Abstract

One-hundred twenty-eight taxa of phytoplankton and 32 taxa of zooplankton were collected from three stations in the Arkansas River near its con- fluence with Red Rock Creek. Samples were taken on six dates between April 1975 and January 1976. Maximum numbers of taxa of both groups occurred in late spring. Minimum variety of phytoplankton existed in summer, while both variety and density of zooplankton were extremely low in fall and winter. Density of phytoplankton ranged from 34.2 to 533.1 cells/ml. Density values were low in winter and early spring and reached maximum levels in early fall. Zooplankton density ranged from 0.5 to 129.3 individuals/i with maximum density occurring in summer. Diversity (d) of phytoplankton in the river was high ranging from 2.3 to 3.8. Values on most dates exceeded three. Diversity of zooplankton was generally low except in late spring. Chlorophyll a ranged from 0.04 to 0.30 mg/m3. Highest values were measured in summer, while lowest values occurred in winter and late spring. Plankton populations have been used as indicators of the degree and severity of organic enrichment. Williams (1963) described spe- cies composition of phytoplankton and zooplankton populations at 65 stations of the National Water Quality Network. He concluded that pollution alters the normal population patterns and the organ- isms serve as important indicators of water quality. Data on the dis- tribution of these organisms in Oklahoma is sparse. It was the objec- tive of this study to describe seasonal variation in species composition and diversity of phytoplankton and zooplankton populations in the Arkansas River near its confluence with Greasy and Red Rock Creeks in northcentral Oklahoma and to measure phytoplankton chlorophyll concentration. METHODS. Three stations were established on the Arkansas River (U, M, and D = Upstream, Middle, Downstream). Station U is located about 15 km south- east of Ponca City, Oklahoma, and about 1.5 km above the confluence of the river with Red Rock Creek. Station M is immediately below the joining of the two streams. Station D is about 5 km below Station M and immediately below

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