Sea-ice algae play important roles in polar ecosystems, both when within sea ice and once they have been released into the ocean during seasonal ice melt. The release of ice algae begins an import path for biogenic fluxes in ice-covered regions. Simple observations indicate that sea-ice algae are attached to the ice on which they live, although the attachment mechanism is unknown. However, the association of ice algae with exopolymeric substances (EPS) and the role of EPS in other forms of microbial adhesion, suggest that EPS are important in algal attachment to ice. This attachment appears to be episodically overcome during seasonal ice melt, based on evidence that algae leave the ice in local pulses. Evidence suggests several physical mechanisms that detach algae from ice, but quantitative data that would allow prediction of algal release from ice do not exist. The proposed research combines controlled laboratory experiments in specially-designed ice tanks with field observations of coastal fast-ice bottom communities near Barrow, AK. The research will address how ice algae attach to ice, the physical processes that may cause flux of algae out of the ice, and the biogeochemical characteristics of particles released from ice under different conditions.
The role of EPS in attachment of algae to sea ice will be assessed in experiments using artificially-created, sea-ice sheets colonized by ice algae. The time course of algal flux from the ice during melting will be compared between ice enriched in EPS and ice with chemically-degraded EPS. Additional experiments will focus on four specific mechanisms that potentially trigger the release of algae from sea ice: a) excess density and loss of attachment sites, b) flushing by surface meltwater, c) local heating due to light absorption, and d) under-ice currents. Each mechanism will be experimentally isolated to determine its impact on algal flux out of experimental ice sheets. For each mechanism, the minimal conditions necessary to trigger an algal pulse from the ice will be quantified for comparison with field observations. During these experiments, biogeochemically-relevant characteristics of the particles released from the ice will also be measured, including: size, sinking rate, and organic C, organic N, chlorophyll, and EPS concentrations. Complementary field studies will be conducted in each year of the proposed research. Time series of algal abundance in the ice and water column will be used to identify release events. Comparison to time series of physical variables relevant to the four proposed detachment mechanisms will allow assessment of the factors that may have triggered the pulses.
