Implications of Long-Term Climate Change for Biogeography and Ecological Processes in the Southern Ocean

Abstract

Understanding the long-term consequences of climate change for Southern Ocean ecosystems is important because this is one of the last remaining wildernesses on the planet and because the Southern Ocean is a major driver of global climate. The Southern Ocean is roughly the size of Africa and experiences exceptional seasonality. Its many habitats include the permanently open ocean, sea ice, frontal systems and neritic waters, and different zonal (east-west) and meridional (north-south) regions are on different trajectories in terms of climate, sea ice cover and biological populations. The Western Antarctic Peninsula has experienced substantial warming, loss of sea ice and declining Adelie penguin populations, while eastern Antarctica has cooled, shown increased ice cover and increasing numbers of Adelies. In the ocean itself, warming seems to be concentrated north of the Antarctic Circumpolar Current and at depth rather than in surface waters. Densities of Antarctic krill are correlated with ice cover in the previous winter and in the south-west Atlantic have decreased over the last century while salps have shown increasing numbers south of the Antarctic Circumpolar Current. Even in this example, the mechanisms involved are uncertain, making predictions difficult. The historic loss of an enormous biomass of consumers through fisheries, led to top-down ecological effects including competitive release among predators. These pelagic food webs are, however, strongly physically forced, making them particularly vulnerable to changes in environmental conditions. The size and species composition of the primary producers affect food chain length and the efficiency of the biological pump. Critically, the primary producer community is profoundly shaped by factors influencing the availability of light (e.g. season, ice melt and water column stability), micro and macronutrients. Changes in these will have deeply important bottom-up effects, and this brings us to defining biogeography. In these pelagic systems, biogeographic provinces are defined by the frontal systems that delineate sharp discontinuities in conditions in the water column and the taxa that dominate primary production. Because these are not geographically fixed, changes in biogeography in this context describe the expansion, contraction or simple displacement of biomes. The associated food webs revolve around a small number of key species that differ among habitats and biomes. They are not simple, exhibit considerable flexibility and include a number of taxa, particularly the cephalopods, that are difficult to sample and remain poorly studied. A major difficulty in understanding how climate change is likely to manifest is the brevity of relevant datasets. We have few physical or biological benchmarks to use in separating short-term noise from long-term signal. As a physical example, Southern Ocean fronts can exhibit short-term meridional 2shifts of >100 km in a matter of weeks. Biologically, regional differences in trajectories of Adelie penguin numbers need to be viewed against the background of substantial variability over the last 45,000 years. Except for ice or sediment cores, such data are available for few variables or species. In addition, research efforts are geographically unbalanced for logistic reasons. Remote sensing and the Argo float programme reduce this problem by increasing spatial coverage enormously for some variables, but not others, and even then offer relatively new time series. Important variables that are undergoing, or will undergo, change include: sea ice cover (essentially habitat loss), sea temperatures, wind and mixing regimes, the positions of fronts, ultraviolet levels and ocean pH. Many of these will have interacting effects, and the areas likely to be exposed to multiple environmental changes far exceed those already experiencing important changes. Species are potentially vulnerable to stressors at all ontogenetic stages and in many cases sublethal effects (e.g. on reproductive success) are likely to affect population dynamics before stressors reach lethal levels. Likely, and complicated, will be possible changes to the microbial plankton, including increased microbe deposition by aeolian dust and greater susceptibility of potential hosts to viral infection because of other stresses. All these stressors can have direct and interacting effects as well as indirect effects. For example, meridional shifts in frontal positions will alter the foraging ranges of top predators that are land-based during the breeding season. The ecosystem-level consequences will depend on individual species reactions, including the rates at which they can respond and stressors to which they respond, with the potential disruption of species interactions through different phenological responses. Perhaps the key characteristic of predictions for the effects of climate change on the Southern Ocean lies in an even greater degree of uncertainty than the norm.