Controls on polar ice algal communities and their coupling to spring phytoplankton communities
Keywords: Environmental Earth System Science

Sea ice algae, primary producers inhabiting sea ice, are a vital food source for upper trophic levels in spring prior to the development of summer phytoplankton blooms. As ice algae melt out of the sea ice they are eaten by zooplankton, exported to the benthos, or hypothesized to remain in the water column and seed phytoplankton blooms. Over the last few decades, ice conditions have dramatically changed on regional scales in the Arctic and Antarctic. This dissertation work seeks to understand how these drastic environmental changes impact early spring primary producers. Even though ice algal measurements have increased in recent years, they are still relatively scarce given the hostile nature of polar regions. In this dissertation, I expand ice algal measurements in the Chukchi Sea pack ice and provide the first measurements of spring ice algae along the west Antarctic Peninsula, while advancing our understanding of the linkages between ice algal and phytoplankton communities in polar oceans using a combination of fieldwork and ecosystem modeling. In the Arctic, I characterized the biomass, physiology, and community composition of the spring ice algal bloom and identified drivers of bloom decline in the Chukchi Sea. Furthermore, I explored the ice algal seeding hypothesis using multivariate statistical analyses and growth model simulations constrained with paired ice and water column taxonomic composition and algal physiology field data (Chapter 1). To go beyond annual snapshots of ice algal communities, I applied a 1-D sea ice ecosystem state model to the Chukchi Sea region and examined how changing sea ice conditions impacted ice algal production over the 1980 to 2015 period (Chapter 2). Results from these studies suggest that ice algal production has decreased 22% over time due to sea ice melting earlier in the spring season. Ice algal production is likely to continue to decline into the future as ice continues to melt earlier in spring. Our field study suggests that declines in Chukchi Sea ice algal communities will have little effect on the timing of under-ice phytoplankton blooms. In the Antarctic (Chapter 3), I characterized the taxonomic composition and physiological characteristics of the high biomass slush ice layer and used a combination of experiments to explore the fate of ice algae following ice melt. Combined, results from field samples and experiments suggest that the sea ice environment along the wAP does act as a reservoir and seeds water column populations of certain taxa that are better adapted to both low and high light conditions than their water column counterparts in spring. However, the dominant taxa seeded by sea ice has a higher sinking rate compared to other phytoplankton groups and therefore sinks following ice melt and does not persist in phytoplankton and contribute to summer phytoplankton blooms. Comparison of the ice algal communities and the linkages between ice algae and phytoplankton communities of the Arctic and Southern Oceans shows lower trophic level responses to environmental change in polar marine ecosystems are diverse and dependent on the system in question. Understanding how physical and biological drivers impact lower trophic levels is important to advance our knowledge on how continued climatic change will impact regional food web processes as well as broader global biogeochemical cycles.