Polar ecosystems vulnerable to increased sunlight

Small changes in the timing of polar sea ice retreat that allow more sunlight to reach sea floor communities, could fundamentally change sea floor biodiversity and ecosystem function.

Research by Australian Antarctic Division and University of New South Wales (UNSW) scientists, published in Global Change Biology in November 2013, predicts biodiversity in some areas of the polar sea floor could be reduced by as much as one third, within decades, as the poles warm.

Already, warming temperatures and changing wind patterns have led to an increasing number of ice-free days over summer in parts of Antarctica and the Arctic, exponentially increasing the amount of light reaching sea floor communities.

This exponential increase in light is due to the Earth’s tilt at high latitudes, such that the sun is above the horizon for considerably longer in summer than winter. As a result, early melt that brings the date of ice loss closer to the summer solstice, when sunlight is at its maximum, greatly increases the annual sunlight exposure of sea floor ecosystems.

In shallow Antarctic coastal waters, this may cause unique invertebrate-dominated communities that are adapted to dark conditions, to be replaced by less biodiverse algal beds, which thrive in light. Invertebrates such as sponges, sea squirts and worms perform important functions such as filtering water, recycling nutrients and providing food for fish and other creatures.

Senior Australian Antarctic Division scientist and a coordinator of the research, Dr Martin Riddle, said the study demonstrates a simple mechanism for a ‘non-linear tipping point’, where relatively small changes in the environment have major effects on the ecosystem.

‘The Intergovernmental Panel on Climate Change Fourth Assessment Report identified thresholds, step-changes and nonlinear interactions as key uncertainties in predicting climate change impacts on polar ecosystems,’ Dr Riddle said.

‘Because of the abrupt state change from liquid to ice, polar regions were thought to be particularly vulnerable to this type of nonlinear response. However, until now, no clear examples of the effect on ecosystems have been demonstrated.

‘Our research has shown that even a slight shift in the date of the annual sea ice departure could cause light reaching sea floor communities to exceed a threshold or tipping point, leading to widespread ecosystem shifts.’

The research team found evidence of such impacts during a study at seven shallow-water sites around Casey station between 1998 and 2004.

The team used light meters to measure seasonal light variation on the sea floor at depths of up to 10m and they photographed the coast at noon every day for two and a half years, to determine sea ice cover. They also measured the growth rates and light sensitivity of algae under different light conditions and surveyed species living on sub-tidal boulders, to see how sea floor communities varied with ice cover.

‘We found that areas that currently lose ice about two months after the summer solstice in late December, are on the cusp of a tipping point,’ Dr Riddle said.

‘If ice breakout occurs only slightly earlier, algae will be able to invade areas now dominated by invertebrates.’

The team also found that the structure of communities living on boulders on the sea floor was strongly related to sea ice duration.

‘The combined cover and diversity of most invertebrates declined steeply as sea ice duration shortened, while algae thrived,’ Dr Riddle said.

‘Such regime shifts have been observed in the Arctic, where macroalgae abruptly invaded rocky reef habitats during a period of gradual sea ice reduction.

‘About one third of the species we sampled could be locally extinct once the coast is regularly ice free for more than half the year.’

While the research demonstrates the vulnerability of shallow water polar marine ecosystems, a similar process may apply to other ecosystems that are seasonally covered by ice or snow, including Antarctic land and lake ecosystems, and alpine regions. Offshore ocean ecosystems, where light penetration to the sea floor is limited by depth, may instead experience phytoplankton proliferation.

UNSW marine biologist and lead author of the research paper, Dr Graeme Clark, said such enhanced primary production could have flow-on effects to higher trophic levels, stimulating populations of species that use plants and algae for food and/or habitat.

‘Changes in phytoplankton abundance, for example, may affect whales, krill, and other planktivores,’ he said.

‘The complexity of ecosystems makes it difficult to predict the full gamut of repercussions, but it is clear that early ice loss will pave the way for light-loving species at the expense of dark-dwellers.’

Clark GF, Stark JS, Johnston EL, et al (2013). Light-driven tipping points in polar ecosystems. Global Change Biology, 19: 3749–3761, doi: 10.1111/gcb.12337.

Wendy Pyper
Corporate Communications, Australian Antarctic Division