An application of the principles of ecological stoichiometry, this preliminary investigation examined the elemental composition of Amphipoda Gammarus spp. (including G. pseudolimnaeus and G. fasciatus) and associated leaf litter collected from disturbed and pristine (reference) streams in five watersheds located in James City County, VA. Disturbed streams received discharge from storm water retention ponds (BMPs) within 100 m of the sampling site, and also displayed varying degrees of channel incision and widening, bankside root exposure, disruptive sedimentation and burial of leaf pack habitat, and potentially conspicuous anthropogenic waste (e.g. rubbish, discarded appliances.) Pristine streams displayed no anthropogenic disturbance, although undocumented contamination may impact such sites. Samples were sorted, with identifiable (larger than quarter-sized) leaf litter and amphipodsremoved from coarse woody debris, sand, pebble, cobble, and unidentifiably small organic detritus. Leaf litter and Gammarus spp. were identified to family and genus, respectively, sorted into size classes, freeze-dried, pulverized, and analyzed for total C, H, and N with a Perkin-Elmer Series II CHNS/O 2400 elemental analyzer. Total P for leaf litter and Gammarus spp. were determined colorimetrically using an ashing/acid technique. Comparisons of C:N, C:P, and N:P were completed by size class and stream type. A repeated measures analysis revealed significant differences by amphipod size class for C:N (MD, SM) and N:P (LG, SM,) and for %P (LG, SM) and %N (LG, MD.) No significant differences occurred in C:N, C:P, N:P, %P, %N, and %C, between disturbed and pristine (reference) streams. A six-size class (LG+, LG, MD+, MD, SM+, SM) sample was prepared from a single pristine (reference) stream, suggesting an inverse relationship between C/N:P and %P. Leaf litter C:N, C:P, N:P, %P, %N, and %C differ significantly from their amphipod values, indicating that amphipoda homeostatically regulate their elemental composition. The elevated %P observed in SM+ and SM size-class Gammarus spp. and significant difference in N:P (LG, SM) suggests greater amounts of nucleic acids, possibly related to greater rates of cell division and therefore a more rapid growth rate in early instars.
For additional documentation Michael Gallisdofer provided a PowerPoint Presentation entitled "Elemental (C, N, P ) Analysis of Gamarrus spp" provided here in PDF form.