Physical-Biological Coupling and Impacts of Global Change

While all animals in the sea are affected to some degree by the dynamics of the waters around them, it seems likely that planktonic animals are the most tightly coupled to the physics of the fluid medium. Since the majority of marine animals spend at least a portion of their life in the plankton, we intend to focus most of our attention on zooplanktonic organisms, both the holoplankton and the meroplankton. The holoplankton, such as numerous copepods and other macro- and microzooplankters, completes its entire life cycle in the plankton. Many marine benthic invertebrates have larval stages that are meroplankters, spending from hours to several weeks in the plankton. Most nektonic animals, the large swimming forms such as fish and squid, also have at least one planktonic life stage during which they too are at the mercy of the fluid motions of the sea. Accordingly, success of recruitment into the adult population for marine animals does not depend on biological processes alone. The transport of organisms into regions that may be either favorable or inhospitable to them plays a major role in dictating the success of recruitment and thus the abundance of marine animals. Moreover, the characteristics of a region that determine its suitability for any given organism depend not only on the availability of food and the abundance of predators but also upon the dynamic physical features of the local environment that influence the success of recruitment, the efficiency of feeding, and the susceptibility to predation.

The potential impact of global change on animal populations in the sea can be approached by addressing how these physical processes of significance to the success of planktonic organisms are themselves likely to shift with changes in global climate. Scientists cannot predict with complete certainty what the impact of global change will be on the various parameters that characterize the oceans and the atmosphere. Nevertheless, reasonable scenarios predict global warming from the "greenhouse effect", altered precipitation pattems, and sea level rise (e.g., Mitchell, 1989). We provide three examples to illustrate, how these global changes could have profound impacts on the physical features and processes of the sea that help dictate the abundance, distribution, and production of marine planktonic animals.

The coastal region of the Gulf of Alaska, for example, could be the site of a large physical and biological response to global change. The high latitudes seem more sensitive than the low latitudes to many parameters of global change (Mitchell, 1989). As precipitation pattems change and as global warming triggers rapid melting of previously persistent ice fields and retreat of glaciers, the volume of freshwater that enters the Gulf of Alaska is likely to be affected substantially. We already know that the input of freshwater from the entire coastline of the gulf is a critical component of the driving forces for the Alaska Coastal Current. Under a different precipitation regime with different pattems and degrees of icemelt, the magnitude and even the direction of this current could shift dramatically. Effects of such a shift on populations of various fish species could be large. For example, transport of eggs and larvae of the Alaskan pollock could be affected. Walsh and McRoy (1986) modeled effects of advective transport on this species in the adjacent Bering Sea, testing the hypothesis that years of reduced temperature (such as a climatic shift) delay the development of copepod nauplii that represent the primary food source for pollock larvae. They concluded that the observed pattern of interannual variability in year-class strength was consistent with the larval starvation hypothesis, and not with the effects of advection. It would be interesting to examine this same hypothesis in the region affected by the Alaska Coastal Current, where freshwater inputs and resultant currents can be substantially higher (Royer, 1982). Implications of such a study would extend far beyond the Gulf of Alaska. For example, coastal currents driven by the buoyancy derived from freshwater inputs can be found off the coast of Norway. Mean currents in the Middle Atlantic Bight may be forced by freshwater injections from the St. Lawrence River estuary. The Mississippi River likewise has a measurable effect on the nearshore circulation of the entire Gulf of Mexico west of the Mississippi Delta, especially in spring.

Another important set of examples of how global change may influence the biology of the sea involves oceanic fronts. Fronts exist where sharp boundaries are present between two neighboring water masses with different properties (e.g., differing temperatures, salinities, chlorophyll concentrations, speeds). Fronts are ubiquitous in the ocean, especially in the coastal zones. One can identify at least five common types of fronts: shelf/slope (or shelf break) fronts, upwelling fronts, tidal mixing (or shallow sea) fronts, estuarine fronts, and advective fronts (see Joyce, 1983 for a thorough set of classifications). Biological activity is well known to be concentrated in frontal regions for a variety of reasons (e.g., Le Fevre, 1986). The locations of several of these frontal types as well as the behavior and dynamics of the fronts, such as downwelling and mixing, may be expected to shift with changing global climate because most fronts can be strongly affected by winds. For example, the position of upwelling fronts will be under the control of coastal wind pattems, which are thought to be subject to climatic variation (Bakun, 1990). Indeed, paleoceanographic evidence indicates that coastal upwelling associated with the Asian monsoon was more intense about 9000 B.P. during the maximum in Northern Hemisphere solar insulation (Mitchell, 1989). It seems reasonable to hypothesize that associated upwelling fronts may have changed their locations in addition to their upwelling regimes. In addition, the positions of estuarine fronts will be under the control of freshwater input to the estuary and consequently subject to variation with shifting precipitation regimes. The ecological importance of the position of any given front in the life history of organisms is not generally known, but we know that this role is significant in certain well-studied cases (Tyler and Seliger, 1978).

Our final example involves the impact of changing sea level. A growing consensus of scientists agrees that sea level is now rising at a rate of 1-3 mm per year and that this rate is likely to increase (Thomas, 1987). Extrapolated over the coming 50-100 years, this rise in sea level promises to have profound impacts on nearshore habitats. The width of the inner shelf environment (the inner shelf is the strip of ocean where the bottom significantly affects flow, or formally where the surface mixed layer approaches the bottom boundary layer) may be expected to increase greatly, especially over gently sloping bottoms. It is likely that the mean surface gravity wave energy reaching the shore may decrease when distributed over a wider shelf. Since transport in the wave zone, both transverse to and along shore, is controlled by surface waves, the transport of planktonic forms including larvae and juveniles of many organisms may be substantially modified. The role of onshore transport in affecting the success of recruitment of many benthic invertebrates is now undergoing intense scrutiny (e.g., Roughgarden et al., 1988). Most of our present understanding of the ecology of intertidal populations and communities is based on an appreciation of the importance of such processes as competition and predation that occur after the time of larval settlement (Connell, 1961; Paine, 1966). Important current research is evaluating the significance of recruitment limitation in this system and the role of larval transport in affecting the supply of recruits to shore. Changing rates of transport due to rising sea level could have profound implications to the resolution of this issue.

The common thread in these three scenarios is that changing global climate affects the physical phenomena in the sea, from the large scale, such as changing inputs of freshwater modifying buoyancy-driven flows in the entire Gulf of Alaska, to the small scale of turbulence, mixing, and transport nearshore and in fronts. Furthermore, because so many marine animals have planktonic life stages, we expect that the changing physics will have profound effects on individual organisms, and by changing their demographic parameters on populations, and by changing populations on the communities and ecosystems of the sea. This sequence, in fact, represents the strategy by which GLOBEC intends to approach the problem of predicting the impacts of global change on animal abundance, distribution, and production in the sea.