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The Open Ocean in General
Terrestrial environments are primarily horizontal and non-moving. Places stay the same over long periods of time, and also stay where they are. The open ocean is nothing like that. Everything is always moving and changing, and all the gradients are vertical. The most important abiotic (non-biological) factors of open ocean ecosystems are light levels, temperature, salinity, density and water movement, and nutrients.
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Light, which primarily comes from the sun, is important to life in two ways: photosynthesis and vision. Light levels are highest at the ocean surface, and decrease logarithmically with depth down to about 150 meters. The amount of light passing through the ocean surface is affected by cloud cover, waves which reflect sunlight away (increases the ocean surface albedo), and sunlight angle of incidence which depends on latitude, season, and time of day. Within the water, how fast it attenuates depends on the amount of particles that scatter and absorb light. Water close to shore carries a lot more particles than water in the middle of the ocean.
There are three photic zones of importance to open ocean ecosystems: the euphotic zone, where photosynthesis is possible, the disphotic zone, where vision is possible, and the aphotic zone, which has no biological useful light and accounts for about 90% of the ocean volume. These correspond roughly with the various pelagic zones.
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Like light, all heat ultimately comes from the sun, with more of it absorbed into the ocean at the equator than the poles. Cloud cover and winds reduce the amount that the ocean absorbs, the latter by making waves which increase surface reflectance (albedo). Heat moves around among different water masses within the ocean as a whole.
Temperature affects chemical processes such as the precipitation of calcium carbonate, which many living things use to make shells. It affects physiological function; proteins have a temperature range where they can function properly. And it affects water density - cold water is more dense than warm.
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Salinity is a measure of the total dissolved solids in water (mostly salt), with an ocean global average of 35 parts salt per thousand parts water. Salts come from land via rivers, where it concentrates as ocean water evaporates.
Ocean salinity confers an average pH of 8.1 which buffers well (that is, maintains itself) and which favors precipitation of calcium carbonate (used to make shells). It decreases the freezing point of water, so that in natural environments on Earth, it doesn't exactly freeze, it becomes more like slush. Salinity also affects water density; saltier water is more dense.
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Density and Water Movement
Salinity and temperature together determine water density. Dense water masses sink toward the bottom (known as downwelling) while less dense water masses rise toward the top (upwelling). Density thus controls water circulation and stratification.
Water moves around in the open ocean due to three main factors: wind, which makes waves and has a strong influence on where surface waters go; gravity, which causes tides and the aforementioned density flows; and the rotation of the Earth, which makes currents as the ocean bottom moves out from underneath the water above it, with the equator moving faster than the poles. Ocean water exists in distinct masses that move around and pass each other, sometimes in stratified layers that don't mix, other times with turbulence that does.
As water in the ocean moves around, it disperses things in it in some places and concentrates them in others, creating environmental heterogeneity (patches of stuff). Abiotic factors (chemicals, organic and inorganic particles) and living organisms (plankton) can travel quite far. Water movement helps keep nutrients from sinking to the ocean bottom.
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Nutrients and Chemistry
Nutrients are dissolved substances that support growth. The most important of these are nitrogen (ammonium, nitrite, nitrate), phosphorus (phosphates), silicon (silicate, used by diatoms), carbon (methane, carbon dioxide), and sulfur (sulfate). Oxygen is a limiting factor in some ecosystems where water is highly stratified (doesn't mix between top and bottom layers). Iron may be an important trace element in some ecosystems. In deep-sea environments far from sufficient light, such as around rift vents, growth can be supported by the presence of reduced compounds that can be used as an energy source for chemosynthesis (as opposed to photosynthesis); examples are methane, hydrogen sulfide, and ammonium.
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Resources in the Open Ocean are Ephemeral
On the whole, open ocean ecosystems are best defined as having patchy and ephemeral resources. All of these abiotic factors play a part in determining where conditions are right for life to grow. When there is more light for photosynthesis near the surface and more nutrients farther down, somewhere in between there will be an optimal depth for primary production, and indeed patches of microplankton often exist in a layer there. The open ocean is filled with these kinds of vertical gradients which are always shifting around and changing.
Carol M. Lalli and Timothy R. Parsons. Biological Oceanography: An Introduction, 2nd ed. Butterworth-Heinemann. 1997.