Due to different oceanographic and geological characteristics, benthic algal communities of Antarctica and the Arctic differ strongly. Antarctica is characterized by high endemism, whereas in the Arctic only a few endemic species occur. In contrast to the Antarctic region, where nutrient levels never limit algal growth, nutrient levels in the Arctic region are depleted during the summer season. Both regions have a strongly seasonally changing light regime, fortified by an ice covering throughout the winter months. After months of darkness, algae are suddenly exposed to high light caused by the breaking up of sea ice. Simultaneously, harmful ultraviolet radiation (UVR) enters the water column and can significantly affect algal growth and community structure. In the intertidal zone, fluctuations of temperature and salinity can be very large. Ice scours can further influence growth and settlement of intertidal algae. The subtidal zone offers a more stable habitat than the intertidal, permitting the growth of larger perennial algae and microbial mats. Polar regions are the areas most affected by global climate change, i.e., glacier retreat, increasing temperature and sedimentation, with as yet unknown consequences for the polar ecosystem.