Influence of a River Plume on the Distribution of Brachyuran Crab and Mytilid Bivalve Larvae in Saco Bay, Maine

Larvae of most coastal marine invertebrates develop through a series of pelagic stages in the inner shelf regions where they are subject to strong velocity fields, including buoyancy-driven flows from river plumes. Taxon-specific larval behavior can interact with flow fields to determine dispersal trajectories. In this study, we examined the hydrodynamic features of the Saco River plume (in the southwestern Gulf of Maine) from July to August and explored how larval behavior may alter the distribution of mytilid (i.e., Mytilus and Modiolus) bivalve larvae and three genera of brachyuran (Carcinus, Hemigrapsus, and Cancer) crab larvae in and around that plume. Hydrographic surveys (via conductivity-temperature-depth casts) and larval sampling (via plankton tows) were conducted to assess temporal and spatial variation in the horizontal and vertical distribution of larvae. The horizontal extent of the Saco River plume varied little during our study and was governed by both inertial and rotational effects. Late stage mytilid larvae were relatively homogeneously distributed in and out of the plume, while the distribution of brachyuran larvae varied among different locations, species, and larval stages. We conclude that mytilid larvae entered the plume through physical entrainment and/or upward swimming processes and could tolerate salinities associated with the plume (< 25). By contrast, brachyuran larvae avoided the plume via downward swimming to avoid osmotic stress, or had perished prior to sampling.