MEPS 307:1-19 (2006)  -  doi:10.3354/meps307001

ABSTRACT: Coasts subjected to upwelling are very advective environments where zooplanktonic organisms face forcible removal from the system. Partition of time between a surface layer, which is under the direct influence of the wind, and a deeper layer, dominated by the compensation counterflow, has been proposed as a mechanism that could enhance retention over the shelf. We used a 3-dimensional numerical model to examine the hypothesis that diel vertical migration (DVM) is able to retain larvae of the littoral crab Carcinus maenas over the shelf of the northwestern Iberian upwelling system. Simulations contrasted 2 different wind regimes typical of winter (non-upwelling) and spring (upwelling), and 4 scenarios of vertical distribution: 2 with nocturnal DVM and 2 without DVM. Larvae were simulated as particles released in the model during neap tides at semilunar intervals, to simulate the hatching behaviour of the species, and were followed for 5 wk, which approximates the average larval development time of the species. The particles from the different migration scenarios showed very different responses to wind forcing. Particles without DVM were more sensitive to wind-driven surface currents and were often advected seaward of the shelf break, even during winter when upwelling events were less frequent and of short duration. Particles with DVM were consistently distributed over the shelf, and were very frequently found in the inner shelf region during upwelling. This was accomplished by a larger proportion of time spent in the onshore underflow, which dragged the particles shorewards, than at the surface. The results of the simulations show that, for larvae of littoral species exhibiting DVM over a large expanse of the water column, upwelling may actually be a mechanism that enhances retention over the shelf.

KEY WORDS: Diel vertical migration · Larvae · Recruitment · Carcinus maenas · Upwelling