By Laura Poppick, Staff Writer | August 05, 2013 09:07am ET
Animals that dwell on the seafloor of the Arctic and Antarctic spend most of their lives in total darkness: Sea ice blocks rays during the spring and early summer, and the sun sets completely in the winter. Late summer and early fall — when the ocean warms up enough to thaw the ice — often marks the only time these critters see light.
But as climate change causes sea ice to begin melting earlier and earlier in the summer, shallow-water ecosystems will soak up increasingly more rays. New research from a team of Australian biologists suggests this could cause a major shift in the seafloor communities along the coast of Antarctica, where invertebrates like sponges, worms and tunicates — globular organisms that anchor to rocks on the seafloor — currently dominate. A manuscript of the report is currently in press at the journal Global Change Biology. [6 Unexpected Effects of Climate Change]
“Some areas where ice breaks out early in summer are already shifting to algal domination,” said Graeme Clark, a biologist at the University of New South Wales who was involved in the study.
Seasons and tipping points
Early-summer ice melt not only lengthens the amount of time photosynthesizing organisms like macroalgae (or seaweed) can thrive under the sun during the summer, but it also increases the intensity of that exposure. The sun sits highest in the sky during the summer solstice — the period when Earth tilts most directly toward the sun — that occurs between June 20 and 23 in the Northern Hemisphere and Dec. 20 and 23 in the Southern Hemisphere, depending on the phase of the Earth’s orbit. Rays travel directly to the seafloor during this time. During spring and fall, however, low-angle rays reflect off the sea surface and often never make it to the seafloor.
This compounding effect of a longer sunlit season and higher-intensity rays could exponentially increase the amount of sunlight hitting benthic, or seafloor, communities in the coming decades and cause major tipping points for those invertebrate-dominated ecosystems, Clark said.
Tipping points occur when relatively minor environmental changes — like sea ice melting several days earlier than usual — cause rapid and significant ecological transformation. In this case, the tipping point would push ecosystems from invertebrate-dominated to algae-dominated.