest that nutrient and sediment inputs associated with urbanization and stormwater management are important variables that affect wetland ecosystem services in urbanizing landscapes.