Nitrogen and Phosphorus in a Stretch of the Guadalupe River, Texas, with Five Main-Stream Impoundments by

Nitrogen and phosphorus were studied in a 168 km stretch of the Guadalupe River that had 5 main stream impoundments. Flow through the study area was controlled by releases from these 5 reservoirs and from Canyon Reservoir, a deep-storage reservoir, located 30 km upstream. Parameters measured monthly on a diel basis at 16 stations were nitrate nitrogen, nitrite nitrogen, ammonia nitrogen, Kjekdahl nitrogen, inorganic phosphate phosphorus, organic phosphate phosphorus and total phosphate phosphorus. Inorganic nitrogen concentrations observed in this study were as high or higher than that previously reported for other bodies of water. Nitrate nitrogen entered the study area in relatively high concentrations from Comal Springs, which was a major source of water for the Guadalupe River. Water from Canyon Reservoir, the other major source of water, was relatively low in nitrate nitrogen. The concentration of nitrate nitrogen was, therefore, dependent in part upon the portion of the total river flow originating from the 2 sources. Increased discharge from Canyon Reservoir and utilization by plants in areas of high chlorophyll alpha resulted in low nitrate-nitrogen levels. Retention of water in reservoirs reduced the concentration of nitrate nitrogen due to increased utilization by plants in areas of low flow. Nitrate nitrogen, in general, reached seasonal minima in summer and maxima in winter. Nitrite nitrogen showed considerable variation with no meaningful pattern except that higher concentrations occurred in association with high chlorophyll alpha and high Kjeldahl nitrogen, regions and periods of low river flow, and large phytoplankton populations. There was no increase in concentration of any form of nitrogen in the vicinity of sewage outfalls and no downstream accrual. Phosphorus levels in the study area were as high or higher than those reported in studies of other bodies of water. Sewage treatment plants at New Braunfels and Seguin, Texas, were major sources of phosphorus to the Guadelupe River. Total phosphate phosphorus was determined to be the most critical phosphate parameter in assessing eutrophication. Seasonally, it ranged from a winter high to a summer low. Concentrations were highest immediately below sewage outfalls and decreased as water progressed downstream. Inorganic phosphate-phosphorus concentrations showed no clear seasonal trend but were clearly associated with sewage outfalls. Since large standing crops of phytoplankton were observed in