Invertebrate colonization of woody debris in coastal plain streams

We studied colonization dynamics and habitat preferences of macroinvertebra tes associated with submerged woody debris in Louisiana coastal plain strea ms. In an in situ experiment, Branches of magnolia Magnolia grandiflora and water oak Quercus nigra were anchored over sand and gravel substrate at tw o sites in the Bogue Falaya River, St. Tammany Parish, Louisiana, U.S.A. Br anches were removed from the units weekly for 4 weeks and biweekly for 1 mo nth to assess colonization. Invertebrate numbers were highest at weeks thre e and six on magnolia over gravel habitat, although numbers of the most dom inant taxa (Baetidae, Hydropsyche sp., Cernotina sp., Oecetis sp. and Hydro ptilia sp.) peaked at different times during the study. Abundances of Hydro psyche sp. and Cernotina sp. were greatest on magnolia branches secured ove r gravel substrate, whereas Oecetis sp. abundances were greatest on oak bra nches secured over sand substrate. We also surveyed six streams in southeast Louisiana to examine the relative importance of wood, gravel and sand as substrate for lotic macroinvertebra tes. In fall 1997 and spring 1998, we took water samples and measured in si tu water quality characteristics at one site on each stream, and then colle cted submerged wood (surface area about 3051 cm(2)) and sand and gravel sub strate (Surber samples totaling 2800 cm(2) for each substrate) to determine the density and taxonomic composition of resident macroinvertebrates. High est numbers of invertebrates were collected in fall and spring from gravel substrate (P=0.0001). Macronychus sp. were more abundant on wood than grave l or sand ( P <0.0003) and Hydropsyche sp. was more abundant on fall wood s amples (P=0.0001). Analyses of water quality parameters revealed associatio ns between Hydropsyche sp. abundance and potassium (P=0.0070) and specific conductance (P=0.0080), although both parameters exhibited interactions wit h substrate type (P <0.0144).