Introduction

Only recently have efforts begun to focus the attention of the scientific community on issues of the status of global marine biodiversity and the potential ramifications on basic ecosystem processes. Two of the current initiatives arc Norse's (1993) coordination of the development of a global marine biodiversity strategy and BIOMAR (Butman and Carlton 1993; NRC 1995), a US effort to establish and fund a research agenda. While both initiatives consider oceanic and coastal ecosystems, neither focussed specifically on open-ocean biodiversity. Our purpose here is to spur discourse specifically on open-ocean biodiversity, and focus attention on the possible links between changes to biodiversity and response at the ecosystem level. This is in contrast to the usual treatment, in which the impacts of ecosystem alteration on biodiversity arc the relevant issue.

The open-ocean biome consists of all marine environments beyond the continental shelf, which extends seawards from ali land masses to a depth of 200 m (Figure 16.1). Pelagic and deep-sea (below 200 m) environments exist here. It is the world's largest biome, covering over 70% of the world's surface area and an even greater percentage of its inhabitable volume. It is also the biome about which we know the least. New families and phyla have recently been described, and newly discovered life forms exist in the open ocean that rely on novel energy pathways in ecosystems significantly different from those on land (Edmond and Von Damm 1983; Cavanaugh 1985; Tunnicliffe 1991). In contrast to many terrestrial systems, and some marine environments which have a characteristic biogenic structure and endogenous properties, oceanic systems are thought to be structured primarily through physical processes (Steele 1985; Holling 1992; Holling et

Functional Roles of Biodiversity: A Global Perspective <5^,

Edited by H.A. Mooney, J.H. Cushman, E. Medina, O.E. Sala and E.-D. Schulzc jfcyVi fjj} © 1996 SCOPE Published in 1996 by John Wiley & Sons Ltd unr

Figure 16.1 A representation of the world's oceans, showing levels of primary productivity in mg C fixed per square metre per day (after map 1.1 in Alias of the Living Resources of the Seas, FAO. Rome. 1972), and the major currents that structure the ocean's basins. The open ocean covers all marine environments away from land and is usually defined by the continental shelf, which extends from land to a depth of about 200 m, or about 200 km from the shore. The actual boundary between coastal and ocean biomes depends on the prevailing currents, and is thus highly dynamic al. 1994). We begin by describing the principal physical forces that structure open-ocean ecosystems, and follow with a brief introduction to the patterns of oceanic biodiversity that map onto this physical template. We then provide a summary of the ways that humans impact open-ocean diversity. A simple theoretical framework is then given that couples ocean biodiversity and ecosystem processes.

We propose that seven general ecosystem processes operate in the open ocean, and then examine where the important functional groups reside. Wc then list where the strongest evidence exists for a relationship between diversity and processes. Although we know little about the functional role of many of the species that reside in this biome, the extent of potential impacts is clearly great.

16.2 PATTERNS OF OPKN-OCEAN DIVERSITY 16.2.1 Structure of the waterscape

Open-ocean environments are principally differentiated by their physical and chemical differences. All ocean ecosystems are closely inter-linked by a dynamic medium (water), which is mixed by currents that operate at all scales and link latitudinal extremes with each other and connect'the deep ocean with pelagic environments. Because of the over-all connectedness of oceanic environments, delimiting major biogeographic regions is not a trivial task (Rex 1983), We present here a hierarchical classification system of the fundamental ecosystems found in open oceans which reflect key processes that structure these ecosystems. These processes are the source of energy for primary producers, the physical heterogeneity of the environment, depth and latitude (Figure 16.2).

Most organic compounds in the marine environment are derived from photosynthesis in the upper layers of pelagic environment. This is not the sole source of energy for ocean life. Organisms which do not depend upon photosynthetic energy were first discovered around geysers, but the most flourishing of these communities occur around hydrothermal vents and cold seeps in the open occan and marginal seas (Edmond and Von Damm 1983; Tunnicliffe 1991), The primary source of energy for these communities is chemosynthesis, generated by free-living bacteria (thermophilous at 100°C) using as electron donors Fe, Mn, S04 and CH4 among others (Huber el al. 1989). These novel communities were discovered less than 20 years ago. and more than 190 new species have been described from them (Grassle 1989; Van Dover 1990). Symbiosis with bacteria is a common characteristic of many of the metazoan species found in hydrothermal vents and cold seeps (Roberts el al. 1991; C'avanaugh 1994).

1. Energy

1. Energy

Chemosynthesis Hoi therm a! vents

Photosynthesis

All other ecosystems

Photosynthesis

All other ecosystems

F.pibe miiic ecosystems

Pelagic ecosystems

2. Physical structure

Infaumil bentliic ecosystems

F.pibe miiic ecosystems surface

200 m

4000 m

6fKH) m surface

200 m

1000 m

4000 m

Mcsuhomhic Builiybemhic

Abyss albctiihic

Pelagic ecosystems

Cpipelagic Meso pclagic Bathypelugic

Abyssal pelagic thidatjichigic

Extreme

Arctic and Antarctic ecosystems

4. Latitude

4. Latitude

Extreme

Arctic and Antarctic ecosystems

Temperate

Figure 16.2 A heuristic view of the mechanisms that structure open-ocean basins (after Chandler ei a). 1996)

Temperate

Tropical

Figure 16.2 A heuristic view of the mechanisms that structure open-ocean basins (after Chandler ei a). 1996)

Most terrestrial and coastal communities are delimited by some dominant form of life (usually but not exclusively an angiosperm: for example, temperate deciduous, boreal coniferous and tropical forests, grasslands, kelp beds and coral reefs). The separation of open-ocean communities is usually by physical or chemical parameters. A subtle boundary separates the epipe-lagic from the mesopelagic ecosystem, defined by a combination of light penetration depth (photic zone), thermocline and wind-driven mixing depth. Usually all of these processes are manifested between 100 and 200 m depth. The epipelagic ecosystem often extends over the edge of the continental shelf, and the lateral distinction between epipelagic and coastal surface waters is highly dynamic. This boundary is sometimes sharpened by the meeting of different water masses (oceanic fronts), for example, at the inner edge of the Gulf Stream off the eastern United States. It may also be greatly blurred, both by periodic littoral incursions of large, highly-mobile pelagic species such as tuna (Thunnus) and leatherback turtles (Eretmochefys), and by warm-core gyres containing translocated epipelagic assemblages in a coastal sea.

In some coastal areas, a narrow continental shelf drops precipitously to extremely deep water, bringing the open-ocean water column into unusually close proximity to the shore and terrigenous influences. This we refer to as the coastal open-ocean ecosystem. The distinction is more than semantic: it is here that land-based impacts are likely to have the most direct and immediate effect on pelagic assemblages. It is also where alterations in pelagic biodiversity and food-web structure have the most immediate effect upon human society. Cold, nutrient-rich waters are upwelled in coastal areas, stimulating primary production that cascades through the mesopelagic and down to the benthic communities. The mesopelagic and abyssal benthic ecosystems arc commonly referred to as the "deep sea". The largest inhabited volume on earth is the mesopelagic realm. Here organisms exist at 1°C, in darkness, at extremely low organic carbon levels and organism densities.

The effect of latitude on the duration and intensity of seasons also plays a major role in structuring oceanic communities. In most tropical oceans, the build-up of a permanent thermocline inhibits the redistribution of nutrient-rich waters from the deep, up into the nutrient-depleted photic layer. In contrast, temperate epipelagic waters arc more productive, but also more variable. Seasonal changes in productivity alter the temporal distribution of nutrients to the benthic community, impacting reproduction, and thus affecting recruitment in the deep sea. The ocean currents which provide boundaries for many of the ocean's regions are to a large degree determined by winds, which themselves are determined by latitude and gravitational forces (Coriolis). These currents play a large roie in the shaping of the ecological communities in the open oceans (Figure 16.1).

Although the pelagic realm dominates the inhabitable volume of the open ocean, the greatest diversity of marine life inhabits the physically structured benthos. Here, often fast-moving, cold currents descend from the poles and move towards the equator, occasionally creating powerful underwater storms which carry vast amounts of sediment across the ocean bottom, disrupting much of the epibenthic life (reviewed by Hollister et al. 1983). Colonization of benthic communities following medium- to largc-scale disturbances has been found to be generally slow for macro-, meio- and micro-fauna in one experiment (Desbruyeres ei al. 1985), and dependent on opportunistic colonization events (Grassle and Morse-Porteous 1987; Grassle 1989).

Open-ocean systems are heterogeneous at all scales (Figure 16.3a; Steele 1985; Colebrook 1991; Kawasaki 1991); hydrodynamic structure predominates and is provided to oceanic systems principally by currents and waves from small-scale eddies, through warm-core rings of water moving across ocean basins, to transoceanic currents (Figure 16.3a,b). Some ocean basins are biologically more self-contained; large-scale circulating masses of water, or gyres, dominate the North Pacific and North Atlantic Oceans. Other basins arc more dynamic or open (e.g. currents; the Atlantic Pacific conveyor belt, the Gulf Stream). Mesoscale structure in the pelagic is provided by floating rafts of weed (e.g. Sargasso Sea) and/or flotsam, sea-ice, storms and large mats of diatoms (Kemp and Baldauf 1993). In the benthos, there are abyssal plains (structured by turbidity currents), rifts and occasional deadfall (wood or animal carcasses). Temporal variation also occurs at all levels, from diurnal fluctuations in light, to tidal cycles and annua! cycles', up to the Milankovitch cycles. Although the dynamics are poorly understood, long-term variability in the abundance of fish and plankton lias suggested important concordance between climate and ecosystem processes (Gushing 1982; Colebrook 1991; Kawasaki 1991).

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