Implications of brine channel geometry and surface area for the interaction of sympagic organisms in Arctic sea ice

Dynamic temporal and spatial changes of physical, chemical and spatial properties of sea ice pose many challenges to the sympagic community which inhabit a network of brine channels in its interior. Experiments were conducted to reveal the influence of the internal surface area and the structure of the network on species composition and distribution within sea ice. The surface of the brine channel walls was measured via a newly developed method using a fluorogenic tracer. These measurements allowed us to quantify the internal surface area accessible for predators of different sizes, at different ice temperatures and in different ice textures. Total internal surface area ranged from 0.6 to 4 m2 kg−1 ice and declined with decreasing ice temperature. Potentially, 6 to 41% of the area at −2°C is covered by micro-organisms. Cooling from −2 to −6°C drastically increases the coverage of organisms in brine channels due to a surface reduction. A combination of brine channel frequency measurements with an artificial brine network experiment suggests that brine channels ≤200 μm comprise a spatial refuge with microbial community concentrations one to two magnitudes higher than in the remaining channel network. The plasticity of predators to traverse narrow passages was experimentally tested for representative Arctic sympagic rotifers, turbellarians, and nematodes. By conforming to the osmotic pressure of the brine turbellaria match their body dimensions to the fluctuating dimensions of the brine channel system during freezing. Rotifers penetrate very narrow passages several times their body length and 57% their body diameter. In summary, ice texture, temperature, and bulk salinity influence the predatory–prey interactions by superimposing its structural component on the dynamic of the sympagic food web. Larger predators are excluded from brine channels depending on the architecture of the channel network. However, extreme body flexibility allows some predators to traverse structural impasses in the brine channel network.