Productivity and nutrient dynamics in an oligotrophic central Texas plateau aquatic ecosystem

The first ever direct effluent discharge permit was recently issued in the contributing zone of the Barton Springs portion of the Edwards Aquifer, a sensitive karst system in the Texas Hill Country. This discharge will change the flow regime and nutrient loads in Bear Creek, the stream, the aquifer it drains to, and Barton Springs, home to an endangered salamander (Eurycea sosorum) and a popular swimming pool in Austin, TX. Field and lab experiments were conducted to assess the potential alteration of the existing algal community and productivity in both Bear Creek and Barton Springs due to the increased nutrient loading. Matlock periphytometers were deployed in the field and sestonic algae growth bioassays were conducted in the lab to determine the limiting nutrient controlling periphytic and sestonic algae growth and to estimate critical nutrient threshold concentrations (Ks) . Results showed that both the stream and springs were co-limited by both nitrogen and phosphorus with a phosphorus threshold concentration of 0.004 mg/l. These results support the hypothesis that these systems are highly oligotrophic and that any nutrient additions of the type projected for the permitted discharge will significantly increase algal productivity, moving these systems to mesotrophic and possibly eutrophic status.