The phytoplankton are algae suspended in the water column and transported by currents.

Whether a river phytoplankton could be self-sustaining was in doubt for some time, because downstream flow would seem to prevent the persistence of their populations. It was suggested that any river plankton was the result of displacement of cells from the benthos, backwaters, and lakes or impoundments along the river's course, and reflected washout and export rather than a true "potamoplankton." These are indeed major sources of phytoplankton in rivers. In small, fast-flowing streams, sloughing of ben-thic algae likely is the primary source of phyto-plankton, and any cells in the water column are simply eroded material in transit (e.g., Swanson and Bachman 1976). However, in sluggish, lowland streams, in side channels and within macro-phyte beds, and in rivers of considerable length, the residence time of a water mass can be sufficient for true plankton to colonize and reproduce. Under these conditions phytoplankton and zooplankton almost always are present, and at times they can develop substantial populations.

It is doubtful that any planktonic organisms are restricted only to flowing water, and so the phytoplankton species found in rivers are drawn from the same pool of species found in standing water. Thus, the presence of lakes, ponds, and backwaters, and more recently the creation of impoundments, can be of great importance in seeding the river with plankton. Diatoms, particularly centric diatoms, have been found to dominate the composition of river phytoplank-ton in a number of studies (Mississippi, Baker and Baker 1979; Nile, Talling and Rzoska 1967; Thames, Lack 1971; Nakdong, Ha et al. 1998; Lower Parana, O'Farrell et al. 1998; Columbia, Sullivan et al. 2001; San Joaquin, Leland 2003). A 300-mile section of the Sacramento River recorded 15 genera of cyanobacteria, 38 genera of chlorophytes, 13 flagellate genera, and 29 diatom genera (Greenberg 1964). Diatoms dominated numerically, however, averaging 75% and occasionally 99% of cell counts. In tropical rivers, desmids (a green alga) have the largest number of species while diatoms dominate the biomass, although to a lesser degree than in temperate rivers (Rojo et al. 1994). In the Baia River, a tributary of the ParanĂ¡, cyanobacteria and diatoms dominate phytoplankton biomass, whereas chlorophytes contribute the most species (Train and Rodrigues 1998). In the Orinoco River, diatoms, flagellates, and cyanobacteria dominate the phytoplankton (Lewis 1988). Cyanobacteria have been reported to form dense blooms in the Nile and during summer in temperate rivers (Talling and Rzoska 1967, Bennett et al. 1986), apparently in response to nitrate depletion. These studies establish that phytoplankton occur in virtually all major rivers examined. They are present throughout the year, even when not apparent, as Sze (1981) demonstrated by collecting samples of Potomac River water from which he successfully cultured several major phytoplankton groups. Interestingly, the order of appearance in culture was centric diatoms, then chlorococcolean green algae, then pennate diatoms, and finally the cyanobacteria, which paralleled their seasonal appearance in the river.

Variation in temperature, light availability, nutrients, and discharge influence seasonal variation in phytoplankton abundance. In temperate rivers, abundance is greatest in spring and summer (Aliakmon River, Greece, Montesanto et al. 2000; Columbia River, the United States, Sullivan et al. 2001; St. Johns River, Florida, Phlips et al. 2000). In the Columbia River, higher nutrient levels, lower flows, and higher light availability probably favor diatom abundance during the spring. In the St. Johns River, seasonal variation is related to light availability, which is primarily influenced by the color of the water. Less color and lower light attenuation are observed during spring and summer, and these conditions correspond with greatest abundance of phytoplank-ton. In tropical rivers, phytoplankton abundance responds to hydrologic seasonality, and is higher during periods of low water. In the Orinoco River and some of its tributaries, highest produc tion and biomass of phytoplankton was observed during the periods of falling and low water, probably related to greater transparency, shallower depths, and lower flows (Lewis 1988). In the Baia River, phytoplankton biomass also peaked during low water and was dominated by heterocystous cyanobacteria. This was followed by an increase in diatoms, probably favored by an increase in rainfall and turbulence (Train and Rodrigues 1998).

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