Vertical Mixing and Stratification (1994)

The water column conditions on Georges Bank are determined, in large measure, by the balance between vertical mixing and surface heating (see Figure 6). The balance between these competing processes shifts both seasonally and with location on the Bank. Turbulence generated by surface wind stress and by stress in the bottom-boundary layer associated with strong semidiurnal tidal currents tend to mix the water column. During spring and summer, the net surface heat flux is positive and increasing. This tends to stabilize the water column, countering the tendency for vertical mixing, and leads to thermal and density stratification over the deeper areas around the perimeter of the Bank, especially over the broad southern flank of the Bank. The transition zone between the well-mixed water over the crest of the Bank and the stratified water over the flank is called the tidal mixing front. The location of this frontal region shifts as the balance between mixing and surface heating changes seasonally.

Observations have shown that when the water column is well mixed, planktonic organisms (phytoplankton, zooplankton and fish eggs and larvae) tend to be distributed throughout the column, and when the column is stratified, the plankton tend to be concentrated within or above the pycnocline. This concentration of food particles is believed advantagous to the growth of larval fish (e.g., Buckley and Lough, 1987). Conversely, increased turbulent mixing also may be advantageous to plankton feeding (e.g., Rothschild and Osborn, 1988). The conditions under which turbulent mixing or its suppression by density stratification are beneficial or detrimental to the feeding success of the target organisms is not known.

As discussed earlier, well-mixed and stratified water columns are characterized by different concentrations of phytoplankton and different assemblages as well. These differences in food web structure may lead to different growth and recruitment rates of copepods, resulting in different zooplankton assemblages in well-mixed and stratified water columns. Of particular interest is the strong link-weak link hypothesis of Runge (1988): large copepod taxa like Calanus finmarchicus are tightly coupled to the timing of phytoplankton blooms because their growth rates are highly dependent upon food concentration, whereas smaller taxa such as Pseudocalanus or Paracalanus are not coupled to variations in food concentration because their growth rates are seldom food-limited. This hypothesis was derived because maximum egg production rates of large copeods like Calanus will only occur at chlorophyll concentrations in excess of 5-10 µg chl-a per liter; such concentrations occur only during blooms and/or in the well-mixed regions of Georges Bank. Growth rates of small taxa such as Pseudocalanus spp. and Paracalanus parvus are probably seldom or ever limited by food supply. Their maximum egg production rates occur at 1-2 µg chl-a per liter, a concentration which is observed in all regions of the Bank during the spring, summer and fall.

The bottom-boundary layer plays a critical role in generating turbulent mixing and in providing an environment with strong vertical shear in the currents. The near-bottom current shear may be important to vertically migrating organisms in determining their location (i.e., their retention on the Bank), and subsequently their feeding, growth and survival.

Questions: Vertical mixing and stratification studies should address the following questions:

  1. What processes control the seasonal development of stratification on Georges Bank, especially over the southern flank of the Bank. How does stratification vary in response to diurnal surface heating, changes in current during a single tidal cycle, fortnightly modulation in tidal amplitude and synoptic scale wind events? Is temporal and spatial variability in stratification over the southern flank of the Bank similar in the along-isobath direction?

  2. What are the dynamics controlling the structure of the tidal mixing front? What is the spatial exent of the frontal region? How does its extent and location vary in time from winter through summer? Are there secondary circulations associated with the front that affect the location or retention of target organisms within the frontal region?

  3. What is the structure of the bottom-boundary layer on Georges Bank? How does the boundary layer change with bottom depth and with the seasonal development of stratified conditions in the water column above? How do the mean Eulerian and Lagrangian currents differ in the bottom-boundary layer? Do surface or internal gravity waves play an important role in the dynamics of the bottom-boundary layer?

  4. How is the vertical distribution and abundance of cod and haddock larvae and their zooplankton prey influenced by the water column structure? How do these organisms respond to the disruption and subsequent re-establishment of stratified conditions by storm events? What is the time scale of this response?

  5. How sensitive is larval growth to water column stratification? Do wind-induced disruptions in the development of stratification significantly affect the growth and survival of larval fish?

  6. How does the sensitivity of larval growth to stratification change as stratification develops and as the larvae grow in size during the spring?

  7. Two mechanisms can lead to successful encounters of larval fish. Copepod prey can become concentrated in fronts or pycnoclines, or, prey can become locally abundant because of enhanced egg production by adult females in food-rich regions, leading to enhancement of naupliar recruitment. Are there situations on Georges Bank where these two mechanisms operate together, or are copepods abundant in fronts or pycnoclines due to physical mechanisms, and in well-mixed water columns because of high local growth rates?

  8. What is the spatial and temporal distribution of turbulent kinetic energy on Georges Bank? Is turbulence suppressed in the pynocline over the southern flank? Does turbulent mixing affect the feeding of target organisms by increasing their contact rate with prey items?
Strategy: The processes of mixing and stratification on Georges Bank are best studied by a combination of moored and shipboard measurements of the major forcing functions and the resulting water column response, and through numerical circulation modeling. For the surface boundary layer, the primary atmospheric forcings are the surface heat and momentum fluxes. Estimation of these fluxes using bulk formulae requires measurements of the long and short wave radiation components at the sea surface, the temperature and humidity of the air above the sea surface, precipitation, and near-surface winds. Estimation of stress in the bottom-boundary layer requires current measurements which are closely spaced in the vertical, pressure measurements to determine the surface wave field and a record of the character of the sediment surface. Within the water column, temperature, salinity, and current need to be measured. Shipboard CTD/ADCP measurements will provide profile information on water structure and current with a vertical resolution of at least 1-2 m. Moored time-series measurements of water structure and current need sufficient vertical resolution to resolve the surface and bottom-boundary layers and seasonal pynocline. The tidal mixing front or transition from stratified to well-mixed conditions occurs over a relatively short cross-isobath distance on the southern flank of the Bank. This situation provides an opportunity to make simultaneous moored measurements and shipboard transects in well-mixed, marginally stratified, and strongly stratified water columns that are exposed to essentially the same atmospheric forcing conditions. This should allow greater insight into the processes controlling mixing and stratification.

Cod and haddock larvae and their zooplankton prey also reside on the southern flank of the bank during the period when stratification is developing. Fine-scale observations of the distribution of the larvae and their zooplankton prey should be made at successive life stages: as the larvae grow in size, as their prey organisms change and as stratification develops in the water column. This study will require measurement of the micro-scale distributions of the larvae and their prey. These observations can be made by a combination of towed net systems and towed, profiling and moored acoustical and optical sampling systems. It is critical that the plankton sampling and growth rate measurements be made concurrent with the physical measurements described above. Acoustical and optical instruments should be placed on the moorings as an aid to obtaining time-series observations of the vertical distribution and abundance of chlorophyll and the zooplankton community. These continuous measurements will provide information on the physical and biological conditions between the periods of the smaller-scale ship observations. By the combination of shipboard and moored sampling, changes in plankton biomass resulting from the changing physical conditions can be determined on a range of time scales from minutes to months.

The feeding success and growth of the larvae can be determined by gut content analyses and a variety of chemical and physiological measures. RNA/DNA ratios and lipid content analysis are indicators of recent growth. The growth rates can be compared to observations of prey availability and water column conditions (temperature and density structure). Otolith increments, both in size and in chemical composition, can provide time histories of growth conditions to be compared with physical oceanographic conditions and prey abundance. Relationships between otolith increments and the time histories of physiological condition, food availability and water column properties need to be investigated. If significant relationships are found, otolith analysis could provide indications of the conditions encountered by the survivors of the larval population during different times in their early life history.

Techniques available for study of copepod growth rates are very limited. Growth rates of adult females can be estimated from measurements of egg production rates in 24-h incubations. Growth rates of the larval and juvenile stages can be estimated from measurements of molting rates in 24-h incubations. Critical for the success of this type of work is the collection of animals in as gentle a manner as possible. See Peterson, et al. (1991) for details of how growth rates may be derived from such measurements and of some of the pitfalls asssociated with this type of work. The development of more precise and more rapid methods for estimation of copepod growth rates is of the utmost importance and is strongly encouraged.

A study of the dynamics of the tidal mixing front on the southern flank of the Bank will require a variety of approaches, including transects of observations across the frontal region. Towed, undulating bodies instrumented with sensors for measuring temperature, salinity and turbulent dissipation rate. Ship mounted ADCP units can provide snapshots of the structure of the front and the flow field in the frontal region at different stages of the tide. In addition, Lagrangian experiments with passive drifters or tracers should be considered to look for secondary circulations associated with the front. To indicate the influence of these physical conditions on the life history of the target organisms, concurrent observations need to be made with acoustical and optical biological instrument systems, as well as multiple opening and closing net sampling along the transect lines. Because of the near-bottom diel vertical migration of cod and haddock juveniles, both pelagic and demersal sampling gear will be needed.

The distribution of turbulent kinetic energy can be estimated by profiling and towed shear probe instruments. The recent feeding success of target organisms can be determined by the gut content and biochemical analyses mentioned above. Comparison of turbulent intensity distributions, feeding rates and prey concentration under varing conditions can indicate the degree of influence turbulent mixing has on planktonic feeding.