Invertebrate colonization of woody debris in coastal plain streams

We studied colonization dynamics and habitat preferences of macroinvertebrates associated with submerged woody debris in Louisiana coastal plain streams. In an in situ experiment, Branches of magnolia Magnolia grandiflora and water oak Quercus nigra were anchored over sand and gravel substrate at two sites in the Bogue Falaya River, St. Tammany Parish, Louisiana, U.S.A. Branches were removed from the units weekly for 4 weeks and biweekly for 1 month to assess colonization. Invertebrate numbers were highest at weeks three and six on magnolia over gravel habitat, although numbers of the most dominant taxa (Baetidae, Hydropsyche sp., Cernotina sp., Oecetis sp. and Hydroptilia sp.) peaked at different times during the study. Abundances of Hydropsyche sp. and Cernotina sp. were greatest on magnolia branches secured over gravel substrate, whereas Oecetis sp. abundances were greatest on oak branches 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 macroinvertebrates. In fall 1997 and spring 1998, we took water samples and measured in situ water quality characteristics at one site on each stream, and then collected submerged wood (surface area about 3051 cm²) and sand and gravel substrate (Surber samples totaling 2800 cm² for each substrate) to determine the density and taxonomic composition of resident macroinvertebrates. Highest numbers of invertebrates were collected in fall and spring from gravel substrate (P=0.0001). Macronychus sp. were more abundant on wood than gravel or sand (P<0.0003) and Hydropsyche sp. was more abundant on fall wood samples (P=0.0001). Analyses of water quality parameters revealed associations between Hydropsyche sp. abundance and potassium (P=0.0070) and specific conductance (P=0.0080), although both parameters exhibited interactions with substrate type (P<0.0144).