Macroinvertebrate abundance and distribution of hydrilla and ceratophyllum habitats in the Atchafalaya River Basin, Louisiana

Submerged aquatic vegetation (SAV) plays an important role in aquatic systems, providing shelter, breeding habitat, and epiphytic forage for numerous fishes and aquatic macroinvertebrates. Since 1960, many lentic habitats in the southern U.S. have been invaded by Hydrilla verticillata, and in the last two decades this aggressive macrophyte has become the dominant species of SAV in the Atchafalaya River Basin in south central Louisiana. Because of its highly branched morphology and characteristically high densities, I found it difficult to quantitatively sample the macrofauna inhabiting hydrilla stands with traditional gears such as sweep nets, particularly under the canopy. As a consequence, I developed a suitcase trap that provided an efficient, quantitative method of sampling hydrilla-associated macroinvertebrates, and compared the abundance and taxonomic composition of samples collected with suitcase and sweep net samplers in dense hydrilla habitat. The suitcase trap is easy to deploy and retrieve, effective in all plant densities, permits estimation of macroinvertebrate densities by plant volume or dry weight, and is more effective than traditional sweep nets in describing the vertical distribution of macroinvertebrates inhabiting hydrilla-dominated littoral habitats. To provide a better understanding of the effects of exotic macrophyte invasions on the ecology of epiphytic invertebrates, and to identify possible management alternatives to mitigate detrimental impacts associated with these invasions on littoral habitat quality, I measured the density of vegetation-dwelling macroinvertebrates on exotic Hydrilla verticillata and native Ceratophyllum demersum in the Atchafalaya Basin. I collected a total of 34,996 macroinvertebrates from hydrilla and coontail habitat from May to August 2001 to determine how exotic hydrilla compares to native macrophytes in terms of macroinvertebrate habitat. Abundant macroinvertebrate taxa included Amphipoda, Decapoda, Diptera, Gastropoda, and Ephemeroptera. Overall, macroinvertebrate abundance between hydrilla and coontail was relatively similar. It is apparent from this study that, at least at the assemblage level, differences between abundance and distribution patterns of macroinvertebrates in different macrophyte species do exist. Principal component analysis displayed differences between macroinvertebrate assemblages, although the relative effects (and interactions) of declining or fluctuating water quality, macrophyte architecture, food resource quantity and quality, and predatory mortality on macroinvertebrate community composition remain to be identified