Study of Source/Retention/Exchange of Plankton (1996)

The primary physical processes which contribute to influx, retention and exchange of water, nutrients and plankton on Georges Bank include the buoyancy- and tide-driven subtidal mean flow, recirculation around the western end of the Bank, wind-forced off- and on-bank flows, and entrainment of Bank water by warm core rings. These advective processes have different time scales varying from an event (a few days) to fortnightly to monthly and seasonal and longer, and different length scales varying from a few kilometers to the bank-wide broad-scale. In addition, higher frequency smaller-scale phenomena such as tide-induced internal waves, internal tides, turbulent mixing and the bottom-boundary layer also may contribute to a net subtidal cross-isobath transport of water and organisms. Whether any of the target species systematically use the vertical shear of the rotary tidal currents to maintain their horizontal position or move onto the Bank is a fundamental question.

The study of these processes will require integrative approaches involving both innovative field observations and modeling. What is needed is a truly Lagrangian description of water and organism trajectories over periods of time varying from one to several tidal cycles to many days, coupled with a new level of understanding of the physical and biological processes that cause the observed Lagrangian motion. New techniques need to be developed to follow both water and key species better. This includes drifters with improved positioning, telemetry, and variable drogue depth, and new optical and acoustical sensors to obtain profiles of target species concentration, biomass, and size distribution from both fixed and moving platforms.

Population phenomena on Georges Bank will be determined in part by the characteristics of populations in the source waters (e.g., Gulf of Maine). The life history characteristics, population density, physiological state, and genetic character of immigrant individuals can be important both as predictors of population dynamics on the Bank and, for the latter two parameters, as "tags" to determine the relative contribution of each source region to production processes on the Bank. Knowledge about the genetic character of planktonic populations of the Gulf of Maine and Slope Water is important for predicting the responses of the population to disturbance, exploitation, or climate change. If the population is homogeneous across the region, re-growth of populations will be rapid and without genetic change. If the population is heterogeneous, recolonization following a reduction in the size of the population may take longer, be less certain, and may be accompanied by genetic changes in the population, because genetically-distinct populations may provide the infusion of new recruits to Bank populations.

Questions: The source, retention and exchange studies should address the following questions:

  1. What is the subtidal circulation over the Bank and how does it evolve in time from December to August? What physical processes (e.g., tidal, buoyancy, wind) determine the density field and drive the subtidal circulation, and do their relative contributions change with time? What time and space scales characterize these processes and the resulting subtidal circulation?

  2. What physical processes dominate the cross-isobath transport over different parts of the Bank, both in the bottom-boundary layer and the rest of the water column? What time and space scales characterize the cross-isobath transport? Does the nature of cross-isobath transport change with stratification?

  3. What physical processes contribute to recirculation of water around the western end of the Bank? Where and on what time scales does recirculation occur? Is recirculation primarily chronic or episodic? What processes disrupt recirculation, and how is it re-established? Does recirculation exhibit a clear seasonal cycle, with little or no recirculation in the well-mixed winter and strong recirculation in the highly stratified summer? Does recirculation occur throughout the year in the well-mixed water over the top of the Bank?

  4. What is the on- and off-bank flow driven by different wind events? Where does this occur and on what time and space scales? Is the directly wind-driven flow concentrated in the surface boundary layer, and if so, how does this interact with the Bank topography? Is there any compensatory flow at depth and, if so, where does it occur? How does the nature of the wind-driven response change with different characteristic storms, season and stratification? Does the interaction between vertical current shear and vertical migration of the target species permit them to be retained on the Bank?

  5. How often do Gulf Stream warm-core rings entrain shelf water from the Bank? Do rings force Slope Water onto the Bank? How large are these cross-isobath transports and are there preferential sites and times for these bank-ring interactions?

  6. How do the physical flow processes identified above influence the retention and exchange of target species on the Bank? Which physical and biological (behavioral) processes are most important for keeping different target species on the Bank during key stages in their early life histories? Which processes are most important in causing exchange of different target species from the Bank? How do these various processes change with time from December to August? Can the exchange of organisms from the Bank be measured and quantified?

  7. What are the life history, physiological, and genetic characteristics of target species populations in the Gulf of Maine, Georges Bank, and Slope Water? How readily can populations from these different regions be discriminated?

  8. Is the Gulf of Maine the source region for seeding populations of major planktonic species on Georges Bank and does the source level abundance play a major role in determining the peak population size on the Bank?

  9. How do physiological and genetic characteristics of planktonic populations in these regions change over time (generationally, seasonally, inter-annually)?
Strategy: The influx, retention and exchange of water and target species from the Bank is perhaps best studied through a combination of remote sensing, in situ Lagrangian, Eulerian, and shipboard measurements, and coupled physical/biological modeling. Remote sensing of sea surface temperature and color should monitor the existence and location of Gulf Stream warm-core rings as they pass close to the Bank, and help identify episodic exchange of Bank water along the southern flank caused by both rings and storms. Remote sensing should also help identify other on- and off-bank surface flow events which may occur around the rest of the Bank perimeter. A variety of Lagrangian studies are needed to observe the movement of water and target species both within and onto and off the Bank. These studies include the deployment of standard drifters designed to follow the horizontal water motion at selected levels, drifters which use acoustical techniques to measure remotely the vertical distribution of organism volume scattering (biomass), numbers, and target strength (size), and new drifters which mimic the vertical diel motion of different target species, and shipboard surveys which follow discrete parcels of water while measuring the horizontal currents and water properties of the surrounding fluid. Moored Eulerian measurements of physical and biological variables cited above would help establish time scales and vertical structure of different processes. Finally, physical and coupled physical/biological modeling efforts are needed to develop insight about retention and exchange processes, and to guide and complement the field measurements. More specific comments on experimental strategy follow.

Lagrangian studies are needed both a) in the western Gulf of Maine to identify source water regions which carry target zooplankton species onto the Bank in winter and early spring and b) over the Bank to help quantify the circulation there during the late winter through summer period. In particular, these studies should attempt to quantify the biological structure of the water flowing onto the Bank, including the species composition and stage structure of target species, the rates of flow, and degree to which water parcels are retained over different parts of the Bank as the seasonal stratification develops.

Lagrangian studies also are needed along the southern flank of the Bank to quantify the exchange of Bank water and target species during strong wind events. The recent development of air-deployed satellite-tracked drifters allows for an efficient seeding of Bank waters during selected storms to learn more about the response of the near-surface Bank waters to strong wind events. The availability of real-time meteorological data from moored platforms on the Bank plus regional real-time satellite infrared and color data and medium range weather forecasts should improve the ability to predict which storms to select for intensive study. These Lagrangian drifter studies can also help direct shipboard measurements of key physical and biological variables on an opportunistic basis (e.g., during a storm event or as soon afterwards as feasible). If new techniques can be developed that allow a ship to make rapid transects of target species concentrations, then this information can be combined with ADCP and CTD or Batfish data to compute horizontal fluxes of shelf water and target species, and can help researchers determine the extent to which organisms are passively transported by the flow field.

The above approach also should be applied to study the influence of warm-core rings on the retention and exchange of target species on the Bank. Remote sensing should provide good guidance to possible ring entrainment events and allow selected events to be surveyed and monitored by ship and seeded with Lagrangian drifters.

Studies of the population density, age structure, and physiological and genetic characteristics of planktonic populations will require quantitative collecting that is fully integrated with the physical characterization of the region. Depths and stations where net tows are taken need to be carefully chosen so as to provide data on the position of the organisms in the flow field. Special collections or fixation methods may have to be employed in some cases. In most cases, it is expected that the planned plankton collections can be split following collection, and either assayed immediately or preserved appropriately.