Sediment-trap experiments on the importance of hydrodynamical processes in distributing settling invertebrate larvae in near-bottom waters

Abstract The hypothesis that planktonic larvae of benthic invertebrates sink through the water like passive particles in turbulent flows near the seabed was tested in the field by exploiting biased sampling characteristics of sediment traps. Traps of several designs were calibrated in a laboratory flume using passively sinking larval mimics having fall velocities similar to those measured on nonswimming polychaete larvae. A priori predictions regarding the rank order in which various trap designs would collect passively sinking larvae in the field were specified by the rank order in which the traps collected larval mimics in the flume. Field experiments were conducted at two sites, 10- and 14-m depth, in Buzzards Bay, Massachusetts, U.S.A., and traps were moored 0.4–1.6 m above the seabed. In experiments during four field seasons, with deployments lasting from several hours to 11 days, trap collections of Mediomastus ambiseta (Hartman) polychaete postlarvae, total bivalve larvae and postlarvae, spionid/sabellariid polychaete larvae (individuals too small to identify definitively to family), spionid polychaete larvae, enteropneust larvae, and gastropod larvae nearly always corresponded to a priori predictions for passive particle collections between sedimenttrap designs. Results were statistically more significant during some collection intervals than during others, but the rank order of larval collections within each group of traps (deployed simultaneously) corresponded to the rank order of passive particle collections by the traps in the flume, with a couple exceptions. Collections of a polychaete, Pectinaria gouldii (Verrill), were more similar between trap designs (i.e., not biased, as predicted for passive particle collections) than the organisms mentioned above. Competent Pectinaria larvae may sink more quickly because of their larger size and reduced surface area (due to construction of a parchment tube while still suspended). There may be no trapping bias for particles sinking this fast. Collections of metamorphosing seastar larvae also were not in the predicted passive rank order, which may be due, in part, to larvae adhering to solid trap surfaces during metamorphosis. The passive sinking hypothesis could not be falsified in most of the field experiments, indicating that hydrodynamical processes may determine distributions of larvae in very near-bottom waters. Passive sinking by larvae is not, however, an explicit result of this study. Other processes that may have produced observed collections, such as chemical, sedimentary or biological differences among trap environments, must be tested against the passive sinking alternative hypothesis. If larvae sink like passive particles to within 0.4-m of the seabed, as results of this study suggest, then it is possible that larvae initially reach the seafloor at sites where particulates, with fall velocities similar to larvae, initially settle. Passive deposition may thus determine the relatively large-scale distribution of larvae, with active or passive redistribution of larvae, post-settlement selection, or post-settlement mortality determining localized distributions.