Light‐driven tipping points in polar ecosystems

Abstract

Some ecosystems can undergo abrupt transformation in response to relatively small environmental change. Identifying imminent 'tipping points' is crucial for biodiversity conservation, particularly in the face of climate change. Here, we describe a tipping point mechanism likely to induce widespread regime shifts in polar ecosystems. Seasonal snow and ice-cover periodically block sunlight reaching polar ecosystems, but the effect of this on annual light depends critically on the timing of cover within the annual solar cycle. At high latitudes, sunlight is strongly seasonal, and ice-free days around the summer solstice receive orders of magnitude more light than those in winter. Early melt that brings the date of ice-loss closer to midsummer will cause an exponential increase in the amount of sunlight reaching some ecosystems per year. This is likely to drive ecological tipping points in which primary producers (plants and algae) flourish and out-compete dark-adapted communities. We demonstrate this principle on Antarctic shallow seabed ecosystems, which our data suggest are sensitive to small changes in the timing of sea-ice loss. Algae respond to light thresholds that are easily exceeded by a slight reduction in sea-ice duration. Earlier sea-ice loss is likely to cause extensive regime shifts in which endemic shallow-water invertebrate communities are replaced by algae, reducing coastal biodiversity and fundamentally changing ecosystem functioning. Modeling shows that recent changes in ice and snow cover have already transformed annual light budgets in large areas of the Arctic and Antarctic, and both aquatic and terrestrial ecosystems are likely to experience further significant change in light. The interaction between ice-loss and solar irradiance renders polar ecosystems acutely vulnerable to abrupt ecosystem change, as light-driven tipping points are readily breached by relatively slight shifts in the timing of snow and ice-loss.

Photosynthetic light requirements and vertical distribution of macroalgae in newly ice-free areas in Potter Cove, South Shetland Islands, Antarctica

“…In this whole context, we believe that future ice retreat will continue to favor macroalgal colonization in new coastal areas (with a reduced vertical distribution in areas close to the glacier run-off), causing higher productivity and carbon-sequestration but lower local biodiversity (due to glacial effect and rapid changes) as postulated by Worm et al (2006) and Clark et al (2013).…”

“…Several studies predict rapid changes in rocky-shore communities due to climate change. Clark et al (2013) predict shifts from predominantly heterotrophic to autotrophic states of shallow polar sea beds, and Kortsch et al (2012) observed marked community shifts with abrupt and persistent increase in macroalgal cover in two Arctic fjords due to the increased duration of ice-free periods. KrauseJensen et al (2012) also reported substantial increases in the productivity and maximal depth distribution of seaweeds for kelps in Greenland.…”

“…Particularly considering that the WAP is one of the most rapidly warming regions on Earth (Turner et al 2005), we could expect warmer summers associated with lower and even more negative CB values in areas close to the glacier run-off. On the other hand, it should also be considered that warmer winters and springs could lead to earlier sea ice melting (Schloss et al 2012, Deregibus et al unpublished data), causing an abrupt increase in light, probably compensating the reduction in PAR in summer or even significantly increasing the annual light budget for macroalgae (Clark et al 2013).…”

Disturbance effects of kelp thalli on structure and diversity of a coastal Arctic marine soft-bottom assemblage

“…The observed significant decline in sea ice cover has already influenced productivity and species interactions in Arctic coastal systems (Post et al 2013). Moreover, warming-induced loss in sea ice earlier to summer solstice has the potential to cause extensive ecological regime shifts due to light-driven tipping points, which may alter dark-adapted communities into those dominated by primary producers (Clark et al 2013). Indeed, increased kelp production and extension of kelp beds to greater depth have been reported where the period of sea ice cover has shortened (Kortsch et al 2012;KrauseJensen et al 2012;Krause-Jensen and Duarte 2014).…”

Managing direct and indirect threats to marine ecosystems to balance multiple objectives

“…Freshening of water (Rintoul 2007); iron & nutrient release from continental ice (Boyd et al 2012;Lannuzel et al 2007). Increased ocean surface layer stratification & increase in light conditions (Clark et al 2013) may cause phytoplankton blooms ) and increased primary production at sea-ice edge during spring & summer sea-ice retreat (Peck et al 2010). Micronutrients may seed water column with ice algae (Lizotte 2001), but decreased sea-ice duration may counteract effects.…”