Feces in Aquatic Ecosystems

Abstract

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...

Fecal production by sea urchins in native and invaded algal beds

“…Marine bacterial assemblages have, on average, C:N ratios ranging from 5 to 7 (Fukuda et al 1998). Therefore, to utilize the organic matter in the kelp feces, bacteria must take up dissolved inorganic nitrogen (nitrate or ammonia) from the surrounding seawater, lowering the C:N ratio of the fecal aggregate and increasing its nutritional quality (Wotton & Malmqvist 2001, Povero et al 2003. Urchins coat fecal pellets with mucus that adds to their organic content (Lawrence & Klinger 2001).…”

The utiquity and many roles of exopolymers (EPS) in aquatic systems

“…Binding of faecal pellets is a vitally important indirect role of EPS, as pellets are dense and move through water columns ). Unless they are broken apart, or become diffuse, pellets sediment or are transported by currents and the movement of faecal pellets within oceans, lakes and rivers represents a significant vertical and/or horizontal movement of carbon Wotton and Malmqvist, 2001).…”

Fate of organic carbon released from decomposing copepod fecal pellets in relation to bacterial production and ectoenzymatic activity

“…Aquat Microb Ecol 33: [279][280][281][282][283][284][285][286][287][288] 2003 cycled (Wotton & Malmqvist 2001, Turner 2002 and that most carbon originating from these fecal pellets remains part of the long-lived organic carbon in the epipelagic (Legendre & Michaud 1998). Hence, the role of fecal pellets from small copepods is that of supplying nutrients to the epipelagic planktonic microbial community rather than maintaining the vertical flux of organic matter.…”