Analyses Mass-balance Models of Oceanic Systems in the Atlantic1

This report describes the methods and data sources used to estimate ecological parameters and to construct mass-balance models of oceanic ecosystem of the Atlantic using the Ecopath with Ecosim (EwE) software. Six models were constructed representing oceanic ecosystems of the North, Central and South Atlantic for the 1950s and the late 1990s (1997-1998). The first section of this report characterizes some of the fundamental characteristics of oceanic ecosystems which were captured in a model template for oceanic areas. The subsequent sections are devoted to individual functional groups, composed of species or group of species that share similar ecological functions, habitats and demographic characteristics, or represent important fisheries resources, such as tunas and swordfish. The last sections describe the approach used to balance the models in EwE and the adjustments made to the late 1990s model so that it could represent the ecosystem state in 1950. A MODEL TEMPLATE FOR OCEANIC ECOSYSTEMS What would be the essential characteristics of oceanic ecosystems that one needs to capture in an ecological model? Oceanic ecosystems present low overall productivity, and are dominated by pelagic (plankton-nekton) species communities. Approximately half of the total area of the open ocean is between the latitudes 25 ̊N and 25 ̊S, and 75% is between 45 ̊N and 45 ̊S, meaning that most oceanic ecosystems are within the tropical and subtropical zones (Mann 1984). Of importance to fisheries management is the fact that a large proportion of oceanic areas are beyond the margins of the continental shelves and mostly beyond the EEZs of countries, meaning that resources in these high seas areas are accessible to fleets of all countries (although some form of access control may apply, such as the quota allocations for tunas and billfishes). The open ocean is characterized by horizontal and vertical physical-biological discontinuities which are useful for defining system boundaries and model structure. Based on geographical discontinuities in the physical processes affecting the stability of the mixed layer, Longhurst (1998) defined four major biomes in the world’s oceans: Westerlies, Trades, Polar and Coastal biomes. According to the author the large oceanic areas of the Atlantic fall mostly within the Westerlies and Trades biomes. While in the first the main physical processes affecting the depth of the mixed layer are winds and irradiance, in the Trades biome it is the geostrophic adjustment to the trade wind regimes that conditions the dynamics of the mixed layer (Longhurst, 1998). Within each biome Longhurst (1998) defines biogeochemical provinces 1 Cite as: Vasconcellos, M., Watson, R. 2004. Mass balance of Atlantic oceanic systems, p. 171-214. In: Palomares, M.L.D., Pauly, D. (eds.) West African marine ecosystems: models and fisheries impacts. Fisheries Centre Research Reports 12(7). Fisheries Centre, UBC, Vancouver. Oceanic systems in the Atlantic, M. Vasconcellos and R. Watson 172 distinguished by their unique patterns of surface chlorophyll fields, primary productivity, mixed layer topography and related physical forcing (climatic and oceanographic), photic depth and surface nutrient fields. Figure 1 present a map with the location of the biomes and provinces that are used here to characterize the oceanic ecosystem of the Atlantic. The names of the biogeochemical provinces are provided in Table 1. Models were constructed to represent three oceanic areas over 200 m depths: A North Atlantic model, composed of provinces of the North Atlantic Westerly Winds biome (NADR, GFST, NASW, NASE); a Central Atlantic model, composed of the provinces of the Trade Wind biome (NATR, WTRA and ETRA); and a South Atlantic model, composed of the only province of the Westerly Wind biome in the southern ocean (SATL). The total area of the oceanic provinces represented in the models is ca. 50 x 106 km2, which is roughly equally divided among the three modeled areas (Table 1). Table 1. Area of models and biogeochemical provinces of the oceanic ecosystems of the Atlantic. Model Province Code Area (km) North Atlantic North Atlantic Drift NADR 3,477,925 Gulf Stream GFST 1,086,696 Northwest Atlantic Sub-tropical Gyral NASW 5,784,896 Northeast Atlantic Sub-tropical Gyral NASE 4,379,757 14,729,274 Central Atlantic North Atlantic Tropical Gyral NATR 7,895,149 Western Tropical Atlantic WTRA 5,199,487 Eastern Tropical Atlantic ETRA 5,324,555 18,419,191 South Atlantic South Atlantic Gyral SATL 17,618,486 Total – – 50,766,952 Vertical discontinuities in the open ocean are determined by vertical gradients of light, temperature and abundance of organisms (Longhurst and Pauly 1987). Legand et al. (1972) and Longhurst and Pauly (1987) distinguished two vertical systems in the open ocean: a superficial one occupying the 0450 m layer of the ocean and consisted of phytoplankton, mesoplankton, micronektonic and nektonic species; and a deeper system, below 450 m depth, which has no phytoplankton, low mesoplankton biomass, and had mesopelagic and intrusive bathypelagic micronekton and nekton. The authors structured the biota into five main components (top predatory fish, cephalopods, micronektonic fish, euphausiids and other large crustaceans) which are further divided according to their vertical distribution. Vertically migrating species (such as mesopelagic fish) are in the deeper system during the day and in the superficial system at night. For this reason mesopelagic fish are often considered important vertical transporters of organic matter (Gjøsaeter and Kawaguchi, 1980). Mann (1984) proposed a vertical structure for the open ocean fish communities according to 4 zones. An epipelagic zone (correspondent to the euphotic zone) extending down to a depth of 200 m. In this zone the main predators are sharks, tunas, billfish and swordfish, although some tuna may feed to a depth of about 400 m. The mesopelagic zone extends from 200 m to about 1000 m. Figure 1. Map of the biogeochemical provinces (Longhurst, 1998) that compose each oceanic ecosystem of the Atlantic as defined in this contribution. West African marine ecosystems, M.L.D. Palomares and D. Pauly 173 Characteristics of this zone is the constant presence of a “deep scattering layer” which is normally dominated by myctophids, gonostomatids and sternoptychids (Mann, 1982). At night a large proportion of the mesopelagic fauna migrates to the epipelagic zone. The bathypelagic zone starts below 1000 m. Bathypelagic fish fauna are characterized by dark colour, small eyes, weak musculature and large mouths, and are best represented by the angler fish (ceratioids) and species of the genus Cyclothone (Mann, 1984). These species are adapted to the low food availability of the environment, i.e., they decreased the amount of energy expenditure associated with feeding and reproduction (e.g., angler fishes have developed elaborate lures to attract prey close to their mouth). Close to the bottom of the ocean, Mann (1984) recognizes two distinct groups of bottom dwelling fish. The “sit and wait” predators (e.g. Bathysaurus and chlorophthalmids), which lack swimbladders and are negatively buoyant, and the benthopelagic fishes including a wide ranging species with swimbladders such as rat-tails (Macrouridae), deep-sea cods (Moridae) and brotulids that live close to the bottom. These organisms are largely supported by the carcasses of dead animals sinking from above. Mann (1984) also cites the existence of a community of large amphipods of the family Lysianassidae, shrimps and other decapods which may serve as food for the benthic fish community. Many authors have contributed to the conceptualization of the trophic relationships and the transfer of organic matter between the vertical layers of the open ocean. Longhurst and Pauly (1987) described three pathways by which organic material is produced and transformed by consumption by larger organisms in the pelagic ecosystem: the first is the microbial loop, in which dissolved organic material, originated mostly from plant cells, is utilized by bacterial and fungal cells of small size, which are consumed by a variety of heterotrophic protists, which are then consumed by larger zooplankton. The second pathway is based on the growth of picoplankton (e.g. cyanobacteria) which is consumed mostly by protists, but also by salps and tunicates. Because copepods cannot graze on cyanobacteria, the authors presumed that these cells are grazed initially by heterotrophic nanoplankton. The third pathway is the classical food chain, based on the consumption by zooplankton of phytoplankton cells such flagellates, coccolithorphores, dinoflagellates and diatoms (Longhurst and Pauly, 1987). Diel vertical migrations are ubiquitous for all organisms in the oceanic ecosystem. In these areas a large fraction of the zooplankton and nekton perform extensive vertical migrations, rising to the surface after dusk and descending to 200-500 meters at dawn. A two layered trophic model has been suggested by Longhurst and Pauly (1987) to represent this pattern in the tropical oceanic ecosystem. Associated with the epiplankton are tunas feeding on smaller fish (Gempylidae and Bramidae) and small euphausiids which feed on smaller components of the epiplankton. Lying deeper, in the daytime below 250 m depth, are interzonal fish (Myctophidae and Gonostomatidae) and large euphausiids. These organisms rise at dusk to feed nocturnally on smaller species of the epiplankton, but are not fed upon by tunas and fish of the surface layer which cease feeding at night. In this two-layered ecosystem energy is mostly transported downward below the euphotic zone, where it is utilized by large bathypelagic, nonmigrating predators and by omnivorous and carnivorous deep zooplankton. As noted by Longhurst and Pauly (1987), only in special cases there is an active transport of energy upward, by feeding excursions of larger surface biota (bigeye tuna, sperm whales) toward deeper zones. The two layered model of Longhurst and Pauly (1987) mirrors the description of pelagic food webs described by (Roger and Grandperrin (1976). Analyzing the stomach content of tunas caught in longlines the authors showed that the micronektonic fishes ingested by albacore and yellowfin tunas are mostly epipelagic fish, and that the contribution of the abundant migratory mesopelagic fish is rather small. Analysing the stomach content of the epipelagic fish found in the stomach of tunas, (Roger and Grandperrin 1976) showed that the euphasids found in the stomachs of the epipelagic fishes were mainly nonmigrating Stylocheiron species which stays in the upper layer during the day. The authors therefore concluded that the epipelagic fish eaten by tunas are also day-feeders, preying upon zooplankton organisms that stay in the upper layer during the day. This study has demonstrated that each link in the food chain leading to tunas has food sources restricted only to the biomass which stays between the upper layers of the ocean (0 to 450 m) during daytime. Roger and Granperrin (1976) proposed that the day-night pattern of feeding in the epipelagic zone is responsible for a large downward flow of energy that supports the large biomass of organisms in the deeper layer of the ocean (the “energy valve” concept). Vinogradov (1970) proposed yet another mechanism by which organic matter is actively transferred from the surface to the deeper layers of the ocean. Studying the pattern of vertical migrations (both diurnal and seasonal) adopted by organisms at different depth layers of tropical and temperate oceans, this author Oceanic systems in the Atlantic, M. Vasconcellos and R. Watson 174 proposed that the animals descending from the more productive surface layers must serve as food for the population of the depths. These species in turn can descend to even grater depths and serve as food for the more deep-sea animals. Thus according to the author organic material is actively transferred downward through a “ladder of migrations” of planktonic and micronektonic animals. At the bottom of the deep oceans life is supported by four main sources of organic matter (Rowen, 1981): the rain of particles from the pelagic environment, dead carcasses, the “ladder of migration” of micronektonic organisms, and turbidity currents that carry organic matter from the continental shelves towards the slope and abyssal zones. The contribution of the latter tends to be small. (Rowe, Smith et al. (1986) showed, for instance, that only a small proportion of the phytodetritus in continental shelves of the Northwest Atlantic is exported to deeper zones. Once in the system, the organic matter is consumed by bathypelagic organisms, by demersal fishes and crustaceans that migrate off the bottom to scavenge, or sink to the bottom into the benthic layer. In the benthic layer the organic matter can be consumed by benthic invertebrates or heterotrophic bacteria. The remaining detritus is buried into the ground (Rowen, 1981). Table 2. Input parameters of the North Atlantic model for the late 1990s. Functional group B (kg·km) P/B (year) Q/B (year) EE Landings (kg·km) Baleen whales 24.634 0.020 4.394 0.000 Toothed whales 51.144 0.020 6.689 0.000 Beaked whales 0.536 0.020 8.806 0.000 Seabirds 0.204 0.078 72.779 0.000 Pelagic sharks 0.390 10.000 0.9 1.731 Yellowfin 0.015 1.050 15.530 0.005 Bluefin 2.030 0.500 4.000 0.731 Skipjack 0.463 1.350 19.610 0.162 Albacore 0.000 0.800 9.600 0.000 Bigeye 26.944 0.750 17.160 9.430 Swordfish 0.059 0.700 4.000 0.030 Billfishes 0.051 0.404 4.690 0.010 Large planktivorous fish 0.112 1.800 0.1 0.006 Large epipelagic fish 2.204 0.690 8.938 0.661 Medium epipelagic fish 1.080 7.671 0.9 15.909 Small epipelagic fish 2.053 12.549 0.9 0.017 Large mesopelagic fish 0.150 3.550 0.9 0.000 Small mesopelagic fish 1724.369 1.983 18.250 0.000 Small bathypelagic fish 1.040 3.650 0.9 0.000 Medium bathypelagic fish 0.190 0.290 0.9 0.000 Large bathypelagic fish 0.270 0.490 0.9 0.662 Small bathydemersal fish slope 45.054 0.345 0.628 21.619 Large bathydemersal slope 53.246 0.175 0.318 10.519 Small bathydemersal abyss 121.430 0.378 0.687 0.000 Large bathydemersal abyss 189.631 0.209 0.380 0.000 Small squids 4.600 36.500 0.9 0.163 Large squids 4.600 36.500 0.9 0.000 Benthic cephalopods 1.150 2.300 0.9 0.000 Meiobenthos 1234.000 2.250 22.650 0.000 Macrobenthos 545.000 1.000 9.850 0.000 Megabenthos 493.000 1.100 6.700 0.000 Heterotrophic bacteria 28167.000 18.450 25.000 0.000 Small zooplankton shalow 118184.639 17.300 57.700 0.000 Large zooplankton shalow 7377.317 8.700 29.000 0.000 Small zooplankton deep 46164.009 17.300 57.700 0.000 Large zooplankton deep 1392.264 8.700 29.000 0.000 Phytoplankton 13500.000 259.274 0.000 Detritus Based on the background information presented above, a model template was developed to serve as tool to both evaluate hypotheses about fluxes of biomass in the oceanic food web and to evaluate the ecosystem West African marine ecosystems, M.L.D. Palomares and D. Pauly 175 impacts of fisheries in oceanic ecosystem of the Atlantic. Characteristics of the model template developed here are: 1) a multilayered structure that represents the plankton and nekton of the epipelagic, mesopelagic, bathypelagic and bathydemersal zones; 2) the detailed representation of benthic fauna and microbial loop; and 3) the detailed representation of swordfish and tuna species, notably the main fisheries resources in oceanic areas. Tables 2-4 present the functional groups and the input parameters for each oceanic ecosystems of the Atlantic. Sources and methods used to estimate each parameter are described in the following sections. Table 3. Input parameters of the Central Atlantic model for the late 1990s. Functional group B (kg·km) P/B (year) Q/B (year) EE Landings (kg·km) Baleen whales 20.642 0.020 4.394 0.000 Toothed whales 42.856 0.020 6.689 0.000 Beaked whales 0.449 0.020 8.806 0.000 Seabirds 0.125 0.078 73.562 0.000 Pelagic sharks 0.000 0.390 10.000 0.9 0.372 Yellowfin 0.078 1.050 15.530 0.027 Bluefin 0.008 0.500 4.000 0.003 Skipjack 2.605 1.350 19.610 0.912 Albacore 0.155 0.800 9.600 0.078 Bigeye 8.942 0.750 17.160 3.130 Swordfish 0.302 0.700 4.000 0.151 Billfishes 1.657 0.416 4.137 0.331 Large planktivorous fish 0.112 1.800 0.1 0.000 Large epipelagic fish 6.753 0.693 8.938 2.026 Medium epipelagic fish 1.080 7.671 0.9 0.800 Small epipelagic fish 2.053 12.549 0.9 0.000 Large mesopelagic fish 0.150 3.550 0.9 0.000 Small mesopelagic fish 3253.854 3.757 18.250 0.000 Small bathypelagic fish 1.040 3.650 0.9 0.000 Medium bathypelagic fish 0.190 0.290 0.9 0.000 Large bathypelagic fish 0.270 0.490 0.9 0.000 Small bathydemersal fish slope 15.938 0.355 0.645 0.576 Large bathydemersal slope 18.835 0.160 0.291 1.075 Small bathydemersal abyss 93.452 0.343 0.623 0.000 Large bathydemersal abyss 145.938 0.202 0.368 0.000 Small squids 4.600 36.500 0.9 0.276 Large squids 4.600 36.500 0.9 0.000 Benthic cephalopods 1.150 2.300 0.9 0.000 Meiobenthos 984.000 2.250 22.650 0.000 Macrobenthos 369.000 1.000 9.850 0.000 Megabenthos 394.000 1.100 6.700 0.000 Heterotrophic bacteria 22542.999 18.450 25.000 0.000 Small zooplankton shalow 76767.265 17.300 57.700 0.000 Large zooplankton shalow 17567.677 8.700 29.000 0.000 Small zooplankton deep 18858.584 17.300 57.700 0.000 Large zooplankton deep 5931.747 8.700 29.000 0.000 Phytoplankton 20250.000 146.382 0.000 Detritus