Single, "snapshot" samples do not reveal much about the rates of cycling of organic matter. As organic matter production and processing is dynamic, sampling must be continuous to take account of changes with time. The locations of samples also need to be selected carefully as the concentration of organic matter varies greatly within water bodies. Despite the problems of sampling, some general trends have emerged from studies.

The amounts of DOM in water vary through 24 hours and it is common to find a peak of DOM concentration in the afternoon with a decrease at night. Polysaccharides and other carbohydrates are shunted from actively photosynthesising cells during daylight and make an important contribution to this pattern of DOM abundance. The change in POM through 24 hours varies with feeding on particles and egestion by organisms, the pattern varying from one water body to another. The quality of living POM also changes, the ratio of carbon to nitrogen being higher during the day and lower at night, a finding from both lakes and the sea. In darkness, photosynthesis ceases and CO₂ is no longer transformed into carbohydrate. At this time nitrogen assimilated from the water is used in the manufacture of proteins.

Seasons bring annual patterns in the availability of DOM and POM. As the production of algae is dependent on light and nutrients, the growth of phytoplankton produces peaks of living POM and DOM in spring and summer. Maxima in DOM often occur after algal blooms, when conditioning and leaching of dead algae, feeding and excretion by animals, and the metabolic activity and numbers of bacteria are all likely to be at their highest.

In rivers and in coastal regions, the production of plants reaches a maximum through spring and summer in temperate regions, with dieback in autumn. Where plants are emergent or growing alongside the water body, the fall of leaves in autumn can produce a major input of POM. This seasonal input may be retained in situ or transported by flow or currents to other areas.

The life cycles of animals also affect the amounts of living POM over a year, especially where reproduction results in the production of huge numbers of eggs and immature stages at certain times.

Superimposed on the 24-hour and seasonal cycles are other cycles, and also random events that control the abundance of organic matter. Tides bring DOM and POM to marine coasts twice daily and spring tides have the greatest effect as material is then swept over the largest area of shoreline. Seiching in lakes also delivers organic matter to shores but the effect is much smaller than that of tides on marine coasts.

Floods have a profound effect on levels of DOM and POM in rivers and coastal margins. Some floods are periodic and follow a rainy season; others are less predictable and are affected by meteorological events in the atmosphere. Floods characteristically carry materials from terrestrial sources on flood plains into rivers and inputs of DOM from the flood plain can make a significant contribution to the total organic matter present in the water. Much organic matter is also transported to the sea and lakes by flooding rivers.

Trophic status refers to the organic content of natural waters and the rate at which that organic matter is metabolised [5.1.]. Oligotrophic waters have low levels of nutrients and low biological productivity and turnover; eutrophic waters have high nutrient levels, high biological productivity and high rates of turnover. It is important to note that there are no absolute definitions of trophic status.

As tropical oceans are stratified permanently there is a loss of organic matter and nutrients by sinking to deeper water, from whence it is not replenished except by upwelling across the thermocline. Tropical oceans are thus characteristically oligotrophic and their low biological productivity results in high light penetration and thus a deep photic zone. Temperate oceans have a higher trophic status but high latitude oceans are the most eutrophic, a continuum in trophic status existing from the tropics to high latitudes. Polar oceans are cold but have high insolation for several months in the summer and have an abundance of nutrients. Deep-water currents bring nutrient-rich water to tropical oceans at sites of upwelling, the naturally rich water of high latitudes also transporting organic matter that has sunk, or carried in downwellings from the surface waters overlying it.

Another continuum in trophic status is between the open water of oceans and of coasts, the latter being the more eutrophic and highly eutrophic in some regions. Runoff from land provides a significant input of both organic and inorganic matter to marine coasts. In coastal zones there is warming of shallow waters and the often good light penetration to the substratum promotes the growth of attached algae, and symbiotic algae within corals where reefs occur. Coastal swamps and marshes also provide high inputs from emergent plants. The combination of algal production, allochthonous inputs, and the abundance of heterotrophic micro-organisms, results in high trophic status as organic matter is in good supply and nutrients are released in quantity during breakdown.

The trophic status of lakes is strongly influenced by the underlying rock strata. Igneous rocks provide few solutes but sedimentary rocks are rich in soluble calcium. High levels of calcium salts aid buffering of fresh waters but calcium is also an essential micronutrient for bacteria. Characteristically, many lakes overlying sedimentary rocks are eutrophic as they have high organic matter loadings and large numbers of heterotrophs to maintain turnover. Organic matter is produced by photosynthetic organisms, and mixing of the water body during periods of overturn ensures a supply of nutrients from the decomposition of earlier production. Some lakes are hyper-eutrophic and have bacterial densities that exceed 5 x 10⁷ ml^-1 and very high organic matter loadings.

Trophic status increases when the total input of nutrients exceeds the amount of organic matter that is sedimented, or metabolised and respired as CO₂. Many lakes become more eutrophic with time, although the trend is usually gradual. The opposite trend does occur, especially in lakes where the activity of heterotrophs is arrested by changes in acidity. This occurs during acid precipitation in poorly buffered lakes. Relatively low levels of organic matter are then inefficiently broken down and a lowering of trophic status results.

Another exception to the progression from oligotrophy to eutrophy occurs in lakes that receive large inputs of humic materials. These lakes thus have high loadings of DOM and POM (especially the former) but acidity reduces the activity of heterotrophic bacteria, and indirectly affects their activity by binding available calcium to humic materials. These lakes are termed dystrophic to indicate their arrested trophic status and they often have reducing conditions over/in the substratum. This provides ideal conditions for the long-term production of coal measures and similar organically-enriched strata.

Whereas oceans and lakes are commonly classified by trophic status, trophic status is less often used as a way of classifying rivers. However, some rivers are markedly oligotrophic (e.g., those draining from snow melt), others are eutrophic (e.g., those of high order draining through grassland), and some are dystrophic (e.g., blackwater rivers with flood plains rich in humic DOM). The trophic status of rivers changes along their length, with streams of low order being usually oligotrophic and those of high order tending to be eutrophic. There is thus a longitudinal continuum that contrasts with the temporal continuum in lakes.

It is eutrophication by humans that has given the term its current high profile [5.2.]. Our "out of sight, out of mind" approach has resulted in the dumping of large amounts of organic matter into water bodies.

Inputs of domestic organic matter produce rapid increases in trophic status so that the natural communities become overwhelmed. There is usually a large increase in the abundance of micro-organisms and sometimes anoxic conditions result. This has a profound effect in small lakes and rivers but it can also occur in the sea. For example, marine sites for dumping of sewage sludge often become depleted of oxygen unless currents disperse the organic matter. Passage of organic matter into rivers is not a major problem if there is a sufficient discharge of water to transport it to the sea but low discharge brings problems with accumulation. Disposal of organic matter into small lakes certainly provides a problem as there is no dispersal mechanism and the biological community must cope with the consequences of rapid eutrophication or become choked. In many countries, treatment of domestic effluent is required before discharge into natural water bodies and this topic is considered further in Chapter 7.

An influx of plant fertilisers also causes eutrophication. Phosphate and nitrate are applied to farmers’ fields to increase the yields of crop plants. Runoff after rainfall brings these nutrients into water bodies where they promote the growth of all plants (phosphate also enters water from effluents, as it is a component of several detergents). It might be supposed that once organic matter or nutrients are locked into sediments they cease to be available, but any process that disturbs the sediment (e.g. dredging, or the activities of bottom-feeding fish) makes these nutrients available once again [5.3.].

The earliest models of trophic dynamics show that a small part of the solar energy entering water is trapped by photosynthetic organisms. These organisms are fed upon by herbivores with consequent transfer of energy up to the higher trophic levels. Carnivores feed upon herbivores and "top" carnivores upon carnivores to complete the vertical transfer of energy. As organisms die, their bodies are added to the detritus pool (which also includes faeces and other shed organic matter) to give lateral shifts of energy. At all trophic levels, metabolism results in the production of CO₂ that is lost to the atmosphere.

Such simple trophic models were useful for understanding energy flow through aquatic systems but measurements of each trophic level were relatively easily recorded only in habitats like springs that were small and where inputs could be recorded easily. These simple trophic models gave rise to two other approaches: food webs and functional models.

Food webs have been a focus for ecologists and those interested in the structure of communities [5.4.]. A body of theory has developed that describes links between populations at different trophic levels and considers how the linkages are related to community stability.

Functional models aim to provide an accurate picture of how energy passes through ecosystems and these models arose from ideas generated by studies on the biology of aquatic insects. Placing an organism into a trophic category often proved difficult, e.g., some larval insects are predaceous throughout their lives, others feed on animals for only part of their life cycle. The former are clearly carnivores but what about the latter? An even bigger difficulty comes with animals feeding on material that coats the substratum as they ingest algal cells, bacterial cells and also dead organic matter. They are therefore herbivores, bacterivores and detritivores (the latter two categories not being separated in the simplest models). To overcome this kind of problem it was proposed that organisms can be placed into functional feeding groups, describing the strategies they use to acquire food.

Using this system, aquatic insects were grouped as predators, shredders, scrapers, or collectors. Predators are carnivores that eat other animals and they ingest captured prey whole, in pieces, or after liquefaction of the body contents. Shredders use mouthparts to cut up food that may be living plants or detritus. Scrapers use mouthparts or other devices to remove the covering on stones, macrophytes or muds. Collectors capture DOM and POM from suspension in the water, or from deposits. Strategies and tactics are summarised in Table 5.1.

Table 5.1. Functional feeding groups, tactics employed, and trophic groups.

This functional approach is used extensively in stream and river biology, extending beyond insects, but it has been little used in studies of lakes, marine coasts, or oceans.

Animals in each functional feeding group use different parts of the available food resource and thus exhibit resource partitioning. It is not only food which is partitioned: so is space and any other limited resource needed to support individuals. By partitioning available resources, individuals within populations lower interspecific competition and the evolutionary pressure of competition has acted as an important mechanism for the selection of different approaches to resource use. The way that populations are linked within communities and the stability of those communities brings us back again to the idea of food webs. Changes in one component of a web have an effect elsewhere as a resource becomes depleted, or made available by the activities of animals at a different point within the food web. The trophic structure of aquatic systems is thus dynamic and is controlled by both biotic and abiotic factors.

Trophic structure is affected not only by the dynamics of aquatic organisms but also by external factors. For example, eutrophication as a result of human activities affects aquatic communities by increasing the abundance of organic matter and its turnover, commonly causing a decrease in oxygen levels. Although this is a modification of a natural process it has catastrophic effects on organisms that are dependent on high oxygen tension. These organisms also risk becoming smothered by the rain-down and sedimentation of organic matter.

The influx of acid rain [5.5.] to lakes that are poorly buffered results in a lowering of pH, the arrest of microbial breakdown of organic matter, and the consequent collapse of trophic structure. As animals of higher trophic levels (e.g., some fish) require energy from herbivores via plants they are the most expensive organisms to produce and their numbers show the most noticeable decline. It is possible to treat the consequences of lake acidification by the introduction of calcium salts to the water but application needs to be continued for as long as acid precipitation occurs. Prevention of acidic discharges is the only long-term strategy to encourage the trophic structure of lakes to return to pre-pollution levels.

While eutrophication and acid rain present major problems, it is usually possible to take some remedial action to restore the water body to normal conditions. Other pollutants have a longer-lasting catastrophic effect on aquatic communities and an indirect effect on terrestrial animals that feed on aquatic organisms.

Chemicals are applied to terrestrial and aquatic environments to control pests and thus diminish effects on crop yield or on the numbers of biting flies, etc. They then pollute ground and surface water [5.6.]. The earliest pesticide available on an industrial scale was an organochlorine, DDT, to be followed by many other chemicals as the negative effects of DDT became known. Resistance of pests to pesticides has been an important factor in the need to develop new formulations. Many organochlorine pesticides are not excreted and this has led to bioaccumulation in non-target animals that retain heavier and heavier loads after feeding on dead or polluted animals from a lower trophic level. Any animal at a high trophic level is therefore feeding on concentrated toxins and these have noticeable effects on processes like reproduction even if they are not always fatal.

Several heavy metals also bioaccumulate [5.7.] and one of them, mercury, was responsible for Minimata disease in Japan in the 1950s. Human settlements around a bay depended on shellfish caught locally for a major part of the diet. Unfortunately, a factory was discharging wastes rich in mercury into the water and this was taken through the food chain and concentrated in the fatty tissues of the shellfish. Mercury concentrations were so high that several humans were killed after eating seafood, and poisoning of the nervous system caused many victims to develop a distorting paralysis.

Not all metals are bioaccumulated (for example, copper is generally not) and some can be excreted or stored safely by aquatic organisms. However, the effects of most heavy metals are mainly damaging and destructive of trophic structure, especially at higher levels. Some, such as tributyl tin (TBT), also have an effect on the sex of animals, with disastrous consequences for breeding stocks. As loadings of metals in water bodies have been greatly increased by factories and by waste disposal, more effective monitoring and prevention of pollution provide the only remedy. However, toxic chemicals and heavy metals will remain at high levels in aquatic systems even if there are no further inputs.

In terms of media coverage, oil is probably the most well known pollutant of aquatic systems [5.8.] and we are all familiar with the accidents that befell the Exxon Valdez in Alaska, the Amoco Cadiz in northern France and the Torrey Canyon off the south-west coast of England. While spillages from tankers and pipelines cause the escape of huge amounts of crude, these are not the commonest sources of oil in natural water bodies. Oil-bearing rocks continuously ooze oil, a natural process that has occurred for millions of years. However, it is human activity that has accelerated the input of oil from shipping, wars, washing of garage forecourts, and from carelessness.

Most oil entering water disperses but crude presents more of a problem. Disastrous spillages of crude affect birds and mammals whose plight is readily shown on television. Spillages from tanker groundings affect coasts, often coating beaches and regions of natural beauty that require immediate treatment [5.9.]. The first strategy is containment and the use of booms to isolate a spill. If this is achieved it is then possible to use "sludge gulpers" to transfer the oil to another tanker. Unfortunately, spills are often carried rapidly by currents and then alternative strategies must be employed. Emulsifying chemicals have been used but more modern approaches use a spray incorporating oil-digesting bacteria, often with an added nutrient to stimulate bacterial growth.

Crude oil contains a number of fractions. Volatile organic matter (VOM) evaporates from the surface and gives spillages their characteristic odour, while heavy fractions submerge to form tar balls that are often washed ashore. As VOM is toxic it has an effect on the communities it impacts but tar balls are less of a problem, although they are troublesome on bathing beaches. The worst effect of spillages is the emulsion of oil and water known as "chocolate mousse" (a reference to its appearance). This clings to substrata and has a devastating effect on communities of organisms, although some snails appear able to survive being covered. Where shores have vigorous wave action, the mousse will eventually be washed away leaving new areas for colonisation by macroalgae. These plants grow in profusion until shredder invertebrates recover and begin to reduce the density of algae.

Although only a tiny fraction of the ocean surface is affected by oil spills, they are large-scale events with consequences of similar scale. Many smaller-scale poisonings of aquatic systems occur. Oil spills into streams and lakes are treated with booms or just left to dissipate and be broken up by natural agents. Persistent spills into lakes can lead to a covering of oil which has disastrous consequences for the biota as gas exchange is no longer possible across the surface film. Where consistent spills of light oils occur there is a danger of fire when volatile fractions are ignited and some water bodies in heavily industrialised regions have suffered this fate.

Poisoning by conservative pollutants can be localised. Flame-retardants leaking from dumped electrical equipment release organochlorine compounds into the water, just as an influx of pesticide does. The consequences for poisoning and bioaccumulation are the same as in major pollution incidents. Many other examples can be given of these small-scale events which occur continuously near human habitation but which are little reported by the media, as they appear to have little impact.

Heterotrophic bacteria are important in conditioning and breaking down organic matter in water. They also have another important role. DOM that is constantly leaching from dead POM and is secreted from living POM is taken up and converted into reproducing microbial biomass, bacteria either being free in the water or attached to surfaces. Bacteria are fed upon by protists which, in turn, are fed upon by collector invertebrates. There is thus a microbial loop that returns DOM lost from trophic metabolism back to the trophic pathways [5.10.]. The evidence for and against the importance of the microbial loop is equivocal and its general significance is debated. In most aquatic systems the microbial loop plays a subsidiary role in energy transfer but it may be more important in blackwater lakes and rivers that abound in DOM.

We can now visualise five trophic pathways. In the first, solar energy is harnessed by plants that are fed upon by herbivores and then by carnivores. In the second pathway, detritus (dead POM of both plant and animal origin) is fed upon by detritivores that are consumed by carnivores. The third pathway shows DOM being taken up by bacteria that are consumed by heterotrophic protists that are then fed upon by collectors. Direct consumption of heterotrophic bacteria by collectors also occurs and DOM is taken up directly by animals that are mostly within the collector functional feeding group. Clearly, all pathways are most efficient where the materials consumed are easy to digest and assimilate.

Protists are within the same size range as many unicellular algae and are thus food for organisms that collect small particles, the reason for their significance in the microbial loop. In addition to living free in the water some protists depend on a surface over which to move. The obvious surface is that provided by the substratum but amoebae and ciliates are found in large numbers at the air-water interface of all water bodies and must also coat bubbles when these are formed. Other surfaces to which protists attach are sand grains, marine and lake snow aggregates, allochthonous and autochthonous detritus, and many others. Protists are ubiquitous components of aquatic systems and play an important role in grazing bacteria, bacteria-sized particles, and contributing directly to primary production in those forms that are able to photosynthesise. When large populations of collectors remove protists from water together with algal and detrital food, bacterial numbers increase markedly. This is a good example of top-down control of population density within communities.

Like their modern descendants, the earliest bacteria took up labile DOM by active transport across the cell wall. Some multicellular, soft-bodied organisms from many different Phyla continue to use direct uptake of DOM and colloids. This is largely confined to the sea where the high salinity of the water does not produce a high osmotic gradient. In fresh waters the high concentration of cell contents results in a constant threat that soft tissues become flooded with the dilute water with which they are surrounded. Mechanisms have thus evolved to prevent the direct uptake of organic molecules in aqueous solution. Nevertheless there are some freshwater organisms that take up DOM directly from the water, although the mechanism for doing so is not well understood.

It was thought that some deep-sea tube-worms (Pogonophora) [5.11.] were the largest organisms dependent on the direct uptake of DOM. However, it is now known that symbiotic bacteria provide much of their food requirements, with direct uptake likely to be supplementary. Many marine animals take up DOM directly but its significance in their metabolism is not well known.

The basis of many trophic pathways is the capture of solar energy by plants in photosynthesis. Resulting carbohydrates are transformed into other essential chemicals needed for life after uptake of nitrogen and phosphorus across cell walls. This explains why these chemicals are frequently the limiting nutrients to plant growth. Other cells fix chemicals from which complex molecules are synthesised.

The earliest example of symbiosis of autotrophs and heterotrophs came with the evolution of single-celled algae. Photosynthesis arose in the cyanobacteria [5.12.] (called blue-green algae until recently) and cyanobacterial chloroplasts were incorporated into other unicells to form the most primitive algae. Evidence for this comes from a comparison of the structure of contemporary cyanobacterial chloroplasts and those of modern algal cells. It is only possible to speculate on the origin of this symbiosis but it resulted in the evolution of the organisms that support the trophic structure of most contemporary water bodies.

More obvious symbioses have also developed to the advantage of both partners, something that is dubious in the unicellular algae. For example, coelenterates commonly contain algae within the cells of the outer body layer and one group of coelenterates, the corals, have become dependent on symbiotic algal cells (zooxanthellae) [5.13.]. The algae benefit by being enclosed and thus free from grazing, and the corals have a means of acquiring carbohydrates and other organic chemicals. The mucus produced impressively by many corals is a by-product of photosynthesis by the zooxanthellae, excess carbon being shunted from cells in the form of polysaccharides. As algae are dependent on light for photosynthesis, corals thrive only in shallow water. However, good light penetration in tropical oceans ensures that they are found in water ten of metres deep. It is important to note that coral polyps also feed heterotrophically and they have a functioning enteron (an enclosed space, opening from a single aperture) into which captured food items are passed and where enzymes are secreted.

Many animals near hydrothermal vents [5.14.] are dependent on nutrients supplied by symbiotic bacteria that take up reduced chemicals. Among these animals are bivalves that have symbionts on the gills and crustaceans that have symbionts in their gill chambers. The best-known animals of hydrothermal vents are the Pogonophora mentioned earlier, which are up to 3 m in length and found in impressive profusion at some sites. As we have seen, these worms contain symbiotic chemical-fixing bacteria that generate nutrients used both by the bacteria and also by the tube-worms. Many other symbioses occur between chemical-fixing bacteria and invertebrates and we should expect to see symbiotic relationships around hydrothermal vents and in cave systems. It is amazing to think that the often teeming invertebrate community of vents is dependent on the presence of bacteria to support their trophic structure, although some animals ingest sulphide particles and POM. Adsorbed DOM coatings on particles may also play an important role.

I will review feeding in multicellular heterotrophs by returning to the concept of functional feeding groups. It should be stressed that the examples used only illustrate each of the feeding strategies, many different solutions to food acquisition having evolved.

Most shredders and scrapers have heads or modified anterior regions of the body that contain the feeding apparatus. As the variation of approach used in each of these two feeding strategies is small I will give one example of each.

Limpets, e.g. Patella [5.16.] are familiar seashore snails. Limpets move over the substratum when the tide comes in and use a radula to scrape algae and biofilm from the rock surface. The radula is covered by many small teeth and is extended from the mouth, drawn over the substratum, and returned to the mouth with the food. Other scrapers use limbs or mouthparts to remove attached biofilm but all devices use the same basic approach.

Suspension-feeding collectors take advantage of existing water movements or generate their own feeding currents.

Most blackfly larvae [5.17.] have two head fans, each of which consists of a hemisphere of about 40 rays that are held into the fast-flowing water of the streams and rivers in which they live. From time to time the fans are folded and material caught on them is removed to the mouth by the sweeping action of the mouthparts. Ingestion is non-selective and larvae therefore have a diet that reflects the abundance of particles in suspension, ranging from colloids and DOM up to large mineral grains.

Larvae of the midge Rheotanytarsus live in tubes of silk and detritus that have extensions at one end. Silk strands are woven between these extensions and these strands trap particles from the stream water passing between them. From time to time the larvae ingest the silk and the POM and DOM attached to it, before secreting new strands. Net-spinning caddisfly larvae [5.18.] spin silk nets that act just like fishing nets, trapping materials from the large volume of stream water that often flows through them. In contrast to Rheotanytarsus larvae, caddisfly larvae select the food they take from their nets, with older and larger larvae usually selecting animal food. Not all net-spinning caddisfly larvae produce nets of broad mesh for use in fast currents. Some have very fine-meshed nets attached to the substratum in gentle stream currents. The nets also form shelters in which the larvae live and the diet consists of captured materials and organic matter that grows/covers the netting tube.

Some brittle stars [5.19.] support themselves on a tripod of three arms and use the tube feet of the other two arms to capture particles from currents passing over the substratum on the continental shelf. They use ciliated tracts to duct material captured on the arms to the mouth for ingestion. The size of particles captured is defined by the distance between tube feet and also by the stickiness of the area on which dissolved and particulate materials impact. Brittle stars appear largely non-selective and the particles captured are bound in mucus for retention and ease of transport to the mouth.

Barnacles are examples of animals that generate their own feeding current but they also utilise existing currents. We are familiar with adult acorn barnacles [5.20.] living attached to rocks or to the shells of bivalves. Close observation under water shows the legs of these crustaceans to kick water containing particles towards the opening of the calcareous body covering where they are captured and ingested. This active feeding current is supplemented by the tidal flow of water that delivers particles. Goose barnacles [5.21.] are larger than most acorn barnacles and often live attached to floating debris. Some goose barnacles attach to the hulls of ships where their particle delivery system is much more rapid than that found naturally in the sea and exceeds the current velocity found in many flowing fresh waters.

Oikopleura is a tunicate that lives within a "house" [5.22.] through which a current of water is pumped by movements of the animal. A coarse-meshed screen excludes particles > 10 mm while the finest particles, colloids and DOM become trapped on a network of fibres forming a filter through which water flows to the exhalant aperture. The animals feed on the material accumulated on the filter and this is passed to the mouth on currents generated by the tail and the ciliated gills. Material from the filter is mixed with mucus in the mouth and then ingested. When the filters become clogged the animal vacates the house and builds a new one (a process that can occur every few hours in some species) and the houses form a component of marine snow aggregates, especially in ocean regions where these tunicates occur in very high population densities.

It thus appears that mucus-based filters are relatively cheap to produce. Another example of their use is found in the benthic worm Chaetopterus. These animals [5.23.] live buried in tubes in the substratum and use muscular extensions of the body (parapodia) to draw water through an inhalant aperture. The current passes over the body and exits from the exhalant aperture. Across this flow of water is secreted a mucous mesh net capable of trapping POM, DOM and colloids and which becomes clogged with time. The net is then folded, ingested, and a new mucous mesh net secreted to take its place.

Mucus is a highly hydrated material, a small amount emerging from the secretory goblet cells soon expanding into large mass. This can be manipulated to produce strings used in net mesh formation or passed on to the surface of feeding structures to aid trapping. This approach is used in bivalve molluscs. Bivalves are the most abundant benthic suspension feeders and are found in all kinds of aquatic habitats from small streams to oceans.

Feeding in bivalves utilises the respiratory current, with mucus secretion again playing a part. Many bivalves have large gills consisting of filaments covered in cilia. Lateral cilia draw water into the mantle cavity between the shell valves; it passes between the gill filaments and exits through an exhalant aperture. Fused bundles of cilia called the latero-frontal cirri overlap to form a meshwork and particles are trapped here, the cirri bending to transfer collected material to the front of the gill filaments against the flow of water. Captured particles are retained on the filaments and join other particles and dissolved material that has impacted or become adsorbed. Mucus is secreted so that the whole food package is transferred along the filament to a food groove at the edge of the gill where further cilia conduct the material towards the mouth [5.24.]. Before ingestion, cilia on the labial palps pick up the mucus-bound food from the gill and sorting occurs, with some material being passed to the mouth for ingestion and the rest rejected, together with any excess particulate matter that has been inhaled. Material that has been rejected is released into the exhalant current or expelled from between the shell valves. This material is bound by mucus into aggregates that resemble faeces and are thus referred to as pseudofaeces. As bivalves are widespread and very common in some locations, their effect on re-packaging organic matter into faeces and pseudofaeces is considerable.

Many other organisms from the small (e.g., ciliate protists and rotifers) to the large (e.g., tubicolous worms) use cilia to capture particles from suspension, often in combination with mucus. Aquatic insects do not secrete mucus but are sometimes able to accumulate CEPs on to their feeding organs. This accumulation functions in a similar way to the mucus secreted by other suspension-feeding collectors.

Particles and aggregates are sedimenting constantly in aquatic habitats and deposit-feeding collectors have evolved a number of methods to gather this material. Animals that move over the surface of the substratum gather deposited material using mouthparts (e.g., larval midges) or limbs (e.g., some starfish) with the gathered bundle being ingested. Animals residing within the substratum can only collect particles surrounding them. Worms such as Arenicola (the common lugworm of marine coasts [5.25.]) live in U-shaped burrows with the head end being a site of erosion into the burrow and the tail end being where the characteristic faecal casts are deposited. Some primitive worms use a mucous bag to capture particles at the eroding entrance of their tubes, thus feeding in a very similar way to the suspension-feeding Chaetopterus. Some midge larvae use a net of silk spun across the tubes in which they live. It should not surprise us that similar designs, providing solutions to similar problems, have evolved in widely different organisms.

Deeply-buried bivalves use a respiratory current in the same way as their surface-dwelling relatives. Deposited particles are drawn into the mantle cavity through extensible inhalant siphons [5.26.] that can be extended far out across the surface in some species to capture particles in a manner reminiscent of a vacuum cleaner.

The greatest array of approaches to acquiring food is found among the predators and we can only touch on the enormous variety of predation strategies found in aquatic animals. One way of categorising predators is into those that are mobile and those that are sedentary or sessile. Mobile predators hunt their prey and therefore need an effective means of location through sight, smell, touch, or a combination of senses. Once located, the prey is captured, subdued and then eaten, sometimes whole and sometimes after being butchered. Among the most well know aquatic predators are sharks which have an acute sense of smell and an awareness of vibration that enables them to home in on prey. As seen in numerous films, sharks then manoeuvre and strike using jaws equipped with very sharp teeth that capture and cut up prey almost instantaneously, pieces being swallowed rapidly. Other mobile predators include prized sport fish that can be tempted on to hooks using baits, lures, flies, and the many techniques devised by rod anglers.

Further examples of mobile predators are found among the invertebrates. Raptorial, catching limbs are found in water boatmen (insects living in fresh waters, [5.27.]) which hold on to prey with the fore limbs and use their needle-like mouthparts to inject an immobilising toxin and enzymes, and then to extract resultant fluids. Water boatmen are effective swimmers and are sensitive to vibrations within the water as a mean of locating prey. Cuttlefish [5.28.] have excellent vision and use extensible arms to capture prey at a distance. They subdue the prey using a toxic bite administered when the prey is held by the oral tentacles while it is butchered by the horny beak driven by powerful muscles. Cuttlefish thus show a range of features that make them effective predators.

Not all prey are captured individually. For example, baleen whales (the largest aquatic predators) use a predation strategy similar to suspension feeding. The predominant food of some of these whales is krill [5.29.], pelagic shrimps often found at very high densities in the Antarctic Ocean. The whales take in a mouthful of water and express it through baleen rays attached to the upper jaw. These are fringed and form an effective filter so that krill are concentrated and then ingested.

Some predators use ambush tactics, with prey coming to them. Ctenophores (often called sea gooseberries from their appearance) are primitive pelagic animals consisting of two cell layers separated by a jelly skeleton [5.30.]. They move by means of cilia fused into plates and have long, retractile tentacles that trap prey in secreted mucus. A similar approach is used by the jellyfish but they have stinging cells that inject a barbed thread into the tissue of the prey, also injecting a toxin to subdue it. The cells trigger on contact and are highly sophisticated structures for animals so primitive. Prey of both ctenophores and jellyfish often swim into their trailing tentacles.

Among the best known ambush predators of fresh waters are larval dragonflies (Odonata) which have modified mouthparts folded under the head in a mask. The mask has two teeth at its extremity and these are used to hold the prey. At a critical distance the hinged mask is rapidly extended and then folded, bringing the prey to the mouthparts for butchery [5.31.]. In this type of predator, encounter with prey is not as random as in the pelagic comb jellies and jellyfish. Dragonfly larvae move to regions where prey are abundant and, indeed, can stalk prey. However, the hunting strategy most commonly employed is one of concealment and ambush.

Ambushing is enhanced in some fish by the development of lures. Angler fish [5.32.] were given their name from their ability to use fishing techniques like those used by rod anglers. Many forms of lure have evolved in different fish but among the most impressive is a modification of the base of the mouth that closely resembles a worm. The fish waits in ambush, concealed by its coloration but with the wide, toothed mouth open and with the "worm" appearing to wriggle from time to time. This attracts the attention of smaller predators that enter the mouth of the angler fish, which is then snapped shut. Other angler fish use lures on modified dorsal spines, or extensions from the roof of the mouth. These are used in the same way as rod anglers presents lures and they are flicked over the wide open mouth to entice fish in.

Predators also hunt in groups and this behaviour is found in several higher aquatic animals. Mackerel (Scomber) and tuna (Thunnus) are fast-swimming fish that hunt other fish, the mackerel [5.33.] being common in coastal waters and tuna [5.34.] in oceanic waters, although tuna have been over-fished in some regions. Shoals of these predators chase shoals of smaller fish and feed on them with rapid strikes based on visual perception of movement. This strategy is exploited by mackerel anglers who use strips of coloured plastic (or coloured feathers) attached to lines of hooks. Tuna fishing is commonly carried out with long nets in which the fish become entangled, but traditional methods capitalise on the shoaling habit by corralling fish between nets joining boats, tuna then being gaffed aboard.

One of the most sophisticated methods of predation by groups is shown by some baleen whales when feeding on fish. As they are air breathers, these whales use streams of bubbles released under water from their blow holes to frighten shoaling fish. Several whales operate together in this strategy so that the shoals are maintained in a dense mass up through which the whales charge to gain many fish in each mouthful.

Just as with other feeding strategies, only a superficial illustration of the range of existing predation techniques can be given here. However, evolution also selects for genotypes that successfully equip organisms with a means of avoiding predation. An arms race has thus taken place between predators and prey, with predators evolving new catching techniques and prey new mechanisms to avoid being caught.

One obvious way in which animals avoid being eaten is to seek refuge. This may involve burrowing into the substratum (as do many bivalves or worms), the construction of tubes or cases (as in many midge and caddisfly larvae), or be behavioural (as when organisms hide from visual predators during daylight). Some snails have a tough shell into which the animal withdraws and these are used secondarily by hermit crabs [5.35.] that inhabit empty snail shells. Such mechanisms are not a perfect foil for predators. Some fish nibble the tops of siphons of bivalves; intertidal animals living in tubes and burrows are predated by birds, especially waders; shells in fresh waters cannot be as readily strengthened with calcium salts as those in the sea and are easier to attack.

Just as predators have developed powerful senses for hunting, the avoidance of predators acts as a strong selection pressure for increased acuity in prey organisms. Mobile prey that sense predators at a distance are more likely to be able to avoid them. In water, the senses used are vision (e.g., the eyes of many fish), vibration (e.g., use of the lateral line system in fish), tactile cues (e.g., the long antennae of shrimps and prawns), and smell (e.g., sense organs in some clams). However, many aquatic prey organisms appear to have only limited awareness of the presence of predators until close contact and the only defence then is to retaliate.

To deter predators, some aquatic animals use toxins just as do several aquatic plants. While some fish have sharp spines that make them difficult to swallow, others have associated poison glands that inject chemicals that produce very unpleasant local irritation of the predator's mouth. The stinging cells used by jellyfish to capture prey also serve to deter predators although some sea slugs that feed on coelenterates are able to sequester stinging cells. These cells are passed in some way to the papillae on the dorsal surface of the slug and serve as its own defence. While secretion, and sequestration, of toxin is probably rather common, sequestration of undischarged stinging cells must be rare.

Cryptic coloration is used both by predators and by prey. Many flatfish are cryptically coloured, blending in well with their background [5.36.]. Coloration may be aided by irregular margins to the body or by covering the fins by swimming just under the surface of the substratum. Some flatfish, and many cephalopod molluscs (octopus and cuttlefish [5.37.]) have chromatophores in the skin, cells which expand, or contract, to induce colour change. This occurs rapidly and ensures even better camouflage than a fixed cryptic coloration.

The general brownness of aquatic animals also serves to camouflage (at least, where light is present) and the tubes of animals like caddisflies and some polychaete fan worms are made of the same materials as those in the surroundings. To maintain crypsis, some crabs decorate their bodies with pieces of shell and detritus and many burrowing mayfly larvae are characteristically covered with detritus when caught during sampling the substratum of ponds. Mayfly larvae have three long cerci (filaments) at the end of the body [5.38.] and parts of these are often missing on specimens collected, supposedly having broken off or sacrificed to a predator. Sacrificing non-vital parts of the body (e.g., by starfish or primitive worms) provides a further means of anti-predator defence.

The final method used by prey animals to avoid predation is to live in groups. This increases the chance of predators being detected and decreases the chance of an individual being predated, but only if the predator hunts for single prey rather than for groups. Dense swimming aggregations have the advantage of confusing predators that search for individual prey and a single prey item is then often difficult to distinguish. Highly evolved aquatic animals such as some fish form shoals that move as one, confusing predators by making sudden changes of direction and then scattering when the predator attacks.

In discussing feeding strategies, we must be aware that some animals are members of several functional feeding groups. This may be a change that occurs during the life cycle or at one point in the life of an animal. For example, some net-spinning caddisfly larvae feed largely on detritus during their early larval life but later select animal food from the wide range of organic matter captured on their nets. They are thus suspension-feeding collectors that change to being predators as they grow larger. Blackfly larvae (Simuliidae) are good examples of suspension-feeding collectors, but several species also scrape algae and biofilm from the substratum with their mouthparts in addition to collecting material from the water current. They also ingest animals that they encounter, so can be regarded as having three feeding strategies. Another example of the concurrent use of different feeding strategies is shown by the small crustacean Corophium [5.39.] which lives in burrows in mud flats. It feeds by raking deposited particles using long modified antennae and also feeds by using limbs to force a current through its burrow, from which it collects suspended particles using other limbs.

Variations in feeding strategy with time are commonplace among aquatic animals and care is needed when assigning positions of animals in food webs and other models.

The earliest unicells took up DOM across their cell walls, a method of uptake retained by present-day unicells and some multicellular organisms. Capture of particles by cells requires part of the cell membrane to be internalised with the particles in a process called phagocytosis [5.40. – shown here for a white blood cell] and this is well illustrated by protists. Enzymes are delivered to the food vacuole to bring about the digestion of large molecules associated with the internalised particles. After digestion, usable breakdown products pass across the membrane into the cell and the remaining contents of the food vacuole pass out from the cell in the reverse of phagocytosis.

The gut of animals surrounds ingested food and its lining is part of the organism. However, the gut contents are external to the body, although retained within it. The gut is thus a means of concentrating food from the external environment into a chamber where the environment is altered markedly to effect digestion. Food is compacted within the gut, the pH is usually changed, and enzymes are secreted. Breakdown of ingested food thus results from abrasion (which must be prevented from affecting the gut wall) and/or hydrolysis (by acids, alkalis or enzymes). Labile molecules resulting from digesting are passed across the gut wall and enter the animal where they are used in metabolism, just as in unicells. The gut thus functions in a similar way to the food vacuoles of protists and some organisms still use phagocytosis by cells lining the gut to take up nutrients. This is a feature retained by cells that were ancestrally free living. After digestion and absorption, the materials remaining in the gut are egested together with the waste products of metabolism.

Through time, evolution has selected for changes in the structure of the gut so that the simple tube of the earliest animals has become much modified. For example, there may be separate regions of the gut for milling, digestion, absorption, harbouring of symbionts, and for extraction of water. The extent to which the gut has evolved varies from organism to organism. Generally speaking, the simplest gut structure is found in collectors that process large quantities of low-quality organic matter, with animals in other functional feeding groups showing variation in complexity.

A good quality diet can be defined as one having a large proportion of nitrogen relative to carbon, and organic compounds that are relatively easy to break down. Thus, collectors that have no selection mechanism have the poorest quality diet and predators the highest quality diet. Shredders and scrapers have diets that range in quality, some components being readily digestible and others refractory to digestion. Plant strengthening compounds are among the most resistant chemicals as they consist of complex carbohydrates that require many cleaving reactions before labile sugars become available. Indeed, many animals do not have the enzymes to cope with this diet and require the assistance of symbiotic micro-organisms that have the capacity to digest chemicals like cellulose before the animals can utilise this food.

If the amounts of material ingested (I) and egested (E) are measured we can calculate the amount that is assimilated (A = I - E) and this useful measure allows us to assess the efficiency of digestion of organisms for different foods. Measures of assimilation efficiency (I - E)/ I (as a percentage) of some animals from different functional feeding groups are given in Table 5.2. These data confirm that predators have the highest assimilation efficiency and collectors the lowest, reflecting differences in the quality and quantity of their diet. For shredders it is conditioned plant detritus that gives the highest assimilation efficiency, with decomposing micro-organisms being assimilated to the highest degree. Scrapers assimilate algae and bacteria more efficiently than detritus and the generalised organic layer, showing a similar response to shredders feeding on conditioned plant detritus.

Table 5.2. Assimilation efficiency of animals within different functional feeding groups

The diet of collectors is of low quality but abundant and little energy is therefore expended in acquiring food. In contrast, the diet of predators is of high quality but scarce, and predators often need to use energy to find, catch and then subdue prey. There is thus a trade-off and the optimal strategy is the one that gives the best return in energy for every unit of energy expended. This also links to the time ingested material spends within the gut.

If poor quality food is very plentiful there is little advantage in retaining it for long periods as it remains of low quality. This is why many collectors have capacious guts and feed almost continuously. Selection mechanisms improve the quality of the diet of collectors and thus a measure of control over what is ingested and a longer gut retention time should therefore be profitable. Predators, in contrast, have long gut retention times to assimilate much of the good quality food captured. However, when prey are abundant, predators will increase their rate of feeding and often have a shorter time for digestion as only the most easily assimilated components of the diet are used. Retention times for shredders and scrapers vary according to the abundance of food and, in the former case, the need for sufficient retention time to allow digestive enzymes of any symbionts to work effectively. Both scrapers and shredders show some selectivity in their diets; the latter having a preference for conditioned plant detritus as it contains a higher ratio of usable food (micro-organisms) compared to that which is refractory (plant strengthening compounds)

All organic and inorganic particles in water have coatings of adsorbed organic matter, in addition to attached micro-organisms. This applies to even the smallest colloidal particles captured by some suspension feeders. Inside the gut, coated particles encounter a changed chemical environment and large shifts in pH often occur within a very short distance along the gut from the mouth. Such shifts affect adsorbed coatings and these may become detached and hydrolysed. The gut lumen therefore contains a column of particles surrounded by an aqueous medium which has been enriched by desorption and hydrolysis. Enzymes continue the process of digestion by breaking down refractory organic compounds into ones that are assimilable so that the aqueous milieu within the gut is rich in labile compounds, while the majority of the gut contents are little affected by enzyme activity. We do not know the significance of coatings in nutrition but it may be considerable in some aquatic animals.

Just as shredders cut up conditioned plant detritus to obtain its microbial coatings, so some collectors may ingest particles to obtain their coatings of adsorbed matter. But what about the ingestion of living organisms? All shredders, scrapers and collectors ingest living micro-organisms together with living plants and animals. Some ingested organisms are killed by the greatly changed physico-chemical environment within the gut and much of their contents digested, but others pass through the gut alive, although subject to abrasion and stress. The surviving organisms produce exudates during normal aquatic life and the stressed environment of the gut into which they are collected is likely to increase exudate production. Amongst the exudates are labile compounds and exopolymers, some of which are easily digested. It is clear that living organisms passing through the gut may be a source of nutrients even though the organisms survive intact.

Materials in the hindgut are usually compacted to produce faecal pellets. These are passed into the water and begin to sink through the water column to join pellets produced by benthic organisms on the substratum. As pellets contain material that is difficult to digest, it is safe to assume that freshly-egested faecal pellets are of lower food quality than the foods that are available within the water column and that have recently been ingested. Freshly-produced faecal pellets are thus best avoided by feeding animals but we should pause to assess just how abundant this material is in many environments. We are all familiar with worm casts on muddy shores but the abundant faecal and pseudofaecal strings of molluscs are less familiar, although much more abundant. The muddy, organic-rich sediments of lake and sea margins are sometimes described as being pelletised without mention that we are talking predominantly about faecal matter. Similarly, the organic matter that accumulates in the lee of rippled sand in rivers and coastal regions often contains large numbers of faecal pellets. Pellets also make up a high proportion of the deposited material along river margins and lake shores.

On egestion, faecal pellets produce a pulse of DOM that leaches out and this probably attracts the first colonising bacteria. In addition, bacteria that have survived passage through the gut are packed together with usable substrates in the bound pellet and this is likely to be an ideal medium for their growth. The food value of faecal pellets increases as refractory substrates are broken down during conditioning, and valuable chemicals such as nitrogen are taken up from the water by attached and bound bacteria.

Non-selective feeders capture and ingest faecal pellets of any age but only conditioned pellets are likely to be eaten by collectors and shredders that select for conditioned detritus. Feeding on faecal matter is termed coprophagy. It must be stressed that this is not like coprophagy in rodents, where the digestive system of one individual attacks refractory plant material twice, pellets being re-ingested upon egestion. Coprophagy in aquatic animals is a component of detritivory but one that has been substantially ignored in most habitats. The exception is in biological oceanography where the role of faecal pellets in conserving organic matter to aid autotrophic metabolism has long been recognised.

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