The Sargasso Sea is a major oceanic gyre, with low nutrients as the main feature of the biogeochemistry of the area. In summer, the region is dominated by the Bermuda High which creates a barrier to further frontal passage and conditions for the formation of shallow, fresh, warm mixed layers, which typically shoal to less than 20 m. Temperatures peak at 28C, and rain forms low salinity layers in the surface 10 m which could stimulate phytoplankton blooms. The area is notable for several features, including its rafts of pelagic Sargassum, the Anguilla eel and the Atlantic Humpback whale which is seen in relatively large numbers. The only land mass in the Sargasso Sea is the small island of Bermuda. Surrounding it is a coral reef ecosystem which extends around the island up to 10 km from the islands. The limestone supports the most northerly coral reefs in the Atlantic, which have a reduced diversity of corals, but which are well adapted to the relatively high latitudes and clear seasonal changes. In some areas there are remarkable, nearly continuous linear sequences of emergent algal-vermetid reefs. The greatest threat to the reefs of Bermuda is siltation, and locally derived contamination effects are minimal. Northwesterly winds from North America often contain contaminants such as anthropogenic sulphur and other trace substances, but apart from this, some tar contamination from shipping traffic has been of concern. Local issues revolve around excessive fishing, some locally generated air contamination and concerns about coral reefs due to dredging and ship grounding. Generally the area is relatively free of pollution.
