An analysis of structural pattern in an eelgrass ( Zostera marina L.) amphipod community

Abstract

The amphipods of eelgrass ( Zostera marina L.) habitats near Beaufort N.C., were studied with the aims of 1. 1) determining the seasonal and spatial patterns of abundance and diversity of amphipods, 2. 2) evaluating the hypothesis that predation is generating observed patterns, and 3. 3) examining the hypothesis that competition among amphipods in eelgrass systems should be relatively unimportant. Four measures of amphipod community structure examined — density, number of species per sample, diversity ( H'), and evenness ( J) — showed basically similar seasonal patterns. These measures are at their highest values during the late winter and decrease sharply during the spring months to summer lows. There are increases in all of them in the fall. Seasonal patterns are generally similar from year to year and are somewhat similar within a single year at two study sites located in different areas of the cstuarine system. Predator inclusion experiments using the pinfish Lagodon rhomboides (L.) were successful in generating significant reductions in amphipod abundances following the two-week duration of the experiment. On the other hand, fish exclusion experiments were mostly unsuccessful at generating significant increases in amphipod density and species diversity as compared with uncaged controls, due to an increased activity of decapod predators. The importance of competitive interactions was evaluated in three ways: 1. 1) the matrix of product moment correlation coefficients was generated for all common species from all available sample data and was examined for significant negative values, 2. 2) gut contents of five common species were examined for food overlap, and 3. 3) laboratory competition experiments were carried out with three of the more common amphipod species. Evidence for competition was slight. In the amphipod assemblage studied, there is no competitive dominant which monopolizes food or space at high densities. Increased predation pressure results in a pattern of monotonic decrease in abundance and diversity. Removal of one level of predator (fish) in this system appears to result in increases in other predatory levels (decapod crustaceans), thus continuing to keep amphipod abundances low.