We noted, in the case of Hubbard Brook, that nutrient cycling within the forest was great in comparison to nutrient exchange through import and export. By contrast, only a small fraction of available nutrients take part in biological interactions in stream and river communities (Winterbourn & Townsend, 1991). The majority flows on, as particles or dissolved in the water, to be discharged into a lake or the sea. Nevertheless, some nutrients do cycle from an inorganic form in streamwater to an organic form in biota to an inorganic form in streamwater, and so on. But because of the inexorable transport downstream, diversity of patterns of nutrient input and output decomposition and nutrient flux ... ...influenced by stoichiometry...
Figure 18.7 Nitrogen available to actively growing roots of the bunchgrass Bouteloua gracilis in shortgrass steppe ecosystems in relation to precipitation in the study period. The values for the six sampling periods are the averages of eight replicate plots. •, downslope plots; o, upslope plots (up to 11m further up the same hillslope). (After Hook & Burke, 2000.)
... and plant defense chemicals nutrient 'spiraling' in streams
Figure 18.8 Nutrient spiraling in a river channel and adjacent wetland areas. (After Ward, 1988.)
spiraling. Nutrients, in organic form, pass on through the food web via invertebrates that graze and scrape microbes from the substratum (grazer-scrapers - see Figure 11.5). Ultimately, decomposition of the biota releases inorganic nutrient molecules and the spiral continues. The concept of nutrient spiraling is equally applicable to 'wetlands', such as backwaters, marshes and alluvial forests, which occur in the floodplains of rivers. However, in these cases spiraling can be expected to be much tighter because of reduced water velocity (Prior & Johnes, 2002).
A dramatic example of spiraling occurs when the larvae of blackflies (collector-filterers; see Figure 11.5) use their modified mouthparts to filter out and consume fine particulate organic matter which otherwise would be carried downstream. Because of very high densities (sometimes as many as 600,000 blackfly larvae per square meter of river bed) a massive quantity of fine particulate matter may be converted by the larvae into fecal pellets (estimated at 429 t dry mass of fecal pellets per day in a Swedish river; Malmqvist et al., 2001). Fecal pellets are much larger than the particulate food of the larvae and so are much more likely to settle out on the river bed, especially in slower flowing sections of river (Figure 18.9). Here they provide organic matter as food for many other detritivorous species.
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