The development of stratification is controlled by a balance between solar insolation, which warms and reduces the density of the surface layer, and mixing induced by tidal currents and surface wind stress. On the deeper portions of Georges Bank stratification begins to develop in the spring, while in the shallow central region strong tidal currents keep the water column well-mixed year-round (Garrett, Keeley and Greenberg 1978). Spawning of cod and haddock stocks on Georges Bank occurs in late winter and spring on the northeastern part of the Bank. The developing eggs and larvae are carried south and west in the mean flow along the southern flank of the Bank as the seasonal stratification begins to develop (Fig. 1).
In the well-mixed region cod and haddock larvae have been observed to grow more slowly and to be in poorer physiological condition, as measured by an RNA/DNA ratio, than larvae at a stratified site on the southern flank of the bank (see Fig. 4 and 5 in Buckley and Lough 1987). The difference in condition is believed to be due to a reduced concentration and vertically more homogeneous distribution of prey at the well-mixed site in comparison to the stratified site. The sensitivity of the larvae to temporal variability in stratification, such as that caused by wind events, is not known, but is hypothesized to be important. In other areas the dissipation of biological structure by wind events has been shown to be detrimental to larval fish feeding and growth (Lasker 1978; Peterson and Bradford 1987).
Stratification also can have negative implications for the feeding of larvae and zooplankton. Recent theoretical and observational studies hypothesize that certain levels of turbulence enhances encounter rate between predators and prey, and promotes growth of the predator (e.g. Rothschild and Osborn 1988; Sunby and Fossom 1990; Davis et al. 1991).
Models of climate change suggest that the NW Atlantic in the future may be characterized by warmer temperatures, increased precipitation and river runoff (Manabe and Stouffer 1980), and reduced wind stress (Manabe and Wetherald 1980). These factors may result in changes in the characteristic development of stratification on Georges Bank. For example, at warmer temperatures the nonlinearity in the equation of state of seawater would result in more buoyancy per unit of heat input to the surface water. Even subtle changes in relationship between buoyancy and heat input might have significant effects on the timing and character of the stratification process and on the availability of food organisms to larval fish.
A simple, one-dimensional model of stratification (which includes wind stress, tidal mixing, surface heating, and uses canonical parameter values from the literature) suggests that an increase in the winter water temperature from 4°C to 8°C would result in an earlier and stronger development of stratification during the spring (Fig. 2). Though preliminary, this result suggests that climate induced changes in stratification would be of a magnitude known to be important to the growth and survival of larval fish. It also suggests that the linkage between stratification and larval survival is an important area of study for U.S. GLOBEC.
A study of stratification variability on Georges Bank and its effect on larval fish survival is being supported by the NOAA Climate and Global Change, Marine Ecosystems Response Program (this MER project has since been merged into U.S. GLOBEC). The objectives of this study are two-fold: (1) to relate spatial and temporal variability in stratification to change in the food availability, growth and survival of larval cod and haddock on Georges Bank; and (2) with this understanding, to evaluate the potential effects of climate change on larval survival. As a first step, cruises were done in May 1992 and are planned for May 1993 to test and intercalibrate biological sampling systems, biochemical techniques and sampling strategies.
Biochemical methods will be used to determine recent growth and physiological condition of fish larvae in relation to water column conditions and prey density. In addition, the recent daily increments in otolith growth will be analyzed to compare with recent physiological condition and water column structure.
The two vessels conducted short (4 km) transects at the different sites on a rotating basis. During a transect the various sampling systems were deployed and, when appropriate, tow-yo'ed to provide information on the distribution of larvae and their prey on small space scales (e.g., VPR, TAPS, CTD). Joint transects by both vessels were done to allow comparisons between systems. These short transects provided repeated simultaneous measurements by different systems under different stratification conditions.
In the second portion of the study, sampling on five 1.5 km x 1.5 km grids was conducted at the different sites. The grid surveys were conducted both individually and jointly by the vessels. The grid sampling attempted to provide a three-dimensional picture of the fine-scale distribution of larvae, their prey, and the hydrographic structure. A grid consisted of six transects spaced about 300 m apart. In joint operations, one vessel followed the other and offset laterally by about 150 m, so that the result was 12 transect lines about 150 m apart.
The 1-m MOCNESS collected larvae for gut content, biochemical and otolith analysis. RNA, DNA and protein content have been analyzed for 244 larvae. The otoliths from these same larvae will be analyzed for daily increment width for estimating larval growth. The prey field for the larvae will be determined from gut contents and compared with suitable prey in the water column estimated from the 1/4-m MOCNESS and plankton pump samples, and from the results of the video and acoustic systems.
Calanus finmarchicus were sorted from the MOCNESS collections at the stratified and the well-mixed study sites for genetic analysis by Ann Bucklin (University of New Hampshire). The genetic similarity of the Georges Bank, Wilkinson Basin (Gulf of Maine) and Gulf of St. Lawrence populations confirms that the Gulf of Maine and Gulf of St. Lawrence are source regions for recruitment of C. finmarchicus to Georges Bank.
Initial data processing indicates that the various acoustic and video sensing systems all functioned well. Sufficient data were collected to intercompare the systems, but those comparisons have not yet been completed. The VPR provided near-real time insights into the plankton taxonomic composition in the water column. Calanus remained largely within the top 10 m of the water column both day and night. Preliminary abundance estimates indicate surface swarms at densities as high as 20,000-50,000 m-3. High densities of larvaceans and pteropods were also observed. Densities of the latter were as high as 106 m-3 and comprised a dense scattering layer observed with the towed acoustic instruments.
The 120 kHz acoustics on BIOSPAR provided volume backscattering in 1 m depth intervals throughout the water column for the seven day mooring period. In addition to storing the data internally, the data were transmitted via VHF radio telemetry to the vessel, and, in reduced form, were transmitted daily to shore via ARGOS satellite. In general, BIOSPAR observed a significant diel cycle which consisted of a layer of high volume backscattering residing at mid-depths (~40 m) during the day and moving up into the surface layer at dusk (Fig. 4). Large transient targets appeared occasionally in both the BIOSPAR and the 420 kHz towed acoustic ("Greene Bomber") data. These targets occurred from mid-depth to just above the bottom and may have been schools of herring or other larger fish.
The towed 420 kHz acoustic system had roughly an order of magnitude greater backscatter than the BIOSPAR 120 kHz system, probably because of the different abundance of targets visible by the different frequencies (120 kHz has a minimum detectable size of ~10 mm; 420 kHz has minimum detectable size of ~4 mm). Preliminary analysis of the differences in the vertical distribution of acoustic backscatter by the two instruments suggests that the differences are real and reflect the difference in vertical distribution of the organisms detectable by the two frequencies.
Identifying the three-dimensional relationships between the biology and the physical structure on a fine spatial scale was an important objective of this study. To maintain the correct spatial relationship of the sampled water columns during the four hours needed to complete the grid required compensating for the net translation of the water by the tidal currents. To accomplish this, six parallel transects spaced 300 m apart were sampled relative to a drifting buoy drogued at about 15 m. This simple technique worked quite well. While the drifter moved about 4 km and the vessel track relative to the earth extended over about a 6 km area, the pattern of sampling, relative to the water (i.e., drifter) formed the desired, well-defined grid. The method assumes that the horizontal current shear on the scale of the grid is small, which is probably a reasonable assumption. The vertical current shear, however, is a problem. The moored current measurements indicate about a 3 to 2 ratio in the displacements expected at 15 and 45 m. With the surface moving about 4 km, the deeper part of the water column would have moved only about 2.5 km--so that the surface and deeper water columns would have been shifted about one grid scale relative to each other during the grid sampling. This problem can be minimized by placing the drogue at the depth where the biological and physical structure of interest is located. Compensation for vertical shear translation of sampled water parcels can also be done using moored or shipboard current measurements to calculate relative locations in a fixed-time reference frame.
The analysis of the data from the various sampling systems is continuing. The data sets will be used for both system intercomparisons and for analysis of the relationships between the larval fish (both abundance and physiological condition), their prey field and the physical structure of the water column. It is hoped that these results will provide a foundation for more intensive studies during the U.S. GLOBEC Northwest Atlantic Program.
The Stratification Group includes: R. Beardsley (Woods Hole Oceanographic Institution, WHOI), M. Berman (National Marine Fisheries Service, NMFS), L. Buckley (NMFS), C. Davis (WHOI), A. Epstein (WHOI), J. Green (NMFS), L. Incze (Bigelow Laboratory of Ocean Sciences), G. Lough (NMFS), J. Manning (NMFS), D. Mountain (NMFS), P. Wiebe (WHOI).
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