Kelps (large, brown macroalgae up to 30m in length) form the basis of these highly productive systems found on shallow, subtidal rocky reefs around cold temperate and polar coastlines. Their forest-like structure and vertical habitat supports diverse epiflora and –fauna living on the kelp itself, as well as rich communities of invertebrates, fish and marine birds and mammals living and foraging in and around these ecosystems. High nutrient requirements mean these ecosystems are often associated with upwelling water, while wave action and currents are important for replenishing oxygen.