Project Summary

Karen Wishner

U.S. GLOBEC: Cross-frontal Distributions and Exchange of Zooplankton on Georges Bank

ABSTRACT

Cross-frontal distributions and exchange of zooplankton on Georges Bank are a primary focus of Phase 3 of the U.S. GLOBEC Georges Bank program, which has the broader goal of understanding the response of marine populations to global climate change, especially interactions between zooplankton and fish. This proposal examines the cross-frontal exchange of zooplankton at two sites important to the biology of GLOBEC-targeted species: the Northeast Peak Front in March (coincident with expected inflow of the copepod Calanus finmarchicus and the presence of cod larvae and high predator abundance) and the Southern Flank Tidal Mixing Front in June (coincident with high concentrations of Calanus and predators). Questions addressed include the interaction between vertically-stratified zooplankton distributions and the cross-frontal exchange at different depths, the temporal variability of these interactions on diel, tidal, and fortnightly scales, and the effects over time of the repeated passage of tidal fronts at particular locations along a bathymetric slope. Zooplankton abundances and vertical distributions will be determined from a Lagrangian sampling scheme utilizing transects relative to frontal position conducted at different phases of the tidal cycle (semidiurnal and fortnightly), and will be related to hydrographic structure, tidal phase, and distance from the front. In a complementary Eulerian sampling scheme employing time series sampling at fixed locations along a bathymetric slope, zooplankton distributions and variability will be related to the time since frontal passage. Vertically-stratified MOCNESS tows will be used to collect zooplankton. Cross-frontal exchange of zooplankton (different species and lifestages with different vertical distributions) will be estimated from concentration gradients across the front at different depths and along isopycnal surfaces, using various estimates of flow and water exchange provided by physical oceanographers.

BACKGROUND

This proposal, on the cross-frontal distributions and exchange of zooplankton on Georges Bank, is one of 3 linked proposals submitted for GLOBEC Phase III and recommended for funding. The other 2 proposals in the group are (1) Hebert / Barth and (2) Gifford / Sieracki / Belkin. Hebert / Barth is a physical oceanography proposal while the Gifford et al. and Wishner proposals combine biology and physics.

The major scientific questions addressed by the three proposals as a whole are:

The work will be done as a coordinated two-ship operation, using one ship (Oceanus) for the SEASOAR work of Hebert and Barth and one ship (Endeavor) for the biological/physical work of Wishner and Gifford, Sieracki, and Belkin. Cross-frontal processes and exchange will be studied at two sites and times important to the biology of GLOBEC-targetted species and known to have strong thermal fronts at those times: the Northeast Peak Front in March (coincident with expected inflow of the copepod Calanus finmarchicus and the presence of cod larvae and high predator abundance) and the Southern Flank Tidal Mixing Front in June (coincident with high concentrations of Calanus and predators). Two 18-day cruises have been requested on the Endeavor by Wishner and Gifford et al., and a 16 and 18 day cruise have been requested on the Oceanus by Hebert / Barth.

Specific cruise activities, which would be done in each area, include:

Sensors mounted on the SEASOAR include dual temperature and conductivity sensors, a fluorometer, transmissometer, and a Microsoar. Sensors mounted on the SCANFISH include temperature and conductivity sensors, a fluorometer, and oxygen sensor. MOCNESS sensors include temperature, salinity, a fluorometer, and transmissometer. The SEASOAR (Hebert and Barth) and SCANFISH (Belkin in conjunction with biologists) will be doing near real-time mapping of cross-frontal distributions of a wide variety of physical, biological, and chemical parameters, which will be used to determine cross-frontal exchanges and mixing of water. These data will be used in conjunction with the species distributional data from the MOCNESS and CTD-Go-FLO rosette to determine cross-frontal exchanges of nanophytoplankton, nanozooplankton, microphytoplankton, microzooplankton, and mesozooplankton. Thus, zooplankton distributions and variability will be analyzed within the context of a sophisticated physical understanding of cross-frontal exchange and dynamics. Our sampling scheme takes account of the semi-weekly and diurnal tidal cycle, day/night cycle, front location, and Bank topography.

Other direct collaborators, who will participate in these cruises, include Barbara Sullivan (cross-frontal distribution and exchange of small predators in MOC-1 samples), Larry Madin (predator distributions and feeding rates), and Ann Bucklin (Pseudocalanus cross-frontal distributions and genetics from MOC-1 samples). We will also coordinate closely with the mooring work (Bob Beardsley: Northeast Peak moorings, Jim Irish: Southern Flank moorings), remote sensing (Jim Bisagni: front detection), and shipboard ADCP analyses (Charlie Flagg), as well as other biologists, physicists, and modelers.