When considering aquatic habitats, many variables come to mind. Obviously, there is a continuum of salinity ranging from essentially distilled water at glacier faces and high mountain streams, to other freshwaters, to estuaries where fresh and salt waters mix, to oceans, to hypersaline environments such as the Great Salt Lake. Current is another factor; water may be still and stagnant, or flow in currents of various velocities. Currents may be unidirectional, such as in streams, or multidirectional, such as when waves wash across a beach. Aquatic habitats may occur in open water, or they may be associated with the bottom of the body of water, and both will be affected by the mechanical and chemical makeup of the local geology. All sorts of daily and seasonal temperature regimes can be expected. Aquatic habitats vary in the amount of light they receive and range in size from tiny pools at the base of a plant to the Pacific Ocean in size.
Freshwater vs. Marine Habitats
The obvious difference when comparing these two extremes is the salinity of the water, and the differences associated with that salinity. One obvious consequence of the difference in salinity is the change in osmoregulatory strategy that must take place. Many organisms in salt water are osmoconformers, essentially isotonic in relation to the seawater, although they may regulate certain ions at levels different from those of the surrounding ocean. A fair number of marine organisms are hypotonic in relation to the seawater and must therefore actively take up water to replace that they lose to the seawater. Of course, in taking on that water, they usually take in too many ions, and they must have some mechanisms for expelling those ions.
Organisms in freshwater have the reverse problem. They tend to take on water from the environment, and, in expelling the excess water, may lose important ions. Since freshwater is too dilute to make a good cytoplasm, it is no surprise that all freshwater organisms are hypertonic to the environment and that in consequence they must be active osmoregulators. Most have some mechanism to pump ions into the body. Of course, the whole problem of osmoregulation can largely be avoided by an impermeable outer body; but this in turn makes O2 uptake from the surrounding medium impossible. The respiratory surfaces thus become major sites of both ion and gas exchange. The linings of the gut and the kidneys also become important sites of ion regulation both in freshwater and in those marine species which are osmoregulators. Only two major groups of organisms with aquatic representatives do not depend on water for oxygen uptake. Both of these groups, the insects and the amniote vertebrates (turtles, snakes and lizards, crocodilians, birds, mammals) evolved on land and breath atmospheric air (some insect larvae take up O2 directly from the water and these comments do not apply to them). Their bodies are largely impervious to water or ion exchange, yet even in these groups we commonly see adaptations similar to those in groups with more intimate contact with the water. For example, both sea birds and marine turtles have salt glands near the eyes which eliminate ions from the body, and marine mammals have highly efficient kidneys.
It can be assumed that the salt content, high or low, of a body of water has relatively little impact on the taxa which are found there, since virtually all taxa have representatives in either freshwater or marine or even hypersaline environments. The only real difference in taxa composition of communities appears when comparing terrestrial and marine habitats, which seem to be the most ecologically distinct (despite the common environmental problem of water loss faced by organisms in both these habitats). Marine habitats are essentially devoid of insects and flowering plants, two groups which coevolved on land and do not seem inclined to move into marine habitats. Competition from organisms already there is often cited as a reason, but it is more likely that there has simply not been enough time for them to evolve into marine niches. In addition, both of the terrestrial groups have life cycle adaptations that are not well-suited for open aquatic habitats in any event. Going the other way, many marine species have not moved successfully onto land. Most cited are the crustaceans, and again, the usual reason given is competition from insects, but it is more likely that crustaceans have not moved onto land simply because they have not evolved breathing mechanisms that are effective on land; the same could be said for the echinoderms. In addition, many marine species have larval stages that could not exist on land even if the adult form could. It seems obvious to say that fish haven't made the transition to land, but this would not be accurate; fish did move onto land over 300 million years ago (MYA); we call their descendants amphibians, reptiles, birds and mammals.
Another difference between freshwater and saltwater, besides the ion concentration, is the density. Although there are probably other explanations, it is true that the largest organisms which have ever lived have been marine. Whether this is due to the greater support offered by dense seawater, or due to the fact that marine systems have larger volumes, or due to some other factor is not clear.
The depth of freshwater systems is usually much shallower than that of marine systems. As we saw earlier, depth is not a critical factor as long as the bottom of the body of water is above the LCP (light compensation point). Many freshwater bodies of water, including both rivers and lakes, have the bottom well within this range. Remember, though, that freshwater is highly susceptible to turbidity caused by soil erosion, thus the LCP might be artificially raised above the bottom. Marine systems, at least those away from the coast, are not usually affected by turbidity. The deepest freshwater lakes are about 2.7 km deep (and this occurs only in Lake Baikal, Siberia); oceans are up to 12 km deep, and the average depth of the oceans - or even the shallow part of the oceans, the continental shelf - is much greater than the average depth of freshwater and is almost always below the LCP.
Temperature relations in marine systems as opposed to freshwater systems are again largely dependent on the relative size of the systems. Generally, the larger marine systems show virtually no diurnal temperature shifts, and very small seasonal ones. On the other hand, small freshwater habitats may experience daily shifts in temperature of over 30 K, and pronounced seasonal temperature changes exist even in bodies of water as large as the Lauretian Great Lakes. Oceanic systems are a large part of the global weather system, which in general moves heat from the warm equator to the cooler poles. Oceanic areas exposed to currents involved in this heat transfer may be much warmer or cooler than would be expected due to their latitude alone, for instance, consider the relative warmth of the ocean near Britain due to the Gulf Stream, or the cold water off the southern California coast.
The effect of temperature, and the allusion to currents brings us to the next topic, the flow of water. 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.
In a lake, water can pile up at one end of the lake due to a consistent wind from one direction. When the wind stops, the piled up water will flow back to the other end. This is known as a seiche, and can easily be replicated in a bathtub by using your body to pile up the water at one end. After the water sloshes back, it usually overshoots by some margin, and ends up piling up at the other end, the sloshing back, etc., for some time (Fig. 2). Of course, each successive slosh is less than then previous one, and in a lake, the only really noticeable differences occur during the wind (the set-up) and during the first seiche. Seiches may become quite complicated in lakes stratified by temperature or salinity differences as the lower levels will slosh at a frequency different than the upper ones. This sloshing of the lower levels is known as an internal seiche.
Figure 2. Action of a seiche. In the upper left drawing, the lake is at its normal level. In the upper right drawing, a consistent wind piles water up at one end of the lake. In the lower left, the wind stops and water flows back in response to gravity. Momentum of the flowing water causes the return flow to be greater than the original volume of water in that part of the basin, resulting in higher than normal levels. Gravity acts on this elevated water and causes it to flow back to the far side of the lake in the lower right picture. The oscillations decrease in magnitude each time. Vertical scale greatly exaggerated.
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 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 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.
Aside from the comparison between marine and freshwater systems, we should also make a distinction between lotic, or running waters, and lentic, or still waters. These distinctions, of course, make sense only in freshwater; the entire ocean is lentic, although small areas swept by strong currents might somewhat resemble a lotic system. We will examine lentic systems first.
Lentic systems form when water is trapped at a level above sea level (or below sea level without an outflow). We call large lentic systems lakes, smaller ones ponds. Ponds and lakes form when something blocks a stream, such as a mudflow, avalanche (cirque lakes), beaver, or human. Even more lakes form in natural depressions such as old volcanic craters (caldera lakes), earthquake rifts (graben or rift lakes) or low-lying areas. Glaciers may scoop out depressions (the Lauretian Great Lakes), or leave behind large blocks of ice in the soil (till) they deposit as they retreat. The block of ice creates a void (hole) in the till, and as the ice melts it fills the hole (kettlehole lake).
Many characteristics of a lake are consequences of its basin and its catchment. The basin, as explained above, determines the lakes size, shape and depth. For instance, glacial lakes tend to be very deep, while beaver ponds are shallow. The catchment is the area of ground surrounding the lake that contributes water to it. All of the streams and/or rivers upstream of the lake (feeders), and the water they drain from the surrounding land contribute to the catchment. Subsurface water (groundwater) may also contribute water to lakes. The nature of the soil and rock in the catchment will have a great impact on the water chemistry of the lake. For instance, glacial lakes are often surrounded by swampy land (bogs), and these bogs produce large amounts of moss, which sinks under the water and begins to decay. Because it decays slowly, under anaerobic conditions, acid conditions prevail, and the water in a glacial lake is often stained with dark brown humic acid. Lakes in areas with lots of limestone are often very alkaline. Land use within the catchment will also affect the water in the lake; farming the land will contribute soil (eventually filling in the lake), fertilizers and pesticides; cities will mean rapid runoff after storms because water flows off pavement rather than sinking in, and oils, salt, etc. from city streets will enter the lake.
Another important concept in lentic systems is residence time. Residence time is simply the average amount of time water spends in the lake. It can range from minutes to years; for instance, in Lake Erie it is about 2.5 years. Another way of looking at residence time is to empty the lake and see how long it takes to fill. This obviously is not practical for most lakes. The residence time affects many things, including water chemistry. If, for instance, a dangerous chemical is accidentally dumped into two lakes, the lake with the shorter residence time will be able to flush out the toxin more quickly. In certain saline lakes, where there is no outflow, residence time may be short for the water (due to high temperatures and rapid evaporation), but no flushing will occur.
Because there is no single, directional flow in a lentic system, stratification may occur. Stratification is the horizontal partitioning of a lake into strata, layers of water that do not mix. The basis for stratification is usually density differences induced by temperature, however, other factors that change density, such as salinity, also may be responsible. In a typical lake in a temperate climate, stratification normally manifests itself in the formation of two layers, a warm upper epilimnion, and a cool, lower hypolimnion.
It is perhaps easiest to see how stratification works by examining a typical temperate lake throughout the year. In the summer, the sun warms the upper levels. Since warm water is less dense, it tends to float, and the warm water concentrates at the surface. Between the warm upper water layer, and the cool, dense water below, there is a zone of rapid temperature change, the thermocline (Fig. 5). The thermocline is not necessarily a sharp plane, often the zone of temperature drop may extend for several meters. The depth and thickness of the thermocline are affected by the lake basin, wind, insolation, and other factors. Obviously, shallow lakes may warm all the way to the bottom and not exhibit a thermocline. In other shallow lakes, the bottom layer may be fed by springs; in such cases the thermocline may be quite abrupt; you may have experienced such thermoclines when swimming in a pond.
The division of the lake into these two zones has a profound effect. The epilimnion is mostly above the LCP for many organisms, and these organisms quickly use up all the nutrients as they photosynthesize. The wind may mix the upper layer, insuring ample O2 throughout, but the hypolimnion will not be mixed because the density difference is too great for the wind to overcome. The hypolimnion, below the LCP for most organisms, will become anoxic. As organisms in the epilimnion die, they will fall to the hypolimnion, carrying essential nutrients with them, and, in the hypolimnion, even more O2 will be used in their decay, exacerbating the O2 deficit there. Anaerobic respiration will become the rule, and bacteria which can survive under these conditions may add things like methane and H2S to the water, making it acidic and smelly.
Figure 5. Graph showing a temperature profile of a stratified lake. In this example, warm surface waters (epilimnion) persist to a depth of about 8 meters, where they meet the cold, dense water of the hypolimnion. The zone where the temperature changes rapidly, from 8 to 10 meters, is the thermocline.
Some organisms take advantage of the differences between the hypolimnion and epilimnion. Because of the lack of O2, most fish avoid the hypolimnion, and thus it becomes a refuge from fish predation for the organisms which can live there. Chironomus larvae (which will grow up to be midges, which resemble mosquitoes but don't bite) have hemoglobin very similar to ours (2 subunits rather than 4); they can absorb O2 from the water even at low O2 levels. The Chironomus larvae (a.k.a. bloodworms) feed on bacteria, which, as mentioned above thrive on the rain of detritus (decaying organic material) from above. Chaoborus larvae (the adults of which look a lot like midges and mosquitoes, and also don't bite) are predators themselves; these bizarre-looking, transparent (they are called phantom midges), larvae hide at the bottom during the day, then rise up to the epilimnion at night to feed on plankton, presumably when capture by fish (which often rely on sight to capture prey) will be less likely. Chaoborus larvae make their vertical migration with the aid of small air sacs in their bodies; they can add or subtract air from these sacs to alter their density. Such diurnal vertical migrations are even more common in marine systems.
As summer ends, the amount of heat gained by insolation during the day will be less than the amount lost by radiation of heat at night. The surface waters will radiate heat to the atmosphere at night, cool, become more dense, and sink. Eventually, the whole water column will be at the same cool temperature. At this point, any wind pushing on the surface water can cause the water to be set in motion, and the water from the bottom is free to mix with that on the surface. This mixing is an important time for life in the lake. It allows the nutrients which have accumulated on the bottom to come to the surface, and it also allows O2 to reach the bottom of the lake.
Stratification of a different type will occur when the lake freezes in the winter. Here the surface water (ice) is much less dense than the other water and thus floats on top. Ice cuts down on O2 exchange, but this is not as critical in the winter when the cold temperature has slowed down the metabolic rates (and thus O2 demands) of most of the organisms in the water. Still, long ice covers may cause fish kills. Snow cover on the ice may drastically reduce light levels also, but, most importantly, the ice and snow cover reduce heat loss from the lake and thus reduce the likelihood of the lake freezing completely. Underwater springs and flowing water coming into the lake also contribute some crucial warmth at this time.
In the spring, the ice melts and the water is again at a constant temperature throughout the water column; in large lakes this temperature is 4o C at both the fall and spring turnover (mixing). Again, this mixing allows nutrients from the bottom to enter surface waters, and allows O2 to reach the bottom. Often a bloom (massive growth) of algae will occur at this point. The city of Akron obtains its drinking water from a lake; you can estimate when spring turnover has occurred by the taste and smell of the water, which is affected by blooms of the alga Dinobryon. Soon, increasing insolation begins the process of stratification by warming the surface waters (although strong storms will cause mixing for some time). If you can measure the levels of plant nutrients in the epilimnion, or the level of O2 in the hypolimnion, you have a pretty good idea of the limits on biological processes that will constrain those two habitats for the summer (but remember that additional plant nutrients can enter the surface waters). The process of cultural eutrophication contributes to both algal blooms in the epilimnion and oxygen deficits in the hypolimnion; such effects are much less common in oligotrophic lakes.
A lake that exhibits two periods of mixing separated by two periods of stratification is known as a dimictic lake. There are also monomictic lakes, usually in warmer climes where the lake doesn't freeze (or in some large lakes in cooler areas); polymictic lakes which mix constantly; and oligomictic lakes which are often found near the equator, remain stratified year-round, and thus rarely mix. Mixing of a lake may be complete (holomixis) or incomplete (meromixis).
Benthic Sediments in Lakes (and streams)
Benthic sediments are often very much like the parent rock of the surrounding watershed. Often, waves or currents will sort the bottom sediments into areas of uniform sizes. The chief means of characterizing sediments is based on size:
Note the small size of the boulder (down to about 1 foot) in terms of what you normally consider to be a boulder! Generally, in a lake, the coarse sediments settle out near the inflows at the edges of the lake, and finer sediments will predominate in the profundal benthos.
In addition to the inorganic material, considerable organic material will settle to the bottom of lakes. This material may be modified by organisms living near the bottom and take on several forms, including gyttja or copropel. This material is largely formed by decaying plankton settling to the bottom, being eaten by bottom dwelling organisms such as Chironomus, and excreted as feces (Kopros Gr.: dung). It is gray or dark brown in color, and may appear gelatinous or as small pellets. The layer of copropel is thicker in eutrophic lakes. Sapropel forms when bottom sediments do not obtain sufficient O2; it is black in color and smells like rotten eggs due to the presences of H2S and methane. Formation of sapropel is another indication of eutrophic conditions.
Overall, several zones can be delineated in a lake (Fig. 6). The benthic habitats include the littoral zone, where enough light reaches the bottom to support plant growth, and the profundal zone, which is below the LCP and often below the thermocline as well. The habitats in the water column include the littoral zone, where plants are present, the open water area above the LCP known as the limnetic zone, and the area below the LCP which is also called the profundal zone. The epilimnion and hypolimnion are also present in stratified lakes and may or may not correspond to the neritic and profundal zones.
Figure 6. Habitats in a lake. The littoral zone rings the lake and includes that portion of the water column where plants emerge from the water, or that portion of the bottom where rooted plants are found. The limnetic zone is the open water above the LCP; the profundal zone is the water below the LCP and the benthic areas below. The thermocline is an area of rapid water temperature change with increasing depth; it exists only in stratified lakes and divides the water column into an upper epilimnion and lower hypolimnion. The epilimnion and hypolimnion correlate with the limnetic and profundal zones only when the thermocline is located at the LCP.
Again, the main distinction between a lotic system and a lentic one is the presence in lotic systems of a unidirectional gravity induced current. Another way of looking at a lotic system is to think of it as a series of overlapping lentic systems with very short residence times. In many ways, lotic systems resemble lentic ones, and we will focus here on the differences.
You are no doubt familiar with many of the names given to lotic systems. Generally, in order of increasing size they are: seeps, springs, streams, rivers. There are also creeks, cricks, rivulets, and so on. Be particularly cautious of the habit in the western U.S. to name every drainage ditch a river in honor of the 2 hours every ten years they actually achieve such flows. This practice no doubt grows out of nostalgia for water, and to compensate for the eastern habit of naming large hills mountains.
The current dominates lotic systems. As mentioned earlier, current speeds can get up to about 3 m/s, but most rivers have currents less than 1 m/s; many "fast" streams are actually flowing about 0.3 m/s, and large rivers have deceptively fast currents. Remember also that the average current speed does not take into account the areas of fast flow (near the surface in deep water) and slow flow (along the bottom and edges, behind obstructions) that may exist. Many small streams are actually series of lotic fast water and small pools that are essentially lentic.
Current meters, both mechanical and the time honored timing of an apple (density near that of water, floats but is not affected by wind, bright red, easily visible, cheap, biodegradable, nutritious) over a measured distance, only measure average current speed, and about the only thing you can say about average speed is that it is the speed that most organisms are least likely to encounter as they crouch behind and under rocks, logs, etc. In addition, there is the boundary layer (Fig. 7). As Stephen Vogel puts it, the boundary layer is perceived by most as a fuzzy notion that it's a discrete region rather than as a discrete notion that it's a fuzzy region. The boundary layer is defined arbitrarily by humans; it refers to the fact that a fluid flowing over a surface tends to "stick" to the surface, so that at the surface the fluid speed is essentially zero, and it increases rapidly. Engineers usually define the boundary layer as the area in which current speed is up to 99% of the undisturbed current speed; biologists typically use 90% as the cutoff point (I told you it was arbitrary).
Figure 7. Graph showing height above the substrate where the current speed is 90% of the mean current velocity (1 m/s). Note that the further back one goes from the front surface of an object placed in the current (moving to the right along the horizontal axis), the thicker the layer becomes. Note also that the layer in this example is always quite thin (up to 2 cm here), and that even within the boundary layer the water is anything but stagnant. Current speed will decrease to zero at the water-substrate interface. A higher current speed would result in a boundary layer curve displaced down and to the right.
The boundary layer gets thicker as current speed gets smaller, and gets narrower as you approach the front edge of a surface exposed to flow. The boundary layer is thus wider at the back of a rock in the stream then it is near the front edge, and, if current speed increases (say, after a rain), the boundary layers in both locations will grow narrower. If you doubt the existence of the boundary layer, look at the blades of a fan, and figure out why the dust there doesn't blow off when the fan is turning. The reason, of course, is that the dust is well within the boundary layer and thus not exposed to the air currents. In streams, only microorganisms are small enough to escape the effects of the current by living in the boundary layer; most multicellular organisms are simply too big even if they are highly flattened (see McShaffrey and McCafferty, 1987 for more on this).
Because of the costs involved in trying to maintain position in a current, most organisms in lotic systems are benthic, and hold onto the bottom in one of the ways mentioned above. In general, benthic organisms in streams have more elaborate holding mechanisms than those in lakes; these holding mechanisms include such things as silk which is used by insects in very fast currents. Organisms in streams also tend to be more streamlined (because of the higher Re - remember) or flattened, however recent studies (McShaffrey and McCafferty 1987, Craig 1990) show other reasons for being streamlined such as the ability to inhabit crevices or to accelerate quickly. Remember too that the direction of the current as it swirls around rocks and other obstructions may not be as unidirectional as the net flow of the stream is. Furthermore, the swirling motion is chaotic, hence unpredictable, for most organisms - they must be prepared for the current to switch suddenly.
A strong current means that plant nutrients and gasses such as O2 will usually be mixed throughout the water column, allowing good gas, nutrient, and waste exchange for those organisms which can keep a grip. Plankton do not do well in lotic systems, however, since they may be swept into areas unfavorable for growth. Attached algae may also suffer since moving water can carry soil particles in suspension, blocking out light and raising the LCP above the bottom. The ability of the water to carry and move particles varies with its speed, and the deposition of those particles, which occurs as the water slows down, affects the distribution of different size particles on the bottom of the stream. Because of the chaotic nature of the currents, soil particles on the bottoms of streams tend to be more patchy and mixed than those in lakes, and will often shift position.
It is possible to divide the stream into different habitats; these habitats differ mainly in current speed and the resulting nature of the substrate (Fig. 8). Areas where the current is capable of lifting particles from the bottom are known as erosional habitats; areas where particles are coming out of suspension are known as depositional habitats. The distinction is a bit fuzzy, since it really depends on what size of particle you are talking about; usually we refer to the erosional or depositional areas in terms of silt or fine sand. Among erosional habitats, we can distinguish riffles, where at least some rocks break the surface and there is active mixing of air and water, and runs, where the water moves faster but is deeper than the rocks, and which has cobble or larger particles for substrate. Depositional areas are typically called pools, with bottoms of sand or silt.
Figure 8. Habitats in a river(above). Riffles occur in shallow areas where rocks penetrate the surface of the water. Runs are deeper areas of swift current, usually upstream or downstream of a riffle. Pools are areas of still water. In rivers, erosion usually takes place on the outer edge of a bend, deposition on the inner edge. Water is usually deeper on the outer edges of a bend. The image below, of Sugar Creek in Indiana, shows a series of riffles and runs. The cobble substrate apparent in the areas with smooth surface waters suggest relatively high current velocity, thus these areas are better described as runs than as pools. Note the importance of vegetation in stabilizing a bank.
Areas of a stream may switch between being depositional and erosional as more or less water enters the stream. It is important to remember that streams are dynamic, constantly changing. Periods of high and low water may be seasonal or daily depending on such factors as climate, nature of the watershed, and the need for electricity in Las Vegas. For instance, streams in wooded areas are more resistant to flooding than streams in urban areas because the trees, with their leaves and roots, slow the movement of water, keeping it from running off and releasing it slowly; water runs right off urban streets.
Streams differ from lakes in terms of the effects of the watershed or catchment on the biological processes. While lakes are largely influenced by their catchment, they are influenced to a lesser degree than are streams, since the lakes can build up reservoirs of important nutrients and other materials. Streams, on the other hand, may rapidly lose any nutrient that escapes into the water. The fact that many nutrients are carefully conserved as they flow through steam ecosystems has opened a "hot" area of stream research into what is known as nutrient spiraling. Streams are often shaded by trees on the banks, and thus little photosynthesis can occur. These streams are highly dependent on outside energy sources (as distinct from the sun); this outside material is known as allochthonous as opposed to autochthonous materials derived within the stream itself. Streams are often much more dependent on allochthonous sources (such as plant leaves each fall) than are lakes. Note too that the presence or absence of trees will affect the temperature characteristics of the stream; clearcutting of forests not only increases the amount of soil that rushes into a stream, it also warms the stream up, reduces the amount of O2 present, increases BOD, and so on.
As we have already mentioned, streams vary in size. Generally speaking, as a stream increases in size, it will carry more water at a higher speed, be more turbid, be deeper, and be more saline. Riffles and pools will be replaced by long runs. The progression from small headwater streams or springs to great rivers with deltas and distributaries carrying the water into the ocean, is orderly. So orderly, in fact, that a deceptively simple scheme for classifying rivers has come about (Fig. 9). River order is calculated as follows: The first discernible stream that forms (from a spring in most parts of the world) is 1st order. When two 1st order stream merge they become 2nd order; when two 2nd order streams merge a 3rd order stream is formed. A first order stream entering a 3rd order stream has no effect. The problem comes in when trying to pin down those first streams, which may dry up from time to time. Since river orders are usually calculated on maps, the resolution of the maps used is also critical. Generally, first order streams are the ones that show up on United States Geographical Survey 7.5o topographical maps or "quads". Drainage patterns of rivers may often be distinctive due to the underlying geology.
Figure 9. Determination of river order. The smallest streams are all 1st order, when two 1st order streams join they form a 2nd order stream; when two 2nd order streams join they form a 3rd order stream, and so on. Note that addition of a lower order stream does not change the order of the stream; stream order increases only when two streams of equal order join.
As you may have guessed by now, the distinction between lotic and lentic systems is very fine. One of the striking features of the natural world is its firm rebuffs to those who want to divide it up and classify it in neat packages. Most systems are not composed of discrete units but instead are continua, and the lotic - lentic continuum is one of those. As we have seen, even in an otherwise lotic system there are small pockets of calm water which are essentially lentic. On the other end of the scale, very large lentic systems can resemble lotic ones. For instance, Lake Erie is lentic, yet the western basin of the lake, which is very shallow, resembles a large river in some aspects of its biota. Many of the species present are more at home in rivers than in a lake, with the wave-swept shores being most river-like in respect to their flora and fauna.
We have already considered many aspects of marine habitats. The open water of the ocean is known as the limnetic or pelagic zone (Fig. 10); 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 10. 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) 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.
Some Other Aquatic Habitats
By now, you should be aware that some of the most biologically interesting habitats come at boundaries. The principle is so general that it extends to terrestrial ecosystems and even to human political systems. Mobile organisms living near boundaries can indeed have the best of both worlds. Vents, estuaries, rocky shores, all benthic habitats, the air-water interface: all of these are productive habitats located on boundaries. Before we leave the subject of habitats, let us consider a number of very small habitats often overlooked, yet extremely interesting biologically.
Temporary pools come in all sizes and shapes, but share in common the fate of being ephemeral - they dry up. Often overlooked, they may contain a diverse assembly of living things. Many of these organisms have unique strategies that allow them to survive long periods of dessication. These might include drought-resistant eggs or spores, the ability to burrow down into the mud and conserve water (found in lungfish and a number of amphibians), and the ability to actually dry out, yet come back to life when rehydrated (cryptobiosis, exhibited by tardigrades and rotifers, among others). Temporary ponds are often shallow, well-lit, predator-free, and rich in nutrients, making them favorable habitats for the organisms which can tailor their life cycles to periodic dessication and large fluctuations in salinity and temperature. Many organisms show drastically decreased life cycle times as compared to other members of their taxon which do not inhabit temporary pools. Temporary pools normally form in depressions during wet seasons, after snowmelt, during flooding, or even just after a rainfall. The nature of the basin is variable: pockets in rock, low-lying areas, holes in tree stumps, pockets formed by leaves, and human structures such as cattle troughs, tire ruts, drainage ditches, bird baths, and so on. In fact, human-created habitats may be particularly important; some of the worst mosquito pests do their best breeding in water that collects in old tires; the tires absorb heat from the sun and form warm, sheltered habitat for the mosquito larvae. The life span of temporary pools may last from hours to months. Organisms which colonize such habitats may travel arrive as resistant spores or eggs carried on the wind or by waterfowl; they may persist in resistant stages in the soil; or they may be carried in by floodwaters. Amphibians are noted for their use of temporary ponds as breeding sites; of course the adult amphibians can traverse the terrain to deposit eggs in the temporary pond.
Kemp, JL, DM Harper, GA
Crosa. 1999. Use of 'functional habitats' to link ecology with morphology and hydrology in river rehabilitation. Aquatic