Trophic Dynamics and Organic Carbon Sources in Washington DC, USA Area Seepage Spring Communities

Abstract

Seepage springs are common ephemeral freshwater habitats in the Washington DC area that are home to a variety of invertebrates including the endemic and rare Stygobromus hayi, the official amphipod of Washington DC, USA. However, little is known about the community structure of these habitats. In this study, we analyzed the general food web dynamics of two seepage springs (Pimmit Run and Goldmine Tract) in the Washington DC area that are known to include a congener of S. hayi, Stygobromus tenuis potomacus. Within these two seepage springs, we identified 29 taxa, with varying degrees of abundances, from 18 field excursions over three seasons: Winter, Spring, and Summer. Using dual abundance stable isotope analysis of δ¹³C and δ¹⁵N, we estimated the trophic positions of Lumbriculidae, Conasellus kenki, S. tenuis potomacus, Crangonyx shoemakeri, Tipula sp., Pseudolimnophila sp., and a species of Platyhelminthes. S. tenuis potomacus was the dominant predator in both locations, with δ¹⁵N and δ¹³C values between 7‰ and 10‰ and −25‰ and −27‰ respectively. C. kenki and C. shoemakeri were consistently lower in the food web, with δ¹⁵N and δ¹³C values between 4‰ and 7‰ and −24‰ and −27‰ respectively and, in some cases, C. kenki may be prey for S. tenuis potomacus. In both seeps leaf material was 4‰ to 5‰ lower in δ¹³C than the shredding/grazing invertebrates suggesting that they derive nutrients from microbes colonizing leaves not from leaf carbon itself.