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 prop erties of sea ice pose many challenges to the sympagic community which inha bit 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 sur face of the brine channel walls was measured via a newly developed method u sing a fluorogenic tracer. These measurements allowed us to quantify the in ternal surface area accessible for predators of different sizes, at differe nt ice temperatures and in different ice textures. Total internal surface a rea ranged from 0.6 to 4 m(2) kg(-1) ice and declined with decreasing ice t emperature. Potentially, 6 to 41% of the area at -2 degrees C is covered by micro-organisms. Cooling from -2 to -6 degrees C drastically increases the coverage of organisms in brine channels due to a surface reduction. A comb ination of brine channel frequency measurements with an artificial brine ne twork experiment suggests that brine channels less than or equal to 200 mu m 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 rep resentative Arctic sympagic rotifers, turbellarians, and nematodes. By conf orming to the osmotic pressure of the brine turbellaria match their body di mensions to the fluctuating dimensions of the brine channel system during f reezing. Rotifers penetrate very narrow passages several times their body l ength and 57% their body diameter. In summary, ice texture, temperature, an d bulk salinity influence the predatory-prey interactions by superimposing its structural component on the dynamic of the sympagic food web. Larger pr edators are excluded from brine channels depending on the architecture of t he channel network. However, extreme body flexibility allows some predators to traverse structural impasses in the brine channel network. (C) 2000 Els evier Science B.V. All rights reserved.