Functional equivalency of natural and excavated coastal plain ponds

Abstract

A major obstacle to assessing functional equivalency of restored or created wetlands is the time needed to develop the functions of natural wetlands. We compared hydrologic, water-quality, and vegetation-composition functions of nine natural coastal plain ponds in the New Jersey Pinelands with those of four small, well-established excavated basins that are at least 50 years old. Our study revealed that well-established (> 50 yr old) excavated ponds achieved a moderate degree of functional equivalency with Pinelands wetlands, representing a range of coastal plain pond characteristics. Based on water-depth-fluctuation patterns and the similarity of most hydrologic indices, including high-water pond area, mean water depth, area of exposed substrate (drawdown), and the presence of a clay lens, the excavated ponds seemed to achieve hydrologic equivalency with the natural reference wetlands. However, steeper bank slopes found at most of the excavated ponds affected nearshore water depths and resulted in the absence of plant zonation that characterizes coastal plain ponds. The water-quality function, represented by pH, specific conductance, and total organic carbon, differed between pond types. The pH and specific conductance of the excavated ponds were higher and total organic carbon concentrations were lower compared with the natural ponds. We attributed these differences to landscape setting, reflected by adjacent vegetation and contrasting plant zonation. Elevated specific conductance values in the natural ponds were likely due to the higher hydrogen ion concentrations. Reduced light transmission due to higher organic carbon concentrations in the natural ponds may have greater ecological importance. However, differences in water-quality functions between the pond types may make excavated ponds more prone to changes in pH if constructed within landscapes with extensive developed or agricultural lands. The excavated ponds met or exceeded most vegetation-composition reference criteria associated with the natural wetlands. Total and herbaceous species richness were greater in the excavated ponds. Most importantly, the excavated ponds supported a native Pinelands species composition, thus preserving regional biodiversity. Because all ponds were acidic and displayed pH values within the range associated with native Pinelands plants, differences in pH may not have been the cause of the greater species richness. Although overall species composition differed between the two pond types, the flora of the created wetlands was similar to that of coastal plain ponds found in other regions and other areas of the Pinelands. The major difference in vegetation composition between ponds was both the lack of distinct vegetation zonation due to steeper slopes and lower patch-type diversity in the excavated ponds. These structural differences can be overcome by constructing ponds with slopes that are comparable to natural ponds. Because the transitional-upland location of the excavated ponds is a more likely location for a mitigation wetland, the effect of landscape setting on water quality may not be as easily remedied as the lack of nearshore slopes.