Report on zooplankton vertical distributions in a changing turbulence field: Measurements during GLOBEC Cruise SJ9508 (June 6-16, 1995)

Lew Incze, Dave Hebert, Nick Wolff, Neal Oakey and Ford Dye

We examined changes in the vertical distribution of Calanus finmarchicus, Pseudocalanus spp., Oithona spp. and Temora spp. copepodites and nauplii before, during and after a moderate wind event (a short period with max. sustained wind speed = 10.5 m/s) in early June on Georges Bank. Pump sampling for zooplankton (retention on 40 m mesh) and microstructure profiling (Epsonde) was conducted at an anchor station on the stratified southern flank of the bank in approximately 84 m of water. During calm conditions before and after the wind event, copepodites and nauplii had maximum concentrations in the upper 10-15 m of the water column. The wind event led to an increase in near-surface turbulent dissipation rates (Epsilon) from 10-8 W/kg (before) to 10-7 W/kg at 15 m and >10-6 W/kg at shallower depths. Copepodites of all four taxa responded by descending below the turbulent surface layer to form new maxima below 20 m where water column turbulence was at a minimum (Epsilon = 10-8 W/kg, similar to their shallower environment before the wind). Response time appeared to be rapid, although sampling frequency was not great enough to resolve this well. For nauplii, responses lagged behind copepodites but were of similar pattern except for Temora, which remained near the surface. Therefore, moderate surface turbulence caused a major reorganization of vertical patterns and a >4x increase in concentration at depth for most copepod stages.

Since we were anchored and water was being advected past us, we had to consider whether observed changes in zooplankton vertical patterns were a function of wind-induced turbulence (that is, a response to physical change related to time) or the result of a particular feature that was advected past us (that is, a spatial feature of the seascape). We did this by back-calculating the trajectory of water at various depths using the ADCP record. From this we reconstructed the distribution of sampled water parcels, including zooplankton, fluorescence and hydrographic data. The deepening of zooplankton observed during the wind event had been sampled near the local high and low tides; therefore, there appeared to be no on-off bank difference or bias in the data. Also, the pattern of zooplankton deepening did not coincide with along-shelf changes in fluorescence, hydrography or species/stage composition -- factors that might suggest other causes for the observed zooplankton movements. Finally, the along-shelf movement between sampling days was small. We conclude that the zooplankton vertical changes were most likely a response to wind-forced turbulence of the upper layer, and that the response can be tied to a well-measured turbulent dissipation rate. If these zooplankton avoid turbulence associated with this modest wind event, this may explain the pycnocline maximum in their abundance often seen in spring studies of larval fish and their prey distributions; it would also help to explain variations in their vertical distributions seen over time and in various studies. In our study, copepods did not show maximum abundance in the fluorescence maximum layer EXCEPT during the wind event, when they apparently went down to avoid the increased turbulence above. The relationship between zooplankton distributions and the subsurface chlorophyll maximum should be reconsidered with the added dimension of surface turbulence as a possible contributing factor. Furthermore, the role of wind-induced turbulence on larval fish feeding rates should now also consider the INDIRECT effects that surface turbulence may have by increasing zooplankton concentrations at depths/light levels where larval cod feed. This, too, should increase contact-rates between predator and prey and enhance feeding rates.