Trends in ice cover and associated pelagic ecosystem consequences at two Antarctic LTER sites

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Sea ice (PAL-LTER) and perennial lake ice (MCM-LTER) are important ecological drivers because they act as intermediaries between the aquatic ecosystem and the atmosphere. Sea ice is the principal physical agent structuring polar marine ecosystems, whereas perennial lake ice covers largely decouple the aquatic ecosystems from atmospheric interactions, permitting the development of unique quasi-stable biochemical systems. Sea ice cover around the western Antarctic Peninsula has been experiencing ice extent and duration decline since the 1980’s. As a result, spring phytoplankton blooms in the northern part of the peninsula have diminished because the fresh water input from melting sea ice has declined, decreasing the stability of the water column allowing for greater water column mixing. Conversely, the declining sea ice extent in the southern part of the peninsula allows for greater light penetration, which promotes phytoplankton growth. The response of zooplankton to the declining sea ice extent is more complex because they have a longer life span than phytoplankton, which integrates a climatic signal over several years. However, it is expected to eventually see zooplankton decline along with the diminishing sea ice extent. The reorganization of zooplankton coupled with diminishing sea ice conditions may negatively impact some predators (such as minke whales) that rely on both high densities of krill and sea ice for survival. The permanent lake ice on the McMurdo Dry Valley lakes thickened from 3 to 4.5 m from the early 1990’s to early 2000’s, followed by an ice thinning to ~3 m at the present time. The ecological response to the trends in lake ice thickness is complex. Even though light penetration through the ice is the main driver of the primary productivity, phytoplankton photosynthesis is also affected on an inter-annual basis by large fluxes of meltwater into the lakes, which can increase light attenuation through the water column.