Macrophyte mediated predation on hydrilla-dwelling macroinvertebrates

Hydrilla verticillata invaded south central Louisiana during the 1970s, and subsequently became the dominant submerged macrophyte in floodplain habitats of the Atchafalaya River Basin. Effects of hydrilla on littoral habitat structure, water quality, fish, and macroinvertebrates have been pervasive. Hydrilla can reduce water column dissolved oxygen, harbor different macroinvertebrate communities compared to native macrophytes, and can negatively affect fish feeding and growth. We hypothesized that dense hydrilla stands would also impact vertebrate predation on resident macroinvertebrates, although predation effects would likely be mediated by bed position. During 2003 and 2004, we conducted exclosure experiments in the Atchafalaya River Basin with artificial substrates to test for variations in hydrilla bed macroinvertebrate communities caused by predation, plant architecture, and bed position. To determine the quantity of invertebrates consumed by fishes, we also examined stomach contents of potentially invertivorous fishes inhabiting these beds. We found that bed position and predation were important in structuring macroinvertebrate communities, whereas plant architecture had little effect. Diet analyses indicated that littoral fishes fell into three categories: 1) those that did not feed on macroinvertebrates; 2) those that fed on small invertebrates (e.g., dipterans); and 3) those that fed on large invertebrates (e.g., decapods) and poeciliid fishes. Further, the diets of the invertivorous fishes coincided with spatial distributions of the littoral macroinvertebrate community. In general, those macroinvertebrates most commonly found in fish diets occurred in greater densities within the bed interior. This study demonstrated that hydrilla beds appear to provide a refuge from fish predation for resident macroinvertebrates, which may be an important aspect of macrophyte management strategies.