IMAGE-BASED ASSESSMENT OF FISH-INDUCED STREAMBED BIOTURBATION IN NESHAMINY CREEK, BUCKS COUNTY, PENNSYLVANIA

Biogeomorphic activity of the invasive common carp (Cyprinus carpio) has been found to have an extensive impact on bottom sediment in the middle reaches of Neshaminy Creek (lower Bucks County, PA). The fish have been observed feeding on the bottom of the creek resulting in feeding pits in muddy substrate derived from Triassic riparian mudstones and historic mill-dam “legacy” sediments. This study is the first attempt to characterize and quantify the density and dimensions of these biogenic structures. Analysis of photographs taken through shallow water shows that the slightly asymmetric depressions occur at densities of up to 40 m^-2 and range from 2.5–9.0 cm in diameter. Whereas it was difficult to determine their depth, a digital ImageJ-based grayscale analysis based on a relative darkness scale related to trapped phytodetritus indicates a range between several millimeters to >1 cm. Because carp have no native predators and grow up to 10 kg even in small creeks, they have the potential for significant impact on fluvial substrate, in addition to their more traditional role of removing aquatic macrophytes and nutrient cycling. Native fish from the catfish (Ictaluridae) and sucker (Catastomidae) genera also feed on the bottom of the creek contributing to bottom sediment redistribution. These feeding pits could be preserved in the geologic record as variations of Piscichnus ichnogenus, with potential of co-occurrence and overprinting by riparian ichnites during periods of droughts or permanent water-level drop. The preservation of streambed and basal streambank traces will be improved through rapid deposition or water-level rise and an influx of fresh mud.