Seasonal and Longitudinal Variation in Nitrate and Chlorophyll a in a Central Texas Reservoir
Date
1998-05
Authors
Cook, Bryan P.
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Abstract
Canyon Reservoir is a subtropical impoundment located on the Guadalupe River in southcentral Texas. A wet February through June, including one of the largest flood events in Canyon's history, occurred during the 1997 sampling year. NO3+NO2-N concentrations were unusually low during the winter months, presumably from reduced river inflows due to a drought in 1996. Surface NO3+NO2-N was higher after a large June 22 storm, which established a decreasing longitudinal gradient in NO3+NO2-N from up-reservoir to the dam throughout the summer. Surface NO3+NO2-N was depleted in the lacustrine epilimnion during the growing season, apparently both from phytoplankton uptake and isolation from upstream and hypolimnetic inputs. Storm inflows rich in NO3+NO2-N tended to increase chlorophyll a concentrations in the transitional zone. Chlorophyll a concentrations were generally highest in the transitional zone throughout the year. Results of nutrient limitation assays suggested that nitrogen was co-limiting, along with phosphorus, to the phytoplankton. Temperature and dissolved oxygen patterns were similar to those in the record wet year of 1992. Cold hypolimnetic water from the winter months was released through the deep-release penstock during the growing season, resulting in a warmer, anoxic hypolimnion and an early fall turnover.
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Keywords
nitrates, chlorophyll, Canyon Lake, phytoplankton
Citation
Cook, B.P. (1998). Seasonal and longitudinal variation in nitrate and chlorophyll a in a central Texas reservoir (Unpublished thesis). Southwest Texas State University, San Marcos, Texas.