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FIGURE 1.6 Nutrient uptake and release in streams is coupled with downstream transport, stretching cycles into spirals. Spiraling length is the sum of the distance traveled by a nutrient atom in dissolved inorganic form in the water column, called the uptake length (Sw, in meters), and the distance traveled within the biota before being mineralized and returned to the water column, called the turnover length (SB). Arrows show uptake and release of nutrient retained within the streambed. (Modified from Newbold 1992.)

FIGURE 1.6 Nutrient uptake and release in streams is coupled with downstream transport, stretching cycles into spirals. Spiraling length is the sum of the distance traveled by a nutrient atom in dissolved inorganic form in the water column, called the uptake length (Sw, in meters), and the distance traveled within the biota before being mineralized and returned to the water column, called the turnover length (SB). Arrows show uptake and release of nutrient retained within the streambed. (Modified from Newbold 1992.)

critical in establishing the link between fertilizer use in the Upper Mississippi Basin and the Gulf of Mexico's anoxic zone, where nutrient enrichment has harmed fisheries over a large region. Nutrient exports are a fairly constant percentage of inputs, which differ among catchments largely because of human influences over agricultural activities, food and crop imports and exports, and atmospheric deposition. Reductions in the mass of nutrients exported by rivers will require either a reduction of inputs, or finding ways to increase internal removal.

The river continuum concept integrates stream order, energy sources, food webs, and to a lesser extent nutrients into a longitudinal model of stream ecosystems (Figure 1.7). In a river system flowing through a forested region, the headwaters (order 1-3) are heavily shaded and receive abundant leaf litter, but algal growth often will be light-limited. Streams of order 4-6 are expected to support more plant life because they are wider and less shaded, and in addition receive organic particles from upstream. The headwaters have more allochthonous inputs, indicated by a ratio of primary production to respiration well below unity, whereas the mid reaches have more autochthonous production and a higher P/R ratio. Higher-order rivers are too wide to be dominated by riparian leaf fall and too deep for algal production on the bed to be important. Instead, organic inputs from upstream and the floodplain, along with river plankton, play a greater role.

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