Swirling larvae settle better

Posted on September 24, 2012

Many types of freshwater invertebrate rely on water movement for the dispersal of their larvae.  For such species, local patterns of distribution are determined by the ability of larvae to exit from the water current and establish themselves on solid surfaces.  Few studies have investigated how the variations in flow that occur close to complex stream bottoms affect larval settlement.  Adopting black fly (Simulium tribulatum) as a test species, researchers used a recirculating flume with a 150 x 30 x 10 cm channel to observe larval settlement in relation to water velocity.  Fine-scale velocity patterns were affected by the flume’s flow setting and the presence of an 8.5-cm diameter PVC hemicylinder on the bottom of the channel.  The hemicylinder’s shape meant that flow accelerated over its upstream surface and boundary layer separation occurred over its downstream surface, producing a recirculating eddy.  Near-bed velocities were measured at four successive points over the upstream and downstream surfaces of the structure.  For the upstream surface, larval settlement rates dropped with increasing near-bed velocity, presumably because high velocities reduced attachment success.  In contrast, over the downstream surface the opposite trend was found and larval settlement increased strongly with velocity.  The researchers  suggest that the eddies created by flow separation behind bottom irregularities help to increase the concentration of larvae and give them multiple chances to contact the streambed.  While further experiments with more complicated bottom structures are warranted, the results so far suggest how the responses of larvae to small-scale patterns of water flow can help to explain differences in distribution within and across habitats.

Reference:  Fingerut, J.T., Hart, D.D. & Thomson, J.R.  2011.  Larval settlement in benthic environments: the effects of velocity and bed element geometry.  Freshwater Biology 56, 904–915. http://onlinelibrary.wiley.com/doi/10.1111/j.1365-2427.2010.02535.x/pdf