1994/95 Georges Bank Summary

## Georges Bank Investigators Summarize 1994 and 1995 Results at Workshop

The U.S. GLOBEC Georges Bank Program Scientific Investigators met at the Woods Hole Oceanographic Institution for two days in October 1995 to discuss the results of the 1994 and 1995 field investigations. U.S. GLOBEC supported 8 cruises (81 days) to Georges Bank in 1994, and 24 cruises (278 days) in 1995. The Coastal Ocean Program of NOAA also supported 4 cruises (47 days) and 5 cruises (61 days) in 1994 and 1995, respectively. The workshop objectives were to enable the scientific investigators to describe their research activities to one another, to exchange data and information, and to work together toward developing the scientific products that will fulfill the program's goals and objectives.

### Goals

Proximate: to understand the population dynamics of target species on the Bank, in terms of their coupling to the physical environment and their predators and prey.

Ultimate: to predict changes in the distribution and abundance of these species as a result of changes in their biotic and physical environment and to anticipate how their populations will respond to climate change.

The workshop was organized around four major themes:

- Bank-wide physical and biological structures
- water column stratification and its effects on biological distributions and processes
- the population dynamics of the target species (cod, haddock, Calanus finmarchicus, Pseudocalanus)
- hot topics: unexpected findings/discussions or interesting tidbits

Each of the sessions was a blend of physics, biology and modeling work. Presentations and discussions were designed to improve our understanding of the coupling of physics and biology of the region. For each session there was a chairman and two rapporteurs. After each session, an open discussion was held. A total of 66 presentations were made over the 2.5 days of the workshop. Abstracts of most of the presentations are available via the U.S. GLOBEC Georges Bank World Wide Web site at: http://globec.whoi.edu/globec-dir/reports/siworkshop.1995/report.html

### Unexpected Findings

Several of the scientific results from the studies conducted on Georges Bank in 1994 and 1995 were unexpected. Prior to this program it was believed that nutrition of larval cod during the first few weeks after hatching was almost exclusively derived from the yolk carried in the yolk sac. This assumption is apparently incorrect; investigators in the program have discovered through a set of experiments that within a couple of hours after hatching, cod larvae are capable of feeding on microzooplankton, and that this energy intake is essential to their survival. A second surprise was Calanus finmarchicus actively feeding and reproducing throughout the summer and autumn. Previously, it was thought that C. finmarchicus entered a deep-dwelling (off the Bank--in the Gulf of Maine or slope water) quiescent (or diapause) state in summer (after the spring bloom) as C5 copepodites, to later complete development and reproduction in late winter or early spring during the period of the phytoplankton bloom. Cruises in late fall and early winter of 1994 and 1995 found some C. finmarchicus awake, feeding and maturing. Moreover, analysis of data collected during the 1977-1987 MARMAP program show that a small fraction of the population continued to reproduce and develop throughout the year. Finally, sessile hydroids, Clytia gracilis, apparently transported from the bottom on Georges Bank by strong tidal currents, were very abundant, up to 25000 per m3, in the water column. Laboratory experiments and gut content analysis suggest that the hydroids could consume 50-100% of the daily production of copepods and may also prey directly upon cod larvae.

### Session I. Bank-wide structures

David Mountain (Chair), Ann Bucklin and Dan Lynch (Rapporteurs)

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.

Hydrographic conditions and physical transports on the Bank and adjacent regions were the subject of a number of presentations (John Loder; Peter Smith; David Mountain; James Bisagni; Christopher Naimie; Glen Gawarkiewicz; Robert Beardsley; Charles Flagg and Robert Houghton). Three presentations addressed biological variability using historical data sets: Steve Bollens used data from Atlantis cruises in 1939-41 to examine the distribution and abundance of zooplankton; Carol Meise and Barbara Sullivan reported on the seasonal, interannual and spatial patterns of copepods and invertebrate predators, respectively, using the MARMAP data set collected from 1977-1987.

Nine presentations were made on broad-scale biological observations obtained during 1994 and 1995. Ted Durbin reported on the abundance, age structure, and distribution of plankton on the bank. Adult C. finmarchicus appeared to be advected onto the bank, encounter higher food levels leading to high egg production, and were advected along the southern flank. Nauplii and young stage copepodites developed rapidly, in some cases near estimated maximum rates, at all sites. Conversely, development of older copepodite stages of Calanus and Pseudocalanus progressed slowly. Jack Green reported that 90% of the larval fish sampled during April and May 1995 had prey in their guts, mostly the naupliar stages of calanoids and Oithona in April, but calanoid copepodites also in May. Ann Bucklin used mtDNA to document regional differences in Meganyctiphanes norvegica populations. Genetically, C. finmarchicus constitutes a single population from the Gulf of St. Lawrence to the Gulf of Maine. mtDNA measures of genetic similarity may be useful for documenting dispersal pathways in the NW Atlantic. April and May conditions on the bank were described by Dave Townsend from (non-GLOBEC) cruises made during 1993 and 1994. Hydrographic, nutrient, chlorophyll and zooplankton data suggest that both primary and secondary production on the Bank are nitrogen-limited; zooplankton production is highest on the Northern Flank and Northeast Peak, where new nitrogen is supplied to the bank. Planktonic hydroids were very abundant on the bank in 1994; in 1995, experiments were done to estimate growth rates, life histories and feeding rates of the hydroids (Barbara Sullivan). Spatial changes in the distribution of larval cod and haddock and their fish predators, and how these distributions shift in response to temperature changes on the southern flank were reported on by William Michaels. Charlie Miller reported on his investigations of the storage lipids of C. finmarchicus, especially the dynamics of transfer between the wax ester and triacylglyceride lipid pools. Liz Clarke's data suggest that the variability in enzyme activities of larval fish and copepods is probably related to small-scale heterogeneity in the local oceanography. Condition varied with season and by year (using 1993 and 1994 samples), and the patterns differed for the Calanus and larval fish. Peter Wiebe found quantitatively comparable estimates of the abundance and size of the most abundant zooplankton taxa (copepods and pteropods) using MOCNESS and VPR sampling. The relative proportions of individual taxa observed in net and VPR samples were used to apportion high frequency acoustic estimates of zooplankton biomass into individual taxa over the gridded sampling region.

The final seven presentations of this session addressed physical, biological and coupled bio-physical modeling of the Georges Bank system. Using climatological hydrography, Frank Bub (and Wendell Brown) developed a box-model of late-summer and autumn transports (advection) and mixing (diffusion) for the Gulf of Maine and Georges Bank region. Cisco Werner presented the results of including turbulence in a coupled trophodynamic-transport model of cod and haddock larvae, which indicate that more detailed information on larval feeding behavior is needed to evaluate the impact that turbulence has on growth; although turbulence may enhance encounter rates with prey (a positive impact), it's impact on search behavior, pursuit and capture may negatively impact larval growth. Dan Lynch summarized efforts to develop realistic dynamic prey fields to support larval fish growth experiments in highly detailed 3-D circulation fields; the specific example presented was a stage-structured population dynamics model of Pseudocalanus in a realistic circulation field. The advection of cod and haddock larvae from the NE peak site of spawning was accurately simulated (compared to the distribution of larvae determined from high resolution sampling during the spring) by a 3-D prognostic model hindcast using observed nearby winds and climatological density fields (James Manning). Using the Lynch model of circulation on the bank, Fred Page showed particle tracking results that indicate that the cod and haddock are spawning in the areas and at times that result in their longest residence times on the bank (see also the article by Sinclair and Page in U.S. GLOBEC NEWS No. 8, March 1995). Greg Lough showed how the distribution and retention of larvae on Georges Bank differed among years (using MARMAP data), and that this may contribute to variable recruitment. They are using a particle tracking 3-D model with observed past physical forcing to examine how large an impact these interannual differences in forcing can have on the distribution and survival of the larvae. Changsheng Chen described an NPZ model coupled with a mixed-layer physics model that produced spatial patterns of biomasses and fluxes of biological variables that agreed reasonably well with field measurements on and off the Bank.

### Session II: Stratification and Its Effects

Bob Beardsley (Chair), Dian Gifford and Ted Durbin (Rapporteurs)

The session on stratification was subdivided into four parts: (1) physical oceanographic setting; (2) biological oceanographic setting; (3) measurement of vital physiological rates; and (4) a panel discussion of the effects of stratification and microturbulence on the biology of Georges Bank.

Physical Oceanographic Setting. The suite of presentations covered scientific ground as extensive as Georges Bank itself. Jeff Van Keuren used measured light fields on the Bank during the development of stratification to estimate the times and depths where larval fish, which are visual predators, are able to feed. Jim Bisagni presented a retrospective analysis of SST and stratification on the Bank, and related interannual variability in stratification to differences in heat flux.

Five presentations dealt with various aspects of the program's long- and short-term mooring programs. Jim Irish gave an overview of the long-term mooring program, including Page Valentine's bottom surveys of the mooring sites and preliminary data from the crest and southern flank moorings. Bob Beardsley described the evolution of vertical stratification at mooring site ST1, including resolution of an ca. 20 day "salt event" of warm salty water intruded from offshore in May 1995 (presumably associated with a warm-core ring). Jim Manning presented preliminary data from mooring site ST2, including measurements of wind stress associated with vertical velocity shear during wind events. Jim Churchill and Sandy Werner described various aspects of the bottom tripod (BASS) system deployed at mooring site ST1, including resolution of stratification events in the bottom five meters of the water column.

Drifter studies were described in two presentations. Jim Churchill reviewed the small-scale drifter program, which assisted the larval fish component in tracking plankton patches and studied small scale fluid dynamics. Peter Garrahan described the short-term drifter deployments performed on the 1994 and 1995 vital rates process cruises.

Two presentations concerned turbulence fields on the Bank. Russ Burgett presented maps of microturbulence measurements done in the spring of 1995 using Neil Oakey's Epsonde probe system. Chris Naimie reviewed model-generated turbulence patterns on the Bank.

Biological Oceanographic Setting. Topics covered ranged from phytoplankton to predators. Mike Sieracki, Scott Gallager, Mark Berman and Ted Durbin described aspects of the biological setting of Georges Bank during the 1994-1995 process cruises, proceeding from the base of the food chain to distributions of crustacean zooplankton. Mike Sieracki described an abundant microbial loop consisting of a diversity of phytoplankton and protozoan taxa which function as prey for larger zooplankton and larval fish. Zooplankton distributions were determined by a variety of techniques including the Video Plankton Recorder (VPR) and the Tracor Acoustic profiling System (TAPS). Scott Gallager described micro- to mesoscale vertical and horizontal distributions of plankton relative to Bank hydrography. The VPR was able to resolve the distributions of a number of fragile forms that are inadequately sampled by more traditional means, including the colonial diatom Chaetoceros socialis, the colonial stage of Phaeocystis pouchetti, and delicate gelatinous zooplankters. Mark Berman reviewed acoustic studies using the TAPS system, emphasizing the abundance of sand grains in the water column on the Bank crest, and the potential problems they cause by interfering with acoustic determination of crustacean zooplankton abundance. Ted Durbin described the age structure of target copepod populations over time, noting that the Bank crest was relatively devoid of copepod naupliar stages. In contrast, nauplii were abundant on the northeast peak and downstream on the southern flank. Irving Kornfeld, a guest from the University of Maine, described genetic effects of fishing on haddock.

Vital Physiological Rates. Presentations concerning vital physiological rates focused on target zooplankton taxa. Ted Durbin, Dian Gifford, Jeff Runge, Bob Campbell, Melissa Wagner, Elaine Calderone, Scott Gallager, Greg Lough, Lew Incze, Michael Moore, Larry Madin and Grace Klein-MacPhee presented data on various aspects of zooplankton vital physiological rates. A number of presentations dealt with rate measurements of target copepod species. Ted Durbin's mesocosm experiments suggested that copepod growth was food-limited in May-June 1994. Dian Gifford's feeding studies showed that Calanus was primarily a carnivore in May-June 1994, obtaining most of its nutrition from planktonic protozoa. Jeff Runge's data showed that egg production rates were high at all stations in 1995 (Figure 1) and that female Calanus produced eggs everywhere on the Bank, but that egg hatching success varied, probably as a function of diet. Bob Campbell's growth rate experiments indicated that younger stages of Calanus grow faster than older stages, but that growth was not related to chlorophyll concentration. Larry Madin described patterns of distribution and abundance of invertebrate predators, with emphasis on the hydroid Clytia sp. and the impact of its feeding activities on crustacean zooplankton populations on the Bank. Grace Klein-MacPhee's data demonstrated the impact of predation by the ctenophore Pleurobrachia on crustacean zooplankton, fish eggs, and fish larvae.

Four presentations covered various aspects of the physiology and ecology of larval cod and haddock. Elaine Calderone used RNA/DNA ratios to determine the condition of cod and haddock larvae on the Bank in 1993 (a year of a pilot project) and 1994 (Figure 2). In 1993 larvae of both species were in good condition overall. In contrast, larvae of both species were in relatively poorer condition in 1994, but condition could not be related to hydrographic conditions. Scott Gallager described the exclusively microzooplankton diet of larval cod before yolk sac absorption. Greg Lough presented a model relating turbulence and larval fish feeding success, and Lew Incze described his May 1992 study of the influence of turbulence on larval cod and their naupliar prey.

Two presentations concerned methods development. Melissa Wagner presented data indicating that RNA/DNA ratios of individual Calanus is related to aspects of nutrition (food availability, perhaps growth rate). Michael Moore reviewed development of his cell proliferation method for measuring copepod growth rates.

### Session III: Population Dynamics of Target Species

Charlie Miller (Chair), Cabell Davis and Dan Lynch (Rapporteurs)

This session included four presentations on models of individual populations or communities interacting with their physical environment, and one presentation on using genetic probes to examine distributions and timing of reproduction. Mike Fogarty described several stage-structured models of fish community dynamics on Georges Bank, which will hopefully lead to models of exploited multispecies systems for fishery management that can accommodate various nonlinear processes and spatial dynamics. Ann Bucklin reviewed work being done to use oligonucleotide probes to discriminate Pseudocalanus spp. with eventual application to examine seasonal timing and location of reproduction in the various Pseudocalanus species. Cisco Werner described an individual based model of cod and haddock growth and mortality coupled to physical circulation (this model was previewed in U.S. GLOBEC NEWS No. 7, September 1994). Andrew Leising described an individual based model of cod larval growth, incorporating variability in prey density, turbulence and temperature, that suggested that the timing and location of cod spawning were crucial to eventual larval survival. Glenn Flierl used empirical orthogonal functions (EOFs) to reduce the complexity of a stage-structured model of the life-cycle dynamics of the copepod, Pseudocalanus spp. With the model, the full (200 variable) system can be adequately reproduced using between 5 and 15 EOF modes.

### Session IV: Hot Topics Discussion

Steve Bollens (Chair), Greg Lough (Rapporteur)

This session served as a forum for investigators to briefly (5 minutes) present novel and intriguing research results that either did not fit readily under the theme areas established for the other three sessions or for which investigators did not have adequate time to present during the other sessions. As such, some of these presentations addressed topics only indirectly related to the GLOBEC target species (sand storms on Georges Bank by Peter Wiebe; the Chaetoceros patch by Dian Gifford et al.; planktonic hydroid growth by Steve Bollens et al.), whereas others addressed more specific aspects of the population biology of the target species (Calanus mating behavior by Charlie Miller; interdecadal zooplankton abundances by Landsteiner et al.; Calanus egg viability by Jeff Runge). But throughout all of this session's diverse presentations was a common theme of trying to elucidate the intricate web of biological-physical interactions that comprise the Georges Bank ecosystem.

(This report summarizes results of the Scientific Investigator's Workshop, which was organized by the U.S. GLOBEC Georges Bank Executive Committee, whose members are Robert Beardsley, Steve Bollens, Ann Bucklin, Cabell Davis, Ted Durbin, Mike Fogarty, Dian Gifford, Greg Lough, Dan Lynch, David Mountain, and Peter Wiebe. Much of the text is directly from or a summary of information contained in an unpublished report of the workshop).