Ocean Habitats

This section will explore habitats in the world's oceans.  The biome concept does not extend into the oceans, but many ocean habitats can be delineated in a way very similar to the way we distinguish biomes on land.  We won't look at all the ocean habitats here, but instead we'll focus on some particular nearshore habitats - coral reefs, rocky shores, mangrove forests and salt marshes.

 

 

 

 

Proceed to Coral Reefs Proceed to Mangrove Swamps
Proceed to Coral Reef Fish Proceed to Sandy Shores
Proceed to Coral Reef Invertebrates Proceed to Rocky Shores

Currents

Understanding any marine system depends on an understanding of the currents that move through the system, because it is these currents that distribute nutrients and move many animals and plants.  Globally, such currents affect all of the biomes on land.

 Water may flow for several reasons, but gravity is at the root of all of them. We are familiar with the simple flow of water downhill that occurs in streams. Remember, though, that water is limited in how fast it can flow. One meter per second is a very fast flow indeed in a stream; even a waterfall usually does not exceed 3 m/s. Over this speed, water separates into smaller droplets, and as the droplets decrease in size they are more easily slowed by the air. Think - rain falls from great heights, yet the speed does reach a maximum. Most freshwater currents are caused by simple gravity pulling water down a slope.

  Most currents are formed more indirectly, however. Density differences, whether due to different salinities or temperatures, cause water to sink or float in relation to the water around it, and the result is a current. Water may also move in response to moving air (waves, surface currents, seiches), the gravitational pull of the Sun and Moon (tides), seismic activity (tsunami), or even the motion of the earth. With the exception of wind induced currents, most of these are more common in marine systems and we will examine them in turn.

  Air is sent in motion by density differences due to differing temperatures. For instance, air heated over a land mass during the day will rise and be replaced by cooler air flowing from a body of relatively cool water nearby (an onshore breeze, the opposite, an offshore breeze, occurs when the water is warmer than the land). As the air passes over the water, it causes the water to move along with it. This effect is strongest at the surface and decreases with depth. Since the water near the surface is moving faster, it piles up in waves that are constantly breaking down as gravity pulls on the water. A stronger wind will be able to pile up more water, thus creating larger waves. Very strong wind can whip the water at the edge of the waves into a frenzy of foam, these waves are known as whitecaps. Waves being actively formed by wind typically have short wavelengths and are known as chop; long, low, smooth waves from storms long past are known as swells. The distance of water over which the wind passes is known as fetch; the greater the fetch, the greater effect the wind will have. A structure (ship, shore, breakwater, etc.) typically has a weather side exposed to the wind and waves; the side away from the winds is the lee side.

  It is generally known but poorly understood that waves cause as much vertical as lateral displacement of the water particles themselves, and that there is little net movement of the water particles. The motion of the average water particle as a wave passes a fixed point is circular; it rises, moves forward, falls, moves backward, and rises again. The circular movement of water near the surface sets up similar, smaller circular patterns in the water below to a depth equal to about 1/2 of the wavelength of the wave (Fig. 1). Actually, what is moving are the high and low points in the water, not the water itself. Still, if all the high points are moving in the same direction, this will cause a net flow of water (a surface current).

 

Figure 1. Water motion in a wave. Wave height is indicated by H; wavelength is the distance between the tops of adjacent crests (or the bottoms of adjacent troughs). The circles indicate the relative motions of water particles at different depths; wave-induced water motion decreases with depth, and is negligible below a depth equal to 1/2 the wavelength. There is some small net movement of the water particles in the direction of the wind.

The waves may bounce off solid objects and be reflected back into the open water; under these conditions the water surface can be a very confused place with waves moving in all directions simultaneously. On other shores, the energy of the wave is dissipated as it breaks on the shoreline. Breaking occurs as the water particles reach shallow water where they cannot complete the bottom part of their circle. They hit the bottom and slow down. As they slow down, more water comes in from behind and the wave grows taller, with the top moving faster than the bottom. This obviously cannot continue for long, and eventually the wave topples over, or breaks. Waves with long wavelengths break in deeper waters, and an experienced eye can judge wavelength and determine depth by where the waves are breaking. Particles proportional in size to the size of the wave may be picked up from the bottom and moved shoreward by the waves, a process known as onshore transport. Water flowing back from breaking waves is known as undertow, and constitutes a current in its own right. If the waves approach the shore at a slight angle, a longshore current will develop along the beach. The longshore current moves parallel to the beach in the opposite direction to that from which the waves approach. The longshore current can be an important factor in shaping the shoreline by distributing the material brought in by onshore transport. At various points, the longshore current will abruptly turn seaward and form a rip current, a sudden return flow to the ocean (lake). Such rip currents may carry unwary swimmers out to sea; it is nearly impossible to swim directly against them. The best strategy is to remember that they are often quite narrow and to swim perpendicular to the rip current until clear, then swim back to shore.

For more on the physics of water, click here.

Tides

Tides occur when the gravity of the Sun and/or Moon acts to slightly offset the gravitational pull of the Earth, allowing the water to rise slightly. It is easiest to think of this as if the sun and moon remained stationary and the Earth revolved under them. Wherever the Sun or Moon is directly overhead, there is a slight bulge in the ocean. A similar bulge is also seen on the opposite side of the planet due to centrifugal force. As the position of the Sun and Moon changes in relation to the surface of the Earth, the bulge seems to move. When the bulge approaches a shore, this results in an apparent high tide; the areas exactly halfway between the two bulges are experiencing low tides at the same moment. Lunar tides are more apparent than the solar tides and thus it is the motion of the Moon, which circles the Earth every 25 hours, that determines the number of high tides (2) which will occur each day.

Figure 3. Tides. The ellipse around the Earth represents a greatly exaggerated profile of the tide. Neap tides occur when the gravitational pull of the Sun and the Moon reinforce each other (every two weeks at full or new moons); spring tides occur when the Moon is at right angles to the Sun (every two weeks at half moons). Obviously not to any kind of scale.

When the Sun and Moon are in alignment (on the same plane) they will reinforce each other's gravitational pull. Thus, whenever there is a full or new moon (every two weeks) the tides will be particularly high and low and are referred to as spring tides (Fig. 3). Whenever the Moon is a half crescent (half moon, every two weeks) the Sun and Moon are at right angle to each other and cancel each other out to some extent. This results in minimal tidal ranges or neap tides (Fig. 3).

The effects of shoreline shape, ocean basin shape, winds, tidal currents, etc., combine to produce variations from the ideal semidiurnal (twice-daily) pattern pictured above. Such a pattern does exist on the eastern coast of North America, and in several other places in the world, but other areas may only experience diurnal (once daily tides) or even no tide at all. The size of the tide is also affected by such patterns; generally open shorelines show less tidal range than funnel-like estuaries such as the Bay of Fundy where the daily tidal range is over 10 m. When such a high tide sweeps up an estuary it can meet the water flowing down to the sea and form a wall of water known as a tidal bore; the Amazon River has a particularly dramatic tidal bore on some of its distributaries.

Tsunami

Tsunami are often called tidal waves because they mimic tides. Tsunami (a Japanese word, the Japanese, living on islands in an area rich with seismic activity have had long experience here) are generated whenever a shift in the Earth's crust displaces water. For instance, when California finally slides into the sea, water will have to move out of the way (this can be simulated by sitting down abruptly in the bathtub, or by doing a cannonball, preferably not in the bathtub, as your mother has no doubt told you). This water travels away at very high speeds, and, at sea, is hardly noticeable. The problem occurs in coastal areas where the tsunami reaches shallow water. The shallow bottom drags on the wave, and the traditional process of wave breaking begins - but with a much larger wave to deal with. As with tides, the effect is pronounced in enclosed areas such as bays. Since people concentrate in such areas, which normally offer protection from wind-formed waves, there is considerable potential for loss of life when a tsunami reaches such a point. Much of the loss of life surrounding major volcanic events in the South Pacific was due to tsunami rather than the volcano itself, since most island people are smarter than to live near an active volcano (sacrificing virgins all the time does not lend itself to population enhancement). Tsunami can also form when a rift opens in the ocean floor as two plates move apart (try separating your hands or legs underwater in the bathtub). On a much smaller scale, the sudden entry into the water of a large chunk of ice from a glacier may cause a small tsunami; occasionally these are of sufficient size to swamp boats or do other localized damage.

The Coriolis Effect and Ocean Currents:

The rotation of the Earth itself will affect the flow of water (or air) once gravity or other forces have put it into motion. Imagine a current heading north in the Northern Hemisphere. The Earth is rotating to the east, and the water picks up that momentum. The Earth rotates fastest at the equator, and slowest at the poles. Therefore, as our water travels north, it moves to a part of the planet that is not moving eastward as fast as the water itself is. Therefore, the water current ends up moving eastward as well, resulting in a current that curves to the right. This effect is called the Coriolis effect; it is most pronounced at the poles and weakest at the equator. It has effects on virtually all moving water or air on the planet. Its effects are the opposite in the Southern Hemisphere, where currents tend to curve left. For somewhat obscure reasons, winds around low pressure zones act differently. Wind around a low pressure zone in the Northern Hemisphere rotate counterclockwise (to the left) because of this force; storms in the Southern Hemisphere rotate clockwise for the same reason. Remember that a storm is simply air moving towards a region of low pressure or away from a region of high pressure; such currents would normally be in a straight line were it not for the Coriolis effect.

The directions of flow do not hold true for bathroom fixtures; for instance, if you take your sink to Australia the flow will not necessarily reverse itself. My sink drains clockwise, but my toilet flushes counterclockwise; obviously basin morphology and design (in my toilet's case the little streams of water are set up to induce counterclockwise flow) can override the Coriolis effect, at least in small containers. Another factor in determining which way the water will flow in a small system is chaos theory. Basically, chaos theory examines the many systems which appear to be random, but in fact are deterministic. Usually, in chaotic systems, oscillations may occur which make the system unpredictable, but, if the physics underlying the phenomenon can be expressed in mathematical equations, it is usually possible to show that for a given set of initial conditions, the outcome (direction of fluid flow, population size, heart rate) will be the same, however messy. The problem with predicting what a chaotic system will do involves the impossibility of knowing (with enough detail) exactly what the initial conditions are. For instance, it has been said that a butterfly flapping its wings in Mexico could have an effect on a hurricane in the Caribbean. Chaos theory is new; it is having a profound effect on the way we see many biological phenomena; and it ultimately explains why weather forecasting will never be an exact science no matter how powerful our computers become.

In the open ocean, the Coriolis effect leads to another phenomenon, the Ekman spiral. As the wind, for instance, blowing north, starts to move the water at the surface in that direction, the Coriolis effect deflects the water at a 45o angle (to the right or left, depending on hemisphere). As this water moves out, it also pulls along the water below it, but, again, the Coriolis effect pulls this water 45o (90o to the original wind), and so on. Summed over depth, the net flow of the water is about 90o to that of the wind. Thus, in the Northern Hemisphere, a wind coming from the east (an easterly) will result in a current to the north. In the oceans, these currents interact to form huge circular currents or gyres (Fig 4).

Disclaimer: Be very careful when doing your homework not to spill water out of the tub. If you get in trouble with your mom, RA, landlord, etc. it is your own fault and I didn't make you do it. You're grown-up now. Try to keep the splashing to a minimum (bubble bath helps hold down the noise). Also, note that the majority of these experiments will not work with two people in a standard American bathtub, so despite my usual admonitions to work together, this isn't a good time to do so. I hereby release myself from all claims of water damage to carpets, floors, rubber ducks and other household artifacts.

Figure 4. Major ocean currents. Note in particular the gyres formed in each of the main oceans (2 each in the Atlantic and Pacific. The gyres do not usually cross the equator, but do set up strong equatorial currents. Note that the Gulf Stream (not labeled) carries warm water from the equator northward and thus warms Europe; similar warm currents include the Japan Current in the northern Pacific, the Aguinas Current warming southern Africa's eastern coast, and the Brazil current warming the eastern coast of Brazil. In the southern hemisphere there is relatively complete circumpolar flow; in the northern hemisphere land masses prevent such flow. Note also the secondary gyre formed north of the Gulf Stream.

Marine Habitats

We have already considered many aspects of marine habitats. The open water of the ocean is known as the limnetic or pelagic zone (Fig. 5); the oceanic zone refers exclusively to waters not lying over the continental shelf and neritic refers to those coastal waters over the shelves. In the oceans, the benthic zones are the littoral, which here means the area between high tide and 100m deep and has nothing to do with plant life; it includes the continental shelf, which extends to about 100 m deep; the bathyl zone which extends to 2000 m (below the photic zone or LCP), the abyssal zone from 2000 to 4000 m, and the deep oceans or hadal zone from 4000 to 10,000 m.

Figure 5. Habitats in the ocean. The benthic habitats are the littoral zone from high tide to about 100m, the bathyl zone from 100 to 2 000m, the abyssal zone from 2 000 to 4 000m, and the hadal zone below 4 000m. The continental shelves usually provide the bulk of the littoral habitat. The open water zones are the neritic zone over the continental shelves, and the oceanic zone over the deeper areas; together the neritic and oceanic zones constitute the pelagic zone. The light compensation point (LCP) delineates the upper photic zone from the lower aphotic zone, the depth of the LCP depends on water clarity.

As a general rule, the oceans do not stratify the way that lakes do. For a variety of reasons, the bottom of the ocean usually does not go anoxic, though exceptions to this exist, especially where human pollution is severe. In other areas, powerful currents sweep the bottom and bring in fresh O2. Oceanic currents are caused by heating of the water at the equator; the warm water flows poleward near the surface in currents like the Gulf Stream, near the poles it cools and returns to the equator along the bottom. These currents are, of course affected by both the Coriolis force and the nature of the ocean basins. Other currents are created by the wind; for instance, strong winds blowing north along the coast of Peru move surface waters away from the coast; the surface water is replaced by nutrient rich water from below in an upwelling current; algae grow; small fish come to feed on the algae; and you get anchovies on your pizza. Of course, the main differences between marine habitats like the open ocean and its bottom and lakes are the salinity, the depth, and the lack of allochthonous inputs. The bottom is nearly lightless, very cold, and utterly dependent on photosynthesis in the surface waters above for any input of energy. In many ways, deep ocean habitats resemble terrestrial caves as ecosystems go.

It has been said that the pelagic ocean, from the surface to the bottom, is a biological desert. This is misleading because deserts are often rich in fauna and flora, while the analogy tries to convey the relative lack of organisms in the open ocean. The open oceans are relatively bare because of the lack of nutrients in the surface waters. The living organisms that do exist here quickly use up the nutrients that are available, and further productivity is limited. The bottom below is impoverished because of the limited productivity above. Another factor is the relative lack of any type of structural complexity in the environment; it is a general axiom that, in the absence of toxins and the presence of the essentials of life, the more complex the environment is spatially, the more species will be present.

The oceans also have several unique habitats not duplicated in freshwater. Coral reefs are among the most productive ecosystems in the world. They are formed by precipitation of CaCO3 (limestone) from the water by small anthozoans. These structures may be only centimeters in length, or they may be thousands of kilometers in length like the Great Barrier Reef of Australia, arguably the largest artifact created by any organism on the planet. Coral reefs form only in clear, shallow, warm water. The coral polyps have endosymbiotic algae which produce much of their food; this accounts for the need for shallow (less than 90 m, corals are most common at depths less than 50m), clear water. Apparently the temperature is also critical, perhaps because of the need for high rates of calcification; reefs do not form below 18o C, and temperatures above 30o C may also have a deleterious effect. Still, this temperature and depth restriction leaves large areas of ocean, particularly in the Pacific and Caribbean, available for colonization by corals. The many types of corals which typically grow together provide a diverse habitat with many crevices and other hiding places for animals, as well as numerous sites for the growth of algae. This leads to an extremely diverse community that is becoming increasingly well studied by use of SCUBA gear. Among other things, the fact that corals only grow under certain conditions gives us important clues about prehistoric climates and sea levels; wherever fossil coral is found (and remember that coral is self-fossilizing!) there was a warm, shallow ocean.

Kelp beds form in waters too cold for coral, but kelp beds are almost as diverse. Kelp is a brown alga (Division Phaeophyta, Genus Nereocystis) which may reach 40 m in length, it provides a habitat for a diverse assemblage of organisms. Other types of large seaweeds such as Sargassum (another brown alga) or Eel Grass, or Irish Moss, etc. all form extensive beds with complex spatial habitats and a relatively high animal diversity.

Shores are another region of the ocean with a good deal of diversity. The nature of the shoreline with respect to the substrate (solid rock, boulders, cobble, sand, silt) and the strength of the waves, along with the tidal range combine to determine what sort of community will develop. Depositional, wave-washed shores of boulders or smaller debris will be too unstable for a rich community to develop in the wave zone, but in areas protected from waves, such as bays or tidal flats, diverse communities can appear, often centered around some type of vegetation such as marsh grasses or seaweeds. Rocky shores provide attachment for a wide variety of organisms, which often arrange themselves in very discrete vertical bands or strata. Rocky shores also may allow tidal pools to form. Tidal pools are extreme environments, yet they support surprisingly diverse communities. Submerged only at high tide (every 12 hours), tidal pools spend the next 12 hours being exposed to rapid warming or cooling, and greatly increased salinity (unless it rains). Organisms which live there must therefore be both euryhaline and eurythermal. Other organisms of the intertidal zone, the area between the high and low tide marks, must be similarly adapted. Of course, all of these shoreline organisms must be able to maintain a grip on the substrate in the face of storm-driven waves. Some are champions at this; the glue holding barnacles to rocks has long been a subject of study in naval research labs desperate to find a strong, waterproof glue.

Estuaries form where rivers enter the ocean in a protected area. As mentioned earlier, estuaries often show complex vertical zonation as waters of various temperatures and salinities mix. In addition, there are often vast beds of vascular plants in the shallow waters to provide additional habitat and cover. The varying temperature and salinity calls for adaptable organisms, and many answer the call, making estuaries another diverse habitat. An abundance of nutrients make estuaries highly productive, and the calm, sheltered waters combines with the nutrients and cover to make estuaries an important nursery area for the larvae of many species. Many of our important food fish start out their lives in estuaries. Humans also find the areas around estuaries attractive places to live, and human impacts on estuaries is severe in many areas. Protecting these crucial habitats is essential for maintaining stability in a number of marine habitats.

Perhaps the weirdest marine habitat are the recently discovered (1977) hydrothermal vents on the ocean floor. These vents form where water, heated and mineralized by contact with volcanic rock, wells up out of the ocean floor. The warm water carries a large amount of H2S and other chemicals that chemosynthetic bacteria can extract energy from. These bacteria act as the base of the food chain, either by being ingested or by living as endosymbionts in other organisms clustered around the vent. A variety of worms, echinoderms, crustaceans, molluscs and other phyla cluster around these vents, where the water temperature may exceed 200o C (it doesn't boil because of the pressure). Obviously, these organisms have found some way to stabilize their proteins at these temperatures. Each vent is like an "island" - separated from other vents by stretches of cold, nutrient poor water that apparently forms an effective barrier to dispersal, since each vent may have a unique community formed of endemic (found nowhere else, as opposed to pandemic, found everywhere) species. You might want to read the fictional account by Arthur C. Clarke in 2010: Odyssey Two of similar life elsewhere in the solar system.

Proceed to Coral Reefs Proceed to Mangrove Swamps
Proceed to Coral Reef Fish Proceed to Sandy Shores
Proceed to Coral Reef Invertebrates Proceed to Rocky Shores
 

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