Icy conditions offer a daunting habitat, but arctic marine plants are tenacious. Because of the constant ice cover and an almost total absence of tides, the waters of the Arctic Basin do not mix as they do in other oceans, so there is no opportunity for the introduction of nutrients from other ecozones. In fact, biological productivity in the ecozone is only 1% of the well-mixed Atlantic Ocean. Despite these obstacles, algae grows on the underside of sea ice in the summer. Biological hotspots consisting of blooms of microscopic plants known as phytoplankton occur in spring and summer along the edges of the pack ice or in stretches of open water, called polynyas, where currents are strong enough to keep the water from freezing. These blooms are the basis of the Arctic food chain.