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Polar regions have experienced significant warming in recent decades. Warming has been most pronounced across the Arctic Ocean Basin and along the Antarctic Peninsula, with significant decreases in the extent and seasonal duration of sea ice. Rapid retreat of glaciers and disintegration of ice sheets have also been documented. The rate of warming is increasing and is predicted to continue well into the current century, with continued impacts on ice dynamics. Climate-mediated changes in ice dynamics are a concern as ice serves as primary habitat for marine organisms central to the food webs of these regions. Changes in the timing and extent of sea ice impose temporal asynchronies and spatial separations between energy requirements and food availability for many higher trophic levels. These mismatches lead to decreased reproductive success, lower abundances, and changes in distribution. In addition to these direct impacts of ice loss, climate-induced changes also facilitate indirect effects through changes in hydrography, which include introduction of species from lower latitudes and altered assemblages of primary producers. Here, we review recent changes and trends in ice dynamics and the responses of marine ecosystems. Specifically, we provide examples of ice-dependent organisms and associated species from the Arctic and Antarctic to illustrate the impacts of the temporal and spatial changes in ice dynamics.

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Researchers often encounter taxonomic problems when analysing data on changes in macrobenthic assemblages due to pollutlon. In response, some authors have suggested that data at famlly and even phylum level may be sufficient to detect such changes. This hypothesis is commonly tested by visual comparisons of ordinabon plots generated by multidimensional scaling (MDS). Our study assessed this approach using a dataset previously shown to contain patterns in species composition that were related to both habitat type and heavy metal pollution. These data allow us to assess the ability of analyses at reduced taxonomic resolutions to detect 'signals' associated with pollution gradients among the 'noise' associated with habitat gradients. Such situations will arise in most studies of the impacts of pollutlon on inshore macrobenthic assemblages. The patterns associated with both habitat and pollution gradients were visible in ordlnations based on data pooled to hlgher taxonomic levels. The similarity among ordinations based on different sets of pooled data was not fully supported by more detailed analyses. Changing the number of dimensions in ordinations, the taxonomic resolution, the attribute:object ratio and the non-zeros ratio of datasets all affected the matrlx of association measures and the results of ordinations and Procrustes rotations. The effects of these changes were complex. Further work is needed to determine the likely effect of using data with reduced taxonomic resolution affect to detect impacts.

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