Sea Ice plays an important role in Antarctic marine ecosystems, notably through the onset of phytoplankton blooms in spring and summer as sea ice retreats, which feeds the food web. Sea Ice also prevents wind disturbance, and mixing of, surface waters affecting the flow of nutrients to shallow seabed ecosystems, as well as affecting light availability.
Significant spatial and temporal variability in sea ice extent and thickness make it difficult to assess long-term, climate-induced trends.
Reductions in winter sea ice extent has altered the abundance distribution and size composition of Antarctic krill. This is reflected in a seasonal shift in the krill fishery with peak operations moving from summer to late autumn.
Sea ice may also be a driver of changes in the breeding success and population growth rates of some Antarctic seabirds/ flying birds; however, it is difficult to attribute these changes as many factors may be involved.
Medium evidence, low agreement
There is a high degree of uncertainty in short, medium and long-term trends in sea ice extent and duration. The regional differences mean that a shift from net gain to net loss of sea ice has only recently taken place. Confidence on the current impact of sea ice change is low due to (i) a lack of long-term ecosystem monitoring, (ii) difficultly in accounting for natural variability of the climate system, and (iii) a mismatch in the timescales between climate projections and those of ecological processes.
If sea ice coverage reduces in time and extent, an ecological tipping point may be reached resulting in a regime shift in shallow water polar communities. Shallow living, dark-adapted invertebrates could be replaced by light-dependent communities such as algae. In deeper waters, less ice and increased storm and wave activity is likely to increase vertical mixing of nutrients in the water column. This could result in a fundamental change in ecosystem functioning.
Sea ice has also been shown to strongly influence benthic biomass, with impacts varying from shallow seas to the continental shelf dependant on factors such as iceberg scour and primary productivity.
A range of impacts on ice-dependant mammals and seabirds have been identified. For example, crab-eater and Weddell seals could be negatively affected by changes in the extent, persistence and type of annual sea ice, whilst Ross and Leopard seal will be the least negatively influenced. Bird species such as emperor penguins and snow petrels may also be adversely affected by sea ice change.
The impact of reduced sea ice may be expected to occur first at some of the lower latitude sites in the polar UKOTs such as South Sandwich Islands and South Orkney Islands.
Medium evidence, medium agreement
There is a degree of uncertainty in sea ice projections. Key knowledge gaps include temporal data series of sea ice duration, thickness and extent. Change is not linear and often reaches critical tipping points. Furthermore, trends vary between regions within Antarctica.