COI Funded Project: Larvae accumulation and transport by internal bores: an experimental study


Project Duration: 6/1/98-12/31/99
Key Words: larval transport, invertebrate, marine reserves, fishery management

Final Report

Bottom dwelling coastal marine animals (e.g. clams, oysters, barnacles) face a complication on their way to establishing new colonies and replenishing old ones. Their early life stages live suspended in the water, drifting at the mercy of ocean currents in a larval phase at the end of which, the larvae must find a suitable habitat where they can mature into adults and complete their life cycles. Knowledge of ocean currents that return larvae to viable coastal habitats is therefore key to understanding the maintenance and dynamics of coastal species. Internal tides and waves propagate within the body of the ocean and are mechanisms that can transport larvae back to shallow coastal habitats. Traveling on the interfaces of layers of different density, internal waves also act like surface waves and break as they shoal producing an internal surf.

Field observations show that large internal tides can result in the exchange of warm inshore water with colder offshore water. This dramatic exchange brings larvae of coastal species shoreward. To predict which types of larvae are accumulated and transported in those fronts, we are evaluating the mechanisms of accumulation in propagating fronts. Our main goal is to test the idea that larvae and other particulate matter accumulate near the leading "nose" of a gravity current.

Our experiments mimic the warm water advancing over colder water with a light, freshwater gravity current moving over heavier saltwater in an 18-meter-long experimental tank. Video cameras looking from the side and top record the motion of buoyant spheres during the surge. The tapes show that particles originating behind the nose of the gravity current sweep to the nose as the front propagates down the tank, and once they reach the nose they remain there. Further study should elucidate the physical effects and ecological consequences of internal tidal bores, a process with profound yet largely unexplored implications for coastal populations.