Spatial Distribution of Macroinvertebrates Inhabiting Hydrilla and Coontail Beds in the Atchafalaya Basin, Louisiana

Hydrilla [ Hydrilla verticillata (L.f.) Royle] became established in the Atchafalaya River Basin (ARB) in south central Louisiana during the 1970s, and now dominates the submergent macrophyte community. We examined the potential effects of this shift in macrophyte composition on the distribution of phytomacrofauna by comparing water quality and macroinvertebrate assemblage structure in canopy and sub-canopy habitats at edge and interior locations within hydrilla and native coontail ( Ceratophyllum demersum L.) beds during the latter stages of the 2001 flood pulse. Both plant species exhibited similar water quality characteristics during the study, with significantly higher temperature, dissolved oxygen, and pH levels in canopy habitats. Principal components analysis of log-transformed macroinvertebrate densities identified four assemblages that together accounted for 63.5% of the variation in the density data. The GastropodaHydrachnida assemblage exhibited higher densities in coontail during May-June (declining river stages), and was more abundant at interior locations in both macrophyte beds during July (stable river stages). The Hemiptera-Amphipoda assemblage exhibited higher densities in the canopies of the two plants during both sampling periods, as did the Decapoda-Odonata assemblage in July. The Diptera-Coleoptera assemblage showed a similar trend in vertical distribution, as well as marginally higher densities in hydrilla beds. The continued spread of hydrilla throughout the ARB has reduced the diversity of macrophyte habitats available to phytophilous macroinvertebrates, and has resulted in pervasive hypoxia in the macrophyte sub-canopy over large portions of available littoral habitat, with significant impacts on the vertical distribution of littoral macroinvertebrates.