THREATS TO AQUATIC ORGANISMS
There are risks to living in water. Organisms can be swept away or abraded by waves or currents. They face the threat of desiccation when exposed by tides or when a water body dries, which may also cause increased salinity. Many aquatic animals show remarkable adaptations to cope with these potential threats. Terrestrial organisms that are dependent on water also face risks when their supply of water dries out, or its composition is changed markedly. Humans are also threatened by several major diseases that are associated with water.
Coping with erosion
Erosion occurs where moving water is in contact with the substratum. It may be flowing downstream (as in rivers or streams), ebbing and flowing (as in depositional and sub-tidal shores), or impacting as waves (as in exposed shores). In addition to their role in shaping the margins of aquatic habitats, erosional forces also threaten the biota. Micro-organisms, unicellular algae and other small plants attach closely to substrata and are less directly affected by erosive forces than larger plants and animals.
Tactics for coping with the erosional force of waves on sea and lake shores
Intertidal organisms are threatened by erosion and also risk becoming dried when the tide is out. Some strategies that have evolved to prevent erosion also limit water loss and thus have a dual protective role.
Seaweeds are exposed to direct wave action in the intertidal zone and also by strong ebb and flow in the subtidal [10.1.]. They cannot move, so their location on the shore is dependent on the attachment of very young algae that fix to the substratum by a holdfast, a stipe (or stalk) supporting the fronds. The holdfast [10.2.] prevents easy removal, but many seaweeds suffer erosion damage to their fronds. Brown seaweeds, and many red seaweeds, are tougher than their green relatives and live for longer periods with regular exposure to wave action. Their fronds become abraded after being washed against the substratum but damage is compensated by rapid growth and some forms have wavy edges that reduce the risk of ripping. Tufting of brown algae is also common, some fronds being protected from rubbing against the rock by fronds further down in the tuft.
Some seaweeds of the subtidal have fronds that are abraded and others (e.g., Postelsia) have the fronds borne on short, rigid stipes that carry most of the plant clear of the substratum during water movement [10.3.]. In some subtidal regions there are forests of seaweeds, particularly of kelps that include species that grow rapidly to tens of metres in length in waters with good light penetration [10.4.]. In these plants there are strong forces acting on the stipe which cause both stretching and bending, and these forces must be counteracted to prevent breakage. For example, the stipe of the kelp Nereocystis [10.5.] has cells arranged in concentric layers of tissue, the thickest being the cortex. This has an outer section with cells arranged radially and an inner section with cells arranged longitudinally, each cell of the inner cortex being strengthened by spirals of cellulose within its wall. The cells of the outer cortex provide flexibility and the cells of the inner cortex the ability to resist stretching, both the result of the orientation of their cells. The spiral strengthening of the cell walls of the inner cortex provides further resistance against pull along the stipe. Despite these effective mechanisms the stipes of kelps do break and the strandline of beaches after storms is littered with the remains of subtidal seaweeds.
The eroding shore of lakes is stony, with algae and mosses attached to these stones. Macrophytes are also found rooted in sediments that accumulate between the stones. However, their numbers are small compared to the dense stands of emergent and submerged plants characteristic of the sheltered shores of lakes and ponds, where erosion is only of limited significance. Unless the lake is very large, wave action is not the dramatic event it becomes during storms on marine coasts.
Many animals colonise the rocky shores of lakes and the sea and a strategy to avoid the effects of erosion that is common to both habitats is that of seeking shelter. When we make a trip to the rocky coast to look for animals it usually involves turning over stones and then replacing them. At low tide, animals seek shelter from exposure to the air, but hiding under stones or algal fronds has the dual role of preventing dislodgement when waves pound the shore. The same applies to lake margins, although tidal influences do not occur here. Turning over stones on lake margins reveals many invertebrates that shelter from wave action, but which also gain protection from predators and occasional exposure resulting from changes in water level.
Other animals attach to the surface of rocks. Among these are familiar residents of rocky coasts such as barnacles, limpets and mussels. When barnacle larvae settle on the shore after their early planktonic life, the small crustaceans cement themselves to the substratum and secrete around themselves their characteristic calcareous plates [10.6.]. This is an effective means of resisting erosion as the barnacles become a continuation of the rock surface. Limpets have conical shells that are held against the surface of the substratum so firmly that a "home scar" is worn on soft rocks by the shells of some limpets [10.7.]. The shape of the shell provides some resistance to the threat of dislodgement and limpets attach powerfully by means of the muscular foot. Like other snails, limpets use pedal locomotion to move over the substratum in search of food but the foot is used solely for attachment when the animals are exposed at low tide and during periods of wave action. The base of the foot is covered by many tiny projections that extend into depressions over the rock, and into secreted mucus, to provide a very large surface area for attachment. This gives a strong resistance to dislodgement as is often discovered when we try to kick a limpet from a rock. Mussels use a third strategy [10.8.], with proteinaceous threads holding them to the substratum by the foot where the secretory byssus gland is located (these threads are the "beards" we clean from mussels prior to preparing them for cooking). Although byssus forms an effective attachment, waves often remove groups of mussels from the most exposed parts of the shore.
Freshwater limpets (Ancylus [10.9.]) are found on lake margins, but these are very small and have thinner shells than their marine counterparts. Most animals living on the exposed shores of lakes shelter under stones and do not show special adaptations for coping with wave action and other erosive forces. However, river animals live in an environment that is highly erosive, at least in fast-flowing sections.
Attachment mechanisms in streams and rivers, life under stones and in the hyporheic
Few rooted plants are found in streams of low order, except at the margins. However, streams of mid order (and lowland streams of low order) that have an open canopy often have extensive growths of submerged and emergent aquatic plants [10.10.]. These plants are rooted between stones and trail in the current that is unidirectional, unlike the wave action affecting seaweeds. Submerged plants then become effective traps of material that provides nutrients and serves to consolidate the plants in the substratum. Rooted plants also shelter aquatic animals, just as do stones.
Many animals in streams and rivers show mechanisms that prevent dislodgement. Flatworms [10.11.] and some insect larvae have a low profile [10.12.], which means they do not extend far from the boundary layer of near-stationary water close to the substratum on which they are moving. Leeches use suckers to attach in addition to their use in locomotion, and some insect larvae secrete silk for attachment. Blackfly larvae [10.13.] anchor into pads of silk, midge larvae build cases of silk and detritus (just as relatives do in filter beds), and some caddisfly larvae build retreats of silk and stones. Other caddisfly larvae build cases of stones or discarded shells that are portable and thus serve as ballast and sinking devices should they become displaced [10.14.]. Yet other animals use claws on limbs to hold on and friction pads of setae are present in some mayfly larvae. Where the threat of erosion is ever present, it is not surprising that such a variety of mechanisms have evolved to prevent dislodgement of animals in streams and rivers.
Rivers carry large quantities of particles in suspension and mineral particles in motion are likely to be highly abrasive. Where there is a high bedload of erosive particles the substratum is often scoured and unsuitable for colonisation. However, streams and rivers have regions of flowing water extending deep within the substratum and many animals occupy this hyporheic zone where they have all the advantages of flowing water but little or no erosive force to contend with. The importance of the hyporheic fauna in the metabolism of streams and rivers has only recently been recognised.
Sandy shores and interstitial life
We are all familiar with the instability of sandy shores as we have paddled on beaches during seaside holidays and had sand swept over our feet by the ebb and flow of water. The instability of the substratum prevents plants from growing on sandy shores, although terrestrial sands and sand dunes are stabilised by colonising plants. Animals live within the sand and burrowing forms are common where the sand is mixed with mud and organic matter. Many of the animals inhabiting sandy beaches are deposit collectors or suspension feeders and their bodies are protected from the abrasive movement of sand by exoskeletons, shells, or tubes constructed from mucus and/or mineral grains [10.15.].
Some sandy habitats become consolidated and interstitial spaces become coated with exopolymer from colonising bacteria. Sometimes spaces are nearly filled [10.16.]. Conditions are thus similar to those in the sand filter beds described earlier. Indeed, the fauna of sand filters is very similar to that of sandy bays in lakes and in slow-flowing areas within rivers.
Salinity, heat and cold
Coping with salinity change
There are daily changes in the salinity of water in estuaries and stream outfalls, and irregular changes in salinity on marine shores resulting from rainfall. Seasonal changes also occur. These are most dramatic in some freshwater lakes in the tropics that fill during the wet season and then evaporate, concentrating salts within the water. Changes in salinity affect aquatic organisms, as the osmotic gradient to which they are exposed is changed. Several mechanisms reduce the risk posed to animals confronted with salinity change: the use of impervious coverings; osmoconforming (with body fluids changing in concentration to match those of the surrounding water); and osmoregulating (with body fluids maintained at a steady concentration by efficient removal of water).
Bivalves are isolated from salinity changes on marine shores if the shell valves remain shut tightly and many snails adhere closely to rocks to isolate themselves from the fresh water that falls on them as rain. However, this threat is one of the lesser challenges faced by intertidal animals. Freshwater animals face the permanent threat of their tissues becoming flooded as water enters their bodies across an osmotic gradient, the body fluids being much more concentrated than the surrounding water. Among the most successful colonists of fresh waters are insects and crustaceans, the former secondarily invading water after evolving on land. The impervious exoskeleton of these animals provides a barrier against the incursion of water and isolates tissues from the surroundings.
Where there is a regular input of fresh water into the sea, as in estuaries, we find osmoconformers like the polychaete worm Nereis [10.17.]. The worms are able to tolerate dilution of the body tissues, excreting water when conditions become more saline with the incoming tide. Excretion is also used to reduce the water content of tissues by permanent inhabitants of fresh waters such as bivalves. These had marine ancestors and bivalves colonised fresh waters successfully because they have a high capacity for excretion, dilute urine being passed from the body by excretory systems with a high retention for essential body chemicals. The mass of urine excreted by freshwater bivalves can be up to 400% of body mass each day, compared to 10% of body mass in marine forms. Migratory fish like salmon also have efficient kidneys and the mucus produced over the surface of their bodies acts as a barrier to the formation of an osmotic gradient. This mucus coating is analogous to the impervious covering of insects, crustaceans and molluscs.
Lake Chilwa in Malawi [10.18.] is an example of a lake that undergoes a dramatic change in salinity during the drying phase. Extensive rains bring in large volumes of fresh water during the filling phase and many organisms are present. The lowest concentration of chemicals is found in the surface waters around the margin of the lake shortly after filling and it is here that animals and plants are especially abundant, having life cycles co-ordinated with the filled phase. As evaporation proceeds in the hot sun, the salinity of the lake water increases from 0.3 to a high of 16.7, until only pools of water remain. These pools are devoid of almost all organisms apart from bacteria and some cyanobacteria that have mucilaginous coatings and are thus not adversely affected by the saline water. Some saline lakes have a higher salinity than the drying Lake Chilwa yet have more complex biological communities of tolerant organisms.
Other habitats where the ionic strength of water is high are hot springs but, like the hydrothermal vents of oceans, their most important characteristic is high temperature.
High temperatures at hydrothermal vents and hot springs
Warming of the environment increases the metabolic rate of most animals, making their metabolism more efficient. However, temperatures can rise too high and heat death then follows as tissues are denatured. In most aquatic habitats the water temperature rarely rises above 35°C, and then only at the air-water interface. Most water is very much cooler and the bulk of oceanic waters are cold.
However, some aquatic habitats do have water at high temperature. Drying lakes are one example, but they don't approach the temperatures of hot springs [10.19.] and hydrothermal vents [10.20.]. In these habitats water can attain a temperature of 100°C, or more in hydrothermal vents where water boils at a higher temperature under the extreme pressures of the deep ocean. Water emerging as plumes from vents cools rapidly in contact with the adjacent cold water but the presence of warm water stimulates growth of hydrothermal vent communities. These are islands of high productivity within very much less productive oceans, in the same way that coral reefs are islands of productivity at the surface. Bacteria that grow using reduced chemicals are found in close proximity to hydrothermal vents and in hot springs and these organisms have evolved biochemical mechanisms to prevent heat death even at very high temperatures.
Very cold temperatures
Many cold-dwelling organisms are able to live in a wide range of temperatures but are largely confined to cold regions as a result of competition. Exceptions are those found in the deep oceans that are not only cold but also have high pressures to which organisms must be adapted. Some bacteria and algae live within ice [10.21., 10.22.] and these are of special interest to scientists investigating ice on other planets, or their moons.
Many animals live in very cold water and these contribute to the high productivity that is a feature of polar oceans. Animals that have their temperature controlled by the external environment maintain a low metabolism, but warm-blooded vertebrates have constant, high metabolic rates and require an insulating coating of fats to be laid down under the skin [10.23.]. In fresh waters there are also animals adapted to living in cold water, especially in regions where conditions are similar to those that must have existed at the edge of ice sheets during periods of glaciation. Fishing in ice-covered lakes is a favourite pastime in many countries with cold winters, holes being bored through thick ice far from the shore to enable catches to be made [10.24.]. Clearly, the biological community is active at this time, even though it is isolated from the atmosphere and the water temperature is very low. Of course, the abyssal depths of oceans are very cold at all times (with the exception of vents) so all organisms living here are cold adapted.
Indirect effects of high temperature
Both very low and very high temperatures do not present a risk for organisms that are adjusted to these temperatures. However, hot weather has an indirect effect on organisms by reducing oxygen tension, as oxygen solubility in water decreases with increase in temperature. Warm water also promotes an increase in metabolic activity that further reduces oxygen levels as a result of aerobic metabolism. Animals use up oxygen faster than it can diffuse into the water from the air and this often means that those that need high oxygen tension cannot survive.
Living at the water-air interface
Effects of light and turbulence at the water surface
Almost all light that enters water bodies does so through the water surface and this includes rays within the visible spectrum, infra-red and ultra-violet radiation. Absorbed light warms the water and, with few exceptions, water bodies are at their warmest at the very surface when solar radiation is high. High light intensity at the surface also ensures a good energy source for photosynthesis and this results in a high production of algal cells. Algae exude extracellular material and this is added to the pool of organic matter at the water surface that results from decomposition and from the production of exopolymer by bacteria. Light-driven reactions like UV-photolysis then break down the organic matter into labile DOM that is taken up by the community.
UV light is lethal for some organisms and stressful for others, so adaptation to high UV light intensity is necessary for organisms exploiting the surface microlayers and the bulk water just below them. Extracellular products are likely to be released as a defence against high solar radiation and thick surface slicks of hydrophobic matter also play a role in the defence of the community. These slicks appear yellow-brown when viewed against a white background and it is possible that they act as partial UV filters.
The surface microlayers [10.25.] and the first few centimetres of bulk water are disrupted by turbulence, wave action, and flow but re-form at the surface when disruption from these forces is reduced. In flowing waters, there is too much disruption for a surface microlayer community to be retained and many organisms and organic matter are washed downstream. However, sections of lowland rivers, and dead water zones in all flowing waters, provide conditions that are not unlike those in standing water bodies. The role of the surface film community is as significant here as in most aquatic habitats.
Negative effects of accumulations at the surface film
Although present in high concentrations in some eutrophic water bodies, the natural surface microlayers are disrupted sufficiently frequently to prevent them being a problem for the biological community. However, oils that accumulate after spills, leaching from oil shales and other sources may produce a more permanent barrier to the diffusion of gases. A coating of oil effectively isolates the water unless the surface film is agitated vigorously, and oxygen is used up within the water. This eventually causes the death of aerobic organisms, although oil slicks on oceans are either local or patchy relative to the size of the water body [10.26., 10.27.].
The threat of desiccation
Some aquatic organisms face the threat of desiccation when removed from water. This may be after evaporation of lakes and ponds, exposure to air during tidal cycles, drying of springs, or many other causes.
Strategies for coping with the short-term threat of desiccation
Several animals and plants of marine coasts live in cracks and tidal pools to escape wave action and exposure to high temperature [10.28.]. Both microhabitats also reduce the threat of water loss as pools are near-permanent bodies of water and cracks have high humidity. Of the intertidal animals that are exposed to the air, barnacles, limpets and mussels all have a means of enclosing the body and this reduces water loss and thus the threat of desiccation. In contrast, chitons [10.29.] tolerate water loss and the consequent change in the concentration of their body tissues. Many seaweeds also tolerate water loss. Brown seaweeds on marine shores become dry [10.30.] and may be crisp to the touch when exposed, becoming re-hydrated when the tide comes in. Under this crisp covering, other fronds remain moist and the tufting growth of seaweeds retains moisture after the tide has gone out, in addition to reducing abrasion of fronds pounded by waves. Invertebrates shelter among the fronds of seaweeds during low tide and thus remove the threat of water loss.
Animals living buried in intertidal sand and mud are able to avoid the threat of drying as interstitial water is present, especially where sediment grain size is small. Often, only the surface of the substratum becomes dried. Becoming buried thus acts to prevent water loss, abrasion, and the attention of many predators.
There is no freshwater equivalent to tidal cycles, so organisms living in streams, rivers and lakes are not faced with the threat of desiccation twice each day. However, unlike almost all marine organisms, some of those living in fresh waters are subject to long-term exposure to drying.
Strategies for coping with the long-term threat of desiccation
If streams and rivers become dried, animals move downstream in the drift, or move down into the wet substratum. Aquatic plants cannot escape in this way and are killed if dependent on being submerged in water. Emergent and terrestrial plants often colonise newly-exposed substratum and grow profusely, just as they do around the margins of some drying lakes. When freshwater habitats remain dried for months, or years, aquatic organisms must have a means of overcoming the long-term threat of desiccation.
On rocky marine coasts some animals live so high on the shore that they must only receive intermittent sea water [10.31.], and then mostly in splashes rather than by immersion. The most obvious animals of splash zones are barnacles that are tightly enclosed and therefore have little water loss, maintaining an aquatic environment within their calcareous covering. While exposed to the air, these animals are unable to feed and this explains their small size and very slow growth rate compared to relatives further down the shore. Some species of barnacles seem especially resistant to water loss and are characteristic of the high shore line.
Retention of water within shells is also a method used by some molluscs. Mussels can live high on marine shores and they retain moisture within the shell valves, just like barnacles. Freshwater molluscs also show adaptations to cope with the threat of desiccation. Aspatharia, an African bivalve [10.32.], is an inhabitant of temporary streams and lives through dry seasons with the shells tightly shut and sealed with dried mucus. This is an effective means of preventing water loss but there are potential problems with storage of the by-products of metabolism. It is not known what happens to the nitrogenous waste produced by these bivalves, but CO2 is stored as CaCO3 within the kidney. Reduced metabolism ensures that lower levels of waste products are produced and this metabolic dormancy is one of the keys to successful tolerance of drying of the habitat. The gastropod Pila [10.33.] closes the aperture of the shell with an operculum sealed with mucus and can remain dormant for over a year. Both Pila and Aspatharia (which can survive for two years out of water), rapidly return to aquatic life when rains bring water to their habitats. Revival takes about 30 minutes.
Among other animals, larvae of the biting midge Dasyhelea [10.34. - upper] build cocoons of mud in the bottom of the rain pools they occupy and these allow the larvae to cope with occasional drying. Cocoons are not only formed by invertebrates. African lungfish of the genus Protopterus [10.35.] live in lakes and swamps, especially those that are deficient in oxygen as these animals are able to breathe air. When their habitat dries, the fish bury themselves in the soft, muddy substratum that then dries around them. They spend the whole of the dry period buried in this way, with the body tightly coiled to present the smallest surface area possible to their surroundings. The lungfish are surrounded by a cocoon of secreted mucus and lipoprotein that dries to form an impervious layer. This is only penetrated by a breathing hole that connects to the surface through a narrow tube, although breathing is very reduced as metabolism drops to less than 20% of normal levels, and the fish may not breathe for several hours at a time. As with the molluscs described above, the waste products of metabolism are stored and urea may make up 3% of total body weight after a long period of dormancy.
Some frogs also become buried in oozy mud and produce a cocoon of sloughed skin and mucus around themselves. Often the bladder of these frogs is filled to provide a source of water to compensate for loss, although many seem able to tolerate some dehydration. Return of water causes the frogs to emerge and re-commence adult life. Their appearance from dried mud at the onset of seasonal rains gave rise to an ancient myth about spontaneous generation - it was believed that mud and rain mixed to produce frogs.
Long-term refuges and life cycles that avoid dry phases
An effective way of overcoming the drying of an aquatic habitat is to use long-term refuges, or to have a life cycle adjusted to seasonal rains. Not all parts of lakes and rivers dry out in dry seasons, and refuges are often found to which animals migrate. For example, some streams and rivers dry except within the hyporheic, and deep pools of standing water are formed within the natural channel. Such water holes [10.36.] provide a refuge for aquatic animals through dry seasons and they are also sought out by many terrestrial animals that visit them to drink. These pools are usually subject to many disturbances for aquatic animals and there are also the risks of increased salinity, high temperature, and low oxygen tension. These habitats are, however, wet when the rest of the water body has dried and such pools often teem with life.
Many aquatic organisms do not require water for the whole of their life cycle. Aquatic insects provide an excellent example of animals that have an aquatic phase and a terrestrial phase. Insects have secondarily invaded water after evolving as terrestrial animals and the majority of adult aquatic insects live a completely terrestrial existence, only eggs and the immature life stages requiring water. They avoid the threat of desiccation by having a life cycle synchronised so that only the eggs and immatures are present during the wet phase, the adults avoiding the dry phase completely.
A similar strategy is used by micro-organisms, algae and macrophytes that grow in the water during the wet phase. When the water body dries they produce spores or seeds that are resistant to desiccation. These are dispersed by wind or remain in/on the substratum of the dried water body, together with dormant vegetative structures like rhizomes. Spores and seeds remain viable for many years and a similar mechanism of coping with drying is found surprisingly among the invertebrates.
The eggs of many animals inhabiting temporary pools survive the dry phase by having highly resistant coverings and a metabolism that approaches zero. The eggs of temporary pool crustaceans like brine shrimps are sold in toy shops (one name is "sea monkeys" [10.37.]) with instructions to place the eggs in water and watch the animals emerge to perform tricks. Once adult fairy shrimps are mature, further eggs are produced and these can be collected, dried and stored for addition to new tanks of water. These eggs have evolved to cope with conditions in pools that become completely dried, only re-filling during seasonal rains.
Interestingly, invertebrate life stages other than eggs become effectively dried in anhydrobiosis. Some rotifers [10.38.], tardigrades [10.39.] and nematodes [10.40.] change shape when their pool habitat dries, with nematodes coiling into tight knots and the others shortening into forms characteristic of each. The purpose of this shortening is to reduce surface area and thus limit the area over which respiration occurs. In addition, the metabolism of the animals is reduced dramatically and free water in the tissues is replaced by chemicals such as inositols, just as in dormant seeds.
Perhaps the most astonishing aquatic invertebrates to show this strategy of coping with the threat of desiccation are larvae of the midge Polypedilum vanderplanki [10.41., 10.42.] which live in African rock pools of very short duration. As the pools dry, so do the midge larvae. Using anhydrobiosis, they replace water with chemicals that maintain the integrity of tissues and larvae are able to withstand long periods of drying in this state. They are also tolerant of a very wide range of temperatures and changes in their chemical environment. Mud cut from ponds can be stored on the laboratory shelf and years later placed into a dish of water. Larvae resume their aquatic life within minutes, swimming in the dish with the vigorous side-to-side body movements characteristic of midge larvae.
Throughout time, many different types of aquatic invertebrates have used their ability to cope with the threat of desiccation as a means of colonising land. This led to the evolution of fully terrestrial forms, some of which must still live in damp microhabitats to retain moisture. Among vertebrates, small fish moved far up the shore (and contemporary mudskippers even climb trees [10.43.]) and the evolution of the amphibians began. Most amphibians are terrestrial for their adult life but need water for egg laying and for larval life, just as aquatic insects do. In time, more advanced vertebrates broke entirely with the need for aquatic habitats. Reptiles and then mammals became completely terrestrial, although some representatives of each group have returned secondarily to life in water, retaining their need for air breathing.
Humans are the dominant terrestrial animals, with our ability to manipulate the environment on a large scale. Although water is essential to human life, and we use water for transport, power generation and disposal of wastes, water also brings threats.