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A conceptual model of the trophodynamical response to river discharge in a large marine ecosystem
JOURNAL
OF
MARINE SYSTEMS ELSEVIER
Journal of Marine Systems 12 (1997) 187-198
A conceptual model of the trophodynamical response to river discharge in a large marine ecosystem Stig Skreslet Department
of Fisheries
*
and Science, Bad@ College,
N-8002
Bad@, Norway
Received 19 July 1995; revised 2 December 1995; accepted 18 April 1996
Abstract Year-class strength in North-East Arctic cod (Gadus morhua), which inhabit the Barents Sea, and commercial landings of juveniles from this population, have been positively correlated with Norwegian meltwater discharge one and three years in advance, respectively. A conceptual model is developed, by empirical data used to investigate how the freshwater signal may be transmitted with time and in space through the food-web. It assumes that interannual variation in discharged volume of meltwater during summer forces planktonic primary production in neritic fronts. The strength of this impulse is transmitted from one organismic system to another, along the north Norwegian shelf, being advected by Calanus finmarchicus, a herbivorous copepod. The population system of this copepod interacts with the survival and growth of juvenile NE Arctic cod, and causes the cod stock size to fluctuate with the strength of the signal. By migration and advection within their respective population systems, NE Arctic cod and C. jinmarchicus possibly transmit the freshwater signal on extensive time and space scales, from the Norwegian shelf to distant parts of the Arctic Mediterranean Ecocystem that contains both population systems. Continued empirical research and numerical modelling is needed to develop this theory. 0 1997 Elsevier Science B.V.
1. Introduction River outflow has repeatedly been suggestedto influence marine fish production, in Norwegian (Helland-Hansen and Nansen, 1909; Gran, 1923; Izhevskii, 1964; Skreslet, 1976, 1988) as well as in Canadian and US Atlantic coastal waters (Sutcliffe, 1972, 1973; Sutcliffe et al., 1977). The suggestions have created debate, partly becauseof the implied effects of regulated river flow (Neu, 1976, 1982a,b; Skreslet, 1981a), and provoked reviews intended to
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clarify the relationship (Skreslet et al., 1976; Skreslet, 1981b, 1986a;Bugden et al., 1982; Kaartvedt, 1984). Most questions on regulation still remain open, but the debate has established that river outflow has profound direct effects on physical, chemical and biological processesin the sea,both in coastal waters and on larger spacescales.There is, however, little knowledge on how biota may transfer effects of river discharge, indirectly by trophic interactions in foodwebs, to commercial fish (Drinkwater, 1986). This contribution summarizesrecent empirical information and presentsa conceptual ecological model of how the meltwater discharge from Norway in May/June seemsto influence production of marine fish, by its effects on the trophodynamical transfer of
0 1997 Elsevier Science B.V. All rights reserved.
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energy on the extensive time and space scales (cf. Skreslet, 1986b) of a large ecosystem.
2. Levels of ecological processes Ecological processes occur on three levels of biological organization (Odum, 1971): (i) organismic systems, (ii) population systems, and (iii) ecosystems. The first level involves individual organisms or communities of species that form groups smaller than the population, interacting with abiotic (physico-chemical) and biotic (intra- and interspecific) factors in their environment. An example is how climate interacts with the feeding of larval cod (Gadus morhua). In tranquil waters cod larvae probably use the energy reserves of their yolk to locate and invade patches of nauplii of the copepod Calanus finmarchicus, to improve their scope for survival at first feeding (Skreslet, 1989). Wind-generated turbulence tends to upset this relationship by dispersing the patches of prey, but the disadvantage may be compensated by micro-turbulence that increases the encounter rates between cod larvae and nauplii (Sundby and Fossum, 1990). Thus, organismic systems which involve cod larvae are probably highly variable on small spatial scales, causing some groups of cod larvae to grow and survive while others starve. The time scales are probably short, because the abiotic and biotic components are non-conservative. For instance, as they grow, each cod passes through a series of organismic systems that have different abiotic and biotic properties that vary with seasonal changes in climate, prey abundance, growth-related preferences by the cod itself, and more or less stochastic encounters with predators. Each of the systems provides both deterministic and random factors that influences the cod’s chances of death or continued life. A population system refers to the geographical distribution of organisms united by the exchange of genes by reproduction, and how the population interacts with abiotic and biotic factors. Reproduction, hatching, larval feeding, adolescent growth and adult feeding, may occur in quite distant and very different habitats. North-East (NE) Arctic cod (G. morhua) spawn along the coast of Norway, most notably in
Fig. 1. Polar projection of the Arctic Mediterrawm Ocean. S: Southern limit of Arctic fauna. N: Northern limit of Boreal fauna. The area between N and S: Sub-Arctic Transitional Zone. L: Lofoten Islands. M: More. Modified from Dunbar (1968).
the waters around the Lofoten Islands in north Norway (Fig. 1). In a couple of months, their offspring are advected into the southern Barents Sea which is the growth habitat of sexually immature cod, until an age of 6-7 years. The sexually mature cod occupy feeding grounds in the north-western Barents Sea (cf. Brander, 1994). The different habitats contain different prey and predators that interact with cod on the level of organisms, as each population may be quite differently distributed in geographical temls. For example, while C. finmarchicus is the main food resource for larval and O-group cod (Wib’org, 1948; Wiborg, 1960; Sysoeva and Degtereva, 1965) sexually immature Norwegian spring-spawning herring (&pea harengus) is a periodical food source for older juvenile cod. Both food sources belong to populations that are also distributed in the oceanic parts of the Nordic (Greenland, Icelandic: and Norwegian) Seas (Fig. l), where cod is not present (Blindheim and Skjoldal, 1993). In the northern Barents Sea, NE Arctic cod feed on adolescent capelin (Mallorus uillosus) that is a Sub-Arctic planktivorous species coupled to the Arctic food chains of the drift-ice margin (Sakshaug, 1992). However, the
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capelin larvae may fall prey to herring in the southem Barents Sea, where it spawns. Thus, the four species form populations that on large scales have different geographical distribution, but overlap on smaller scales forming organismic systems that provide specific habitats for each population, All respond to interannual changes in the position of the polar drift-ice. In years with inflow of warm water from the Norwegian Sea, larger parts of the Barents Sea are opened for feeding of NE Arctic cod, probably increasing the population’s scope for growth (Midttun and Loeng, 1987). The ecosystem is a term used quite indiscriminately in marine science, unfortunately often for systems with open boundaries that exchange mass and energy (e.g. Skreslet, 1986a). Mann and Lazier (1991) do not identify marine ecosystems in geographical terms, but do so over the range of scales in ecological processes. Recent attempts have been made to identify the large marine ecosystems of the world ocean (Sherman, 1993). The criteria for identification of these ecosystems seem to be based on the distribution of managed adult fish, rather than conservation of mass and control of energy flow in a self-contained unit (cf. Odum, 1971). A classical ecosystem concept that demarcates the geographical framework of the system’s food-web, may be useful for modelling purposes, by closing the model’s flow and dissipation of energy, from sunlight attenuated by primary producers and energy that forces physical processes, to loss of heat from macro- and micro-consumers. However, this is an idealized approach. Any marine ecosystem is rather open and may communicate with others by exchange of advected biomass, for instance by the large-scale latitudinal migration of organisms like whales and seabirds. Tchemia (1980) introduced the Arctic Mediterranean as a term for the marine geophysical system enclosed by the Eurasian and North American continents. It receives freshwater from both continents, and oceanic water is advected into the system by Atlantic water through the Faroe Channel. Outflow of Polar surface water by the East Greenland and Labrador Currents, and deepwater outflow from the Nordic Seas to the North Atlantic, balances its water budget. Biogeographically, the Arctic Mediterranean contains Arctic, Sub-Arctic and Boreal species (Ek-
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man, 1967). Some of them may be spatially separated (Fig. 1) by their adaptations to temperature or other factors. However, some Arctic and Boreal species may co-exist in the Sub-Arctic Transitional Zone (Ekman, 19671, together with endemic SubArctic species like capelin, and exchange biomass through the food-web. To obtain a concept that conserves most of the mass and energy crossing the biogeographical borders, the concept of the Arctic Mediterranean Ecosystem will be adopted here.
3. Freshwater
outflow
and cod recruitment
It is generally accepted that the year-class strength in NE Arctic cod is largely determined at the larval stage (Ellertsen et al., 19871, and is a function of the production of copepod nauplii (Van der Meeren, 1993). Taking the ratio between three-year old recruits and the parent stock size as an index for year-class strength, Skreslet (1976) found that the log-transform was positively correlated with the annual 30 day maximum river discharge from different regions along the Norwegian west coast. The correlation occurred when the year-class index lagged 1 year after the compared discharge, and the best correlation occurred with discharge data from the More region (Fig. 1). The relationship was significant for 13 years until 1962, when the correlation broke down. However, at the end of this period, the landings of sexually immature cod from the southern Barents Sea became positively correlated with freshwater outflow three years in advance (Skreslet, 1988). The reason why the freshwater signal expressed in the cod population shifted from one relationship to another, remains unexplained. The basic question rising from the correlations, is whether and how freshwater outflow may force biological systems to influence the abundance of cod. The freshwater outflow to the neritic province of north-west Norway is highly seasonal, due to the accumulation of snow in Norwegian mountains during winter. It melts in May and June and the freshwater is discharged into numerous fjords, causing a diffuse outflow of brackish plumes that merge in the Norwegian shelf waters (Svendsen, 1986). Geostrophic forcing causes the coastal water to flow northwards between the coastline and the 200 m
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depth contour of the shelf, as the Norwegian Coastal Current (NCC). Its seaward margin converges with saltier water being advected northwards from the North Atlantic Ocean by the Norwegian Atlantic Current (NAC). The NAC mainly flows in parallel with the shelf break, which is situated at about 400 m depth. At More and the Lofoten Islands where the Norwegian Shelf is narrow (Fig. 4), the NAC and the NCC converge to form a distinct front along me shelf break. Over the wide mid-Norwegian Shelf, the two currents diverge and the shelf topography tends to induce eddies with complicated thermo-haline frontal structures (Satre and Ljoen, 1971). C. finmarchicus is the staple diet of NE Arctic cod larvae, both during larval first feeding on nauplii at the Lofoten Islands in April/May (Ellertsen et al., 1987), and while the larger larvae are advected by the NCC along the north Norwegian shelf (Wiborg, 1948; Wiborg, 1960). The abundance of each new year-class is established by the autumn, as O-group cod in the SW Barents Sea (Anon, 1994). Thus, the production of C. jknarchicus at the Lofoten Islands would appear to play a key role in the production of NE Arctic cod. The occurrence of a strong O-group year-class of NE Arctic cod in the Barents Sea, is likely to be accompanied by a large biomass of C. finmarchicus (Wiborg, 1978), possibly being proportional to the production of nauplii at the Lofoten Islands. The imported biomass of zooplankton and O-group cod to the Barents Sea, probably also supports the growth in older adolescent year-classes that increase their biomass by planktivorous and cannibalistic (Bogstad et al., 1993) feeding.
4. Shelf production
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proportional to freshwater outflow during the summer, as it appears to be high when salinities are low at the surface, high in the intermediate layer above sill level, and low in the basin water (Fig. ,2). Despite the sparsity of the observations and the low significance of the relationships, they do suggest three different processes of advection into the wintering habitats in fjords. (i) An annual large-scale flux of shelf surface water into the fjords during September/October (cf. Haakstad, 1979) may explain the negative dependence of copepod abundance on surface salinity which expresses effects of freshwater outflow. (ii) Advection of shelf water to compensation currents of the estuarine circulation in fjords may explain the positive dependence of copepod abundance on the salinity in intermediate depths. The force of compensation currents is a function of freshwater outflow, probably causing the intermediate layer’s salinity to increase in years with large outflow. (iii> Deep overflows of Atlantic water across the shelf to fjord basins, probably occurs frequently during winter in north Norwegian fjords (cf. Skreslet and Loeng, 1977). Thus, the negative dependence of copepod abundance on basin water salinity indicates that both copepod production and dilution of NAC water may be proportional to the meltwater discharge during summer. The deep inflows of Atlantic water to the fjords causes the basin water temperatures to be: stable at
of C. finmarchicus
The C. finmarchicus females that produce the nauplii eaten by cod larvae at the Lofoten Islands in April and May, emerge from two wintering habitats prior to their spawning. The first habitat is the 300-900 m deep fjord basins situated between the Lofoten Islands and the Norwegian mainland (S@nme, 1934). The recruitment to this wintering stock starts in the autumn and early winter, by advection from shelf waters (Skreslet and Rod, 1986). The abundance of recruits in the autumn seems to be
Fig. 2. Abundance of Calanus finmarchicus (lo-’ per m* sea surface) plotted against average salinity 6, So) in three depth ranges in the Saltfjord near the Lofoten Islands. The c:opepod was collected in October for seven years, from 1983 to 1991, by five replicate vertical hauls from bottom to surface by a 180 micron 0.1 m2 Juday net. Observations are jittered to show overlapping observations. Regression lines are drawn by the method of least squares. The dependencies are significant ( p = 0.05) according to the Fisher exact test (Skreslet, unpublished).
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7°C. Thus, the basins are hospitable for a variety of carnivorous plankton (Skreslet, 1993) and fish species (Skreslet, 1994) known to prey on C. fznmarchicus. Early recruits to the copepod’s wintering stocks may therefore have little chance to survive to maturity as spawners in late winter. The fjord spawning stocks may therefore consist mainly of late recruits advetted by deep inflows across the shelf, in late winter. The second wintering habitat of C. j%mlarchicus is the cold basin water of the Nordic Seas where they stay at 600-loo0 m depth ((astvedt, 1955; Lie, 19681, mainly as copepodite stage V (CV>, the last stage before sexual maturation to CVI. The low temperatures at these depths may exclude a number of predators and reduce the consumption of the copepods’ energy stores (cf. Richter, 1995). Those that accumulate outside the slope of the Lofoten shelf possibly recruit to reproduction habitats on the shelf, in a manner comparable to the advection of C. jbnarchicus from wintering habitats at the Scottish slope to reproduction habitats in the northern North Sea (Backhaus et al., 1994). The fjord stocks of C. finmarchicus spawn locally in April (Somme, 1934; Wiborg, 1954; Skreslet and Rod, 1986), providing nauplii as prey for cod larvae at the Lofoten Islands (Ellertsen et al., 1987). This organismic system seems to be terminated when the new generation of C. jkmarchicus becomes advetted to the outer shelf in May/June, probably as a consequence of increased freshwater outflow (Skreslet and Rod, 1986). Much of the stock is probably advected rapidly into the SW Barents Sea by the NCC (cf. Bjorke and Sundby, 19881, but it may be speculated that some are retained as females along the shelf break, possibly spawning in concert with recruits from the deep wintering habitat outside the continental slope. Ruud (1929) found C. Jinmarchicus to spawn vigorously at the More shelf break (Fig. 4) in June/July. Skreslet and Rod (1986) found that this was also the case in early July at the shelf break outside the Lofoten Islands, as indicated by the presence of spawners (female CVI), the high abundance of nauplii (NI-VI) and the presence of young copepodids (CI-III). A larger abundance of CI-III, but fewer nauplii and very few females, were located farther away from the shelf. The authors inferred
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rapid advection of the offspring from the shelf edge spawning habitat, across the core of Atlantic water and subsequent accumulation in a marginally stratified frontal zone established by thermal differences in the Norwegian Sea. Thus, C. finmarchicus originating from the Norwegian shelf may use the epipelagic zone of the Nordic Seas as a growth habitat, and the lower mesopelagic zone as a wintering habitat, which may eventually provide recruits back to the neritic spawning habitats. The spawning habitats at the shelf break may be rather specific, occurring only at the narrow shelves outside More and Lofoten. Much less is known about the conditions for production of C. jinmarchicus in between these two areas, where the shelf is much wider (Fig. 4). In this region Audunson et al. (1981) found that the NCC formed a frontal zone close to the shore in March, apparently following the 200 m depth contour, far from the deeper shelf break. However, with decreasing salinity due to river discharge, the haline frontal zone moves farther off-shore, and the current becomes more shallow (Hjort and Gran, 1899; Helland-Hansen and Nansen, 1909:; Ljoen, 1962; Saetre and Ljoen, 1971; Rey, 1981a). In July/August the coastal water still maintains a haline horizontal gradient at the 200 m depth contour, but may extend as a marginally stratified layer over the shelf and even cross the core of Atlantic water that runs along the shelf break (Ljeen, 1962). Considering the large distance from the shelf break to the 200 m depth contour, a shelf break front may possibly be recognized as a thermal rather than a haline front. Troughs and banks on the shelf are likely to induce fronts as well, but little is known about these structures. They may be major reproduction and growth habitats for C. jbnarchicus, but so far no investigation have been carried out to establish knowledge on this issue. However, Skreslet (1995) found that in an exceptional year with delayed and small freshwater discharge, the spawning of C. finmarchicus outside the Lofoten Island was not associated with the shelf edge, but with mesoscale frontal structures on the inner shelf. This situation may be similar to conditions on the wide mid-Norwegian shelf, where the ratio between freshwater flow and the shelf width, is small, even in years with large discharge.
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C. finmarchicus is essentially a herbivorous batch spawner with low fecundity, depending on algal biomass to produce eggs (Marshall and Orr, 1962). Hay (1995) found an exceptionally high average fecundity of 76 eggs per female C. jinmarchicus and day to be caused by a considerable bloom of phytoplankton associated with freshwater influence of the Firths of Forth and Tay. Besides being a function of the primary production rate, the size of a new generation also depends on its duration. The reported on-shelf spawning at the Lofoten Islands (Skreslet, 1995) had not started by the end of May, but lasted for more than two months, until the end of August. Thus, the summer generation of C. jknarchicus produced in the Norwegian neritic province may become large in relation to the spring generation, even in years with little outflow.
5. Shelf primary
production
The phytoplankton production in the NCC starts to increase along its frontal zone in April, being larger there than in its median parts (&an, 1929; Braarud and Nygaard, 1980; Peinert, 1986). Rey ( 198 1b) has studied planktonic primary production across the NCC and found that the highest averages occurred in May, with decreasing production during summer. Sakshaug et al. (198 1) found that in June the Norwegian coastal current was nutrient depleted and in a post-bloom situation, but he observed phytoplankton to be in an initial stage in the Atlantic water outside the NCC. However, Skreslet (1995) reported from monthly continuous surface observations across the Lofoten shelf that the fluorescence increased from June to July over the shelf, associated with increased freshwater influence. Farther south on the Lofoten shelf, Peinert (1986) observed low phytoplankton biomass in August, attributed to a high abundance of grazing zooplankton. Berge (1958) and Paasche (1960) reported high primary production in the Nordic Seas during summer. There is little information on how the meltwater discharge which normally occurs from mid-May to late July, influences this oceanic system by its interaction with physical processes in the shelf break front. However, tides are supposed to generate internal waves that provide energy to turbulent diffusion
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of plant nutrients in shelf break fronts, resulting in primary production controlled by the M, ,tide (Mann and Lazier, 1991). In the NCC front, Mork (1981) found that lateral waves generated by variable wind stress, develop eddies governed by baroclinity and bottom topography. Experiments with rotating scale models of the NCC have demonstrated ho,w cyclonic eddies may generate in the frontal zone (McClimans, 1986). Pingree et al. (1979) observed that eddy formation in a marginally stratified front in British coastal waters generated phytoplankton production. Similar conditions are likely to develop along the Norwegian shelf as well. The diatom spring bloom in Norwegian fjords is normally terminated after a few weeks in April due to nutrient shortage, much like a batch culture. The estuarine circulation, which arises from meltwater discharge to Norwegian fjords in May, initiates a second bloom which functions more like a chemostat culture (Sakshaug, 19761, being fed a rnixture of nutrients from both the freshwater and the entrained seawater. However, high flushing rates of brackish water are likely to advect phytoplankton species into shelf waters (Sakshaug, 19761, leaving only those which are able to produce in the shear zone between brackish outflow and the compensation current (Erga and Heimdal, 1984). The retained system may possibly feed biomass into shelf waters, but ,so far the hypothesis is not supported by evidence.
6. Synthesis To suggest how freshwater outflow may influence organismic systems that support year-class strength and growth in NE Arctic cod, the available empirical information has been coupled in a conceptual model represented by two energy flow diagrams. One shows how energy flows across the shelf (Fig. 31, and the other (Fig. 4) the flow along it. The spring production of C. jkmarchicus nauplii that determines the year-class strength in NE Arctic cod, occurs in organismic systems found in fjords and shelf waters (Fig. 3). The reproduction rate of C. jinmarchicus is proportional to the spawning stock size (Tande and Slagstad, 1992) that emerge from wintering habitats at the Lofoten area, being a function of the stock abundance in the previous autumn,
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local predation during winter, and phytoplankton production rates during the spring bloom. Local freshwater outflow occurs later and seems to terminate the reproduction in fjords, by flushing the copepods into shelf waters (Skreslet and Rod, 1986). However, the outflow may be one of the forces that regulates the reproduction of C. jkmarchicus in an organismic system associated with the shelf break front. The production of nauplii in this system may not be important to year-class formation in NE Arctic cod, as it occurs after the hatching period of cod larvae. A fraction of the new generation of copepods enter growth habitats in the Nordic Seas, where they subsequently enter deep wintering habitats, or are advected back, on-shelf to wintering habitats in fjord basins, providing biomass to the next year-class of cod. The cross-shelf fluxes of energy presented in Fig. 3 is supposed to occur at More, as well as at the Lofoten Islands. The summer production of phytoplankton at More is supposed to be a function of river discharge, regulating the reproduction of C.
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finmarchicus. The copepod spawning stock is probably recruited by advection from sources elsewhere, like fjords to the south, the North Sea, or the Norwegian Sea. The summer generation of C. finmarchicus is probably advected away from More, both northwards to the mid-Norwegian shelf, and into the epipelagic zone of the Norwegian Sea. These copepods probably do not become sexually mature in their first summer, but seek wintering habitats as CV, in deep fjord basins and in the Nordic Seas basin (Figs. 3 and 4). At this stage, the abundance of C. finmarchicus appears to be a function of freshwater outflow at More (Fig. 2). After the winter diapause, C. Jinmarchicus CV recruit to the Lofoten spring spawning stock, producing nauplii falling prey to cod larvae (Fig. 4). The number of cod larvae is a function of the NE Arctic cod spawning stock biomass, and the abundance of their prey a function of the number of spawning female copepods. The life-span of this copepod generation, being close to one year, is supposed to
SUMMER
El NORWEGIAN
SEA
0 0
Consumer
>)
Interaction
Storage
1,
Heat
sink
Fig. 3. Cross-shelf flow of energy between organismic systems producing Calanus finmnrchicus, in fjord and shelf habitats during spring, and in the shelf break frontal zone in summer. For simplicity, heat sinks have been omitted from points of interaction. NI-NW: naupliar stages. CI-CV: adolescent copepodites. CVI: sexually mature copepodites. Symbols: adopted from Odum (1971).
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Barents
Sea
Fig. 4. Energy flow between organismic systems, from Mere to the Barents Sea over four subsequent years. The 400 m depth contour indicates the shelf break. O-group cod: NE Arctic cod fry. I-V-group cod: age groups of juvenile NE Arctic cod. Cod SSB: NE Arctic cod spawning stock biomass. Spring and summer producers refer to phytoplankton. Other legends as in Fig. 3.
explain the one-year lag in the correlation between freshwater outflow from More, and year-class strength in NE Arctic cod (Skreslet, 1976). The maturation of gonads in C. Jinmarchicus occurs during winter, depending on internal energy reserves, but the act of spawning seemsto require food (Tande and Hopkins, 1981). Thus, the phytoplankton spring bloom in the Lofoten region is likely to determine the total egg production of each individual copepod female, and thus, influence the growth rates of the advanced cod larvae and metamorphosedO-group fry. While juvenile cod are advected from the Lofoten Islands to the Barents Sea, they may fall prey to a
number of planktivorous species,ranging from carnivorous zooplankton to fish, whales and seabirds. Larvae of several fish species may play changing roles in relation to each other, being prey, competitors or predators, dependingon interspecific changes in advantagesgained from interannual differences in hatching period and growth rates, as they do in NW Atlantic waters (Paz and Larraneta, 1992a.,b).However, there is no information from Norwegian waters that such factors influence the survival of cod larvae. Copepodids surviving the advection into the southern Barents Sea possibly fall prey to I- and II-group cod that may also feed as cannibals on the O-group cod, both items being complementary to cod
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food produced in the Barents Sea, itself. Two years later, a number of II-, III-, IV- and V-group cod, being a function of year-class strength and group specific growth rates, recruits to the stock of exploited immature cod (Fig. 4). According to the present conceptual model (Figs. 3 and 41, a year-class of large II-group cod, having grown fast on large food rations in their first year, and older year-classes gaining enhanced growth in the same year, may recruit to the fisheries simultaneously, explaining the occurrence of substantial landings of sexually immature cod three years after large freshwater outflow from More (cf. Skreslet, 1988).
7. From hypothesis towards theory In essence, Helland-Hansen and Nansen (1909) and Izhevskii (1964) formulated the same null hypothesis: natural vernal discharge of freshwater from Norway forces physical processes that stimulate biological productivity in the food-web of the Arctic Mediterranean Ecosystem. One way to falsify the hypothesis would be to observe that interanuual variation in freshwater outflow was not correlated with growth parameters in marine populations. This was attempted by Skreslet (1976, Skreslet, 1988) who tested correlations between outflow from unregulated rivers and year-class strength and landings of NE Arctic cod, and fatness in minke whales (Balaenoptera acutorostrata) and Norwegian springspawning herring. Significant correlations were observed and did not allow for a rejection of the hypothesis. Similar tests made in north American waters (Sutcliffe, 1972, 1973; Sutcliffe et al., 1977) did not reject that freshwater outflow influences the production of fisheries resources. The next step, to test the hypothesis, would be to use the regression algorithms as predictive models, observing whether or not results calculated from observed freshwater outflow might reproduce observed population data. However, several of the rivers used in the correlation analyses now have regulated flow due to hydro-electric energy production. In fact, after 1970 the construction of large hydroelectric plants have influenced most large water courses in Norway, preventing further use of natural river flow as representative freshwater input to the sea. Further
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testing of the hypothesis will therefore require regional budgets of both natural and regulated run-off. At present, such budgets do not exist. So far, the hypothesis has been tested on the ecosystem level, by relating river flow to population parameters being distant in both time and space. Its falsification can also be attempted by breaking it down to testable lower-order hypotheses on the levels of populations and organisms. To gain foot-hold for this strategy, it is necessary to develop a theory that explains how freshwater outflow influences the transfer of energy through the food-web of the Arctic Mediterranean Ecosystem. The present conceptual model is intended to be a step in this direction. Several testable hypotheses directed towards the population level, may be formulated from theoretical deductions. One can be that interannual variation in the production of C. fintnurchicus covaries with salinity in Norwegian coastal waters. Such an analysis is underway, represented by the preliminary data in Fig. 2. Another hypothesis that ought to be tested, can be that year-class strength in NE Arctic cod is a function of the spawning stock biomass of C, jinmarchicus, prior to the spawning of cod. Still another to be tested, is that C. $nmarchicus is advected from the Norwegian shelf into the Norwegian Sea and back, constituting one single, large population. The effects of river flow on primary production in the NCC need scientific attention. Numerical. models of phytoplankton production generated in river plumes appear to be strong tools for the deduction and testing of hypotheses (see this volume). However, a proper understanding of the coupling between physics and biology may require studies on dimensions smaller than those resolved by the present generation of models. Therefore, classical field studies on phytoplankton production is still in demand, for testing of hypotheses and for conceptual modelling, as well as for validation and verification in numerical modelling. At present, numerical models used in ecological oceanography addresses organismic systems, mostly involving planktonic primary production and to some extent, herbivorous zooplankton. There is little connection between such models and those addressing nektonic population systems, and ecosystems. Hopefully, the present conceptual model may provide some ideas on how different models can be nested.
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8. Conclusion Trophic interaction between the herbivorous copepod C. jbnarchicus and carnivorous juveniles of NE Arctic cod (G. morhua) occurs in different organismic systems along the north Norwegian shelf, where the two population systems overlap. The relationship is subject to interannual variation, probably forced by freshwater outflow in the previous year, causing the year-class strength of cod to vary. Due to their large size, both population systems play major roles in the Arctic Mediterranean Ecosystem, and may be expected to transfer the freshwater signal to other parts of its food-web as well. A conceptual model has been established to explain the initial transfer between organismic systems along the coast of north Norway, and identify key processes in the ecosystem. The model may also provide ideas for nesting of numerical models, eventually resulting in a basin-scale model for the Arctic Mediterranean Ecosystem.
Acknowledgements I would like to express my gratitude towards colleagues who I have met in different scientific fora, to discuss matters of common interest. In particular, I acknowledge the free flow of unrestricted information shared with me at the annual meetings of the Association of Norwegian Oceanographers. Without the constructive feed-back and encouragement from fellow members, I would never have been able to persue the present problem over so many years. Several of my research efforts in this direction have received financial support from the Norwegian Research Council.
References Anon, 1994. Report of the Arctic fisheries working group. ICES CM 1994/Assess:2, 240 pp. Audunson, T., Dalen, V., Krogstad, H., Lie, H.N., Steinbakke, O., 1981. Some observations of ocean fronts, waves and currents in the surface along the Norwegian coast from satellite images and drifting buoys. In: Se&e, R., Mork, M. (Eds.), The Notwegian coastal current. University of Bergen, pp. 20-56. Backhaus, J.O., Harms, I.H., Krause, M., Heath, M., 1994. A
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hypothesis concerning the space-time succession of Calanus finmarchicus in the northern North Sea. ICES J. Mar. Sci. 51, 169-180. Berge, Cl., 1958. The primary production in the Norwegian Sea in June 1954, measured by an adapted 14C technique. Rapp. P.-v. Reun. Cons. Perm. Int. Explor. Mer 144, 85-91. Bjerke, H., Sundby, S., 1988. Production and abundance indices of postlarval and O-group cod. In: Loeng, H. (Ed.), The Effect of Oceanographic Conditions on Distribution and Population Dynamics of Commercial Fish Stocks in the Barents Sea. hoc. 3rd Soviet-Norw. Symp., Murmansk 1986, pp. 127-144. Blindheim, J., Skjoldal, H.R., 1993. Effects of climatic changes on the biomass yield of the Barents Sea, Norwegian Sea, and West Greenland Large Marine Ecosystems. In: Sherman, K., Alexander, L.M., Gold, B.D. (Eds.), Large Marine Ecosystems -Stress, Mitigation, and Sustainability. AAAS Press, Washington, pp. 185-198. Bogstad, B., Lilly, G.R., Mehl, S., Palsson, 6.K.. Stefansson, G., 1993. Cannibalism and yearclass strength in Atlantic cod (Gadus morhua L.1 in the Arcto-boreal ecosystems (Barents Sea, Iceland, and eastern Newfoundland). ICES Mar. Sci. Symp. 198, 576-599. Braarud, T.. Nygaard, I., 1980. Phytoplankton observations in offshore Norwegian coastal waters between 62% and 69”N, 2. Diatom societies from More to Vestersen, March/April, Sarsia 65, 93-l 14. Brander, K. (Ed.), 1994. Spawning and life history information for North Atlantic cod stocks. ICES Coop. Res. Rep. 205, l-149. Bugden, G.L., Hargrave, B.T., Sinclair, M.M., Tang, C.C., Therriault, J.C., Yeats, P.A., 1982. Freshwater runoff effects in the marine environment: the Gulf of St. Lawrence example. Can. Tech. Rep. Fish. Aquat. Sci. 1078, l-89. Drinkwater, K., 1986. On the role of freshwater outflow on coastal marine ecosystems-a workshop summary. NATO ASI Ser. G 7, 429-438. Dunbar, M.J., 1968. Ecological Development in Polar Regions-a Study in Evolution. Prentice-Hall, Englewood Cliffs, NJ, 119 PP. Ekman, S., 1967. Zoogeography of the Seas. Sidgwick and Jackson, London, 417 pp. Ellertsen, B., Fossum, P., Solemdal, P., Sundby, S., Tilseth, S., 1987. The effect of biological and physical factors on the survival of Arcto-Norwegian cod and the influence on recruitment variability. In: Loeng, H. (Ed.), The Effect of Oceanographic Conditions on Distribution and Population Dynamics of Commercial Fish Stocks in the Barents Sea. Proc. 3rd Soviet-Norw. Symp., Murmansk 1986, pp. 101-1126. Erga, S.R., Heimdal, B.R., 1984. Ecological studies on the phytoplankton of Korsfjorden, western Norway: the dynamics of a spring bloom seen in relation to hydrographical conditions and light regime. J. Plankton Res. 6, 67-90. Gran, H.H., 1923. Snesmeltingen som hovedaarsak til den rike produktion i vort kysthav om vaaren. Samtiden 34, 606-613 (in Norwegian). Gran, H.H., 1929. Investigations of the production of plankton outside the Romsdalsfjord 1926/1927. Rapp. P.-v. Reun. Cons. Perm. Int. Explor. Mer 56 (5). l-50.
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Haakstad, M.. 1979. The reexchange of water between the Norwegian coastal current and the fjords in the autumn. Bode College, Mat/Nat fagseksjon 1979 (4), l-40. Hay, S.. 1995. Egg production and secondary production of common North Sea copepods: field estimates with regional and seasonal comparisons. ICES J. Mar. Sci. 52, 315-327. Kaartvedt, S., 1984. Effects on fjords of modified freshwater discharge due to hydroelectric production. Fisken Havet 1984 (31, l-104. Helland-Hansen, B., Nansen, F., 1909. The Norwegian Sea. Fisk. Dir. Skr., Ser Havunders. 2 (2). l-390. Hjort, J., Gran, H.H., 1899. Currents and pelagic life in the northern ocean. Bergen Mus. Skr. 6, 1-24. Izhevskii, G.K., 1964. Forecasting of Oceanological Conditions and the Reproduction of Commercial Fish. (Translated from Russian by the Israel Programme for Scientific Translations, Jerusalem, 1966). 95 pp. Lie, U., 1968. Variations in the quantity of zooplankton and the propagation of Calanus finmarchicus at station “M” in the Norwegian Sea 1959-1966. Fisk. Dir. Skr. Ser. Havunders. 14, 121-128. Ljoen, R., 1962. The waters of the western and northern coasts of Norway in July/August 1957. Fisk. Dir. Skr. Ser. Havunders. 13 (2), l-39. Mann, K.H., Lazier, J.R.N., 1991. Dynamics of Marine Ecosystems. Blackwell, Boston, 466 pp. Marshall, S.M., Orr, A.P., 1962. Food and feeding in copepods. Rapp. Cons. Explor. Mer 153, 92-98. McClimans, T.A., 1986. Laboratory modeling of dynamic processes in fjords and shelf waters. NATO ASI Ser. G 7. 67-84. Mork, M., 1981. Circulation phenomena and frontal dynamics of the Norwegian Coastal Current. Philos. Trans. R. Sot. London 302, 635-647. Midttun, L., Loeng, H., 1987. Climatic variations in the Barents Sea. In: Loeng, H. (Ed.), The Effect of Oceanographic Conditions on Distribution and Population Dynamics of Commercial Fish Stocks in the Barents Sea. Proc. 3rd Soviet-Norw. Symp., Murmansk 1986, pp. 13-27. Neu, H.J.A., 1976. Runoff regulation for hydropower and its effects on the ocean environment. Hydrol. Sci. Bull. 21, 433-444. Neu, H.J.A., 1982a. Manmade storage of water resources-a liability to the ocean environment? I. Pollut. Bull. 13, 7-12. Neu, H.J.A., 1982b. Manmade storage of water resources-a liability to the ocean environment? II. Pollut. Bull. 13, 44-47. Odum, E.P., 1971. Fundamentals of Ecology. Saunders, Philadelphia, PA, 574 pp. Paasche, E., 1960. Phytoplankton distribution in the Norwegian Sea in June, related to hydrography and compared with primary production data. Fisk. Dir. Skr. Ser. Havunders. 12 (11). l-77. Paz, J., Larraiieta, M.G., 1992a. Cod year-class variations and abundance of other commercial fish in NAFO divisions 3N0. J. Northw. Atl. Fish. Sci. 14, 129-134. Paz, J., Larraiieta, M.G., 1992b. Year-class variations of American plaice (Hippoglossoides platessoides) and yellowtail
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flounder (Limanda ferruginea) in the Grand Ba& and the abundance of other commercial fish. Fish. Res. 15, 121-133. Peinert, R., 1986. Production, grazing and sedimentation in the Norwegian coastal current. NATO AS1 Ser. G 7, 361-374. Pingree, R.D., Holligan, P.M., Mardell, G.T., 1979. Pbytoplankton growth and cyclonic eddies. Nature 278, 245-247. Rey, F., 1981a. Primary production estimates in the Norwegian coastal current between 62”N and 72”N. In: Saetre, R., Mork, M. (Eds.), The Norwegian Coastal Current. University of Bergen, pp. 640-648. Rey, F., 1981b. The development of the spring phytoplankton outburst at selected sites off the Norwegian coast, In: Sretre, R., Mork, M. (Eds.), The Norwegian Coastal Current. University of Bergen, pp. 649-680. Richter, C., 1995. Seasonal changes in the vertical distribution of mesozooplankton in the Greenland Sea Gyre (75%) distribution strategies of calanoid copepods. ICES J. Mar. Sci. 52, 533-539. Ruud, J.T., 1929. On the biology of copepods off More 19251927. Rapp. P.-v. Reun. Cons. Penn. Int. Explor. Mer 56 (8). l-84. Sakshaug, E., 1976. Dynamics of phytoplankton blooms in Norwegian fjords and coastal waters, In: Skreslet, S., Leinebo, R., Matthews, J.B.L., Sakshaug, E. (Eds.), Fresh Water on the Sea. Association of Norwegian Oceanographers, Oslo, pp. 139-143. Sakshaug, E., (Ed.), 1992. Okosystem Barentshavet. Universitetsforlaget, Oslo, 304 pp. (in Norwegian). Sakshaug, E., Myklestad, S., Andersen, K., Hegseth, E.N., Jorgensen, L., 1981. Phytoplankton off the More coast in 1’375-1979: distribution, species composition, chemical composition and conditions for growth. In: Smtre, R., Mork, M. (Eds.), The Norwegian Coastal Current. University of Bergen, pp. 681-711. Sherman, K., 1993. Large marine ecosystems as global units for marine resources management-an ecological perspective. In: Sherman, K., Alexander, L.M., Gold, B.D., (Eds.), Large Marine Ecosystems-Stress, Mitigation and Sustainability. AAAS Press, Washington, pp. 3-14. Skreslet, S., 1976. Influence of freshwater outflow from Norway on recruitment to the stock of Arcto-Norwegian cod (G&us morhua), In: Skreslet, S., Leineb@, R., Matthews, J.B.L., Sakshaug, E. (Eds.), Fresh water on the Sea. Ass. Norw. Oceanogr. Oslo, pp. 233-237. Skreslet, S., 1981a. Importance of natural freshwater outflow to the coastal marine ecosystem of Norway and possible effects of large scale hydroelectric power production on year-class strength in fish stocks. Rapp. P.-v. Rtun. Cons. It. Explor. Mer 178, 79-80. Skreslet, S., 1981b. Informations and opinions on how freshwater outflow to the Norwegian coastal current influences biological production and recruitment to fish stocks in adjacent seas. In: Setre, R., Mork, M. (Eds.), The Norwegian Coastal Current. University of Bergen, pp. 712-748. Skreslet, S., 1986a (Ed.), The Role of Freshwater Outflow in Coastal Marine Ecosystems. Springer, Berlin, 453 pp. Skreslet, S., 1986b. Freshwater outflow in relation to space and
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time dimensions of complex ecological interactions in coastal waters. NATO ASI Series G 7, 3-12. Sk&et, S., 1988. Fat indices and stock features of marine carnivores in relation to freshwater inflow to Norwegian coastal waters. In: Wyatt, T., Larraneta, M.G. (Eds.), Long Term Changes in Marine Fish Populations, Int. Symp. Long Term Changes Mar. Fish Pop, Vigo 1986. pp. 69-82. Skreslet, S., 1989. Spatial match and mismatch between larave of cod (Gadus morhua L.) and their principal prey, nauplii of Calanus finmarchicus. Rapp. P.-v. Reun. Cons. Int. Explor Mer 191, 258-263. Skreslet, S., 1993. Zooplankton carry climate signals to cod year-classes. ICES 1993/CCC Symposium/No. 33. 21 pp. (Mimeol. Skreslet. S., 1994. Arsvariasjoner i fiskefaunaen pl reketrafelt i Salten. Bode College Report 1994:5, 38 pp. (in Norwegian). Skreslet, S., 1995. Spawning of Calanusjinmarchicus related to meso-scale environmental structures in a Norwegian shelf break front. ICES CM 1995/Q:8, 9 pp. (Mimeo). Skreslet, S., Matthews, J.B.L., Leinebo, R., Sakshaug, E., 1976 (Eds.), Fresh water on the Sea. Ass. Norw. Oceanogr., Oslo, 246 pp. Skreslet, S., Loeng, H., 1977. Deep water renewal and associated processes in Skjomen. a fjord in north Norway. Est. Coast. Mar. Sci. 5, 383-398. Skreslet, S., Rod, N.A., 1986. Advection of C. jinmarchicus between habitats in Norwegian coastal waters. NATO AS1 Ser. G 7, 375-387. Sundby, S., Fossum, P., 1990. Feeding conditions of ArctoNorwegian cod larvae compared with the Rothschild-Osbom theory on small-scale turbulence and plankton contact rates. J. Plankton Res. 12, 1153-1162. Sutcliffe, W.H. Jr., 1972, Some relations of land drainage, nutrients, particulate matter, and fish catch in two eastern Canadian bays. J. Fish. Res. Bd. Can. 29, 357-362. Sutcliffe, W.H. Jr., 1973. Correlations between seasonal river discharge and local landings of American lobster (Homarus americanus) and Atlantic halibut (Hippoglossus hippoglossus) in the Gulf of St. Lawrence. J. Fish. Res. Bd. Can. 30, 856-859. Sutcliffe, W.H. Jr., Drinkwater, K.F., Muir, B.S., 1977. Correlations of fish catch and envoronmental factors in the Gulf of Maine. J. Fish. Res. Bd. Can. 34, 19-30. Svendsen, H.. 1986. Mixing and exchange processes in estuaries, fjords and shelf waters. NATO AS1 Ser. G 7, 14-45.
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Sysoeva, T.K., Degtereva, A.A., 1965. The relation between the feeding of cod larvae and pelagic fry and the distribution and abundance of their principal food organisms. ICNAF Spec. Publ. 6, 411-416. Saetre, R., Ljoen, R.. 1971. The Norwegian coastal current. In: Wetteland, S.S., Bruun, P. (Eds.1, Proc. POAC conference. Technical University of Trondheim, pp. 514-535. Somme, J.D., 1934. Animal plankton of the Norwegian coast waters and the open sea, I. Production of Calanusfinmurchicus Gunner and Calanus hyperboreus (Kroyer) in the Lofoten area. Fisk. Dir. Skr. Ser. Havunders. 4, l-163. Tande, K.S., Hopkins, C.C.E., 1981. Ecological investigations of the zooplankton community in Balsfjorden, northern Norway: the genital change in copepodite stage IV and V in Cakrnus jinmarchicus. Mar. Biol. 63, 159-164. Tande, K.S., Slagstad, D., 1992. Regional and interannual variations in biomass and productivity of the marine copepod, Calanus finmurchicus, in subarctic environments. Oceanolog. Acta 15, 309-321. Tchemia, P., 1980. Descriptive Regional Oceanography. Pergamon, Oxford, 253 pp. Van der Meeren, T., 1993. How does cod (Gadus morhua) cope with variability in feeding conditions during early larval stages? Mar. Biol. 116, 637-647. Wiborg, K.F., 1948. Investigations on cod larvae in the coastal waters of northern Norway. Fisk. Dir. Skr. Ser. Havunders. 9 (3), l-27. Wiborg, K.F., 1954. Investigations in coastal and offshore waters of western and north-western Norway. Fisk. Dlir. Skr. Ser. Havunders. 11 (I), l-246. Wiborg, K.F., 1960. Investigations on pelagic fry of cod and haddock in coastal and offshore areas of northern Norway in July/August 1957. Fisk. Dir. Skr. Ser. Havunders. 12 (8). 1-18. Wiborg, K.F., 1978. Variations in zooplankton volumes at the permanent oceanographic stations along the Norwegian coast and at weathership station M(ike) in the Norwegian Sea during the years 1949-1972. Fisk. Dir. Skr. Ser. Havunders. 16, 465-487. Ostvedt, O.J., 1955. Zooplankton investigations from the weather ship M in the Norwegian Sea, 1948/49. Hvalr&d. Skr. 40, l-93.
OF
MARINE SYSTEMS ELSEVIER
Journal of Marine Systems 12 (1997) 187-198
A conceptual model of the trophodynamical response to river discharge in a large marine ecosystem Stig Skreslet Department
of Fisheries
*
and Science, Bad@ College,
N-8002
Bad@, Norway
Received 19 July 1995; revised 2 December 1995; accepted 18 April 1996
Abstract Year-class strength in North-East Arctic cod (Gadus morhua), which inhabit the Barents Sea, and commercial landings of juveniles from this population, have been positively correlated with Norwegian meltwater discharge one and three years in advance, respectively. A conceptual model is developed, by empirical data used to investigate how the freshwater signal may be transmitted with time and in space through the food-web. It assumes that interannual variation in discharged volume of meltwater during summer forces planktonic primary production in neritic fronts. The strength of this impulse is transmitted from one organismic system to another, along the north Norwegian shelf, being advected by Calanus finmarchicus, a herbivorous copepod. The population system of this copepod interacts with the survival and growth of juvenile NE Arctic cod, and causes the cod stock size to fluctuate with the strength of the signal. By migration and advection within their respective population systems, NE Arctic cod and C. jinmarchicus possibly transmit the freshwater signal on extensive time and space scales, from the Norwegian shelf to distant parts of the Arctic Mediterranean Ecocystem that contains both population systems. Continued empirical research and numerical modelling is needed to develop this theory. 0 1997 Elsevier Science B.V.
1. Introduction River outflow has repeatedly been suggestedto influence marine fish production, in Norwegian (Helland-Hansen and Nansen, 1909; Gran, 1923; Izhevskii, 1964; Skreslet, 1976, 1988) as well as in Canadian and US Atlantic coastal waters (Sutcliffe, 1972, 1973; Sutcliffe et al., 1977). The suggestions have created debate, partly becauseof the implied effects of regulated river flow (Neu, 1976, 1982a,b; Skreslet, 1981a), and provoked reviews intended to
* Corresponding author. Fax: +47 [email protected] 0924-7963/97/$17.00 PII SO924-7963(96)00097-8
75 51 7496; e-mail:
clarify the relationship (Skreslet et al., 1976; Skreslet, 1981b, 1986a;Bugden et al., 1982; Kaartvedt, 1984). Most questions on regulation still remain open, but the debate has established that river outflow has profound direct effects on physical, chemical and biological processesin the sea,both in coastal waters and on larger spacescales.There is, however, little knowledge on how biota may transfer effects of river discharge, indirectly by trophic interactions in foodwebs, to commercial fish (Drinkwater, 1986). This contribution summarizesrecent empirical information and presentsa conceptual ecological model of how the meltwater discharge from Norway in May/June seemsto influence production of marine fish, by its effects on the trophodynamical transfer of
0 1997 Elsevier Science B.V. All rights reserved.
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energy on the extensive time and space scales (cf. Skreslet, 1986b) of a large ecosystem.
2. Levels of ecological processes Ecological processes occur on three levels of biological organization (Odum, 1971): (i) organismic systems, (ii) population systems, and (iii) ecosystems. The first level involves individual organisms or communities of species that form groups smaller than the population, interacting with abiotic (physico-chemical) and biotic (intra- and interspecific) factors in their environment. An example is how climate interacts with the feeding of larval cod (Gadus morhua). In tranquil waters cod larvae probably use the energy reserves of their yolk to locate and invade patches of nauplii of the copepod Calanus finmarchicus, to improve their scope for survival at first feeding (Skreslet, 1989). Wind-generated turbulence tends to upset this relationship by dispersing the patches of prey, but the disadvantage may be compensated by micro-turbulence that increases the encounter rates between cod larvae and nauplii (Sundby and Fossum, 1990). Thus, organismic systems which involve cod larvae are probably highly variable on small spatial scales, causing some groups of cod larvae to grow and survive while others starve. The time scales are probably short, because the abiotic and biotic components are non-conservative. For instance, as they grow, each cod passes through a series of organismic systems that have different abiotic and biotic properties that vary with seasonal changes in climate, prey abundance, growth-related preferences by the cod itself, and more or less stochastic encounters with predators. Each of the systems provides both deterministic and random factors that influences the cod’s chances of death or continued life. A population system refers to the geographical distribution of organisms united by the exchange of genes by reproduction, and how the population interacts with abiotic and biotic factors. Reproduction, hatching, larval feeding, adolescent growth and adult feeding, may occur in quite distant and very different habitats. North-East (NE) Arctic cod (G. morhua) spawn along the coast of Norway, most notably in
Fig. 1. Polar projection of the Arctic Mediterrawm Ocean. S: Southern limit of Arctic fauna. N: Northern limit of Boreal fauna. The area between N and S: Sub-Arctic Transitional Zone. L: Lofoten Islands. M: More. Modified from Dunbar (1968).
the waters around the Lofoten Islands in north Norway (Fig. 1). In a couple of months, their offspring are advected into the southern Barents Sea which is the growth habitat of sexually immature cod, until an age of 6-7 years. The sexually mature cod occupy feeding grounds in the north-western Barents Sea (cf. Brander, 1994). The different habitats contain different prey and predators that interact with cod on the level of organisms, as each population may be quite differently distributed in geographical temls. For example, while C. finmarchicus is the main food resource for larval and O-group cod (Wib’org, 1948; Wiborg, 1960; Sysoeva and Degtereva, 1965) sexually immature Norwegian spring-spawning herring (&pea harengus) is a periodical food source for older juvenile cod. Both food sources belong to populations that are also distributed in the oceanic parts of the Nordic (Greenland, Icelandic: and Norwegian) Seas (Fig. l), where cod is not present (Blindheim and Skjoldal, 1993). In the northern Barents Sea, NE Arctic cod feed on adolescent capelin (Mallorus uillosus) that is a Sub-Arctic planktivorous species coupled to the Arctic food chains of the drift-ice margin (Sakshaug, 1992). However, the
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capelin larvae may fall prey to herring in the southem Barents Sea, where it spawns. Thus, the four species form populations that on large scales have different geographical distribution, but overlap on smaller scales forming organismic systems that provide specific habitats for each population, All respond to interannual changes in the position of the polar drift-ice. In years with inflow of warm water from the Norwegian Sea, larger parts of the Barents Sea are opened for feeding of NE Arctic cod, probably increasing the population’s scope for growth (Midttun and Loeng, 1987). The ecosystem is a term used quite indiscriminately in marine science, unfortunately often for systems with open boundaries that exchange mass and energy (e.g. Skreslet, 1986a). Mann and Lazier (1991) do not identify marine ecosystems in geographical terms, but do so over the range of scales in ecological processes. Recent attempts have been made to identify the large marine ecosystems of the world ocean (Sherman, 1993). The criteria for identification of these ecosystems seem to be based on the distribution of managed adult fish, rather than conservation of mass and control of energy flow in a self-contained unit (cf. Odum, 1971). A classical ecosystem concept that demarcates the geographical framework of the system’s food-web, may be useful for modelling purposes, by closing the model’s flow and dissipation of energy, from sunlight attenuated by primary producers and energy that forces physical processes, to loss of heat from macro- and micro-consumers. However, this is an idealized approach. Any marine ecosystem is rather open and may communicate with others by exchange of advected biomass, for instance by the large-scale latitudinal migration of organisms like whales and seabirds. Tchemia (1980) introduced the Arctic Mediterranean as a term for the marine geophysical system enclosed by the Eurasian and North American continents. It receives freshwater from both continents, and oceanic water is advected into the system by Atlantic water through the Faroe Channel. Outflow of Polar surface water by the East Greenland and Labrador Currents, and deepwater outflow from the Nordic Seas to the North Atlantic, balances its water budget. Biogeographically, the Arctic Mediterranean contains Arctic, Sub-Arctic and Boreal species (Ek-
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man, 1967). Some of them may be spatially separated (Fig. 1) by their adaptations to temperature or other factors. However, some Arctic and Boreal species may co-exist in the Sub-Arctic Transitional Zone (Ekman, 19671, together with endemic SubArctic species like capelin, and exchange biomass through the food-web. To obtain a concept that conserves most of the mass and energy crossing the biogeographical borders, the concept of the Arctic Mediterranean Ecosystem will be adopted here.
3. Freshwater
outflow
and cod recruitment
It is generally accepted that the year-class strength in NE Arctic cod is largely determined at the larval stage (Ellertsen et al., 19871, and is a function of the production of copepod nauplii (Van der Meeren, 1993). Taking the ratio between three-year old recruits and the parent stock size as an index for year-class strength, Skreslet (1976) found that the log-transform was positively correlated with the annual 30 day maximum river discharge from different regions along the Norwegian west coast. The correlation occurred when the year-class index lagged 1 year after the compared discharge, and the best correlation occurred with discharge data from the More region (Fig. 1). The relationship was significant for 13 years until 1962, when the correlation broke down. However, at the end of this period, the landings of sexually immature cod from the southern Barents Sea became positively correlated with freshwater outflow three years in advance (Skreslet, 1988). The reason why the freshwater signal expressed in the cod population shifted from one relationship to another, remains unexplained. The basic question rising from the correlations, is whether and how freshwater outflow may force biological systems to influence the abundance of cod. The freshwater outflow to the neritic province of north-west Norway is highly seasonal, due to the accumulation of snow in Norwegian mountains during winter. It melts in May and June and the freshwater is discharged into numerous fjords, causing a diffuse outflow of brackish plumes that merge in the Norwegian shelf waters (Svendsen, 1986). Geostrophic forcing causes the coastal water to flow northwards between the coastline and the 200 m
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depth contour of the shelf, as the Norwegian Coastal Current (NCC). Its seaward margin converges with saltier water being advected northwards from the North Atlantic Ocean by the Norwegian Atlantic Current (NAC). The NAC mainly flows in parallel with the shelf break, which is situated at about 400 m depth. At More and the Lofoten Islands where the Norwegian Shelf is narrow (Fig. 4), the NAC and the NCC converge to form a distinct front along me shelf break. Over the wide mid-Norwegian Shelf, the two currents diverge and the shelf topography tends to induce eddies with complicated thermo-haline frontal structures (Satre and Ljoen, 1971). C. finmarchicus is the staple diet of NE Arctic cod larvae, both during larval first feeding on nauplii at the Lofoten Islands in April/May (Ellertsen et al., 1987), and while the larger larvae are advected by the NCC along the north Norwegian shelf (Wiborg, 1948; Wiborg, 1960). The abundance of each new year-class is established by the autumn, as O-group cod in the SW Barents Sea (Anon, 1994). Thus, the production of C. jknarchicus at the Lofoten Islands would appear to play a key role in the production of NE Arctic cod. The occurrence of a strong O-group year-class of NE Arctic cod in the Barents Sea, is likely to be accompanied by a large biomass of C. finmarchicus (Wiborg, 1978), possibly being proportional to the production of nauplii at the Lofoten Islands. The imported biomass of zooplankton and O-group cod to the Barents Sea, probably also supports the growth in older adolescent year-classes that increase their biomass by planktivorous and cannibalistic (Bogstad et al., 1993) feeding.
4. Shelf production
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proportional to freshwater outflow during the summer, as it appears to be high when salinities are low at the surface, high in the intermediate layer above sill level, and low in the basin water (Fig. ,2). Despite the sparsity of the observations and the low significance of the relationships, they do suggest three different processes of advection into the wintering habitats in fjords. (i) An annual large-scale flux of shelf surface water into the fjords during September/October (cf. Haakstad, 1979) may explain the negative dependence of copepod abundance on surface salinity which expresses effects of freshwater outflow. (ii) Advection of shelf water to compensation currents of the estuarine circulation in fjords may explain the positive dependence of copepod abundance on the salinity in intermediate depths. The force of compensation currents is a function of freshwater outflow, probably causing the intermediate layer’s salinity to increase in years with large outflow. (iii> Deep overflows of Atlantic water across the shelf to fjord basins, probably occurs frequently during winter in north Norwegian fjords (cf. Skreslet and Loeng, 1977). Thus, the negative dependence of copepod abundance on basin water salinity indicates that both copepod production and dilution of NAC water may be proportional to the meltwater discharge during summer. The deep inflows of Atlantic water to the fjords causes the basin water temperatures to be: stable at
of C. finmarchicus
The C. finmarchicus females that produce the nauplii eaten by cod larvae at the Lofoten Islands in April and May, emerge from two wintering habitats prior to their spawning. The first habitat is the 300-900 m deep fjord basins situated between the Lofoten Islands and the Norwegian mainland (S@nme, 1934). The recruitment to this wintering stock starts in the autumn and early winter, by advection from shelf waters (Skreslet and Rod, 1986). The abundance of recruits in the autumn seems to be
Fig. 2. Abundance of Calanus finmarchicus (lo-’ per m* sea surface) plotted against average salinity 6, So) in three depth ranges in the Saltfjord near the Lofoten Islands. The c:opepod was collected in October for seven years, from 1983 to 1991, by five replicate vertical hauls from bottom to surface by a 180 micron 0.1 m2 Juday net. Observations are jittered to show overlapping observations. Regression lines are drawn by the method of least squares. The dependencies are significant ( p = 0.05) according to the Fisher exact test (Skreslet, unpublished).
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7°C. Thus, the basins are hospitable for a variety of carnivorous plankton (Skreslet, 1993) and fish species (Skreslet, 1994) known to prey on C. fznmarchicus. Early recruits to the copepod’s wintering stocks may therefore have little chance to survive to maturity as spawners in late winter. The fjord spawning stocks may therefore consist mainly of late recruits advetted by deep inflows across the shelf, in late winter. The second wintering habitat of C. j%mlarchicus is the cold basin water of the Nordic Seas where they stay at 600-loo0 m depth ((astvedt, 1955; Lie, 19681, mainly as copepodite stage V (CV>, the last stage before sexual maturation to CVI. The low temperatures at these depths may exclude a number of predators and reduce the consumption of the copepods’ energy stores (cf. Richter, 1995). Those that accumulate outside the slope of the Lofoten shelf possibly recruit to reproduction habitats on the shelf, in a manner comparable to the advection of C. jbnarchicus from wintering habitats at the Scottish slope to reproduction habitats in the northern North Sea (Backhaus et al., 1994). The fjord stocks of C. finmarchicus spawn locally in April (Somme, 1934; Wiborg, 1954; Skreslet and Rod, 1986), providing nauplii as prey for cod larvae at the Lofoten Islands (Ellertsen et al., 1987). This organismic system seems to be terminated when the new generation of C. jkmarchicus becomes advetted to the outer shelf in May/June, probably as a consequence of increased freshwater outflow (Skreslet and Rod, 1986). Much of the stock is probably advected rapidly into the SW Barents Sea by the NCC (cf. Bjorke and Sundby, 19881, but it may be speculated that some are retained as females along the shelf break, possibly spawning in concert with recruits from the deep wintering habitat outside the continental slope. Ruud (1929) found C. Jinmarchicus to spawn vigorously at the More shelf break (Fig. 4) in June/July. Skreslet and Rod (1986) found that this was also the case in early July at the shelf break outside the Lofoten Islands, as indicated by the presence of spawners (female CVI), the high abundance of nauplii (NI-VI) and the presence of young copepodids (CI-III). A larger abundance of CI-III, but fewer nauplii and very few females, were located farther away from the shelf. The authors inferred
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rapid advection of the offspring from the shelf edge spawning habitat, across the core of Atlantic water and subsequent accumulation in a marginally stratified frontal zone established by thermal differences in the Norwegian Sea. Thus, C. finmarchicus originating from the Norwegian shelf may use the epipelagic zone of the Nordic Seas as a growth habitat, and the lower mesopelagic zone as a wintering habitat, which may eventually provide recruits back to the neritic spawning habitats. The spawning habitats at the shelf break may be rather specific, occurring only at the narrow shelves outside More and Lofoten. Much less is known about the conditions for production of C. jinmarchicus in between these two areas, where the shelf is much wider (Fig. 4). In this region Audunson et al. (1981) found that the NCC formed a frontal zone close to the shore in March, apparently following the 200 m depth contour, far from the deeper shelf break. However, with decreasing salinity due to river discharge, the haline frontal zone moves farther off-shore, and the current becomes more shallow (Hjort and Gran, 1899; Helland-Hansen and Nansen, 1909:; Ljoen, 1962; Saetre and Ljoen, 1971; Rey, 1981a). In July/August the coastal water still maintains a haline horizontal gradient at the 200 m depth contour, but may extend as a marginally stratified layer over the shelf and even cross the core of Atlantic water that runs along the shelf break (Ljeen, 1962). Considering the large distance from the shelf break to the 200 m depth contour, a shelf break front may possibly be recognized as a thermal rather than a haline front. Troughs and banks on the shelf are likely to induce fronts as well, but little is known about these structures. They may be major reproduction and growth habitats for C. jbnarchicus, but so far no investigation have been carried out to establish knowledge on this issue. However, Skreslet (1995) found that in an exceptional year with delayed and small freshwater discharge, the spawning of C. finmarchicus outside the Lofoten Island was not associated with the shelf edge, but with mesoscale frontal structures on the inner shelf. This situation may be similar to conditions on the wide mid-Norwegian shelf, where the ratio between freshwater flow and the shelf width, is small, even in years with large discharge.
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C. finmarchicus is essentially a herbivorous batch spawner with low fecundity, depending on algal biomass to produce eggs (Marshall and Orr, 1962). Hay (1995) found an exceptionally high average fecundity of 76 eggs per female C. jinmarchicus and day to be caused by a considerable bloom of phytoplankton associated with freshwater influence of the Firths of Forth and Tay. Besides being a function of the primary production rate, the size of a new generation also depends on its duration. The reported on-shelf spawning at the Lofoten Islands (Skreslet, 1995) had not started by the end of May, but lasted for more than two months, until the end of August. Thus, the summer generation of C. jknarchicus produced in the Norwegian neritic province may become large in relation to the spring generation, even in years with little outflow.
5. Shelf primary
production
The phytoplankton production in the NCC starts to increase along its frontal zone in April, being larger there than in its median parts (&an, 1929; Braarud and Nygaard, 1980; Peinert, 1986). Rey ( 198 1b) has studied planktonic primary production across the NCC and found that the highest averages occurred in May, with decreasing production during summer. Sakshaug et al. (198 1) found that in June the Norwegian coastal current was nutrient depleted and in a post-bloom situation, but he observed phytoplankton to be in an initial stage in the Atlantic water outside the NCC. However, Skreslet (1995) reported from monthly continuous surface observations across the Lofoten shelf that the fluorescence increased from June to July over the shelf, associated with increased freshwater influence. Farther south on the Lofoten shelf, Peinert (1986) observed low phytoplankton biomass in August, attributed to a high abundance of grazing zooplankton. Berge (1958) and Paasche (1960) reported high primary production in the Nordic Seas during summer. There is little information on how the meltwater discharge which normally occurs from mid-May to late July, influences this oceanic system by its interaction with physical processes in the shelf break front. However, tides are supposed to generate internal waves that provide energy to turbulent diffusion
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of plant nutrients in shelf break fronts, resulting in primary production controlled by the M, ,tide (Mann and Lazier, 1991). In the NCC front, Mork (1981) found that lateral waves generated by variable wind stress, develop eddies governed by baroclinity and bottom topography. Experiments with rotating scale models of the NCC have demonstrated ho,w cyclonic eddies may generate in the frontal zone (McClimans, 1986). Pingree et al. (1979) observed that eddy formation in a marginally stratified front in British coastal waters generated phytoplankton production. Similar conditions are likely to develop along the Norwegian shelf as well. The diatom spring bloom in Norwegian fjords is normally terminated after a few weeks in April due to nutrient shortage, much like a batch culture. The estuarine circulation, which arises from meltwater discharge to Norwegian fjords in May, initiates a second bloom which functions more like a chemostat culture (Sakshaug, 19761, being fed a rnixture of nutrients from both the freshwater and the entrained seawater. However, high flushing rates of brackish water are likely to advect phytoplankton species into shelf waters (Sakshaug, 19761, leaving only those which are able to produce in the shear zone between brackish outflow and the compensation current (Erga and Heimdal, 1984). The retained system may possibly feed biomass into shelf waters, but ,so far the hypothesis is not supported by evidence.
6. Synthesis To suggest how freshwater outflow may influence organismic systems that support year-class strength and growth in NE Arctic cod, the available empirical information has been coupled in a conceptual model represented by two energy flow diagrams. One shows how energy flows across the shelf (Fig. 31, and the other (Fig. 4) the flow along it. The spring production of C. jkmarchicus nauplii that determines the year-class strength in NE Arctic cod, occurs in organismic systems found in fjords and shelf waters (Fig. 3). The reproduction rate of C. jinmarchicus is proportional to the spawning stock size (Tande and Slagstad, 1992) that emerge from wintering habitats at the Lofoten area, being a function of the stock abundance in the previous autumn,
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local predation during winter, and phytoplankton production rates during the spring bloom. Local freshwater outflow occurs later and seems to terminate the reproduction in fjords, by flushing the copepods into shelf waters (Skreslet and Rod, 1986). However, the outflow may be one of the forces that regulates the reproduction of C. jkmarchicus in an organismic system associated with the shelf break front. The production of nauplii in this system may not be important to year-class formation in NE Arctic cod, as it occurs after the hatching period of cod larvae. A fraction of the new generation of copepods enter growth habitats in the Nordic Seas, where they subsequently enter deep wintering habitats, or are advected back, on-shelf to wintering habitats in fjord basins, providing biomass to the next year-class of cod. The cross-shelf fluxes of energy presented in Fig. 3 is supposed to occur at More, as well as at the Lofoten Islands. The summer production of phytoplankton at More is supposed to be a function of river discharge, regulating the reproduction of C.
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finmarchicus. The copepod spawning stock is probably recruited by advection from sources elsewhere, like fjords to the south, the North Sea, or the Norwegian Sea. The summer generation of C. finmarchicus is probably advected away from More, both northwards to the mid-Norwegian shelf, and into the epipelagic zone of the Norwegian Sea. These copepods probably do not become sexually mature in their first summer, but seek wintering habitats as CV, in deep fjord basins and in the Nordic Seas basin (Figs. 3 and 4). At this stage, the abundance of C. finmarchicus appears to be a function of freshwater outflow at More (Fig. 2). After the winter diapause, C. Jinmarchicus CV recruit to the Lofoten spring spawning stock, producing nauplii falling prey to cod larvae (Fig. 4). The number of cod larvae is a function of the NE Arctic cod spawning stock biomass, and the abundance of their prey a function of the number of spawning female copepods. The life-span of this copepod generation, being close to one year, is supposed to
SUMMER
El NORWEGIAN
SEA
0 0
Consumer
>)
Interaction
Storage
1,
Heat
sink
Fig. 3. Cross-shelf flow of energy between organismic systems producing Calanus finmnrchicus, in fjord and shelf habitats during spring, and in the shelf break frontal zone in summer. For simplicity, heat sinks have been omitted from points of interaction. NI-NW: naupliar stages. CI-CV: adolescent copepodites. CVI: sexually mature copepodites. Symbols: adopted from Odum (1971).
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Barents
Sea
Fig. 4. Energy flow between organismic systems, from Mere to the Barents Sea over four subsequent years. The 400 m depth contour indicates the shelf break. O-group cod: NE Arctic cod fry. I-V-group cod: age groups of juvenile NE Arctic cod. Cod SSB: NE Arctic cod spawning stock biomass. Spring and summer producers refer to phytoplankton. Other legends as in Fig. 3.
explain the one-year lag in the correlation between freshwater outflow from More, and year-class strength in NE Arctic cod (Skreslet, 1976). The maturation of gonads in C. Jinmarchicus occurs during winter, depending on internal energy reserves, but the act of spawning seemsto require food (Tande and Hopkins, 1981). Thus, the phytoplankton spring bloom in the Lofoten region is likely to determine the total egg production of each individual copepod female, and thus, influence the growth rates of the advanced cod larvae and metamorphosedO-group fry. While juvenile cod are advected from the Lofoten Islands to the Barents Sea, they may fall prey to a
number of planktivorous species,ranging from carnivorous zooplankton to fish, whales and seabirds. Larvae of several fish species may play changing roles in relation to each other, being prey, competitors or predators, dependingon interspecific changes in advantagesgained from interannual differences in hatching period and growth rates, as they do in NW Atlantic waters (Paz and Larraneta, 1992a.,b).However, there is no information from Norwegian waters that such factors influence the survival of cod larvae. Copepodids surviving the advection into the southern Barents Sea possibly fall prey to I- and II-group cod that may also feed as cannibals on the O-group cod, both items being complementary to cod
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food produced in the Barents Sea, itself. Two years later, a number of II-, III-, IV- and V-group cod, being a function of year-class strength and group specific growth rates, recruits to the stock of exploited immature cod (Fig. 4). According to the present conceptual model (Figs. 3 and 41, a year-class of large II-group cod, having grown fast on large food rations in their first year, and older year-classes gaining enhanced growth in the same year, may recruit to the fisheries simultaneously, explaining the occurrence of substantial landings of sexually immature cod three years after large freshwater outflow from More (cf. Skreslet, 1988).
7. From hypothesis towards theory In essence, Helland-Hansen and Nansen (1909) and Izhevskii (1964) formulated the same null hypothesis: natural vernal discharge of freshwater from Norway forces physical processes that stimulate biological productivity in the food-web of the Arctic Mediterranean Ecosystem. One way to falsify the hypothesis would be to observe that interanuual variation in freshwater outflow was not correlated with growth parameters in marine populations. This was attempted by Skreslet (1976, Skreslet, 1988) who tested correlations between outflow from unregulated rivers and year-class strength and landings of NE Arctic cod, and fatness in minke whales (Balaenoptera acutorostrata) and Norwegian springspawning herring. Significant correlations were observed and did not allow for a rejection of the hypothesis. Similar tests made in north American waters (Sutcliffe, 1972, 1973; Sutcliffe et al., 1977) did not reject that freshwater outflow influences the production of fisheries resources. The next step, to test the hypothesis, would be to use the regression algorithms as predictive models, observing whether or not results calculated from observed freshwater outflow might reproduce observed population data. However, several of the rivers used in the correlation analyses now have regulated flow due to hydro-electric energy production. In fact, after 1970 the construction of large hydroelectric plants have influenced most large water courses in Norway, preventing further use of natural river flow as representative freshwater input to the sea. Further
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testing of the hypothesis will therefore require regional budgets of both natural and regulated run-off. At present, such budgets do not exist. So far, the hypothesis has been tested on the ecosystem level, by relating river flow to population parameters being distant in both time and space. Its falsification can also be attempted by breaking it down to testable lower-order hypotheses on the levels of populations and organisms. To gain foot-hold for this strategy, it is necessary to develop a theory that explains how freshwater outflow influences the transfer of energy through the food-web of the Arctic Mediterranean Ecosystem. The present conceptual model is intended to be a step in this direction. Several testable hypotheses directed towards the population level, may be formulated from theoretical deductions. One can be that interannual variation in the production of C. fintnurchicus covaries with salinity in Norwegian coastal waters. Such an analysis is underway, represented by the preliminary data in Fig. 2. Another hypothesis that ought to be tested, can be that year-class strength in NE Arctic cod is a function of the spawning stock biomass of C, jinmarchicus, prior to the spawning of cod. Still another to be tested, is that C. $nmarchicus is advected from the Norwegian shelf into the Norwegian Sea and back, constituting one single, large population. The effects of river flow on primary production in the NCC need scientific attention. Numerical. models of phytoplankton production generated in river plumes appear to be strong tools for the deduction and testing of hypotheses (see this volume). However, a proper understanding of the coupling between physics and biology may require studies on dimensions smaller than those resolved by the present generation of models. Therefore, classical field studies on phytoplankton production is still in demand, for testing of hypotheses and for conceptual modelling, as well as for validation and verification in numerical modelling. At present, numerical models used in ecological oceanography addresses organismic systems, mostly involving planktonic primary production and to some extent, herbivorous zooplankton. There is little connection between such models and those addressing nektonic population systems, and ecosystems. Hopefully, the present conceptual model may provide some ideas on how different models can be nested.
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8. Conclusion Trophic interaction between the herbivorous copepod C. jbnarchicus and carnivorous juveniles of NE Arctic cod (G. morhua) occurs in different organismic systems along the north Norwegian shelf, where the two population systems overlap. The relationship is subject to interannual variation, probably forced by freshwater outflow in the previous year, causing the year-class strength of cod to vary. Due to their large size, both population systems play major roles in the Arctic Mediterranean Ecosystem, and may be expected to transfer the freshwater signal to other parts of its food-web as well. A conceptual model has been established to explain the initial transfer between organismic systems along the coast of north Norway, and identify key processes in the ecosystem. The model may also provide ideas for nesting of numerical models, eventually resulting in a basin-scale model for the Arctic Mediterranean Ecosystem.
Acknowledgements I would like to express my gratitude towards colleagues who I have met in different scientific fora, to discuss matters of common interest. In particular, I acknowledge the free flow of unrestricted information shared with me at the annual meetings of the Association of Norwegian Oceanographers. Without the constructive feed-back and encouragement from fellow members, I would never have been able to persue the present problem over so many years. Several of my research efforts in this direction have received financial support from the Norwegian Research Council.
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