Fauna of Europe of the Baltic Sea has led to the evolution of many slightly divergent species, such as the Baltic Sea herring, which is a smaller variant of the Atlantic herring. The most frequent benthic species are "Saduria entomon" and "Monoporeia affinis", which is originally a freshwater species. A great part of its bottom is anoxic and without animal life. The Baltic Sea and North Sea are also home to a variety of marine mammals (common seals, grey seals). Europe contains several important freshwater ecoregions, including the heavily developed rivers of Europe, the rivers of Russia, which flow into the Arctic, Baltic,
North Atlantic Gyre the central role it plays in the thermohaline circulation, bringing salty water west from the Mediterranean Sea and then north to form the North Atlantic Deep Water. The North Atlantic Gyre traps man-made marine debris in the North Atlantic Garbage Patch, similar to how the North Pacific Gyre traps debris in the Great Pacific Garbage Patch. The North Atlantic Gyre forms the Sargasso Sea, noted for its still waters and dense seaweed accumulations. As with many oceanographic patterns, the North Atlantic Gyre experiences seasonal changes. Stramma and Siedler (1988) determined that the gyre expands and contracts with a seasonal variance;