The session had 29 presentations that addressed Bank-wide structure of the physics and biology as observed in recent field studies, in historic data sets and in modeling efforts. From the presentations two issues were identified for discussion. Winter time observations in the program found Calanus finmarchicus actively involved in growth and reproduction in the near surface layer. This finding diverged from the conventional Calanus life cycle paradigm, which would have the organisms in diapause at depth in the western Gulf of Maine in winter. Charlie Miller summarized the knowledge of the Calanus life cycle and led a discussion on the implications of the recent findings. The second issue identified was how to evaluate the relative importance of predation by different predator groups and the combined importance of predation compared to other factors in the life history of the target species. Michael Fogarty led a discussion of this issue. Summaries of both issues are included with the abstracts for the presentations in this session.

Large-Scale Hydrographic and Circulation Variability in the Georges Bank Region

John Loder, D.A. Greenberg and C.G. Hannah

Bedford Institute of Oceanography, CA

Model solutions indicate that, in contrast to outer Scotian Shelf banks, Georges and Browns Banks are located in the equatorward evolving barotropic pressure field arising from along-shelf wind stress, resulting in a significant barotropic as well as Ekman-layer current response. Topographic channelling results in most of the cross-shelf Ekman transport on the Scotian Shelf being compensated by barotropic flow through Northeast Channel and along the continental slope off Georges Bank. At storm-band periods (2-5d), the Northeast Channel flow excites a topographic wave along the southern Georges slope. These and other aspects of the barotropic response to wind stress are described in Greenberg et al. (1995).

The climatological seasonal variations in hydrography and baroclinic circulation are examined using 3-d seasonal-mean temperature, salinity and density fields obtained from the historical database, using 4-d optimal linear interpolation with anisotropic correlation scales related to topography (Loder et al. 1995). The spatial extent of various features is described, including low-salinity surface waters from the Gulf of St. Lawrence and coastal runoff, high-salinity Slope Water inflows in deep
channels, the seasonal upper-ocean temperature cycle, and increased summertime bottom temperatures in energetic tidal-mixing areas such as Georges Bank. Diagnostic model solutions indicate that the associated baroclinic circulation is the major contributor to the shelf-scale seasonal circulation in the Scotia-Maine region, including a seasonally-varying equatorward throughflow steered by the bank-basin topography (Hannah and Loder 1995). These fields are an important input to high-resolution model simulations in the Georges Bank region (Naimie et al. 1994; Naimie 1995) which include additional locally-important forcings such as tidal rectification. The fields are also a baseline for climatic variability evaluations.

As an initial evaluation of a decadal-scale climatic variation, 3-d seasonal-mean (composite) hydrographic fields are computed for groups of years representing the relatively-cool period in the
Scotia-Maine region during the late 1950s and 1960s, and the relatively-warm period during the 1970s and early 1980s. Average temperature and salinity differences between the two periods are
largest at depth along the shelf-break and in channels (e.g. Northeast) into shelf basins, and near the surface in the offshore Slope Water, with magnitudes about 5 degrees C and 1 unit respectively. Southwestward baroclinic flow along the shelf break is stronger during the cool period, particularly in winter, by about 2 Sv and 1 Sv on sections off Halifax and Georges Bank respectively. These results support the Petrie and Drinkwater (1993) suggestion that the anomalous conditions during the late 1950s and 1960s have a Slope Water origin related to increased strength of the Labrador Current. The present fields indicate that the anomalous conditions extend along the shelf break beyond Georges Bank, with widespread influences on the shelf hydrography, particularly in deep basins. Average temperature differences on southern Georges Bank are 1-4 degrees C, depending on season and location. Such significant decadal-scale changes during recent times with oceanographic and fisheries datasets provide the potential for evaluating the implications for the Georges Bank ecosystem of other possible climatic changes in the North Atlantic subpolar gyre.

References:

Greenberg, D.A., J.W. Loder, Y. Shen, D.R. Lynch and C.E. Naimie. 1995. Spatial and temporal structure of the barotropic response of the Scotian Shelf and Gulf of Maine to surface wind stress. J. Geophys. Res. (submitted).

Hannah, C.G, and J.W. Loder. 1995. Seasonal variation of the baroclinic circulation in the Scotia-Maine region. Proc. 7th Intern. Bienn. Confer. Phys. Estuar. Coastal Seas (submitted).

Loder, J.W., G. Han, C.G. Hannah, D.A. Greenberg and P.C. Smith. 1995. Hydrography and baroclinic circulation in the Scotian Shelf region: winter vs summer. Can. J. Fish. Aquat. Sci. (submitted).

Lynch, D.R., F.E. Werner, D.A. Greenberg and J.W. Loder. 1992. Diagnostic model for the baroclinic, wind-driven and tidal circulation in shallow seas. Cont. Shelf Res. 12: 37-64.

Naimie C.E. 1995. Georges Bank residual circulation during weak and strong stratification periods - prognostic numerical model results. J. Geophys. Res. (submitted).

Naimie, C.E., J.W. Loder and D.R. Lynch. 1994. Seasonal variation of the three-dimensional residual circulation on Georges Bank. J. Geophys. Res. 99: 15,967-15,989.

Petrie, B.D., and K. Drinkwater. 1993. Temperature and salinity variability on the Scotian Shelf and in the Gulf of Maine 1945-1990. J. Geophys. Res. 98: 20,079-20,089.

Three physical oceanographic cruises (Oct. '93; June '94, '95) have been conducted in the eastern Gulf of Maine for the purpose of: 1) placing and recovering long-term moorings in Northeast Channel and off southwest Nova Scotia, 1) measuring the structure of inflow currents in Northeast Channel, and 3) surveying the hydrographic fields of the region. Basic processing of all current meter and CTD data has been completed, indicating a 90% data return from the moored data. Record mean currents from the moored instruments show persistent inflows of 5-10 cms-1 off Cape Sable and 5-15 cms-1 on the eastern side of Northeast Channel. Outflows of 10-20 cms-1 are found in the western side of the Channel, but these records are short. The expected annual cycles are also found in the inflows, featuring a fresh inflow maximum off Cape Sable in February and a pronounced, salty inflow in the Channel in August. However, strong interannual variability is also
noted in these signals, in part associated with the presence of warm core rings at the mouth of Northeast Channel. Furthermore, detided ADCP "snapshots" of the mean flow in the Channel indicate net transports consistent with previous estimates and maximum inflow and outflow currents of 40+ and 30+ cms-1 on the east and west, respectively.

Future plans include analyses of the wind-driven response, causes of interannual variability, the nature of "cross-over" events between Browns and Georges Banks, and seasonal variability in the baroclinic pressure field.

The eastern portion of the Bank was directly influenced by waters from the Scotian Shelf from February through June. In addition during May Slope Water encroached onto the southern flank of the Bank to near the 80m isobath at the surface and the 70m isobath along the bottom. The Slope Water occupied the region of the Bank where the highest concentrations of cod and haddock larvae
are commonly found in May. The encroachment of Slope Water is believe to have been caused by a large warm core ring south of the Bank. By mid-June the Slope Water was gone and typical Georges Bank water covered the southern flank of the Bank.

Comparison with historic data indicates that the Georges Bank waters began the year somewhat warmer and saltier than normal. Through the spring the temperature and salinity anomalies progressively became cooler and fresher than normal, likely due to the presence of Scotian Shelf water.

Figure 1. Satellite-derived, optimally-interpolated sea surface temperature (SST) map from 8 October 1993 (yearday 280.5) showing a meandering southward extension of the cold Maine Coastal Current flowing onto Georges Bank. Ekman velocity at z = 10 m depth is close to zero. Also shown is a 10 cm/s reference velocity vector and a SST color bar.

Figure 2. Daily-averaged positions for drifter 22502 released within the Maine Coastal Current in northern Gulf of Maine on 29 April 1994 by N. Pettigrew, University of Maine, Orono, Maine. Drogue depth is 10 m. Also shown is the 200-m isobath (dashed). (Data courtesy of N. Pettigrew)

During the weakly-stratified March-April period, the prognostic and diagnostic models predict similar global structure (with a recirculating transport on Georges Bank of 0.05 Sv), although there are local differences which result in the prognostic simulation agreeing more closely with the observed circulation (see Naimie (1995) for a more complete discussion).

During the strongly-stratified July-August period, the newly modeled processes intensify the residual circulation and result in recirculating transport on Georges Bank of 0.25 Sv, which is twice the diagnostic result. The level of agreement between modeled and observed residual circulation improves significantly for this period, with the prognostic model predicting around-bank currents as high as 46 cm s-1 in the frontal zone on the Northeast Peak. Analysis of the tidal-time density and current structure in the frontal zone on the Northeast Peak indicates important tidal frequency dynamics and features which are supported by observational evidence (see Naimie (1995) for a more complete discussion).

References

Naimie, C. E., J. W. Loder, and D. R. Lynch, Seasonal variation of the three-dimensional residual circulation on Georges Bank, J. Geophys. Res., 99(C8):15967--15989, 1994.

Naimie, C. E., Georges Bank residual circulation during weak and strong stratification periods - Prognostic numerical model results, J. Geophys. Res., in press, 1995.

Figure 1. Prognostically modeled winter (March-April) circulation on Georges Bank:

* The 26.25 Ut isopycnal intersects the topography on the southern flank of Georges Bank at roughly the 100 m isobath (white isosurface).
* The climatological wind stress of 0.0472 Pa is directed toward 121.4 degrees clockwise from true north (arrow above Nantucket Shoals indicates magnitude and direction).
* The vertical eddy viscosity varies from above 0.12 m2 sec-1 , at mid-depth on the crest of Georges Bank, to below .02 m2 sec-1, in the deeper surrounding regions (vertical cross-bank section).
* The strength of the around bank circulation peaks at 15-20 cm sec-1 for the northern flank, southern flank, and Northeast Peak (four vertical sections around the bank and legend (m sec-1)).

Figure 2. Prognostically modeled summer (July-August) circulation on Georges Bank:

* The 25.50 Ut isopycnal intersects the topography on the southern flank of Georges Bank at roughly the 80 m isobath (white isosurface).
* The climatological wind stress of 0.0138 Pa is directed toward 51.0 degrees clockwise from true north (arrow above Nantucket Shoals indicates magnitude and direction).
* The vertical eddy viscosity varies from above 0.12 m2 sec-1 , at mid-depth on the crest of Georges Bank, to below .02 m2 sec-1 , in the deeper surrounding regions (vertical cross-bank section).
* The strength of the around bank circulation varies significantly around the bank, reaching a maximum of 30-40, 20-30, and 40-50 cm sec-1 at the northern flank, southern flank, and Northeast Peak; respectively (four vertical sections around the bank and legend (m sec-1)).

Background structure of the shelfbreak front on the south flank of Georges Bank will be described using a climatology developed from NODC data. Cross-shelf sections of temperature, salinity, and density were obtained using time averages of two months and by binning the stations by depth. Stations are averaged over roughly 100 km in the along-shelf direction, and in addition to Georges Bank fields for the shelf south of New England and off New Jersey were also computed. Because of the large gradients in bottom depth near the shelfbreak, relatively narrow spacing of the bins can be used in the frontal zone just offshore of the shelfbreak.

Despite the averaging over large space and time scales and hence various processes and frontal positions, the front shows fairly large near-surface velocities throughout the year, with velocity maxima ranging from 10 to 30 cm/s from the purely density-driven flow. The Georges Bank velocities in general are larger than those of the New England and New Jersey sections.

Mean positions of the front from the April/May time period were described and compared with a buoy trajectory provided by R. Limeburner and R. Beardsley. The buoy velocity was roughly 20 cm/s along the shelf edge as the buoy passed from the south flank of Georges Bank into the Middle Atlantic Bight.

Preliminary analysis plus previous drifter results from SCOPEX suggests that drifter behavior over the Bank is characterized by three regimes: wind-dominated, non-dispersive, and recirculation. In early February, 1995, strong northeast winds shifting to the northwest forced all (5) drifters over the Bank off-shelf where they became entrained in a warm-core ring. In March, strong northwest winds forced another drifter southwestward to the 100m isobath where it became entrained in the shelf/break frontal jet and moved quickly along the 100m isobath into the Middle Atlantic Bight . Many of the drifters deployed on Georges Bank between late February and June, 1995, remained within the region defined by the 60m isobath. The combined tracks of all these drifters generally describe the limits of the 60m isobath, except over the eastern side of the Great South Channel. Beginning in July, 1995, many drifters started to recirculate clockwise around the Bank along a narrow band generally located near the 60m isobath.

An animation of the drifter position data is available from the WWW page http://globec.whoi.edu/globec-dir/misc-data.html.

There have been some 25 cruises on a variety of ships, on a variety of missions, transiting over and around Georges Bank during the GLOBEC Northwest Atlantic Program. On all the cruises ship-board acoustic Doppler current profilers (ADCP)s collected current velocity and acoustic backscatter intensity data. Both narrow (NB-) and broad-band (BB-) ADCPs were used in the study. Four ships have been used in the program, the Albatross, Endeavor, Seward Johnson, and Iselin. The ADCP types and arrangements differ on each of the ships; the Johnson had three different ADCPs all by itself. Some of the ADCPs were the older "narrow-band" variety, while some were the newer "broad-band" ADCPs. This diversity of equipment meant that we were unable to use the same data collection and processing methodology throughout as we had planned. For those ships equipped with the narrow-band ADCPs we were able to use, for the most part, RD Instruments' venerable ADCP DAS, version 2.48 (Endeavor and the last cruise on the Seward Johnson). On some of cruises with narrow-band ADCPs (Iselin and Seward Johnson) and for all those where the broad-band ADCPs were used (Iselin, Seward Johnson, and Albatross), data collection was controlled by RD Instrument's TRANSECT program.

Now that the flood of data is slacking off, the processing and analysis of this vast data set is underway. Of the 25 GLOBEC cruises of all sorts completed, the ADCP data has been processed for approximately 15 cruises. The processing is being done at both WHOI and BNL; all the TRANSECT data files are being processed at WHOI while all the PINGDATA files produced by the ADCP DAS are being processed at BNL. As the processing progresses, data products in the form of plots have been made available to GLOBEC investigators through the World Wide Web (http://www.oasdpo.bnl.gov/~oasdpo/mosaic/globec/).

Some preliminary results from calculations of diurnal and semi-diurnal tidal components, the large-scale circulation over the Bank and the acoustic volume reverberation are presented here. Since Georges Bank currents are dominated by tides, one of the difficulties with the combined space/time character of ADCP data is to extract the tides from the mean and synoptic scale current fields. For this purpose we are using the detiding methods of Candela, Beardsley, and Limeburner (1992) which can decompose the observed currents into smoothly varying mean and tidal current fields. About three quarters of the current spatial and temporal variability from the ADCP observations over the Bank can be described using this type of model. The remaining variance represents smaller spatial and temporal variations due to topography, and seasonal and synoptic scale forcing for which the ADCP data will prove particularly useful. Figure 1 shows the application of the methodology to two 1995 Albatross cruises, 9505 and 9506. As the figure shows, the background current field is completely overwhelmed by the tidal velocities. We are able to extract mean and semi-diurnal tidal current fields which at least qualitatively conform to the existing Eulerian estimates. The semi-diurnal tidal magnitudes and orientations appear to be closer to the observations in than the mean field which seems too small. We will be combining the from all the cruises as well as moored instrumentation to construct a much more accurate semi-diurnal and diurnal tidal description for detiding the ADCP currents.

One of the more useful measurements made possible by the ADCP is an estimate of the acoustic volume reverberation all along the cruise track. It has been shown in a number of studies that the volume reverberation from a single frequency instrument like the ADCP can be used to estimate the zooplankton biomass. These estimates are not as accurate as those obtained with higher frequency and multi frequency acoustic devices, optics or nets, but these limitations are compensated for by greater range and ease of use. We have continued to refine the processing techniques and the need to use the backscatter intensity data over the large shallow areas of the Bank has led to a re-examination of processing methods originally developed for deep water applications. A provisional method relaxes the percent good criterion progressively toward the surface above 50 meters while tightening the percent good criterion below that depth, from our usual 92% to 95%. Because of potential miss-alignments of the ADCP AGC amplifier, the top bin is also being rejected.

On the Scotian Shelf, Georges Bank, and in the Gulf of Maine isotope-salinity values fall along a mixing line between Labrador Slope Water (LSW) and freshwater at delta 18O = -17 0/00. Although values for Georges Bank Water (GBW) at the Bank crest appear to fall along this mixing line analysis of Gulf of Maine data (Fairbanks, 1983) shows that GBW predominantly has the water properties of Wilkinson Basin and is the result of complex mixing processes of SSW, LSW, and local river sources within the Gulf of Maine. These appear as distinct mixing lines in isotope-salinity plots in contrast to the more confused patterns in equivalent temperature-salinity plots. The isotope-salinity value of GBW undergoes seasonal and interannual variations. From Al9508, we find that GBW in July 1995 was fresher than the MARMAP 1981 and 1982 observations. An objective of planned research is to determine if these changes are due solely to changes in the SSW incident directly onto the bank or to changes in the circulation and mixing processes within the Gulf of Maine.

The apparent absence of a distinct signature of St. Lawrence River water seems curious since estimates of freshwater flux (Loder et al., 1995) suggest that the St. Lawrence River contributes 1/3 to 1/4 of the freshwater entering the Scotian Shelf. It can be shown that mixing within the Gulf of St. Lawrence of this fraction of freshwater with water on the Labrador-Gulf Stream mixing curve would produce a water type that appears to have a freshwater source with delta 18O = -17 0/00. This becomes the end member for the mixing line on the Scotian Shelf. Thus the identification of Labrador as the sole freshwater source on the outer shelf in the MAB is misleading.

On the NE continental margin the oxygen isotope is a powerful tracer for distinguishing freshwater sources. It can distinguish SSW directly incident onto Georges Bank from freshwater derived from the Gulf of Maine. It can also distinguish saltier USW, with a Gulf Stream-Sargasso Sea origin, from LSW.

References:

Chapman, D.C. and R.C. Beardsley, 1989. On the origin of shelf water in the Middle Atlantic Bight. J. Phys. Oceanogr., 19, 389-391.

Chapman, D.C., J.A. Barth, R.C. Beardsley, and R.G. Fairbanks, 1986. On the continuity of mean flow between the Scotian Shelf and the Middle Atlantic Bight. J. Phys. Oceanogr., 16, 758-772.

Fairbanks, R.G., 1982. The origin of continental shelf and Slope Water in the New York Bight and Gulf of Maine: evidence from H218O/H216O ratio measurements. J. Geophys. Res., 87, 5796-5808.

Fairbanks, R.G., 1983. Isotopic origins of Georges Bank water. Special document for the National Marine Fisheries Services (NOAA), Woods Hole, MA.

Loder, J.W., B. Petrie and G. Gawarkiewicz, 1995: The coastal ocean off Northeastern North America: A large-scale view. (submitted for publication in THE SEA)

Figure 1: Representative binary mixing curves, with the zero salinity intercept indicated, for the Scotian Shelf to the Middle Atlantic Bight region from Fairbanks, 1982 (solid line); Chapman et al., 1986 (dotted line); Fairbanks, 1983 (dashed line). Depth averaged isotope-salinity values for particular years and seasons are shown for SSW (squares) and GBW (triangles).

* Bank-wide horizontal distributions varied considerably between seasons, years and region, and often did so in a species-specific manner (e.g., Figure 1). For instance, regions of higher than average abundance for the dominant species were as follows: Sagitta and Centropages hamatus <60 m; Calanus and C. typicus between 60-100m; Pseudocalanus <100m; Metridia > 100m. These patterns were similar to those described by other investigators using more recent (e.g., MARMAP) data.

* Vertical distributions of zooplankton on the bank during 1939-41 also varied enormously - between species, between developmental stages, between cruises, and between regions of the bank - with changes in DVM behavior (e.g., reverse vs. normal) occurring in several species. For instance, daytime depth of Calanus increased with developmental stage such that early copepodid stages were typically non-migratory in the surface but later stages were undertaking normal DVM, although a statistically significant reverse migration in adult Calanus was observed on at least one date. For Pseudocalanus, changes in DVM behavior occurred between seasons (e.g., during spring 1940, reverse DVM in March-April, non-migratory in April-May, and normal DVM in June) and between regions (reverse DVM in waters < 100 m and normal DVM in waters >100m during May-June 1941).

* Horizontal patchiness (e.g., correlation length scales [CLSs]) of zooplankton on Georges Bank appear to be affected by storms (wind events). For instance, daily wind speed cubed, a common proxy for mixing intensity, averaged over 30 days prior to each cruise, was positively significantly correlated with the CLSs of Calanus, Centropages, and all species combined (Figure 2). The five other abundant taxa considered all showed positive but insignificant correlations. Thus storminess would appear to break down (homogenize) small-scale patchiness and thereby increase the length scales at which zooplankton abundances are correlated.

* Population growth rates of zooplankton on Georges Bank also appear to be affected by storms (wind events), with those species dominant in the shoal regions of the bank being most strongly influenced. For instance, realized rate of population increase between cruises was significantly negatively correlated with the magnitude of average daily wind speed calculated over the same periods for PseudoCalanus and C. hamatus (Figure 3), both of which are shoally distributed on or near the peak of the bank; species more deeply distributed off-bank (e.g., Calanus and Metridia) showed no such dependence. However, specific events might be expected to effect all species - a comparison of the May-June inter-cruise period for 1940 and 1941 shows that in the high wind year (1940; mean = 5.6 m s-1) population growth rates for the 10 dominant taxa were in all cases lower than in the low wind year (1941; mean = 3.4 m s-1). Thus storminess would appear to negatively affect zooplankton abundances on Georges Bank, perhaps by advecting animals off the bank.

Figure 1. Means (circles) and variances (vertical bars = 2 SE) of the abundance of Calanus and Pseudocalanus for each of fifteen districts on Georges Bank.

Figure 2. The dependence of correlation length scale (km) on the magnitude of averaged daily wind speed cubed. Lines are fitted by functional regression, with coefficients (r) depicting slopes differing significantly from zero (* = p<0.05; ** = p<0.01).

Figure 3. The dependence of population growth rate on the magnitude of averaged daily wind speed. Lines are fitted by functional regression, with coefficients (r) depicting slopes differing significantly from zero (* = p<0.05; ** = p<0.01).

Spatial and seasonal patterns in abundance and age-composition of Calanus finmarchicus and Pseudocalanus spp. on the northeast continental shelf, based on 333 mesh MARMAP surveys (1977-1987) were described. A large part of the spatial and seasonal variation in C. finmarchicus abundance and age structure appears to be tightly coupled to major hydrographic regimes and to major circulation patterns in the region. There was a sharp ecotone between well-mixed Georges Bank and the Gulf of Maine as defined by C. finmarchicus abundance patterns and life history distributions. The ecotone is present year-round, but is most apparent during the stratified season (May-October), when thermohaline density gradients and the near-surface current jet along the northern flank are generally strongest. The Gulf of Maine had the highest abundances of C. finmarchicus, and lowest spatial and seasonal variation in the region while tidally mixed Georges Banks displayed the opposite pattern. This indication of stable population centers in the Gulf of Maine would make it a major source of Calanus in the region, particularly during March-April. Distributional patterns also suggest a strong Calanus influence from Scotian Shelf water in northern Gulf of Maine and on the southern flank of Georges Bank. Calanus distributional patterns do not display a sharp ecotone between well mixed Georges Bank and the GOM. In-shore GOM has the highest abundances of Pseudocalanus, while lower abundances are found in deeper GOM waters. Pseudocalanus abundance on Georges Bank is lowest from Sept-Dec and peaks in May-June.

Seasonal abundance of copepodites stages 1-5 for Pseudocalanus and 1-3 for C. finmarchicus from 165-053 um mesh are described for the Georges Bank-Gulf of Maine region. These data show a continual (throughout the year) presence of younger stage copepodites.

Chaetognaths, primarily Sagitta elegans, were the most abundant and widespread predator on Georges Bank during 1977-1987 MARMAP surveys. They were present in 79% of samples collected and on average were an order of magnitude more abundant than other predators. Cnidaria
(probably hydrozoan and scyphozoan medusa), euphausiid shrimp and gammarid amphipods were the next most abundant groups. Ctenophores and adults of large, rapidly swimming crustacean predators were probably underrepresented in MARMAP samples. Diversity and abundance of predators was highest in the shoal well-mixed region of the bank. In this region predator abundance was positively correlated with abundance of Calanus finmarchicus but was inversely correlated with Calanus abundance in deeper waters.

Chaetognaths and cnidaria were more abundant in 1978-1979 than in other years; there was a statistically significant decline in abundance of chaetognaths over the 10 year period. Abundance of chaetognaths in summer was inversely correlated with average temperature during the previous winter; abundance of cnidaria in summer decreased with increasing summer temperatures. (See Figure 1, following page, and Figure 2, below.)

Population dynamics of the dominant copepods on Georges Bank were investigated during late May-early June 1994 as part of the US-GLOBEC Georges Bank Program. ARGOS/GPS drifters were deployed on the northeast peak, in the shallow well-mixed region, and on the southern flank of Georges Bank. These were used to track zooplankton populations for up to three days at each site. MOCNESS and plankton pump sampling provided a complete description of the age structure of the dominant copepods at each site. Differences in egg production rates and age structure of Calanus
finmarchicus between the northeast peak and the southern flank, coupled with the drifter tracks, suggest a pattern of advection of copepods onto the bank, increased egg laying as they encounter higher food levels, and then the subsequent advection of this developing cohort along the southern flank. Stage durations measured in mesocosms on board ship varied with stage at a given site. Nauplii and young copepodites generally developed quite rapidly, and in some cases near estimated maximum rates, at all sites. In contrast, late stage copepodites of Calanus and Pseudocalanus were developing very slowly. Results indicate that environmental conditions for growth and production on Georges Bank vary significantly for different developmental stages of the dominant copepods.

During April and May 1995 an array of 24 stations was sampled after the wide area coverage had been completed under the GLOBEC Georges Bank Broadscale sampling plan. These stations, arrayed in a 'starburst' pattern approximately 30 nm in diameter, provided samples for analyses related to the growth and survival of larval cod and haddock. Twelve 1m2 MOCNESS samples were analyzed for larval cod and haddock gut contents. Samples (150 m mesh) were sorted for larvae and the guts examined for identification and enumeration of prey items along with a variety of larval morphological measurements to characterize changes in prey preference with larval development.

The April 'starburst' was located well to the west of the highest larval fish distribution as determined by the Bank-wide survey but coincided with the location of the center of larval fish distribution located in the May cruise; in effect providing a resampling of the same location one month later. In April the abundance of cod and haddock larvae in the samples was much higher than in May. The size ranges of cod and haddock in April are similar, with most of the fish ranging between 3 and 10mm SL and a peak for both species between 6 and 7 mm SL. By May the abundance was much lower and the size range increased to 4-23mm for cod and 4-10mm for haddock larvae. Length frequencies observed for both cruises are shown in Figure 1.

Although 90 percent of the larvae examined contained prey in the guts, there was no correlation between the diel distribution of larvae and occurrence of food in the guts. Gut contents were identified to the lowest possible taxon and showed a wide range of prey (Figure 2). In general, it appears that both species were eating the same sorts of prey with naupliar stages of calanoids and Oithona as well as invertebrate eggs comprising the most abundant categories in April. By May, the larger size of the larvae appears to allow predation on larger organisms as is shown by the increased occurrence of copepodites in the gut. However there is still significant predation on smaller groups indicating that the range in size of the prey has increased rather than shifting from smaller to larger prey. Veligers, which are likely to be indigestible, Coscinodiscus, and sand grains were found in a few of the stomachs.

The complete study of the stomach contents of larval cod and haddock, will include analysis of the entire sample array for all 1995 Broadscale cruises and will include analysis of prey availability and selectivity as a function of larval development and environmental conditions.

Figure 1:

Figure 2:

Analysis of the base sequence variation of mitochondrial DNA (mtDNA) for the copepod, Calanus finmarchicus, and the euphausiid, Meganyctiphanes norvegica, revealed differences in the levels of variation among three gene portions, based on the numbers of haplotypes and the genetic distance between haplotypes. 16S rRNA was most conserved, cytochrome oxidase I (COI) was somewhat more variable, and cytochrome b (CYB) was most variable. Calanus finmarchicus in the NW Atlantic is a single, genetically homogeneous population based on 16S rRNA (Bucklin and Kocher, 1995). The validity of mtDNA as a marker of population structure was borne out by parallel analysis of the copepod, Acartia tonsa, found in estuaries along the east coast of the U.S. This species comprised three deeply divergent groups of lineages, exhibited higher nucleotide diversity than C. finmarchicus or M. norvegica, and showed significant differences in haplotype frequencies between adjacent estuaries. Meganyctiphanes norvegica revealed significant differences in COI haplotype frequencies in the NW Atlantic (Figure 1). CYB haplotypes were signficiantly divergent on either side of the N. Atlantic based on an analysis of molecular variation (AMOVA). MtDNA genes differ in patterns of evolution in ways that affect their usefulness as markers of ecological processes in marine populations. Genes of moderate variability may prove most useful for population genetic studies, since using a highly variable marker results in many unique individuals that are not geographically informative.

Thus, C. finmarchicus constitutes a single population (genetically defined) from the Gulf of St. Lawrence to the Gulf of Maine. Within this homogeneous domain, dispsersal pathways will be traced by patterns of genetic similarity among the sampled individuals and knowledge of circulation patterns. This approach may help test our hypothesis that C. finmarchicus of the Gulf of St. Lawrence is a primary source of recruitment for the species in the NW Atlantic.

References

Bucklin, A. and T.D. Kocher. 1995. Source regions for recruitment of Calanus finmarchicus to Georges Bank: evidence from molecular population genetic analysis of mtDNA. Deep-Sea Res. (in press)

Figure 1. Haplotype frequencies for the euphausiid, Meganyctiphanes norvegica, in samples from Norway, Gulf of St. Lawrence, Scotian Shelf, and the Gulf of Maine from 1991 to 1994. A 400 base-pair region of cytochrome oxidase I (COI) was sequenced for 97 individuals. [Sample sizes are indicated by numbers on the bottom of haplotype frequency diagrams. Haplotype "P" is the pooled frequency of all unique haplotypes.]

Shipboard surveys of larval cod and hydrography, including measurements of phytoplankton chlorophyll a and nitrite+nitrate, were conducted on the R/V Columbus Iselin on 20-29 April and 17-26 May of 1993, and 7-15 April and 12-20 May of 1994. The temperature and salinity fields in 1993 were influenced along the southern edge of the Bank by the close proximity of warm, salty Gulf Stream ring (or Slope) water, and on the Northeast Peak by an intrusion of relatively fresh, cold, low-nutrient and low-chlorophyll surface waters of Scotian Shelf origin. The Scotian Shelf Water (SSW) could be tracked between the April and May 1993 surveys, as it advected clockwise around the perimeter of the Bank. The larvae were concentrated in two patches, which were also advected around the Bank (patterns of larval drift are discussed in Townsend and Pettigrew in the Deep Sea Research volume). Surface nutrients in May 1993 (>2.0 µM-N; not measured in April) extended as a broad band running north-south along the Northeast Peak. Chlorophyll a was highest on the Northern edge and Northeast Peak, in both April (>10 µg L-1) and May (>6 µg L 1) of 1993; larval cod densities appeared to co-vary with the chlorophyll distributions in both months, in that larvae were more abundant in the low-chlorophyll waters, perhaps associated with the edges of SSW. Larval densities were reduced by about 75% between surveys in 1993, and by more than 90% in 1994. Intrusions of SSW was also observed in April of 1994, but it was less dramatic than the previous year. There was again Gulf Stream ring/Slope Water on the southeastern edge of the Bank. Nutrients in April 1994 were concentrated along a band (4-8 µM N) that ran seaward of the 60m isobath, from the Northern Edge to the Southern Flank. In May nutrients were similarly distributed, but they were reduced to about two thirds their concentrations in April. Chlorophyll was highest over the shallower portions of the Bank in both April (6-20 µg L-1) and May (1-4 µg L-1) of 1994. Cod larvae in April 1994 were most abundant on the southeastern flank of the Bank, where they formed a single broad patch. In May the larvae were much less abundant and more diffuse about the Bank; they were not concentrated in coherent patches, and their transport between surveys could not be followed.

These cruise results demonstrate that winter concentrations of nutrients on the top of the Bank (inside the 60m isobath) become depleted by the spring phytoplankton bloom, which likely begins in the late winter (March-April) and does not depend upon vertical water column stability (sensu Townsend et al., 1994; Deep Sea Res. I, 41: 747-765). Following the bloom (late April - May), chlorophyll distributions are limited to the northern edge and frontal periphery of the Bank, between the 60 and 100m isobaths; those phytoplankton populations are most concentrated on the Northeast Peak, and appear to be supported by cross frontal fluxes of new nutrients that upwell onto the northern edge and Northeast Peak, and then flow around the Bank toward the Southern Flank. Although patches of relatively high chlorophyll concentrations are also found in May on the Southern Flank and on the top of the Bank, they appear to be supported by regenerated nutrients. Cross-Bank vertical sections on the Southern Flank suggest that off-Bank sources of "new nitrogen" (NO3) there are quickly utilized as those waters are entrained and upwelled onto the relatively broad southern flanks of the Bank, and those fluxes do not reach the central portions of the Bank.

These results suggest that primary and secondary production on Georges Bank is nitrogen-limited, and that this limitation can be characterized by region on the Bank. As has been suggested earlier by Horne et al. (1989; Scient. Mar. 53: 145-158), "new primary production" [that dependent on NO3] is more important on the northern half of the Bank and in the frontal regions; overall primary production is greatest there as well. Primary production on the top of the Bank and on the southern half, on the other hand, is more dependent upon "regenerated production" [that utilizing NH4]. For this reason, it follows that secondary production on Georges Bank is also nitrogen limited, and that zooplankton production (addition of new biomass) is higher on -- and perhaps limited to -- the Northern Flank and Northeast Peak, where new nitrogen [NO3] is supplied. Zooplankton biomass, on the other hand, is likely greater downstream in the "older" waters on the Southern Flank and on the top of the Bank, where primary production is lower, and nitrogen limited. The higher standing stocks of zooplankton at these locations are likely limited to maintenance metabolism only, which is fueled by regenerated primary production; these populations are unlikely to produce new protein. It then follows that successful cod spawning on Georges Bank depends upon larvae being transported to such regions that are enriched with high standing stocks of their zooplankton prey items, which are in turn produced some distance upstream.

As a result of the 1994 field season, planktonic hydroids, primarily of the species Clytia gracilis, were newly recognized as an important component of Georges Banks zooplankton community. Laboratory experiments and gut content analyses indicated that they could consume 50-100% of the daily production of copepods and may also directly prey upon cod larvae.

During 1995 intensive studies of both planktonic and benthic distribution of hydroids were designed to determine how hydroids accumulate and are maintained in the plankton. Planktonic hydroids were
broadly distributed over the southern flank of the Bank with highest abundance over the central shoal region, especially where the shelf/Slope Water front intersects the bottom. Benthic colonies exhibited a very different distribution being most abundant on the North East Peak. These observations are consistent with the idea of there being an upstream source of benthic hydroids that interacts with the clockwise gyre circulation on the Bank to seed and maintain a large population of planktonic hydroids in the central, shoal region. Additionally, laboratory investigations on growth and life history characteristics of planktonic hydroids indicate an important role for mixing in addition to food availability. Clytia gracilis seems peculiarly adapted to thrive in the plankton of Georges Bank. (See






Figure 4. Calanus finmarchicus, C5. Video estimates of oil sac volume vs. prosome length for all specimens recorded in June. Other months showed a similar pattern. Dashed line is an approximate upper limit on oil sac size as a function of body length.