HYDRODYNAMIC ENHANCEMENT OF INVERTEBRATE LARVAL SETTLEMENT IN MICRODEPOSITIONAL ENVIRONMENTS - COLONIZATION TRAY EXPERIMENTS IN MUDDY HABITAT

To test whether the distribution of settling larvae in muddy habitats is influenced by near-bed hydrodynamics, colonization trays with diffe rent trapping characteristics were deployed flush with the ocean botto m at 20-m depth in Buzzards Bay, Massachusetts. The goal of these expe riments was to determine whether different densities of settling larva e would be collected under different hydrodynamic conditions. Before d eployment, trays were filled with pre-frozen, muddy sediment collected from the site; some trays (Flush Trays) were filled so that the sedim ent 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 almost-equal-to 8 cm below that of the surrounding ocean bottom when deployed. This latt er treatment created a hydrodynamic regime that would trap passive par ticles, permitting a test of whether settling larvae at the site would be entrained like passive particles, and thus occur in higher densiti es in Depression Trays compared with Flush Trays. Experiments were dep loyed at five different times during the summer of 1990, and were reco vered after 3 or 4 days depending on the sampling date. Total densitie s of organisms were higher in Depression Trays compared with Flush Tra ys on each sampling date, and of the five taxa that were consistently abundant, four were significantly more abundant in Depression Trays (b ivalve larvae, gastropod larvae, juvenile Mediomastus ambiseta (Hartma n) polychaetes, and nemerteans). Juvenile spionid polychaetes were abu ndant on only one date, and on that date they were significantly more abundant in Depression Trays. The only abundant taxon that was not sig nificantly more abundant in Depression Trays was Capitella spp. polych aetes. 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 samplin g dates. Densities of organisms in Thalassiosira Trays were either com parable 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 exper iments. Furthermore, several of the taxa that were more abundant in De pression Trays are common at the site and might therefore be expected to find Flush Trays a suitable environment in which to settle. Thus, t he 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 pass ive entrainment of settling larvae, indicating that hydrodynamic, as w ell as behavioral, factors may determine where larvae in muddy habitat s initially settle.