Hydrodynamic enhancement of invertebrate larval settlement in microdepositional environments: colonization tray experiments in a muddy habitat

Abstract

To test whether the distribution of settling larvae in muddy habitats is influenced by near-bed hydrodynamics, colonization trays with different trapping characteristics were deployed flush with the ocean bottom at 20-m depth in Buzzards Bay, Massachusetts. The goal of these experiments was to determine whether different densities of settling larvae would be collected under different hydrodynamic conditions. Before deployment, trays were filled with pre-frozen, muddy sediment collected from the site; some trays (Flush Trays) were filled so that the sediment surface would be flush with that of the ocean bottom when in situ, and other deeper trays (Depression Trays) were filled with a similar volume of sediment so that the sediment surface was a 8 cm below that of the surrounding ocean bottom when deployed. This latter treatment created a hydrodynamic regime that would trap passive particles, permitting a test of whether settling larvae at the site would be entrained like passive particles, and thus occur in higher densities in Depression Trays compared with Flush Trays. Experiments were deployed at five different times during the summer of 1990, and were recovered after 3 or 4 days depending on the sampling date. Total densities of organisms were higher in Depression Trays compared with Flush Trays on each sampling date, and of the five taxa that were consistently abundant, four were significantly more abundant in Depression Trays (bivalve larvae, gastropod larvae, juvenile Mediomastus ambiseta (Hartman) polychaetes, and nemerteans). Juvenile spionid polychaetes were abundant on only one date, and on that date they were significantly more abundant in Depression Trays. The only abundant taxon that was not significantly more abundant in Depression Trays was Capitella spp. polychaetes. To determine whether higher numbers in Depression Trays was an active response by settling larvae to elevated organic matter that is often associated with trapping environments such as depressions, some Flush Trays were enriched with Thalassiosira sp. on one of the sampling dates. Densities of organisms in Thalassiosira Trays were either comparable to or lower than those in Flush Trays, suggesting that higher levels of organic matter do not necessarily promote larval settlement of dominant colonizers at this site over the time scale of these experiments. Furthermore, several of the taxa that were more abundant in Depression Trays are common at the site and might therefore be expected to find Flush Trays a suitable environment in which to settle. Thus, the most parsimonious explanation for these results is that larvae were passively entrained in Depression Trays. These field experiments are consistent with results from earlier flume studies suggesting that the microdepositional environment of small depressions may result in passive entrainment of settling larvae, indicating that hydrodynamic, as well as behavioral, factors may determine where larvae in muddy habitats initially settle.