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Nutrient and Energy Transfer within a Large River Ecosystem 243 Table 9.3. Zooplankton estimates from standing waters in floodplains of various northern temperate and tropical river systems' River Mean number of zooplankton per litert Amazon Danube Dvina Magdelena Mekong Ping and Nan Rivers Missouri Nile Parana aka Sokoto Zambesi 107-738 2-10,000 248-400 25-5,380 39-122 136 6.7 0.2-59 12-24,000 950-1,281 16-27 110 'Summarized from Table 4.2 and text by Welcomme (1985). tRanges of mean numbers are provided when available. However, the values in the table do not necessarily reflect true ranges because of variation in the numbers of samples. gear type. and season. ,~ ;.l........ c' ~ l 4 chemically desorbed or actively mobilized by microbiota. is not readily available in rivers with a high sediment load because most of the phos- phorus becomes bound to the sediments (Ellis and Stanford, 1988). The piedmont reaches of large rivers are often turbid from suspended fine sediments that reduce primary and secondary production. Sediments settle in low-velocity floodplain habitats to provide nutrients and allow sunlight to penetrate clearer water, providing favorable conditions for de- velopment and maintenance of a food web (Hynes, 1970; Bayley, 1991; Allan, 1995). Water temperatures become warmer in low-velocity flood- plain habitats that aid in food production (Kaeding and Osmundson, 1988). The combination of nutrients, sunlight penetration into the water column, and warmer water temperatures in low-velocity floodplain habitats pro- vide better conditions for primary and secondary production than adjacent turbid and cooler main river channels. Low-velocity habitats in the shallow littoral zone of floodplains pro- duce higher densities of zooplankton (i.e., higher secondary production) compared with the main river channel (Hynes, 1970; Allan, 1995). Various studies reported zooplankton densities that were 30 (Welcomme et al., 1989) to 100 (Hamilton et al., 1990) times greater in floodplain habitats than the adjacent river channels. WeIcomme (1985) reported zooplankton densities in floodplains of temperate and tropical rivers that ranged between 0.2 and 24,000 organisms per liter (Table 9.3). Seasonal pulses of total zooplankton numbers occur because of the cyclic nature of nu- trients and differences in life cycles of zooplankton species (Welcomme, 1985). Zooplankton produced in floodplains provide an autochthonous food source to main channels in regulated rivers (Eckblad et aI., 1984; Kallemeyn and Novotny, 1977). Various factors affect the micro distribution of benthic insects in streams, but the most important factors are water velocity and substrate size (Blinn . et a1.. 1995; Minshall et al., 1985; Rabeni and Minshall, 1977; Weisberg 'd i 1