ArticlesT hink 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.
What are feces?Almost all aquatic animals have guts into which gathered, or captured, food is ingested. Within the gut the food is subjected to abrasion, changes in pH, and the action of enzymes, all of which contribute to the breakdown of organic matter into labile compounds that are taken up across the walls of cells lining the gut. The gut lumen is thus best regarded as a space where material is concentrated and converted to molecules that can pass across the gut wall and thus be used in metabolism. Viewed in this way, ingested food is "external" to the animal, as only breakdown products enter animal tissues, and materials excreted into the gut lumen are also passed out from the animal with the egested feces. The egesta usually look very different from the materials ingested...
