Roger S. Wotton

Feces in Aquatic Ecosystems

Think of trophic levels, and what probably comes to mind is an illustration from a biology text showing a pyramid, with solar energy trapped by photosynthesizing plants on the bottom, plants fed upon by herbivores in the middle, and herbivores eaten by carnivores on top. These models may also show arrows indicating that feces and dead plants and animal bodies provide inputs to the detritus pool, illustrating how this organic matter is recycled by microorganisms, and how at all trophic levels, aerobic respiration results in the loss of energy from the ecosystem. Such conceptual models provide a basis for energy budgets to be investigated, but the focus of early studies was on energy production and consumption by plants and animals. In many contemporary studies scientists continue to concentrate on feeding, investigating food webs, optimal foraging, predator‐prey interaction, or the dynamics of functional feeding groups. Partly this is because the diet of animals, and the mode of food capture, can be used to model complex processes, and the strategies and tactics differ among populations. Much less attention has been paid to the role of feces in ecosystems, yet fecal pellets are often very abundant, represent a repackaging of available organic matter, and are readily transported. Aquatic plants photosynthesize only where light penetrates, and the photic zone makes up only a tiny fraction of the depth of oceans, but a greater fraction of the depth in most lakes. Primary production in the surface waters supports the biotic community of the photic zone, and feces and dead matter produced there descend through the water column. Feces thus provide an important flux of carbon from surface to deeper waters in oceans, and a similar vertical flux occurs in lakes. Unlike oceans and lakes, rivers receive much organic matter from terrestrial sources, and animals capture this from suspension or after it becomes deposited. Feces are carried horizontally by the current in rivers, and the significance of this transport has only recently been recognized. In this article, we discuss the fate of fecal pellets in aquatic ecosystems, particularly with respect to vertical and horizontal flux. First we need to know more about the feces of aquatic animals.

Black flies in the boreal biome, key organisms in both terrestrial and aquatic environments: A review

Abstract: The boreal biome is rich in running waters suitable for the development of black flies. Here we review the ecological roles of black flies and the options available for their management. Large numbers of these insects play quantitatively important roles in a number of processes, not only in the running-water habitats of the larvae but also in the terrestrial environments of the adults. Black flies suck blood from mammals, including humans, and from birds, with negative effects including occasional death of animals. Black flies also transmit parasites, including filarial worms and haemosporidians. In the last two decades, programs to control black flies by treating the larvae, using Bacillus thuringiensis var. israelensis, have been developed, with predominantly positive results but with limited understanding of the indirect impacts to the aquatic and terrestrial ecosystems. On a more positive side, adult black flies are food for a variety of predators and may favour pollination, as well as helping in nature conservation by deterring people from visiting wilderness areas. Black fly larvae feed on suspended particles, linking this energy source to invertebrate and vertebrate predators (e.g., salmonids). Because larval densities often are high, considerable amounts of sedimenting faecal pellets increase the local retention of organic material, and this provides nutrition for invertebrates and micro-organisms, and potentially fertilizes river margins. Larvae also produce silk, the role of which in the ecosystem is poorly known but potentially important.

Organic matter processing by chironomid larvae (Diptera: Chironomidae)

In laboratory experiments, we used fluorescent dye markers to investigate processing of organic matter by larvae of Psectrocladius limbatellus (Holm.) (Diptera: Chironomidae). 59% of the organic matter used was incorporated into tubes, 39% was present in faecal pellets (both after 24 h), and 2% was found in the larval gut at the end of experiments. Ingested matter passed through the gut rapidly, resulting in the gut being emptied more than 20 times each day. Further 24-h experiments using dye-marked faecal pellets showed that 6% of pellets produced were re-ingested and 12% were incorporated into tubes. There was no preference for conditioned faecal pellets as food over those that had recently been egested and tubes also provided a food reserve on which larvae feed. Chironomid larvae recycle organic matter resulting in its mineralization and their ‘engineering’ has a dramatic effect on the substratum.

Do benthic biologists pay enough attention to aggregates formed in the water column of streams and rivers

Abstract Aggregation of organic matter occurs commonly in the water column of streams and rivers. Aggregates are formed by physicochemical or biological mechanisms, and these aggregations result in the transformation of suspended matter into much larger units. Microaggregates are formed by spontaneous assembly of organic molecules and resemble single particles. In contrast, macroaggregates (flocs and snow) are formed by binding of components with exopolymers (EPS), and they vary in size, shape, and porosity. Fecal pellets are a 3rd type of aggregate and usually are bound by EPS but are more compact than flocs or snows. Transport of aggregates to the substratum plays a significant role in the use of organic matter by microorganisms and other benthic biota, and we need to integrate these processes into our studies of lotic ecosystems. Understanding aggregation is essential to basic science, and it plays a role in planning for conservation and management at a time when flowing waters and their catchments are increasingly threatened.

PARTICLE-SIZE AND CHIRONOMID (DIPTERA) FOOD IN AN UPLAND RIVER

The food ingested by a number of chironomid species in a stony-bottom stream was examined. Temporally co-occurring species ingested closely similar food types and sizes, but considerable differences were found between species having different phenologies. Furthermore, different larval instars of Cricotopus trifascia Edw. fed on different food particle types and sizes, smaller larvae consuming, on average, smaller particles than larger larvae. This tendency recurred when comparing larvae of different sizes belonging to different species. Field and laboratory studies showed that larvae tend to ingest particles in the proportions available in the environment and that populations apparently were food-limited. The amount and size distribution of food particles therefore strongly affect the distribution of stream-living chironomids in space and time.

Growth of Anopheles mosquito larvae on dietary microbiota in aquatic surface microlayers.

Abstract. Hydrophobic organic matter accumulates under the surface film of water bodies to form the surface microlayers. Heterotrophic microorganisms use this organic matter for growth, and they, in turn, are fed upon by Anopheles mosquito larvae and other animals. From laboratory experiments we show that two species of mosquito larvae, Anopheles gambiae and An.quadrimaculatus, grew most rapidly where surface micro‐layers were present and, especially, where labile dissolved organic matter was added to promote growth of microorganisms. The importance of microorganisms was confirmed by the addition of gentamicin antibiotic, which suppressed the microbiota and reduced the growth of larvae feeding on surface microlayers. Anopheles larvae grew well on a suspension of finely ground fish food to which the antibiotic had been added, showing that reduced growth was not due to gentamicin itself. Because sub‐surface microorganisms are the components of the larval diet that most affect growth, we discuss their relevance to strategies for larval control of Anopheles mosquitoes.

Surface Films: Areas of Water Bodies That Are Often Overlooked

Abstract Material accumulates at the water–air interface of all natural water bodies to form a surface film. The interface is a dynamic environment, so surface films are altered by water movements, solar radiation, and biological processes. These films consist of a complex of organic matter and microorganisms, some of which are harmful. Researchers have often overlooked surface films when studying water bodies, and their importance is only now being recognized.

Water purification using sand

Slow sand filters are used to purify drinking water. Each filter consists of a large tank containing a bed of sand through which water passes at typical rates of 0.1–0.3 m h−1. Water is cleaned by physico–chemical and biological processes occurring at the air–water interface, within the bulk water, over the surface of the sand, and within the bed of sand. The processes found in sand filters replicate many of those found in natural sand banks and sandy beaches.

Feeding behavior of aquatic insects: Case studies on black fly and mosquito larvae

The study of feeding behavior in aquatic insects requires integrating a number of techniques. Light and scanning electron microscopy give three-dimensional images of the feeding apparatus needed to understand their structure, while cinematography allows frame-by-frame analysis of movements of the food collecting organs necessary to explain their function. Videography is used in two ways: to provide a record of feeding over time and, with microscopy, to show patterns of flow around the insects. This facilitates the construction of catalogs of feeding behaviors and aids in the interpretation of relationships between functional morphology and hydrodynamics. Experiments are then conducted to determine which foods are ingested from those available in the environment. We use examples of investigations on black fly larvae and mosquito larvae to illustrate this integrated approach to the study of feeding in suspension-feeding aquatic insects.

Shredder fecal pellets as stores of allochthonous organic matter in streams

Abstract The freshwater amphipod Gammarus pulex produces cohesive, cylindrical fecal pellets that are found in large numbers in the hyporheic sediments of UK chalk streams. We investigated the rate of fecal pellet accretion (deposition + in situ egestion) in chalk streams using benthic fine particulate organic matter traps. We also assessed how long pellets remain intact by observing them over time in the laboratory at 3 different temperatures (4, 10, and 20°C). Traps were sampled seasonally (winter, spring, summer, and autumn), and their vegetation cover (marginal Nasturtium, mid-channel Ranunculus beds, or no vegetation) was recorded to see if accretion rate was affected by the presence of in-stream macrophytes. Accretion rate was strongly seasonal (10 to 20× higher in autumn than in winter, spring, and summer), but was not affected by vegetation cover. Laboratory experiments showed that fecal pellets remained intact for 81 d at 4°C and 10°C, whereas they began to break apart after 40 d at 20°C. Chalk streams remain at ∼10°C most of the year. Thus, pellets transferred to hyporheic sediments during autumn remain intact over winter and into spring. In autumn, Gammarus feed on allochthonous C in the form of dead leaves and this C is transferred to hyporheic sediments where it is stored, providing a substrate for microorganisms and a food resource for detritivorous invertebrates at other times of the year.