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<br />Phytoplankton
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<br />Phytoplankton or algae are common and normal inhabitants of surface water
<br />and are encountered in every water supply that is exposed to sunlight (Palmer,
<br />1977). Phytoplankton, though usually inconspicuous, are extremely important
<br />in lakes because phytoplankton are the primary producers of organic matter and
<br />oxygen on which most aquatic animals depend. Phytoplankton provide a source
<br />of food for herbivorous zooplankton and fish. Thus, algae often are an asset
<br />in raw water. However, numerous water problems may result as the abundance of
<br />algae increases or the occurrence of certain kinds of algae increases.
<br />Extensive accumulations of algae at or near the lake surface are called algal
<br />blooms. Algal blooms may occur when there are sufficient nutrients available,
<br />when there are warm water temperatures, and when there is adequate sunlight.
<br />When algae die as a result of overproduction that depletes the available
<br />nutrients needed for growth and reproduction, the dissolved oxygen may become
<br />depleted, which may result in fish kills or in severe mortality of certain
<br />aquatic insects. Decomposing phytoplankton can cause an unpleasant odor; the
<br />fishy smell associated with very productive water is actually the odor of
<br />decomposing algae (McCoy, 1982). Excessive concentrations of algae can be
<br />troublesome because they clog screens, produce slime, and produce poor taste
<br />and bad odor, particularly as anaerobic decomposition occurs.
<br />
<br />The groups of algae that occurred in Pueblo Reservoir during the summer
<br />and fall of 1985 were diatoms, green algae, blue-green algae, cryptomonads,
<br />golden-brown algae, euglenoids, and dinoflagellates. Diatoms, green algae,
<br />blue-green algae, and cryptomonads comprised the majority of the phytoplank-
<br />ton. The average concentrations of phytoplankton collected from the euphotic
<br />zone in Pueblo Reservoir varied from more than 41,000 cel1s/mL during July to
<br />about 11 ,000 cells/mL during October 1985 (fig. 18). During July and August,
<br />blue-green algae comprised about 70 percent of the phytoplankton in the reser-
<br />voir. During September, blue-green algae comprised about 50 percent of the
<br />phytoplankton and, during October, diatoms comprised the greatest percentage.
<br />
<br />The distribution and concentrations of the major algal groups for July
<br />and September 1985 are shown in figures 19 and 20. During July, the greatest
<br />concentrations occurred at transect 3 where more than 100,000 cel1s/mL of
<br />algae were measured. During September, the greatest concentrations occurred
<br />at transect 2 where more than 50,000 cel1s/mL were measured. During July,
<br />blue-green algae were the dominant algal group throughout the reservoir
<br />(fig. 19). But in September, blue-green algae comprised the greatest percent-
<br />age of phytoplankton at transects 1, 2, 3, and 4, and diatoms comprised the
<br />greatest percentage of phytoplankton at transects 5, 6, and 7 (fig. 20).
<br />
<br />The large concentrations of phytoplankton, especially the blue-green
<br />algae, could cause substantial water-quality problems if specific conditions
<br />occur. Given the right conditions--warm water, large rate of algal respira-
<br />tion, low light levels, and small quantities of oxygen entering the reservoir
<br />from either the Arkansas River or the atmosphere--the overabundance of phyto-
<br />plankton could cause substantial oxygen depletion in Pueblo Reservoir in a
<br />relatively short period of time.
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