Processes controlling abundance of dominant copepod species on Georges Bank:

Local dynamics and large-scale forcing

PIs: Cabell Davis (WHOI), Robert Beardsley (WHOI), Changsheng Chen (UMassD),

Rubao Ji (WHOI), Edward Durbin (URI), David Townsend (UMaine),

Jeffrey Runge (UNH), Charles Flagg (SUNY), Richard Limeburner (WHOI)

Project Summary

A fundamental goal of Biological Oceanography is to understand how underlying biological-physical interactions determine abundance of marine organisms. For animal populations, it is well known that factors controlling survival during early life stages (i.e., recruitment) are strong determinants of adult population size, but understanding these processes has been difficult due to model and data limitations. Recent advances in numerical modeling, together with new 3D data sets, provide a unique opportunity to study in detail biological-physical processes controlling zooplankton population size. We propose to use an existing state-of-the-art biological/physical numerical model (FVCOM) together with the recently-processed large 3D data set from the Georges Bank GLOBEC program to conduct idealized and realistic numerical experiments that explore the detailed mechanisms controlling seasonal evolution of spatial patterns in dominant zooplankton species on Georges Bank. We will examine a series of hypotheses that address how dominant copepod species populations are maintained on the bank, including local dynamics and large-scale forcing. Specifically we will determine whether the observed characteristic seasonal-spatial pattern of each species (long-term and inter-annual) is predictable from the interaction between its characteristic life-history traits and physical transport. The extent to which the copepod populations are controlled by food-availability (bottom-up) or predation (top-down) processes will be examined, including the influence of Warm Slope Water versus Labrador Slope Water (NAO-dependent) on nutrient influx through the Northeast Channel and subsequent upwelling and biological enhancement on the bank. Self-sustainability of each species population on the bank itself and in the Gulf of Maine will be studied by controlling immigration from specific source regions. Large-scale forcing including NAO and catastrophic global warming (e.g. complete polar ice melt) will be examined explicitly by forcing the model at the boundaries, using scenarios based on basin-scale data and from concurrent basin-scale modeling efforts. Intellectual Merits: The proposed modeling study will provide new insights into the role of local and large-scale processes controlling zooplankton abundance in the ocean. The dominant copepod species to be studied include small species that are the dominant prey for larval cod and haddock in this region, thus providing critical information for concurrent larval fish modeling studies. This detailed, process-oriented, regional-scale modeling with boundary forcing will lay the groundwork for integration with models of the entire ocean basin. The resulting model will be a legacy of the GLOBEC Georges Bank program by providing a powerful new tool for understanding how local and large-scale forcing interact to control plankton production in the sea. Broader Impacts: Results of the proposed work will be broadly disseminated to the general oceanographic community, the fishing industry, K-12 institutions, and to the population at large, through web-based servers using existing infrastructure at the proposers’ institutions. Web-based users will be able to access model results and run the model using chosen parameter settings to obtain predictions of currents, hydrography, and plankton abundance patterns given selected climate forcing scenarios. Collaboration with the WHOI/UMASS COSEE program will foster communication with K12 students and the public both nationally and internationally.

1. Background

The GLOBEC approach — Understanding complex marine ecosystems requires use of simplifying assumptions, which historically has involved trophodynamic analysis of energy or mass flow by measurement and modeling (Lindeman, 1942; Teal, 1962; Odum, 1957; Steele, 1974). In regions of high diversity such as the oligotrophic ocean this approach remains the only feasible method of analysis (e.g., Sarmiento et al., 1993). As an alternative in low diversity regions, it is possible to model the population dynamics of a few dominant species to understand processes controlling their abundance and to obtain information about system level functioning (e.g., Davis, 1987a). The trophodynamic concept has been used to relate primary and secondary production to fish production (e.g., Clarke, 1946; Cushing, 1975; Sissenwine et al., 1984; Cohen and Grosslein, 1987; Nixon, 1988), but this method is of limited utility in predicting abundance of particular fish species. It is well known that adult fish populations are dominated by a few strong year classes that are set by recruitment from early life stages (Hjort, 1914; Grosslein and Hennemuth, 1973). The GLOBEC program recognizes recruitment as the dominant factor controlling population abundance in marine animal species (GLOBEC, 1991a). The regional GLOBEC programs focus on target species and their dominant prey, with emphasis on individual organisms, population dynamics, and interactions with the physical environment, especially as it relates to global climate change (GLOBEC, 1991b, 1992, 1996, 1997, 2000).

Georges Bank GLOBEC: dominant zooplankton species — Georges Bank was chosen as the first GLOBEC study site due to its sensitivity to climate change, definable populations, importance as a fishing ground, and significant historical database (GLOBEC, 1992). The goal of this program is to understand the biological and physical processes controlling abundance of cod and haddock larvae and their dominant prey species. In the program implementation plan we emphasized the copepods Calanus finmarchicus and Pseudocalanus spp. as target species, since these are dominant prey items for larval cod and haddock. Subsequent studies have found that small copepods (Pseudocalanus spp., Oithona similis, Centropages spp., Temora longicornis) are dominant prey items for cod and haddock larvae on Georges and Western Banks (Lough and Mountain, 1996; Lough et al, 1996; McLaren and Avendano. 1995; McLaren et al., 1997). This contrasts sharply with the situation in the eastern Atlantic (North Sea and Norwegian Sea) where cod larvae feed almost solely on Calanus finmarchicus (Sundby, 1999), and decadal shifts in C. finmarchicus abundance affect larval cod survival (Beaugrand et al., 2003). This difference in larval fish prey preference appears due to prey availability since the larval fish diets reflect ambient copepod species composition, and the shallow banks have high concentrations of small copepod species (Davis, 1987; Sherman et al., 1987; Meise, and O’Reilly, 1996., McLaren et al. 2001; Reiss et al, 2003). In the shallow Baltic Sea, cod larvae also feed on small copepod species (Hinrichsen et al, 2002). In general, small copepods are an important component of marine ecosystems (e.g., Davis, 1987b; Turner, 2004). The present proposal focuses on understanding the biological-physical processes controlling the abundance of dominant copepod species in the Georges Bank/ Gulf of Maine region, including Calanus finmarchicus and the smaller species, Pseudocalanus spp. (moultoni and newmani), Oithona similis, Temora longicornis, Centropages spp. (typicus and hamatus).

1.1. The Georges Bank Physical Environment

Local dynamics — Georges Bank (GB), the Gulf of Maine (GOM), and Scotian Shelf (SS) are part of a single regional coastal current system, driven in large part by upstream mass and buoyancy forcing (Fig. 1). The bank itself is a quasi-flow-through system, with water from offshore and upstream sources arriving on the bank, being modified locally by surface forcing and tidal mixing before moving off-bank to the Mid-Atlantic Bight or re-circulated northward along the Great South Channel.

The strongest currents over the bank are of tidal origin, and turbulent mixing associated with the tidal bottom boundary layer is most intense over the shallow cap of the bank, effectively eliminating local vertical temperature and salinity stratification throughout the year. As seasonal stratification increases on the flanks of the bank, the tidal mixing front forms with associated secondary flow. The clockwise around-bank residual flow increases with seasonal stratification, becoming partially closed from June through

Figure 1. Circulation: 1 flow across GSC into the north flank jet, 2 tidal-pumping of deep water onto GB, 3 wind-driven near-surface flow, 4 small-scale cross-frontal processes, 5 SS cross-over

Figure 2. Schematic of the western North Atlantic shelf-break current system in summer,

(Fratantoni and Pickart, 2005)

September until fall storms and surface cooling destroy the local stratification. Surface heating drives the development of the seasonal thermocline. Salinity on the bank is controlled by advective and mixing processes along the northern and southern flanks. On the southern flank salinity is influenced by on-bank intrusions of saline shelf-break frontal water and very saline warm-core ring water (Fig. 1). Along the northern flank salinity is controlled by advection from the western Gulf across the northern Great South Channel (Fig. 1, 1), tidally-driven near-bottom residual flow (the “tidal pump”, Fig. 1, 2), wind-driven near-surface flow (Fig. 1, 3), small-scale cross-frontal processes (Fig. 1, 4), and intermittent cross-over of low salinity SS surface water (Fig. 1, 5). The tidal pump in particular provides a strong mechanism for bringing deep water from Georges Basin up onto the bank (Chen and Beardsley, 1998; Chen et al., 2003).

Figure 3. Schematic showing the strong westward penetration of LSW and northward position of the Gulf Stream following low winter North Atlantic Oscillation (NAO) index (left) and weak penetration of LSW and more southward position of Gulf Stream following high winter NAO index. (Drinkwater et al, 2003)

Large-scale forcing — Water enters the GOM via two primary paths: (1) the flow of relatively fresh water above 100m from the SS and (2) warmer, more saline Slope Water (SW) at depths greater than 100m through the Northeast Channel (NEC). The primary source of SS water is the West Greenland/Labrador Current system, with additional input from the St. Lawrence system (Fig. 2). As the Labrador Current flows around the Grand Banks, the large shoaling in shelf-break depth from ~300m in the north to ~100m to the southwest helps force the deeper Labrador Current water to flow along the upper slope, thus forming Labrador Slope Water (LSW), which flows west into the Laurentian Channel and along the Scotian upper slope. The westward extent of LSW depends on its source strength, thought to depend on basin-scale forcing (NAO) (Fig 3), and degree of mixing with ambient Warm Slope Water (WSW) of Gulf Stream origin. Since WSW is warmer, more saline, and nutrient rich than LSW, the relative mix of these two end members entering the NEC, and its transport relative to the inflow of SS water, strongly influences circulation, water property distributions, and nutrient content in the GOM/GB system.

Fig. 4. GB salinity anomaly (Mountain, 2005)

Fig. 5. Percent LSW in bottom water (150-

200 m) in 1998. (Drinkwater et al, 2003)

Data from the 1995-1999 GB GLOBEC field study provide an excellent example of the flow-through nature of the GB/GOM system and its linkage to larger basin-scale forcing. The salinity on GB exhibited two significant freshening events between early 1996 to early 1997 and between late 1997 through 1998, with a net drop of ~1 psu (Fig. 4). These two events also were observed in surface (0-30m) GOM waters, suggesting significant increases in freshwater influx from the SS (Smith et al., 2001). In the NEC, WSW was replaced by cooler, fresher LSW in January 1998 as the leading edge of LSW flow extended west due to an increase in the Labrador Current associated with a low NAO. As LSW entered the GOM during early 1998, it mixed with resident GOM water (Fig 5). By early 1999, WSW was again flowing into the GOM through the NEC. Since the tidal pump mechanism can carry deep water up on the northern flank of GB, advection of LSW in Georges Basin onto the bank can occur on relative short time scales (>=1 month), suggesting that part of the freshening on GB observed during 1998 was due to the influx of LSW into the GOM. Clearly, variations in the primary upstream sources (the SS Water, the mix of WSW versus LSW) linked to basin-scale forcing strongly control the water properties (including heat, salt and nutrients) through the GOM and onto GB.

Fig. 5a. Scenario of nutrient input onto GB from the NEC (Townsend et al., 2004).

1.2. Connection between NAO and plankton productivity on GB

NAO-dependent intrusions of LSW and WSW through the NEC greatly influence the nutrient (N, Si) input into the GOM. It is believed that the tidal pumping mechanism along the northern edge of the bank can quickly transport nutrients from Georges Basin up onto the NE peak of GB, impacting the productivity of the bank very rapidly (Fig. 5a). The ratio of N and Si in the inflow water can influence the duration of the diatom bloom. In general, the NEC inflow can affect the total salt and nutrient budget of the GB/GOM, determining the thermohaline circulation and influencing upwelling along the western GOM and subsequent nutrient transport to western GB. The potential impact of this inflow will be modeled in the proposed study.

1.3. Characteristics of GB zooplankton

GB zooplankton is dominated by Calanus finmarchicus, Pseudocalanus spp., Oithona similis, Centropages spp., and Temora longicornis, and Paracalanus parvus (Bigelow, 1926; Davis, 1984, 1987b; Sherman et al, 1987; Durbin et al., 2003; Durbin and Casas, submitted). Each species exhibits a characteristic life cycle and seasonal/spatial pattern in the GB/GOM region. Calanus finmarchicus and Pseudocalanus spp. are cold-water species that avoid the warm surface layer (>10-12^oC) during summer and fall and produce large spring populations. Centropages spp, Temora, and Paracalanus are warm water species and are most abundant during late summer and fall. Oithona is plentiful throughout the GB/GOM region year round.

Fig. 6. Calanus finmarchicus abundance (log₁₀(#/10m²) in the GB/GOM, mean 1977-1987, (MARMAP data redrawn from Meise and OReilly, 1998).

Calanus finmarchicus— This species spends the warmer months in a state of diapause as stage CV in cooler waters (5-7 ^oC) at depth in the GOM (Fig. 6). During late December, it emerges from diapause (mechanism unknown), swims to the surface and molts to adult. Subsequent egg production depends on availability of phytoplankton (Durbin et al., 2003), with the first generation born in late December-early January (Durbin et al., 1997). Generation time is ~2 months at the cold winter/spring temperatures (~5 ^oC), so G1 adults appear in March, and there is time for a total of 3 generations by the end of its growing season in July. Overwintering females produce eggs for a prolonged period, smearing out cohorts (Durbin and Casas, submitted). This annual cycle in the GB/GOM appears stable, having persisted for many decades (Bigelow, 1926; Clarke et al, 1946; Meise-Munns et al., 1991), but the extent to which this population is self-sustaining is unknown.

Fig. 7. Calanus finmarchicus abundance in the N Atlantic from CPR data (Spears 2005) and nets/OPC (Heath et al. 2004). Note high densities in the Labrador and Norwegian Seas.

Calanus finmarchicus is an open ocean species, occurring throughout the northern North Atlantic from the eastern US to the Barents Sea, with centers of population abundance in the Norwegian and Labrador seas (Fig. 7). Immigration into the GB/GOM population from upstream sources may contribute to the apparent stability of the population. C. finmarchicus enters the GOM from the SS and possibly from the SW through the NEC. C. finmarchicus in the SW originate from the Labrador Sea and to an unknown extent from spill-over of the productive shelf populations. Scotian Shelf C. finmarchicus originate from Labrador Sea via the SW (Head et al., 1999) and to some degree from the productive Gulf of St. Lawrence population (Zakardjian et al. 2003). It is unlikely that immigration directly determines the large spring abundance peak in C. finmarchicus in the GB/GOM, since the water mass turnover time is long relative to the generation time, but a seeding type of immigration during the “off-season” could be important in determining the size of the startup population of CVs in December (Saumweber and Durbin, submitted). Since the diapausing SW population resides at a depth of 500m (below the NEC sill depth), and high concentrations are not observed in the upper water column until April (Miller et al., 1991), well-after initiation of the GB/GOM population growth, it is unlikely that a SW population entering through the NEC contributes significantly to the GB/GOM C. finmarchicus population.

It may well be that the resident diapausing GOM population is sufficient to produce the large spring GOM/GB population. At the end of the growing season, downward migrating diapausing CVs cannot reach their normal open ocean depths of 500-2000m, and they become trapped in the GOM basins. A similar effect has been observed on the SS basins (Sameoto and Herman, 1990) and for C. pacificus in the Santa Barbara basin (Osgood and Checkley, 1997). Lagrangian modeling studies have found retention of diapausing CVs in the GOM to be high, especially if the animals stay below 150 m (Johnson et al., submitted). Thus it appears possible that the GB/GOM population could be self-sustaining, but further 3D modeling work is needed, using a concentration-based approach to quantify population dynamics and transport continuously throughout this region during the year. This self-sustainability is in sharp contrast with the situation in the North Sea, which has no deep basins and is dependent on annual input from offshore waters (e.g. Heath et al., 1999).

Fig. 8. GLOBEC monthly mean Calanus abundance

on GB, showing abundance “hole”

Fig. 9. Pseudocalanus abundance in the GB/GOM. MARMAP bi-monthly means 1977-1987 (from McGillicuddy et al., 1998).

Although C. finmarchicus may be able to sustain itself in the GB/GOM system, it does not maintain itself on GB, since the bulk of the population disappears from the bank during the off-season (Fig. 6) and even during the growing season its abundance appears to be driven by GOM concentrations, with a well-defined “hole” in abundance in the center of the bank (Fig. 8, Durbin and Casas, submitted). This hole results from a combination of advection around the bank of the large GOM population and possibly high predation over the crest. A huge literature exists for C. finmarchicus egg production, development and growth as a function of food and temperature including several studies done as part of the GLOBEC GB process work (Campbell and Head, 2000; Campbell et al., 2001a,b; Runge et al. submitted). Such data can be incorporated into the population model for this species and used together with the large field data base to conduct targeted forward modeling to examine biological/physical processes controlling observed patterns.

Pseudocalanus spp.— Like Calanus, the growth season for Pseudocalanus is winter/spring (Fig. 9, 10). Its abundance is higher in shallower areas (<100m) and is highest in the crest region of the bank in June. Pseudocalanus does not overwinter in the GOM as does Calanus and is not normally present in the central Gulf during winter. This genus is an egg carrier and consequently has lower egg production and egg mortality rates (Corkett and McLaren, 1978; Ohman et al., 2002). Pseudocalanus in the GB/GOM

Fig. 10. GLOBEC data for Pseudocalanus

Fig. 11. A) Temora, B) Centropages hamatus C) Centropages typicus, D) Oithona similis. Mean monthly GLOBEC data for 1995-1999, A,B,D May; C January; log₁₀(#/m³+1)

region comprises two species: P. newmani and P. moultoni (Frost, 1989; McLaren et al., 1989a; Bucklin et al., 1998, 2001; McGillicuddy and Bucklin, 2002). P. moultoni appears to be a colder water species and more abundant during winter/spring, while P. newmani is more plentiful during spring/summer (McLaren et al., 1989a,b). P. moultoni is a coastal species and P. newmani an offshore one (Frost, 1989). Thus P. moultoni may be carried onto the bank from western GOM coastal waters (e.g., Cape Cod Bay), while P. newmani may have a source from SS water, either crossing over the NEC, or indirectly through the central GOM (McGillicuddy and Bucklin, 2002). Inverse modeling, representing the biology by a single source/sink term, revealed that the two species are likely to have different source locations on the bank but intermingle by June (McGillicuddy and Bucklin, 2002). The use of a simple cost function instead of a population model, however, meant that the independent species-specific life history information could not be included in the model. Substantial information on development, growth and egg production as a function of food and temperature is available for Pseudocalanus (e.g. Corkett and McLaren, 1978; Vidal, 1980; Davis, 1983, 1984a,b; McLaren et al., 1989b; Ban et al., 2000; Lee et al., 2003; Dzierzbicka-Glowacka, 2004). Use of these laboratory and shipboard derived rates together with field data on distributional patterns of the combined species (plus a subset of species-specific data, Bucklin et al., 2001) can be used in numerical models to explore the biological/physical processes controlling seasonal development of the spatial distribution patterns. Targeted forward modeling experiments are required to determine the potential source locations and interactions controlling subsequent spatial patterns of each population, given their distinctive temperature-food dependent growth and reproductive rates.

Other Dominant Copepod Species — Each dominant copepod species on GB has its own characteristic temporal-spatial patterns and life histories (e.g., Davis, 1987a). Copepods that lay bottom resting eggs, including Centropages hamatus and Temora longicornis, have well defined populations on the crest of the bank (Fig. 11A, B) (Davis, 1987; Sherman et al., 1987). It has been suggested that these species use resting eggs as a strategy for “gluing” their populations to regions that are favorable for growth (Davis, 1987; Lindley and Hunt, 1989; Lindley, 1990; Marcus, 1996; Marcus and Lutz, 1998), but the necessary biological/physical dynamics have yet to be verified via modeling. Centropages typicus abundance is highest during late summer and fall and decreases markedly during winter and spring. It is most abundant in the warm surface layer above the thermocline and not restricted to the crest of GB like C. hamatus. Oithona similis is abundant year round and is not restricted to GB but has a pattern similar to C. finmarchicus, with a large off-bank population. A substantial literature on life history information is available for these species, including egg production, development and growth rates as functions of food and temperature (e.g., Davis and Alatalo, 1992; Klein Breteler and Gonzalez, 1986; Klein Breteler et al., 1982, 1996; Sabatini and Kiørboe, 1994; Maps et al. 2005).

Fig. 12. Abundance trends in GB copepod, GLOBEC 1995-1999.

Abundance trends during the GLOBEC years — Abundance of the dominant copepod species, except Calanus, increased significantly during the five-year GLOBEC GB field program (Fig. 12.). While this trend may reflect a change toward smaller species, indicative of a warming trend, no concomitant increase in temperature was observed. These changes were negatively correlated with salinity (Durbin and Casas, submitted.). High abundances in 1999 appear to be related to a phytoplankton bloom that took place during winter in the central GOM, leading to high reproductive rates and abundances (Durbin et al 2003). The elevated abundances may have been advected onto GB. It is possible that low surface salinity during winter increased stability of the water column and led to the bloom. A negative correlation between salinity and chlorophyll on GB also was found at this time (Durbin and Casas, submitted), the reason for which is unknown. The proposed modeling work will examine the potential causes of the zooplankton increase and its relationship to large-scale forcing and climate change.

1.4. Available Data Sets

GLOBEC GB — The GLOBEC GB field program was conducted from 1995-1999 and included a combination of monthly broadscale and process-oriented cruises (GLOBEC, 1992; Wiebe et al, 2002).  Broadscale cruises provided 3D maps of the plankton species based on net tows (1-m² MOCNESS, Wiebe et al., 1985) at a set of 41 standard stations covering the bank and adjacent waters, and plankton pump sampling (Durbin et al 1997, 2000) at ~18 of the standard stations. CTD bottle casts were made at all stations. MOCNESS (0.15 mm mesh nets) zooplankton samples were collected from 0-15, 15-40 m, and 40-100 m or the bottom (if shallower than 100 m) and 100 m to the bottom or 450 m (if >100 m). Pump (0.035-mm nets) samples were collected over the same depth ranges as the MOCNESS in the upper water column but the maximum depth it was deployed to was 70-100 m depending on wind and tides, or to the bottom on shallower parts of the bank. For complete description of collection and processing methods see (Durbin et al. 1997; 2000). Samples were sorted, and processed data now are available for all life stages of Calanus finmarchicus and Pseudocalanus spp. (N1-N6, C1-Adult).  For other copepod species life stages were sorted as adult males and females, copepodids, and nauplii. These data are stored in an Oracle database together with all of the CTD and chlorophyll data (http://globec.gso.uri.edu). The complete GLOBEC GB data set for broadscale zooplankton includes >3500 net samples (41 stations, 3 depths, 30 cruises), plus >1500 pump samples (18 stations, 3 depths, 30 cruises). In addition, vertically stratified data from several other cruises have been collected in deeper regions of the GOM.

Other Data Sets — Data from the bimonthly cruises of the MARMAP program 1977-87 and its follow-on program, ECOMON (1992-present), include shelf-wide distributions of hydrography and zooplankton (0.333mm bongos). The complete data set is available to us via an NMFS Oracle database (D. Mountain, pers. comm.) Although the zooplankton data are from integrated hauls, these data cover a broader area than the GLOBEC GB data, and, together with GLOBEC vertical data from deeper GOM, will allow us to approximate 3D spatial patterns of each species throughout the GOM region. Further data from the GOM CPR transect, transatlantic CPR, UNH Coastal Observing Center, the 1939-41 GB study, and Canadian AZMP (see Head letter), allows the 1995-1999 GLOBEC data to be viewed in a larger context.

1.5. Previous modeling

Several previous biological/physical models of the GB/GOM region have been developed. Davis (1984) showed that the interaction between around-bank advection/diffusion and temperature-dependent development of the copepod Pseudocalanus could explain observed spatial patterns in population structure. Subsequent studies have shown that this scenario is most likely for P. moultoni, with an admixture of P. newmani later in the season (McGillicuddy and Bucklin, 2002). McGillicuddy et al. (1998) had used the same inverse approach to locate source/sink areas for the Pseudocalanus population. Klein (1987) modeled the 2D NPZD-advection-diffusion on the bank and found that phytoplankton production was removed from the bank via detritus rather than grazing. Lewis et al., (1994, 1991) used 3D modeling to show that strong wind forcing can cause a partial washout of plankton from the bank, increasing the phytoplankton/zooplankton ratio (NPZ model), and causing spatially-temporally sensitive larval populations (particle-based) to be lost from the bank, affecting recruitment. The Calanus finmarchicus population in the GB/GOM has been modeled using the Dartmouth model to show that the deep basins and SS can contribute to the bank population; these models used 2D transport of concentration-based (Lynch et al. 1998), individual-based (Miller et al., 1998), and particle trajectories (Hannah et al 1998). The C. finmarchicus population throughout the Gulf of St. Lawrence/SS/GOM region was modeled to examine local dynamics and exchange between regions within the model interior (Zakardjian et al., 2003). The latter model was a 3D concentration-based 13-stage population model with temperature-dependent development and was run for a full-annual cycle. The results indicated that horizontal transport was dominant on the SS, but that the population is self-sustaining in the Gulf of St. Lawrence. Food-dependence effects on population dynamics during the active season were not examined, however.

1.6. Combining existing models and data

Over the past decade, the GLOBEC GB program has acquired and processed an exceptional 5-year 3D data set of plankton and physical variables, while at the same time developing a high-resolution state-of-the-art prognostic 3D biological/physical model of this region (FVCOM). Data processing and model development recently have reached the point where they can be effectively combined. By the start of the proposed study, the FVCOM model will have been ported to a massively-parallel supercomputer. We now have the unique opportunity to use the data and model together to study the detailed mechanisms controlling zooplankton abundance patterns on GB, explicitly including boundary forcing determined by basin-scale dynamics.

We have assembled a team of PIs who are leading experts on the plankton and physical dynamics of this area. Beardsley and Flagg are physical oceanographers with a long background in this region. Chen is a physical oceanographer and developer of the FVCOM model. Durbin is the scientist in charge of the broadscale data acquisition and processing. Runge is an expert in copepod biology specializing in Calanus fertility and population dynamics. Townsend is the lead scientist studying nutrient-plankton production in the GB/GOM region. Davis is a zooplankton biologist who has done field, laboratory, and biological-physical modeling studies of copepod population dynamics in the GB/GOM region. Davis and Beardsley helped formulate the theoretical underpinnings and approach for the GLOBEC GB program (GLOBEC, 1992).

2. Proposed Research

2.1. Hypotheses

Working Hypothesis – The seasonal evolution of characteristic mean spatial abundance patterns of each dominant copepod species on GB is predictable from the interaction between its characteristic life-history traits and physical transport. These life-history traits include egg production, development, and growth rates (temperature/food dependent) as well as other traits such as vertical migration and diapause. Both the long-term, multi-year, mean and year-to-year variations in seasonal-spatial patterns are predictable by these interactions. Within this working hypothesis, we will address three specific null hypotheses:

H1₀: The abundance of copepod species on the bank is controlled by food availability (bottom-up control). Here we will examine the scenario that GB productivity, and thus food availability for the dominant copepod species, is controlled by nutrient input into the GOM through the NEC, which is determined by the intrusions of Labrador Slope Water versus Warm Slope Water. Alternative hypotheses include: 1) predatory control of copepod seasonal cycles (top-down control), 2) a combination of food-limitation and predation (time-space dependent), and 3) purely physical control by direct effects of temperature on vital rates or advection. In 3), we will examine causes of the observed increase in warm-water copepod species during the GLOBEC years (1995-1999).

H2₀: Copepod populations on GB and/or the GOM region are not self-sustaining. We will examine the need for immigration from different sources to maintain the copepod populations over multiple years. Key source regions will be examined including the SS and SW. For self-sustainability on GB itself, we will examine potential sources from the coastal regions (e.g., Cape Cod Bay, Penobscot Bay), the GOM, and from bottom resting eggs.

H3₀: Catastrophic global warming (e.g., total polar ice melt), parameterized as a lack of Labrador Sea water at the NEC, causes a regime shift on GB from cold-water copepod species to warm-water ones.

2.2. Objectives

The overall goal of the proposed study is to understand the underlying biological-physical mechanisms controlling the seasonal development of spatial patterns of dominant copepod species on GB. Our specific objectives are: 1) to examine how local-dynamics and external forcing control the abundance of these species on GB, 2) to determine the degree to which top-down versus bottom up processes control the dominant copepod species on GB, and 3) to use existing state-of-the-art 3D physical/biological numerical models together with existing high-quality 3D data sets from the GLOBEC GB field program (and other historical data sets), to conduct targeted numerical experiments that explore the likelihood of the hypotheses listed above.

2.3. Methods

2.3.1. The Integrated Model System