Marine habitats The marine environment supplies many kinds of habitats shifting and ephemeral. Swimming organisms find areas that support marine life. Marine life depends in some way by the edge of a continental shelf a good habitat, but only on the saltwater that is in the sea (the term marine comes while upwellings bring nutrient rich water to the surface. from the Latin mare, meaning sea or ocean). A habitat is Shellfish find habitat on sandy beaches, but storms, tides an ecological or environmental area inhabited by one or and currents mean their habitat continually reinvents it- more living species.[1][2] self. Marine habitats can be divided into coastal and open The presence of seawater is common to all marine habi- ocean habitats. Coastal habitats are found in the area that tats. Beyond that many other things determine whether a extends from as far as the tide comes in on the shoreline marine area makes a good habitat and the type of habitat out to the edge of the continental shelf. Most marine life it makes. For example: is found in coastal habitats, even though the shelf area occupies only seven percent of the total ocean area. Open • temperature – is affected by geographical latitude, ocean habitats are found in the deep ocean beyond the ocean currents, weather, the discharge of rivers, and edge of the continental shelf. by the presence of hydrothermal vents or cold seeps Alternatively, marine habitats can be divided into pelagic • sunlight – photosynthetic processes depend on how and demersal habitats. Pelagic habitats are found near deep and turbid the water is the surface or in the open water column, away from the bottom of the ocean. Demersal habitats are near or on • nutrients – are transported by ocean currents to dif- the bottom of the ocean. An organism living in a pelagic ferent marine habitats from land runoff, or by up- habitat is said to be a pelagic organism, as in pelagic fish. wellings from the deep sea, or they sink though the Similarly, an organism living in a demersal habitat is said sea as marine snow to be a demersal organism, as in demersal fish. Pelagic habitats are intrinsically shifting and ephemeral, depend- • salinity – varies, particularly in estuaries or near ing on what ocean currents are doing. river deltas, or by hydrothermal vents Marine habitats can be modified by their inhabitants. • dissolved gases – oxygen levels in particular, can Some marine organisms, like corals, kelp, mangroves and be increased by wave actions and decreased during seagrasses, are ecosystem engineers which reshape the algal blooms marine environment to the point where they create fur- ther habitat for other organisms. • acidity – this is partly to do with dissolved gases above, since the acidity of the ocean is largely con- trolled by how much carbon dioxide is in the water. 1 Overview • turbulence – ocean waves, fast currents and the agi- tation of water affect the nature of habitats • cover – the availability of cover such as the adja- cency of the sea bottom, or the presence of floating objects • the occupying organisms themselves – since organ- isms modify their habitats by the act of occupying them, and some, like corals, kelp, mangroves and seagrasses, create further habitats for other organ- isms. Only 29 percent of the world surface is land. The rest is ocean, home to the marine habitats. The oceans are nearly four kilome- The ocean occupies 71 percent of the world surface, aver- ters deep on average and are fringed with coastlines that run for aging nearly four kilometers in depth. There are five ma- nearly 380,000 kilometres. jor oceans, of which the Pacific Ocean is nearly as large as the rest put together. Coastlines fringe the land for nearly In contrast to terrestrial habitats, marine habitats are 380,000 kilometres. 1 2 2 OCEAN CURRENTS but must drift instead with the currents. If the current carries the right nutrients, and if it also flows at a suitably shallow depth where there is plenty of sunlight, then such a current itself can become a suitable habitat for photo- synthesizing tiny algae called phytoplankton. These tiny plants are the primary producers in the ocean, at the start of the food chain. In turn, as the population of drifting phytoplankton grows, the water becomes a suitable habi- tat for zooplankton, which feed on the phytoplankton. While phytoplankton are tiny drifting plants, zooplank- ton are tiny drifting animals, such as the larvae of fish and marine invertebrates. If sufficient zooplankton establish themselves, the current becomes a candidate habitat for the forage fish that feed on them. And then if sufficient Land runoff, pouring into the sea, can contain nutrients forage fish move to the area, it becomes a candidate habi- tat for larger predatory fish and other marine animals that feed on the forage fish. In this dynamic way, the current Marine habitats can be broadly divided into pelagic and itself can, over time, become a moving habitat for multi- demersal habitats. Pelagic habitats are the habitats of the ple types of marine life. open water column, away from the bottom of the ocean. Demersal habitats are the habitats that are near or on the bottom of the ocean. An organism living in a pelagic habitat is said to be a pelagic organism, as in pelagic fish. Similarly, an organism living in a demersal habitat is said to be a demersal organism, as in demersal fish. Pelagic habitats are intrinsically ephemeral, depending on what ocean currents are doing. The land-based ecosystem depends on topsoil and fresh water, while the marine ecosystem depends on dissolved nutrients washed down from the land.[10] Ocean deoxygenation poses a threat to marine habitats, due to the growth of low oxygen zones.[11] 2 Ocean currents This algae bloom occupies sunlit epipelagic waters off the south- ern coast of England. The algae are maybe feeding on nutrients from land runoff or upwellings at the edge of the continental shelf Ocean currents can be generated by differences in the density of the water. How dense water is depends on how saline or warm it is. If water contains differences in salt content or temperature, then the different densities will initiate a current. Water that is saltier or cooler will be denser, and will sink in relation to the surrounding wa- ter. Conversely, warmer and less salty water will float to the surface. Atmospheric winds and pressure differences also produces surface currents, waves and seiches. Ocean currents are also generated by the gravitational pull of the sun and moon (tides), and seismic activity (tsunami).[12] Ocean gyres rotate clockwise in the north and counterclockwise in the south The rotation of the Earth affects the direction ocean cur- rents take, and explains which way the large circular In marine systems, ocean currents have a key role deter- ocean gyres rotate in the image above left. Suppose a mining which areas are effective as habitats, since ocean current at the equator is heading north. The Earth rotates currents transport the basic nutrients needed to support eastward, so the water possesses that rotational momen- marine life.[12] Plankton are the life forms that inhabit tum. But the further the water moves north, the slower the ocean that are so small (less than 2 mm) that they the earth moves eastward. If the current could get to the cannot effectively propel themselves through the water, North Pole, the earth wouldn't be moving eastward at all. 3 To conserve its rotational momentum, the further the cur- rent travels north the faster it must move eastward. So the effect is that the current curves to the right. This is the Coriolis effect. It is weakest at the equator and strongest at the poles. The effect is opposite south of the equator, where currents curve left.[12] 3 Marine topography Coastlines can be volatile habitats 5 Coastal See also: Coast, Coastal biogeomorphology and Coastal fish Marine coasts are dynamic environments which con- stantly change, like the ocean which partially shape them. The Earth’s natural processes, including weather and sea level change, result in the erosion, accretion and resculp- turing of coasts as well as the flooding and creation of continental shelves and drowned river valleys. The main agents responsible for deposition and erosion Map of underwater topography (1995 NOAA) along coastlines are waves, tides and currents. The for- mation of coasts also depends on the nature of the rocks they are made of – the harder the rocks the less likely Marine topography refers to the shape the land has when they are to erode, so variations in rock hardness result in it interfaces with the ocean. These shapes are obvious coastlines with different shapes. along coastlines, but they occur also in significant ways underwater. The effectiveness of marine habitats is par- Tides often determine the range over which sediment is tially defined by these shapes, including the way they in- deposited or eroded. Areas with high tidal ranges allow teract with and shape ocean currents, and the way sun- waves to reach farther up the shore, and areas with lower light diminishes when these landforms occupy increasing tidal ranges produce deposition at a smaller elevation in- depths. terval. The tidal range is influenced by the size and shape of the coastline. Tides do not typically cause erosion by Marine topographies include coastal and oceanic land- themselves; however, tidal bores can erode as the waves forms ranging from coastal estuaries and shorelines to surge up river estuaries from the ocean.[18] continental shelves and coral reefs. Further out in the open ocean, they include underwater and deep sea fea- Waves erode coastline as they break on shore releasing tures such as ocean rises and seamounts.