Climate change and Southern Ocean ecosystems I: how changes in physical habitats directly affect marine biota

Abstract

Antarctic and Southern Ocean (ASO) marine ecosystems have been changing for at least the last 30 years, including in response to increasing ocean temperatures and changes in the extent and seasonality of sea ice; the magnitude and direction of these changes differ between regions around Antarctica that could see populations of the same species changing differently in different regions. This article reviews current and expected changes in ASO physical habitats in response to climate change. It then reviews how these changes may impact the autecology of marine biota of this polar region: microbes, zooplankton, salps, Antarctic krill, fish, cephalopods, marine mammals, seabirds, and benthos. The general prognosis for ASO marine habitats is for an overall warming and freshening, strengthening of westerly winds, with a potential pole-ward movement of those winds and the frontal systems, and an increase in ocean eddy activity. Many habitat parameters will have regionally specific changes, particularly relating to sea ice characteristics and seasonal dynamics. Lower trophic levels are expected to move south as the ocean conditions in which they are currently found move pole-ward. For Antarctic krill and finfish, the latitudinal breadth of their range will depend on their tolerance of warming oceans and changes to productivity. Ocean acidification is a concern not only for calcifying organisms but also for crustaceans such as Antarctic krill; it is also likely to be the most important change in benthic habitats over the coming century. For marine mammals and birds, the expected changes primarily relate to their flexibility in moving to alternative locations for food and the energetic cost of longer or more complex foraging trips for those that are bound to breeding colonies. Few species are sufficiently well studied to make comprehensive species-specific vulnerability assessments possible. Priorities for future work are discussed.

Southern Ocean phytoplankton physiology in a changing climate

“…Together these factors divide the SO into distinct provinces ( Fig. 1) and generate functional assemblages of microbes that are predictable in space and time (Boyd, 2002;Constable et al, 2014). The principal dichotomy defining these functional assemblages is the presence of sea ice.…”

“…Melting sea ice releases fresh, buoyant, iron enriched (winter accumulation of aeolian input), water and phytoplankton into the water column, initiating a phytoplankton bloom which retreats southward in the wake of the receding ice edge over the austral summer (Lannuzel et al, 2007;Constable et al, 2014). The meltwater stabilises the water column, shallowing the mixed layer depth (MLD) and entraining cells in a high light, high nutrient environment.…”

“…At its maximum extent in December, the MIZ covers ∼6 million km 2 , or approximately 39% of the SSIZ (Fitch and Moore, 2007). The magnitude, extent and timing of SSIZ blooms is profoundly affected by the timing of sea ice disappearance, prevailing wind speed and wave action (Fitch and Moore, 2007;Massom and Stammerjohn, 2010;Constable et al, 2014). As a result, only a relatively small proportion of the entire MIZ (17-24% in December and February, respectively) actually supports the development of phytoplankton blooms (Fitch and Moore, 2007).…”

“…The succession of environmental changes imposed on phytoplankton by recession of the sea ice in the SSIZ generates a predictable sequence of events from bloom formation, growth and production, to grazing, senescence and flux, and a predictable succession of phytoplankton functional assemblages and even taxa (e.g. Kang and Fryxell, 1993;Kang et al, 2001;Constable et al, 2014) (Fig. 2).…”

“…Surface waters of the SO are expected to become warmer, fresher and more stratified with shallower MLD, reduced ice thickness, higher CO 2 concentrations, less sea ice and stronger westerly winds (Boyd et al, 2008;Constable et al, 2014). Together these spatially heterogeneous changes will differentially affect phytoplankton productivity and carbon sequestration by altering ambient temperature, total irradiance and wavelength structure, nutrient availability and trophodynamics (Bopp et al, 2001;Boyd et al, 2008;Caron and Hutchins, 2013;Lewandowska et al, 2014).…”