Characterization of a Central Texas Reservoir with Emphasis on Factors Influencing Algal Growth

Monitoring data collected between June 1996 and December 1999 were used to characterize water quality within Lake Waco with particular emphasis on factors influencing algal growth. A spatial analysis of surface water quality for Lake Waco established two different longitudinal patterns within the reservoir based on distance from major tributary inflows. For physical characteristics, such as conductivity and Secchi depth, riverine, transition, and lacustrine zones were identified along the northern and southern arms of the reservoir with increasing distance from major tributary inflows. For chemical and biological characteristics, such as chlorophyll-α (CHLA) and nitrogen (N) concentrations, a longitudinal gradient within the reservoir was followed based on proximity to either the northern or southern tributary inflows. The highest CHLA concentrations occurred nearest the inflow of the North Bosque River, while the highest total N and dissolved inorganic N concentrations occurred nearest the inflow of the Middle-South Bosque River. Over time, reservoir concentrations of soluble nutrients showed a strong positive correlation with the amount of inflow from the major tributaries. A strong depletion of soluble nutrients with increasing CHLA concentrations was apparent between inflow events, particularly during summer months when warmer temperatures allowed more active algal growth. The reservoir showed very little temperature stratification, although a decrease in dissolved oxygen and pH with depth was indicated during summer months. A slight release of ammonia from bottom sediments was also indicated during the summer, but no corresponding release of orthophosphate was apparent. During the summer, Lake Waco was characterized as eutrophic using a trophic state index based on CHLA or Secchi depth measurements. Phosphorus was identified as the limiting nutrient for algal growth within the reservoir with potential N limitation occurring in late summer or early fall.