Direct and indirect effects of wind-induced turbulence on larval cod feeding: results of a 1-D model for Georges Bank

L. S. Incze, F. E. Werner, N. Wolff and F. Dye

This study is based on results from a turbulence and zooplankton profiling cruise to Georges Bank in June 1995 (GLOBEC Cruise SJ9508). Results from that cruise show that moderate wind-induced turbulence stimulated most of the abundant copepod taxa to descend from shallower to deeper depths (Incze, Hebert, Wolff, Oakey and Dye, submitted March 2000, Marine Ecology Progress Series). In our study, wind speeds increased from near-zero values to 10 m/s and produced modest turbulent kinetic energy dissipation rates on the order of 10-6.5 to 10-7.5 W/kg near the surface. Most of the copepod taxa had a maximum concentration at 10-15 m depth under calm conditions, but migrated downward to 20-25 m, into the pycnocline, during the period of wind-induced turbulence. In doing this, both copepodite and naupliar stages maintained their vertical position in areas of low turbulence ( = 10-8 to 10-9 W/kg). Other taxa had fewer stages in the upper water column, but these also shifted downward when surface turbulence increased, and the downward shift resulted in higher concentrations than before.

Both the shift in vertical position and the apparent preference for lower turbulence (if it is available at an acceptable depth) should impact encounter rates by larval cod. Our observations suggest that INDIRECT effects (through concentration shifts) must be considered in addition to DIRECT effects of turbulence (via velocity enhancements to encounter rates).

To test these ideas, we used a 1-D model of the physics at the experimental site (stratified region, 74 m of water on the southern flank of Georges Bank) and coupled it with a trophodynamic model of larval cod feeding on copepod prey. In the simulations, encounters are modulated by turbulence and light. We initialized the model with T/S conditions from the field experiment and then introduced observed wind data and prey fields. Turbulence in the model at 10 m closely followed observed turbulence levels. We conducted these runs with the copepodite and naupliar stages of Calanus finmarchicus, Pseudocalanus spp., Oithona spp. and Temora spp. We obtained similar patterns of change for all taxa, except for the naupliar stages of Temora which, curiously, did not deepen in response to wind-generated turbulence.

The accompanying figure (jpg or powerpoint version available) illustrates the results for Pseudocalanus spp. nauplii. We use the patterns here without light effects in order to concentrate on just prey concentration and turbulence. Copepodite depth distributions (upper panel, from observations) showed a rapid deepening in response to increased turbulence (model results, middle panel), followed by a return to shallower depths afterward (these are all daytime samples). The third panel plots the encounter rate between a cod larva and this prey at two depths. Encounter rate increased markedly at 25 m due to changing concentrations of prey (see trace at 25 m in the upper panel); this is an indirect consequence of the wind-induced turbulence near the surface. The predicted encounter rate increased initially at 10 m, but then declined abruptly as the potential prey left the shallow mixed depths. This has implications for feeding and for considering the application of turbulence theory to this interaction. If such shifts are common, then the behavior also helps to explain some of the variable depth distribution patterns of zooplankton on the stratified portion of the bank. Light and predator satiation and the vertical behavior of predators have additional effects on encounter and feeding rates which we are considering in further modeling work.