10 V

106Um

107)im lO^im

Mictophytoplankton

- archaea

- coccolithophonds

- cyanobacteria

Mesophyioplankton

- dinaflagcllates (some)

Macrophytoplamaori • dialoius (some)

- rttnoHagcllaies (some)

■ filamentous cvanobactena

Mtcrnwoplankmn

- copcpod naupUi

- foranumierjns

- radiolanans

- trocophore larvae of polychaetes

- valiger larvae pf molluscs Mesozooplatftton

- aduJt copepods

- cladocerans

- larv ae of many bcnlhic species

- rolifers

Macroaoopjantton

- amphipods

- chaetognaths

- ctenophores

- eupflairSiJsiinnlp

- isopods

- Jarval lish

- mysid and true shrimp

Micrtictmsumcn;

fishes (myciophids. capebn, anchovies) ■ molluscs (squid)

Secondary consumers -lishes (ccxi, hemng. sardines, dogfish. skates) - mammals (dolphins, seals, baleen wha(es) • reptiles (leaiherback turtles)

Tertiary Consumers

- mammals (killer whales, sperm whales)

- molluscs (giant squid)

Primary producers

Herbivores and Primary Consumers

Secondary amd Tertiary Consumers

Figure 16.8 The size structure of an cpipelagic food web. The information was adapted from Kennish (1989). Three more size classes of even smaller phytoplankton have been proposed, i.e. picoplankton (0.2-2^m), femtoplankton (0.02-0.2 ¡am) and nano-plankton (2-20 pni), which have been subsumed into the microplankton in this figure for the sake of simplicity dispersed, pianklotrophic larvae yield an ecosystem with high overall stability at large scales.

Another area of contrast between pelagic and terrestrial ecological processes is in the relationship between biodiversity and food-web structure. In general, the food-web structure of open oceans is similar to that found in terrestrial systems. There are important differences, however, particularly in mean chain length and species density across trophic levels. Schoener (1989) found a median of five trophic links in pelagic food webs, as compared with four in terrestrial, sea bottom, lake and upper estuarine webs, and only three links in marine estuaries and rivers. The proportion of primary producer and top carnivore species is low in the pelagic realm, and that of intermediate species is comparatively high. On a broad scale, biomass production is influenced principally by factors related to latitude and depth (Mann and Lazier 1991). Local variation in productivity is driven by (a) spatial dynamics of wind and current creating an underlying template for patch structure and (b) succession within patches due to tracking of primary producers by consumers on temporal scales of weeks and seasons (Valiela 1984).

One aspect of primary production that does appear to be very important is the size threshold at which phytoplankton can be more efficiently consumed by the primary consumers. Nutrient-rich environments, such as upwelling areas, tend to have a greater proportion of larger phytoplankton, and thus move energy up the food chain more efficiently (Valiela 1984, 1991). The phytoplankton communities of nutrient-poor areas, such as the tropica! seas, tend to be dominated by many small nanoplankton. These phytoplankton are too small to be consumed by most zooplankton, and a succession of zooplankton is needed as the energy is channeled to larger consumers. Alternatively micropredators or parasites may recycle the energy back into primary nutrients and the microbial loop.

Anthropogenically derived changes to biodiversity can have significant impacts on food web configuration and the size distribution of organisms within the food web at small to moderate scales. For example, there have been major intra- and inter-specific shifts in life-history profiles of the dominant marine taxa due to overextraction in fisheries (Nelson and Soule 1987; Rijnsdorp 1993; Stokes et al. 1993). Whether these impacts result in the overall productivity of the open ocean changing at moderate to large scales is unclear. At the interspecific level, human fisheries have systematically reduced (in some cases to extinction) all species of large organisms from the epipelagic zone. Because body size limits the range of prey available and turnover time, changes in the frequency distribution of size classes will alter pelagic food webs, as well as the si2c distribution of organic deadfall to deep-sea communities (see below).

Recently, a entirely new group of primary producers have been discovered that could substantially alter our estimates of primary productivity in some regions of the ocean (Chrisholm et al. 1988, 1992; Olson et al. 1990). These discoveries of major new contributors to the oceanic food web makes predicting human impacts on food chain dynamics speculative at best. Resolving the issue of functional similarity will have to wait until we understand the taxonomic breadth of these new groups.

Undoubtedly, the greatest direct impact that humans are having an open-ocean biodiversity is the overexploitation of major vertebrate stocks, causing the current collapse of most of these stocks worldwide (Manire and Gruber 1990; Groombridge 1992; Anonymous 1994). Many of the organisms exploited by humans play pivotal roles in the food web, and because many of the top-level species are simultaneously being exploited, substantial changes in the composition of oceanic communities can be expected (Laws 1985; Weber 1986; Katona and Whitehead 1988; Manire and Gruber 1990). The shift from a bony-fish to a cartilagenous-fish dominated community in the north-western Atlantic is one good example of the reconfiguration of an occan community (Figure 16.9). Examples of major shifts in fish populations (ecosystem flips) are known from the Norwegian - Barents Sea system (Blindheim and Skojdai 1993; Bamre 1994), the Baltic-North Sea system (Hammer 1993), the Yellow Sea in China (Tang 1993), the Okhotsk Sea in Russia (Kuznetsov et al. 1993), and the Humboldt and California Current systems (Soutar and Isaacs 1974; Alheit and Bernal 1993). Many of these "flips" are driven by density-independent shifts in physical oceanic conditions, but several were apparently influenced by human predation (Sherman 1989; Hammer 1993; Kuznetsov et al. 1993; Tang 1993). The shifts in abundance of non-cetacean planktivores in Antarctic waters is probably a consequence of the over-exploitation of cetaceans there (Beddington and May 1982; Vaiiela 1984). Although principally driven by physical factors, these ecosystem shifts may be exacerbated by human fishing effort. In the Okhotsk Sea system, near Russia, human predation produced an initial increase in walleye pollock production by decreasing cannibalism and thus "rejuvenating" the population. However, the narrowed range of size and age classes ultimately decreased the overall stability of the population, and rendered it more vulnerable to environmental perturbations (Kuznetsov et al 1993).

The attributes of ocean systems may make apex predators especially vulnerable to ecosystem effects pursuant to biodiversity impacts (Manire and Gruber 1990). The first is pelagic recruitment. The larvae are subject to highly stochastic determinants, and when disrupted sufficiently the outcome is highly unpredictable on small scales. Large-scale ecosystem resilience is predicted from the prevalence of pelagic larva! dispersal and a broad diet found among many pelagic organisms that includes the young of even their

Yellow Sea 1959 Yellow Sea 1986

Yellow Sea 1959 Yellow Sea 1986

Georges Bank 1990

Georges Bank 1963

Georges Bank 1963

Skaic

Dogfish

Flounder

Cod-like fish

Skaic

Dogfish

Flounder

Georges Bank 1990

Figure T6.9 The change in fish community composition in the Yellow Sea, China (after Tang 1993), and the Georges Bank system, northwest Atlantic (after World Resources Institute 5994). Initial fisheries were concentrated on the most desirable species (e.g. cod and flounder off the Georges Bank), but now "less desirable'" fish species dominate.

own species. The effects of removal of large epipelagic species should cascade through the rest of the water column, as these are principal agents of nutrient transport, both as living individuals undergoing frequent vertical migration, and as deadfall (Smith et al. 1989; Pfannkuche and Lochte 1993). These systems arc ordinarily very resilient, so that the complete elimination of a dominant species may make system reconfiguration extremely difficult to reverse.

Recent simulations of oceanic systems suggest the effects of altering species composition are very difficult to predict, may be highly counterintuitive, and are dependent on the time frame and spatial scale involved (Yodzis 1988).

Eutrophication and introductions Ocean productivity may be further altered by two other human impacts, the indirect effects of increasing the nutrient inputs to oceanic systems, and the introduction of novel organisms to the system. Human impacts on the total productive capacity in the open ocean will be principally primary i.e. nutrient loading within coastal regions, rather than impact on open-ocean biodiversity (Young et ai. 1985; Suchanek 1994). Recent evidence suggests that humans have helped contribute to the eutrophication of such large basins as the Mediterranean and Black Seas (Caddy 1993). Nutrient enrichment has been postulated to increase the frequency of red tides (Smayda 1989; Anderson 1994). The production of toxins by dinoflagellates in so-called red tides, is one example of a localized disturbance by one or a few taxa that often has direct links to human activity (Smayda 1989; World Resources Institute 1992; Halle-graeff 1993; Anderson 1994). These toxins percolate their way through the food web, altering the ecosystem through mortality of many other species, including humans. In addition to the "red tide" dinoflagellates, other toxins are produced by a wide variety of plankton taxa. The factors that trigger the ephemeral blooms of these organisms and the subsequent impact on marine ecosystems are unknown. In most of the oceans, the large-scale mixing of water masses is sufficient to reduce the impact of any one taxa upon overall water quality. However, as one moves down in spatial scale, significant impacts probably do occur.

The open ocean is thought to be more immune to introductions than coastal systems because global currents have already created a cosmopolitan distribution of species. However, some introductions can have dramatic effects! The invasion of the comb jelly, Mnemiopsis leidyi into the Black Sea, most probably from ballast water, is one example of an introduction with dramatic effects (Vinogradov et al. 1989; Shushkina and Musayeva 1990). Up to a 10-fold decrease in zooplankton biomass. a 90% decrease in the jelly fish Aurelia biomass, and a 90% decrease in the pelagic fishery could be attributed to the introduction of this novel predator.

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