379 Yy B( /Io. 7^89 a COMPARISON of PREDICTED and ACTUAL TROPHIC STATUS of LAKE RAY ROBERTS, TX BASED on CHLOROPHYLL a THESIS Pr

379 yy B( /io. 7^89 A COMPARISON OF PREDICTED AND ACTUAL TROPHIC STATUS OF LAKE RAY ROBERTS, TX BASED ON CHLOROPHYLL A THESIS Presented to the Graduate Council of the University of North Texas in Partial Fulfillment of the Requirements For the Degree of MASTERS OF SCIENCE By Lili Lisa Lytle, B.S. Biol. Denton, Texas May 1999 Lytle, Lili, A comparison of predicted and actual trophic status of Lake Rav Roberts. TX six years after impoundment based on chlorophyll a. Master of Science (Environmental Science), May, 1999, 46pp., 3 table, 11 illustrations, references, 15 titles. Two years before impoundment, the trophic status of Lake Ray Roberts was predicted by applying the total phosphorus input into the Organization for Economic Cooperation and Development (OECD) eutrophication model. Predicted mean summer epilimnetic (MSE) chlorophyll a of Elm Fork arm, Isle duBois arm and Main Body were in the eutrophic category of the OECD model. Observed MSE chlorophyll a two years after impoundment of Elm Fork arm, Isle duBois and Main Body had not reached their predicted means and were at the mesotrophic-eutrophic boundary of the OECD model. Six years after impoundment, observed MSE chlorophyll a for Main Body, was closer to itOs predicted mean and in the eutrophic category of the OECD model. Six years after impoundment, Elm Fork arm was the most productive area of Lake Ray Roberts. Observed means of chlorophyll a, total phosphates, suspended solids and turbidity were often highest in the Elm Fork arm. Wastewater effluent from Gainesville and Valley View, TX, had an impact on productivity in Elm Fork arm. 379 yy B( /io. 7^89 A COMPARISON OF PREDICTED AND ACTUAL TROPHIC STATUS OF LAKE RAY ROBERTS, TX BASED ON CHLOROPHYLL A THESIS Presented to the Graduate Council of the University of North Texas in Partial Fulfillment of the Requirements For the Degree of MASTERS OF SCIENCE By Lili Lisa Lytle, B.S. Biol. Denton, Texas May 1999 TABLE OF CONTENTS Page LIST OF TABLES v LIST OF ILLUSTRATIONS vi Chapter 1. INTRODUCTION 1 Impoundments Phosphorus and nitrogen Vollenweider phosphorus loading model OECD eutrophication model Nutrient loading to Lake Ray Roberts Objectives 2. MATERIALS AND METHODS 12 Study area Sample sites Water Quality Analyses Statistical Analyses 3. RESULTS 21 May 1993 June 1993 July 1993 August 1993 September 1993 October 1993 4. CONCLUSIONS 66 5. APPENDIX 70 Mean values and standard deviations for all parameters of each sample date 6. BIBLIOGRAPHY 73 IV LIST OF TABLES Page 1. Predicted annual P load to Lake Ray Roberts and predicted mean summer epilimnetic chlorophyll a 8 2. Water quality parameter means and standard deviations May-October 1993 70 LIST OF ILLUSTRATIONS Page 1. Vollenweider P loading-mean depth/hydraulic residence time 6 2. Total phosphorus load-chlorophyll a line of best fit for U.S. OECD waterbodies with 95 % confidence limits 10 3. Study area of Lake Ray Roberts 15 4. Drawdown of Lake Ray Roberts 17 5. Sample sites for Lake Ray Roberts, 1993 19 6. Comparison of predicted and observed mean summer epilimnetic chl a 22 7. Water quality parameters May 1993 24 8. Water quality parameters June 1993 27 9. Correlation of mean turbidity and mean secchi depth June 1993 30 10. Water quality parameters July 1993 32 11. Correlation of mean turbidity and mean suspended solids July 1993 36 12. Correlation of mean total phosphates and mean suspended solids July 1993 38 13. Correlation of mean turbidity and mean secchi depth July 1993 40 14. Water quality parameters August 1993 42 15. Correlation of mean turbidity and mean secchi depth August 1993 45 16. Water quality parameters September 1993 47 17. Correlation of mean chlorophyll a and mean suspended solids September 1993 50 vi 18. Correlation of mean turbidity and mean suspended solids September 1993 52 19. Correlation of mean suspended solids and mean secchi depth September 1993 55 20. Correlation of mean turbidity and mean secchi depth September 1993 57 21. Water quality parameters October 1993 59 22. Correlation of mean chlorophyll a and mean secchi depth October 1993 62 23. Correlation of mean turbidity and mean secchi depth October 1993 64 VII CHAPTER I INTRODUCTION There are 5,700 impoundments on the Texas river system; seventy-four of which supply 98% of the state's surface water (Texas Water Board, 1991). Impoundments are typically dendritic with the majority of nutrients and water flowing in from a single tributary. As a result, gradients in water quality and trophic condition form from the inlet to the dam (Ryding and Rast, 1989). A common threat to the life and water quality of impoundments is cultural eutrophication. Normally, eutrophication is a gradual process of increased productivity as a waterbody ages. Cultural eutrophication is accelerated productivity from increased nutrient loading by anthropogenic forces, resulting in a profusion of algal biomass and macrophytes (Wetzel, 1983). Accumulations of algal biomass can degrade water quality with offensive tastes and odors, high turbidity, low levels of dissolved oxygen, and diminished recreational appeal. Bioavailable forms of phosphorus (orthophosphate) and nitrogen (nitrate) are the main nutrients required for algal growth. Most freshwater bodies are phosphorus limited. Lakes with hydraulic residence times longer than a few months can have 80-90% of their phosphorus load trapped in their sediments. Phosphorus in sediments can be released during turnover, but the internal cycling of phosphorus in a stratified lake has a limited influence on the accumulation of algal biomass (Lee et a!., 1978). Wastewater treatment plants' discharges are often the greatest external loading source of phosphorus. According to Bartsch (1970), 20-50% of nitrogen and phosphorus may be removed from conventionally treated wastewater, but it is often not enough to prevent potential enrichment. The Vollenweider phosphorus-load relationship, based upon the ratio of mean depth over hydraulic residence time, quantifies the amount of phosphorus loading that may cause a waterbody to become eutrophic (Rast and Lee, 1978). In the Vollenweider phosphorus load diagram (Fig. 1), permissible total phosphorus loads that would maintain oligotrophic conditions are 10 mg/m3 TP or less; excessive loads that result in eutrophic conditions are 20 mg/m3 TP or greater, and concentrations from 10-20 mg/m3 TP would categorize the waterbody in a mesotrophic or transitional state (Rast and Lee, 1978). The Vollenweider phosphorus-loading relationship was the basis for the OECD (Organization for Economic Cooperation and Development) eutrophication model (Fig. 2). This model estimates Mean Summer Epiiimentic (MSE) chlorophyll a concentrations proportional to the total phosphorus load of a waterbody. Waterbodies with mean summer chlorophyll a concentrations above 10 ug/L chl a are classified as eutrophic, and concentrations below 5 ug/L chla would be considered oligotrophic (Archibald and Lee, 1981). Between 5-10 ug/L chla , the waterbody would be in a mesotrophic state. Oligotrophic waterbodies have low nutrient flux, average depths greater than 15m, and high dissolved oxygen levels in their hypolimnion year round (Rast and Lee, 1978). Eutrophic waterbodies have high nutrient flux, higher productivity, algal blooms, shallower depth, dissolved oxygen depletion in the hypolimnion during stratification, oxygen saturation in the surface water, and greater turbidity with secchi depths of 3m or less. Pillard (1988) developed a nutrient mass balance for Lake Ray Roberts, TX. He found the Elm Fork of the Trinity River to be the greatest source of mean annual total areai phosphorus (98.67 x 106 g/m2/yr) compared to Isle duBois Creek (60.89 x 106 g/m2/yr) and runoff into the Main Body (55.19 x 106 g/m2/yr) (Table 1) (Pillard, 1988). Pillard (1988) predicted the mean depth of Lake Ray Roberts to be 27 feet (8.2 m) and hydraulic residence time of 4.5 years. According to the Vollenweider relationship, mean annual total phosphorus in both arms and Main Body were excessive (Fig. 1). Mean areal P loads for Elm Fork arm (3480 mg/m2), Isle duBois arm (920 mg/m2), and Main Body (1875 mg/m2) (Table 1) were plotted into the OECD eutrophication model. Pillard (1988) calculated mean summer epilimnetic (MSE) chlorophyll a concentrations for Elm Fork arm (38 ug/L), Isle duBois arm (29 ug/L), and the whole lake (47 ug/L). Predicted MSE chlorophyll a indicated both arms and main body would be eutrophic. Observed MSE chlorophyll a two years following impoundment (1989) for the Elm Fork arm (12 ug/L), Isle duBois arm (11 ug/L) and the Main Body (10 ug/L) fell below their predicted concentrations (Cairns, 1993) (Fig. 5). Observed MSE chlorophyll a for the three areas of Lake Ray Roberts were at the boundary of mesotrophic and eutrophic categories. The Elm Fork arm of Lake Ray Roberts receives the majority of it's nutrients from the waste water treatment plant upstream, in Gainesville, TX, via the Elm Fork of the Trinity River. During times of low flow, sewage effluents can comprise up to 100% of the Elm Fork's discharge, and as other creeks dry up from July to September, the Elm Fork of the Trinity River can constitute up to 80 to 90% of the flow into Lake Ray Roberts (Pillard, 1988). Higher nutrient loads to the Elm Fork arm of Lake Ray Roberts result in higher chlorophyll a concentrations than the Isle du Bois arm and main body (Pillard, 1988). Additional sewage effluent enters Lake Ray Roberts through Spring Creek, from the wastewater treatment lagoons in Valley View, TX, further enhancing productivity in the Elm Fork arm. Nutrient loading to Isle duBois Creek is from non-point sources during storm events (Pillard, 1988). Landuse of the Elm Fork of the Trinity river is comprised of agricultural and pasture/grassland areas.

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