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. The watershed of Isle duBois Creek has a larger percentage of pasture/grasslands in addition to upland/transitional woods and bottomland hardwoods.
Lake Ray Roberts, TX has been the subject of a long term study by the
U.S. Army Corps of Engineers in cooperation with the Institute of Applied
Sciences at the University of North Texas in Denton, TX. The first environmental assessment was in 1986, before impoundment. Subsequent studies were two and six years after impoundment in 1989 and 1993, respectively.
The purpose of this investigation was to determine if predictions of Lake
Ray Roberts trophic status made by Pillard (1988) using the Vollenweider and
OECD eutrophication models were achieved six years after impoundment (1993) of Lake Ray Roberts. Specific objectives were: Objective 1: Determine if observed Mean summer epilimnetic (MSE) chlorophyll
a six years after impoundment (1993) supported a eutrophic state
as predicted by Pillard (1988) for Main Body of Lake Ray Roberts.
Objective 2: Compare observed MSE chlorophyll a for the Main Body two and
six years after impoundment to determine whether or not there was
a change.
Objective 3: Determine if there were any significant differences in means of
chlorophyll a, total phosphates, ortho-phosphates, nitrates,
suspended solids, turbidity, and secchi depth between Elm Fork
arm, Isle duBois arm and Main Body from May-October, 1993. Figure 1. Vollenweider P loading-mean depth/hydraulic residence time relationship. I Excessh re Loading 3 C 45 s R a a to o .c a. Permissible Loading
1 10 100 1000 Mean Depth/Hydraulic Residence Time 8
Table 1. Predicted annual phosphorus loading to Lake Ray Roberts and predicted mean summer epilimnetic chlorophyll a (Pillard, 1988). CO 9 X o (0 #
•K CO 5 4 4.7 8 207 0 i< 143.2 6 (A 00 O CO CD * * CO!
CO 33 5 X— i 4.7 8 3 19.1 0 CN \*0
0) 6 8 0) c (0 0 9 *
o * 2 9 4.7 8 92 0 65.6 7
CN X O 10 * # CO 10 2 1.9 3 CO 1257 0
361.0 9 CO < I* o * * 00 I 44 0 1.9 3 10.8 1 UL 12.7 4 E \U1 c Z98 6 o (0 * * CD d 3 8 1.9 3 o 348 0 N- 9 6 o ^r 00 S Z o CO 11 2 2 00 o 590 0 143.2 9 361.0 9 h- >% I'D o o CN 96' Z CD CO | CD CO 38 9 10.8 1 c 46.1 9 'c5 6 8 (0 9 6 h- 0 9 ©(0 CO Z CO 4 7 187 5 CD 55.1 9 222.7 1 0)| 3 >* 0 jglj -Q "D CO Figure 2. Total phosphorus load-chlorophyll a line of best fit for U.S. OECD waterbodies with 95 % confidence limits. 11 1000 T -=- 100 10 100 1000 Total Phosphorus Load (mg P/m3) CHAPTER II MATERIALS AND METHODS Study Area Lake Ray Roberts dam was completed by the U.S. Army Corps of Engineers in June, 1987. The reservoir was constructed to provide for flood control, water supply, and recreation. Lake Ray Roberts (Fig. 3) is located 10 miles north of Denton, at river mile 60.0 (96.6km) of the Elm Fork of the Trinity River, 30 miles (48.3km) upstream from Lake Lewisville. Ray Roberts dam is in Denton county, but the reservoir spans into Cooke and Grayson counties. Lake Ray Roberts impounds runoff from the Elm Fork of the Trinity River and Isle duBois creek, with the total watershed above the dam encompassing 692 square miles in the north central basin of the Trinity River. Lake Ray Roberts conservation pool is 29,350 ac (11,878 ha), and it's flood pool is 36,900 ac (19,566 ha) (IAS., 1995). In May 1993, the U.S. Army Corps of Engineers began to drawdown the level of Lake Ray Roberts in order to repair the dam. The water level decreased a total of 12 ft msl. The lowest depth was in September 1993 at 622 ft msl (Fig. 4). The drawdown continued until January 1994. Sample Sites Water samples were collected in triplicate from open water of Lake Ray Roberts on a monthly basis during the growing season from May-October, 1993. 12 13 A description of the eight sampling sites (Fig. 5) are as follows: SPC- Spring Creek EFO- uppermost reach of the Elm Fork arm. Not sampled from August to October, 1993, due to low depth. EF1- timbered part of the Elm Fork arm approximatelylOO meters north of Farm Market Road 3002 bridge. EF2- open water in the Elm Fork arm 1.0 km north of the boat ramp located where old highway 455 enters the reservoir. MB- 0.5 km north of the dam. ID0- uppermost reach of the Isle duBois arm. Not sampled from August to October, 1993, due to low depth. ID1- timbered part of the Isle du Bois arm directly east of Wolf island. ID2- open water of the Isle du Bois arm where old highway 455 enters the reservoir. Main Body was the only site that achieved thermal stratification beginning in June and ending in September 1993. Sites SPC and EFO were added to the sampling regime during the 1993 study. There were no data available for May 1989. Water Quality Analyses Secchi (20 cm diameter disc) depth measurements were recorded to the nearest centimeter. Water samples were collected at 0.5m, in 2 liter, acid washed Nalgene bottles and analyzed following Standard Methods for the Examination of Water and Wastewater for total and ortho phosphates, nitrates, chorophyll a, suspended solids, and turbidity (American Public Health 14 Association, 1985). Mean summer epilimnetic chlorophyll a was calculated for the Main Body by averaging chlorophyll a concentrations from May to October 1993. Main Body was the only site that was stratified from June through September. Statistical Analyses Means and standard deviations of each trophic parameter (phosphates, nitrates, chlorophyll a, suspended solids, turbidity, and secchi depth) were calculated at each sample site for each month during the growing season, May- October 1993. Signficant differences were determined using Tukey's MRT (a=0.05) or Kruskal-Wallis (a=0.05), dependent upon whether data sets were normal (Shapiro Wilkes, a=0.01) and if variances were homogenous (Hartley's, a=0.01). Correlation analyses were conducted for all trophic parameters each month to detect trophic relationships. Only significant correlations (p<0.05) with an R=0.90 or greater are discussed. 15 Figure 3. Study area of Lake Ray Roberts. 16 Gainesville Woodbini e Elm Fork Trinity \ River ollinsville Isle duBois rallev Vi mmicA Point 5 kilometers ^Krum -i ^ 17 Figure 4. Drawdown of Lake Ray Roberts (Courtesy Dr. Doyle). 18 CM CM c3 CO o> O) m 0> 0 o> _ > (|SIU y) |0A8~] 9>|B"| 19 Figure 5. Sample sites for Lake Ray Roberts, 1993 (IAS, 1995). 20 Elm Fork(Arm Isle duBols Arm y. 5 kilometers CHAPTER RESULTS The ranges of predicted mean summer epiiimnetic (MSE) chlorophyll a concentrations of Elm Fork arm, Isle duBois arm and Main Body of Lake Ray Roberts included both eutrophic and mesotrophic categories of the OECD eutrophication model (Fig. 6). Main Body had the highest predicted MSE chlorophyll a (47 ug/L) with the greatest range (14-112 ug/L). Elm Fork arm had a lower predicted MSE chlorophyll a (38 ug/L) within a lower range of concentrations (8-102 ug/L). Isle duBois arm had the lowest predicted MSE chlorophyll a (29 ug/L) and the narrowest range (13-54 ug/L). Main Body was the only site stratified throughout the 1993 growing season. Observed MSE chlorophyll a for Main body (17 ug/L) was in the eutrophic category within it's predicted range (Fig. 6). Observed MSE chlorophyll a for Elm Fork arm (12 ug/L Chla) two years after impoundment (1989) was at the low end of it's predicted range and Isle duBois arm (11 ug/L Chla) and Main Body (10 ug/L Chla) were below their predicted ranges (Cairns, 1993). May 1993 In May, 1993, the upper Elm Fork arm was turbid with significantly high chlorophyll a (Fig. 7). Mean chlorophyll a at SPC (87 ug/L Chla) and EFO (60 21 22 Figure 6. Comparison of predicted arid observed mean summer epilimnetic chlorophyll a. 23 i < < U) o *s mO E E oa £ = .i c E •i E • axt a> ."E=: | i « 5 5 Si I £ § S ui 2 <2. %o © s> % % *<£L ° * * + $ |i '4. ° * % \ h o a \< & 2 % \ %\ % "* \ © o o o CM 00 CO CM (l/6n) e ||Ai|dojO|i|0 3SW 24 Figure 7. Water quality parameters May 1993. 25 BKgJ.V; o Q H O o CD $111 CO (0 CM £ Q 3 a) H E SJi CO 5 O CO CO CO Z Q_ CD CD • V- >> o CO sz CO CM LL_ •c 13 LU O O L. CL +-*0 15 «•-» CO o LU I— 0 03 O JC LL O 111 O CL CO 26 ug/L Chla) were significantly different (Tukey's, a=0.05) from each other and all other sites (9-24 ug/L Chi a). Site EFO (0.15 mg/L P) had more than twice the mean total phosphorus of any other site (0.03-0.06 mg/L P), with the exception of the main body (0.11 mg/L P) (Fig. 7). Mean orthophosphates were only detected at sites in the Elm Fork arm and Spring Creek (0.01-0.03 mg/L P). May mean nitrates were the highest of the growing season (4.0-6.1 mg/L). Mean turbidity at SPC (17 NTUs) and EFO (45 NTUs) were significantly different (Tukey, a=0.05) from each other and more turbid than all the other sites (4-8 NTUs) during May 1993 (Fig. 7). Mean secchi depths were significantly different (Tukey's, a=0.05) among all the sites, either individually or in a group: ID1 (157 cm); ID2 (140 cm) and the main body (144 cm); ID0 (124 cm); EF1 (94 cm) and EF2 (95 cm); EFO (21 cm); and SPC (34 cm). In May 1993, significantly greater chlorophyll a at SPC and EFO indicated an accumulation of algal biomass coincident with detectable concentrations of mean orthophosphates in the Elm Fork arm (Fig. 7). There were no significant correlations among the above water quality parameters. June 1993 High concentrations of mean chlorophyll a, total phosphates, and nitrates were coincident with high turbidity at the upper reaches of the the Elm Fork arm in June, 1993 (Fig. 8). The range of mean chlorophyll a concentrations (6-32 ug/L Chla) had increased since June, 1989 (IAS, 1992) (7-10 ug/L Chi a). Mean total phosphates at SPC (0.08 mg/L P) and EFO (0.09 mg/L P) were significantly greater (Tukey's, a=0.05) than the rest of the sites (0.01-0.04 mg/L P) (Fig. 8). 27 Figure 8. Water quality parameters June 1993. 28 oQ o o 0 w CO 0) CM 0 Q u I- E co L_ CO CO CO CO o Q_ G) z O) • >> Q. • mmmm CD o c CM JC 15 13 •d •i ljl a LU O 0 Ql 75 Ll_ CO LU o JS O sz LL O LU O CL CO 29 Mean orthophosphates were below detectable concentrations with the exception of SPC (0.05 mg/L P). In June, 1989, all sites had mean orthophosphate concentrations (0.03-0.06 mg/L P), with the exception of site ID1. Mean nitrates ranged from 0.39-1.24 mg/L, with highest concentrations in the Elm Fork arm. Turbidities for Lake Ray Roberts in June, 1993 (2-12 NTUs) and 1989 (4- 16 NTUs) were similar, with the exception of site EF0 (30 NTUs) in 1993. Mean secchi depths in 1993 (103-155 cm) were greater than in 1989 (58-109 cm), with the exceptions of EF0 (32 cm) and SPC (45 cm). All sites within the Isle duBois arm (148-155 cm), and the main body (137 cm), were significantly different (Tukey's, a=0.05) from all other sites (Fig. 8). Sites EF1 (103 cm) and EF2 (106 cm) were significantly different (Tukey's, a=0.05) from EF0 (32 cm) and SPC (45 cm). In June 1993, mean orthophosphates were below detectable levels for all sites within Lake Ray Roberts, with the exception of Spring Creek, possibly due to it's proximity to the inlet of the Trinity river. High turbidity at EF0 may have suppressed algal productivity and the uptake of orthophosphates. Mean turbidity and mean secchi depth were significantly correlated with an f?=-0.92 (Spearman, p=0.0001) (Fig. 9). July 1993 In July, 1993, site EF0 was turbid, with high concentrations of mean chlorophyll a, total phosphates, nitrates, and suspended solids (Fig. 10). 30 Figure 9. Correlation of mean turbidity and mean secchi depth June 1993. 31 Secchi Depth (cm) o o o o 10 o o CM 10 o -1 Q. • -4 o sz+-* (>0 CD •&g n ~o 3 Id L_ Zf h- C CO A 0 • ^ • A • ^ O lO o to m CO CM CM (sfllN) Anpfqjnx 32 Figure 10. Water quality parameters July 1993. 33 o Q O O Q 13 H CO CM £ Q 0 o E CO to 2 3 CO 09 CO CO a. CD H >» CO o> O Z "c5 CM .>» Li- • HI O o sz •c 0 O «4—' Ll_ CO 111 -I—(0« O o U- I- LLl ro sz O QO_ CO 34 The mean chlorophyll a concentration at site EFO was five times greater (124 ug/L Chla) and significantly different (Tukey's, a=0.05) from all the other sites (13-24 ug/L Chla). Mean chlorophyll a concentrations were lower in July, 1989 (7-14 ug/L Chla). Site EFO (0.13 mg/L P) was significantly greater in mean total phosphates (Tukey's, a=0.05) than the rest of the sites (0.03-0.08 mg/L P) (Fig. 10). The range of mean total phosphates in July, 1989 (0.02-0.13 mg/L P) was similar to that in 1993, but without localization in any particular area of the lake. Mean oirthophosphates in July, 1993 (0.00-0.07 mg/L P) did not deviate from the range of concentrations in 1989 (0.00-0.06 mg/L P). July mean orthophosphates were the highest recorded for the growing season, 1993. Site EFO had more than double the mean nitrate concentration (0.90 mg/L) of the majority of the other sites (0.18-0.48 mg/L) (Fig. 10). The range of mean nitrate concentrations in July, 1989 was narrower (0.33-0.45 mg/L) than in 1993. In July 1993, mean suspended solids of the Elm Fork arm (14-39 mg/L) were greater than concentrations in the Main Body and the Isle duBois arm (0-2 mg/L), but the difference was not significant (Fig. 10). Mean suspended solids in July, 1989 (4-18 mg/L) were similar to 1993 concentrations with the exception of sites EFO (39 mg/L) and SPC (24 mg/L). Mean turbidity at sites EFO (20 NTUs) and SPC (14.3 NTUs) were at least two times greater than all other sites in July 1993 (2-7 NTUs) (Fig. 10) and the range of turbidities in July 1989 (3-5 NTUs). Site ID2 (150 cm) had a significantly greater mean secchi depth than the rest of the sites sampled (Tukey's, a=0.05). Mean secchi depths of the main body (142 cm), ID1 (144 cm) and ID0 (146 cm) were significantly different (Tukey's, 35 3=0.05) from all other sites. Mean secchi depths of EF1 (61 cm) and EF2 (93 cm), were each statistically distinct (Tukey's, a=0,05), EFO (34 cm) and SPG (31 cm) had the significantly lowest (Tukey's, a=0.05) mean secchi depths of Lake Ray Roberts, In July, 1989, mean secchi depths (132-160) for the entire lake were greater than those of the Elm Fork arm in 1993, There were significant associations among mean phosphates, mean suspended solids, mean turbidity and mean secchi depth in July 1993. Mean suspended solids correlated with mean turbidity (Spearman, p=Q.QQ01, R=0,93) and mean total phosphates (Spearman, p=0.0001, R=0,91) (Fig, 11 s 12). Mean turbidity correlated with mean secchi depth (Spearman, p=0,0001, R=-0,90) (Fig, 13), Chlorophyll a and turbidity were high at EFO in July 1993, Algal biomass, in addition to suspended solids, may have contributed to the turbidity at EFO which may have caused shading effects. High orthophosphates and a decrease in nitrates may indicate a switch to nitrogen limitation, August 1993 In August, 1993, SPC was the most turbid site with significantly greater suspended solids than all other sites (Fig, 14). EF1 had more than twice the mean chlorophyll a concentration (50 ug/L Chla) than most of the other sites (6- 27 ug/L Chla), Mean chlorophyll a was lower in August, 1989 (8-16 ug/L Chla) than in 1993. Mean total phosphates (0.002-0.06 mg/L P) and mean orthophosphates (0.00-0,02 mg/L P) in August 1993, had increased to detectable levels at most sites in Lake Ray Roberts from undetectable 36 Figure 11. Correlation of mean turbidity and mean suspended solids July 1993. 37 Suspended Solids (mg/L) o o o o 00 CO •sr CM {/) "U • MM O CO <*• 13 0 <• Uc M • 0 4 • GL • CO < • CO o> ^ • 0 >» £ A • tj 3 ^ • la (/) -3 3 > 4 • < "g • _q CO 0 o COO CM (sniN) AlPiqjni 38 Figure 12. Correlation of mean total phosphates and mean suspended solids July 1993. 39 Suspended Solids (mg/L) CO ;u o o o o o CO U CD "O 0 QL CO 3 CO •(/g) CD CD CO o O) CO o T— >. 5 (0 3 o CD -D •+—» < • CD ^ • < • Q. ^ • CO O CL • < ro • < 4-1 • < o • <4 h- c CO in o m o m o m o CD CO CO CM CM T— o o o o (-|/6iu) d |e*oi 40 Figure 13. Correlation of mean turbidity and mean secchi depth July 1993. 41 Secchi Depth (cm) o o LO o o to • < QL • A CD Q o >» :> D T3 JQ v_ 3 m mmmm <« nL_ 3 <« h- C (0 M 0 A • < • < JL lO O lO iO CM CM (spun) ^iPiqjni 42 Figure 14. Water quality parameters August 1993. 43 O Q O X2 s_ 3 CM CO Q (O B jo 0 o E 05 c/> 2 00 CD 3 c5 CD 05 Q. CB1 CO CO O •& mmmm •I Z 15 D) • 13 3 CM <: LL o LU o jc "C 0 O 05 •'•J Q. •'•J LL •'•v uu 15 -I—' o I- • 05 JC O Ql O 05 44 concentrations in 1989. Mean nitrates in July 1993 had escalated (1.27-1.69 mg/L) since 1989 (0.18-0.25 mg/L). SPC was significantly greater (Tukey's, a=0.05) in mean suspended solids (13 mg/IL) than all other sites (2-5 mg/L) in August 1993 (Fig. 14). Consequently, SPC was also more turbid (11 NTUs) compared to the other sites (2-5 NTUs). Mean suspended solids and turbidity had not changed since 1989, with the exception of SPC. ID1 (160 cm) and ID2 (161 cm) were significantly (Tukey's, a=0.05) deeper than all other sites. Mean secchi depths of the main body (126 cm), EF2 (100 cm) and EF1 (86 cm) were each significantly different (Tukey's, a=0.05). Secchi depth at site SPC was not recorded for August, 1993. Mean secchi depths for August, 1989 (100-170 cm) were similar to those recorded in 1993. Mean secchi depth correlated with (Spearman, p=0.0001, R=-0.92) mean turbidity (Fig. 15). In August 1993, mean orthophosphates began to decline with an increase in nitrates; an indication of nitrogen fixing bluegreen algae. Turbidity at SPC caused by high suspended solids content limited algal growth. High mean chlorophyll a at EF1 may have been due to a combination of incoming nutrients and higher water clarity. September 1993 During September, 1993 (Fig. 16), mean chlorophyll a, mean suspended solids, and mean turbidity were greatest in the Elm Fork arm, but mean total phosphates were highest in the Isle duBois arm. Mean chlorophyll a at SPC (42 ug/L Chla), EF1 (30 ug/L Chla) and EF2 (27 ug/L Chla) were significantly greater 45 Figure 15. Correlation of mean turbidity and mean secchi depth August 1993. 46 Secchi Depth (cm) o o o o m o o CM lO r CL Q0 O O c CO 0 O lO lO CM (SfllN) AHPiqjnjL 47 Figure 16. Water quality parameters September 1993. 48 o o <1> CO 3 CM H CS) Q S— 0 (/> 5 0 o E CO C/) CO CD CO CD CO 3 CO CO Q_ 0 CO X) E Oz CO 0) CM • Q. Li_ Q_ o 0 UJ CO O 0 jC •t: O 05 LL LU CO •4-o< ro O CL O (/) 49 (Tukey's, a=0.05) than all other sites (8-13 ug/L Chla). Mean chlorophyll a concentrations ranged from 6-20 ug/L Chla in 1989. Mean total phosphorus concentrations at sites ID1 (0.53 mg/L P), ID2 (0.63 mg/L P), and MB (0.54 mg/L P) were ten times the concentration and significantly greater (Tukey's, a=0.05) than all other sites (Fig. 16). Mean total phosphates in the Elm Fork arm (0.03- 0.05 mg/L P) were in agreement with 1989 concentrations (0.01-0.06 mg/L P). No orthophosphates were detected during September, 1989 nor 1993. Mean nitrates had increased (0.66-1.69 mg/L) since two years after impoundment (1989) (0.35-0.55 mg/L). Sites SPC (18 mg/L) and EF1 (10 mg/L) were significantly greatest (Tukey's, a=0.05) in mean suspended solids than all other sites (Fig. 16). The range of mean suspended solids of the other sites (0-4 mg/L) during September 1993 was lower than in 1989 (2-9 mg/L). Mean turbidities at SPC (13 NTUs), EF1(8 NTUs), and EF2 (6 NTUs) were significantly different from each other and all other sites (Tukey's, a=0.05). Mean turbidity in the main body (4 NTUs) was significantly different (Tukey's, a=0.05) from ID1 (2 NTUs), but not ID2 (3 NTUs). Mean turbidities in 1989 ranged from 1 -2 NTUs. There were significantly different (Tukey, a=0.05) mean secchi depths among ID1 (174 cm), MB (149 cm) and ID2 (142 cm), EF2 (80 cm), EF1 (66 cm), and SPC (32 cm). Mean secchi depths in 1989 (150-185 cm) were similar to those of the Isle duBois arm in September 1993. In September 1993, there were significant correlations among mean chlorophyll a, mean suspended solids, mean turbidity and mean secchi depths. 50 Figure 17. Correlation of mean chlorophyll a and mean suspended solids September 1993. 51 Suspended Solids (mg/L) io O m CM CM in CO T3 O < CO A "D < 0) "O c a) CL (0 3 ; oa. o C CD CD o O O o o o CD UO ^r CO CM (-|/6n) b ||AL|d0J0|L|0 52 Figure 18. Correlation of mean turbidity and mean suspended solids September 1993. 53 Suspended Solids (mg/L) o CO h~ O CO TT CO CM CM CO 13 O < • CO "D < • CD ^ • •D C < • Q0 _ < CO CO to 3 O) •g G) o CO CO CO CD < CD X2 £• E «4 T> la +0j w. Q. 3 5 CD CO "D !d L. 3 I- CO CD < • i CM CO CD CM (sfllN) ^!P!QJni 54 Mean chlorophyll a correlated with mean suspended solids (Spearman, p=0.0001, R=0.94) (Fig. 17). Mean suspended solids correlated with mean turbidity (Spearman, p=0.0001, R=0.92) (Fig. 18) and mean secchi depth (Spearman, p=0.0001, R=-0.92) (Fig. 19). Mean turbidity significantly correlated with mean secchi depth, R=-0.90 (Spearman, p=0.0001) (Fig. 20). In September 1993, Lake Ray Roberts returned to phosphorus limitation. Elm Fork arm was more turbid than the main body and the Isle duBois arm, and had higher chlorophyll a concentrations and suspended solids. Significantly greater mean total phosphates were found in the Isle duBois arm possibly due to runoff from a grass farm on Buck Creek. Lack of a coincident increase of orthophosphates may be explained by the predominance of particulate and unavailable forms of phosphorus in the Isle du Bois arm. October 1993 In October, 1993, the Elm Fork arm of Lake Ray Roberts was turbid with high chlorophyll a (Fig. 21). Mean chlorophyll a at SPC (82 ug/L Chla) and EF1 (41 ug/L Chla) were each significantly different (Tukey's, a=0.05) than all other sites (15-28 ug/L Chla). Mean chlorophyll a within Lake Ray Roberts during October 1993 had increased since 1989 (8-12 ug/L Chla). The resurgence of mean total phosphates (0.14-0.40 mg/L) at all sites in October 1993 was ten times greater than concentrations recorded in 1989 (0.02-0.04 mg/L P). Mean orthophosphate concentrations had increased to detectable amounts in October 1993 (0.01-0.02 mg/L P) since 1989. Mean nitrate at MB (0.94 mg/L) was significantly different (Tukey's, a=0.05) from ID2 (0.79 mg/L), 55 Figure 19, Correlation of mean suspended solids and mean secchi depth September 1993. 56 Suspended Solids (mg/L) r-. o CO (o uo CO CL CD Q o CD CO C CO o> CO > Q. CD (uuo) ifldaa IMOO0S 57 Figure 20. Correlation of mean turbidity and mean secchi depth September 1993. 58 Secchi Depth (cm) o o o o to o o CM m o -1 CL c CO a> «« o CNI lO lO (SfllN) Aiipiqjni 59 Figure 21. Water quality parameters October 1993. 60 O O tSSBS8?*r 0 CO fl: •.••v.-.-.-.TO jQ t_ 3 CM I— CO Q 0 tn "jO "S 0 E CO 01 CO =3 vC_O CD CD CO 05 CD CL >> CO 0 _Q O • mmmm . z O 15 -4—» CM • o LL. Q_ LLi O O O 1 € EF2 (0.74 mg/L), and ID1 (0.72 mg/L), which were significantly different from EF1 (0.57 mg/L) and SPC (0.53 mg/L) (Appendix). Mean nitrate concentrations for Lake Ray Roberts October, 1989 (1.25-1.43 mg/L) were higher than observed in 1993. The range of mean suspended solids during October, 1989 (6-35 mg/L) was greater than in 1993 (2-9 mg/L). Mean turbidity at SPC (14 NTUs) and EF1 (9 NTUs) were each significantly different from EF2 (6 NTUs), which was significantly different from ID1 (3 NTUs) and ID2 (4 NTUs), but not from MB (4 NTUs) (Fig. 21). EF1 was the only site that increased in turbidity since October, 1989 (4-6 NTUs). Mean secchi depth at ID1 (147 cm) was significantly deeper than ali other sites. ID2 (130cm) and MB (130 cm) had significantly different (Tukey's, a=0.05) secchi depths than all other sites. All the Elm Fork arm sites (55-81 cm) and SPC (36 cm) were each significantly different from each other and all other sites (Tukey's, a=0.05). October secchi depths in the Elm Fork had decreased since 1989 (120-175 cm). Mean chlorophyll a correlated with mean secchi depth, R=-0.93 (Spearman, p=0.0001) (Fig. 22). There was also a significant correlation between mean secchi depth and mean turbidity (Spearman, p=0.0001, R=-0.92) (Fig. 23). Fall turnover in October 1993, dispersed nutrients throughout Lake Ray Roberts, which resulted in increased mean total phosphates and chlorophyll a at all the sites. 62 Figure 22. Correlation of mean chlorophyll a and mean secchi depth October 1993. 63 Secchi Depth (cm) CO lO LO CNI CO o> CO CO Q. ^ • 0 Q !oc o • ^ 0 • < CO (OLD c "O CD CO 0 o> CB 8 (D CO 5 JD >» CO O ts szQ . O O .c o. x: o o < O c < CO 0 • < o m o in o o h- io CM (l/6n) e iiAqdojomo 64 Figure 20. Correlation of mean turbidity and mean secchi depth October 1993. 65 Secchi Depth (cm) o o O •sr CM O O o o O 00 CO M- CM < • Q. 0 O o • M o 0) Q. CO CO 0 c O) "O CO CD Z c CD 0 < •4 o CM lO lO (sfllN) ^ipiqjni CHAPTER IV CONCLUSIONS Pillard had predicted that Lake Ray Roberts would be eutrophic from the relationship of total phosphorus loading to mean summer epilimnetic (MSE) chlorophyll a exhibited by the OECD eutrophication model. An objective of this study was to determine if observed mean summer epilimnetic (MSE) chlorophyll a for Lake Ray Roberts six years after impoundment (1993) supported a eutrophic state as predicted by Pillard (1988). In addition, observed MSE chlorophyll a for Main Body two (1989) and six years after impoundment (1993) were compared to determine whether or not there was a change. Only the Main Body stratified from June through September 1993. Other sites remained mixed, possibly as a consequence of the drawdown of Lake Ray Roberts. Observed MSE chlorophyll a for Main Body six years after impoundment (1993) (17ug/L) was higher than observed two years after impoundment (1989) (10 ug/L), but did not reach it's predicted concentration of 47 ug/L. Observed MSE chlorophyll a concentrations for Elm Fork arm (12 ug/L), Isle duBois arm (11 ug/L) and Main Body (10 ug/L) two years after impoundment (1989) occurred at the low end of their ranges at the mesotrophic- eutrophic boundary of the OECD model. MSE chlorophyll a for Main Body of Lake Ray Roberts had increased in 1993. A switch to nitrogen limitation in July and October 1993 may have obscured any phosphorus/chlorophyll a 66 67 relationships. High nonalgal turbidity may have also depressed the assimilation of phophorus to algal biomass, particularly in the Elm Fork arm. Lind, et al. (1992) suggested that algal productivity in an inorganically turbid, tropical lake was limited more by light than nutrients. Resuspension of sediments at shallow depths were suspect in perpetuating high turbidity. Sites in the shallower upper Elm Fork arm recieved high concentrations of phosphorus and nitrogen, but accumulations of algal biomass were limited by turbidity. Another objective of this study was to examine significant differences in means of chlorophyll a, total phosphates, ortho-phosphates, nitrates, suspended solids, turbidity, and secchi depth. Above trophic parameters were investigated to determine if the Elm Fork arm and Isle duBois arm were significantly different due to differences in the nutrient loads of the Elm Fork of the Trinity River and Isle duBois Creek, respectively. Pillard (1988) had predicted a difference in the two arms of Lake Ray Roberts from the results of the nutrient budgets of the aforementioned tributaries. Elm Fork arm of Lake Ray Roberts receives sewage effluent from wastewater treatment plants in Gainesville and Valley View, TX, transported by the Elm Fork of the Trinity River and Spring Creek, respectively. Nutrients, principally phosphorus, present in the effluent continously fueled algal productivity in Elm Fork arm. Elm Fork arm often had higher, often significant, concentrations of mean chlorophyll a, total phosphorus, suspended solids and turbidity than Main Body and Isle duBois arm. Differences between Main Body and Isle duBois arm were small compared to their differences with Elm Fork arm. 68 Vollenweider arid OECD models were not meant for exact predictions, but to monitor change in algal productivity. Comparison of observed MSE chlorophyll a of Main Body of studies two and six years following impoundment indicated an increase in algal productivity, but not to the level of it's predicted concentration. Effluent from wastewater treatment plants in Gainesville and Valley View, TX, influenced productivity in the Elm Fork arm with high concentrations of nutrients, particularly phosphorus and nitrogen. As phosphorus became abundant from loading, resuspension from the sediments, and light limitation (low assimilation), algal productivity became nitrogen limited. High turbidity, algal and nonalgal, may have also limited algal productivity. Higher chlorophyll a concentrations were often localized in the upper Elm Fork arm, indicating some shading effects. High turbidity and solids content were typical in the upper Elm Fork arm. Due to the high loading from the Elm Fork of the Trinity River, in the abscence turbid conditions, observed MSE chlorophyll a at a stratified Elm Fork arm may have achieved it's predicted concentration. High suspended solids in the Elm Fork arm may have trapped greater concentrations of nutrients than anticipated, diminishing the nutrient load to the rest of Lake Ray Roberts. Pillard (1988) had predicted that Lake Ray Roberts would be eutrophic. In their chemical and trophic classification of Texas reservoirs, Ground and Groeger (1994) separated the state's reservoirs into geographical regions of East, West and Central Texas. Lake Ray Roberts was not included in the study but is located in the Central region. Central and West groups of lakes had the highest percentage of oligotrophic lakes. Calcium carbonate (CaCOa) 69 precipitate in these regions bind up and coprecipitate phosphorus, depressing algal productivity. Lakes in the central group were also found to have comparably low nitrate concentrations, which resulted in nitrogen limitation. Elm Fork arm has the greatest potential of becoming problematic and displayed symptoms of cultural eutrophication, with high chlorophylls and turbidity. A more stringent treatment regime should be applied to wastewater effluent that removes greater concentrations of phosphorus and nitrogen. Costs of implementation would be outweighed by extended life and improved water quality for drinking and recreation, thereby increasing the value of Lake Ray Roberts. APPENDIX WATER QUALITY PARAMETER MEANS AND STANDARD DEVIATIONS MAY-OCTOBER 1993. 70 Water quality parameters (means and standard deviations) 1993. CHLA TOTAL P ORTHO P Nitrate SS TURB SECC ug/L mg/L mg/L mg/L mg/L NTU cm May SPC 87 3 0.06 0.02 0.02 0.01 5.16 0.14 X X 17 1 34 2 EFO 60 13 0.15 0.01 0.03 0.00 4.78 0.32 X X 45 2 21 1 EF1 20 6 0.03 0.01 0.01 0.00 4.69 0.04 X X 7 1 94 5 EF2 19 5 0.03 0.01 0.01 0.00 4.65 0.05 X X 7 1 95 2 MB 24 1 0.10 0.12 0.00 0.01 5.02 0.14 X X 4 1 144 2 ID2 9 5 0.04 0.02 0.00 0.01 4.78 0.20 X X 5 1 140 3 ID1 9 2 0.02 0.00 0.00 0.00 6.06 0.92 X X 4 1 157 3 IDO 10 3 0.02 0.01 0.00 0.01 4.06 0.28 X X 8 1 124 3 June SPC 20 1 0.08 0.00 0.05 0.01 1.11 0.06 X X 12 6 45 1 EFO 32 3 0.08 0.01 0.00 0.00 1.24 0.04 X X 30 2 32 3 EF1 11 6 0.04 0.01 0.00 0.00 1.09 0.03 X X 7 1 103 3 EF2 9 1 0.03 0.01 0.00 0.00 1.13 0.13 X X 5 0 106 4 MB 23 13 0.03 0.01 0.00 0.01 1.06 0.03 X X 3 1 137 4 ID2 12 1 0.01 0.01 0.00 0.00 0.39 0.50 X X 2 1 154 4 ID1 27 18 0.02 0.03 0.00 0.00 0.80 0.03 X X 2 0 148 4 IDO 7 9 0.01 0.01 0.00 0.00 0.90 0.20 X X 2 1 155 13 July SPC 24 10 0.08 0.02 0.07 0.01 0.32 0.11 24 2 14 3 31 EFO 124 22 0.13 0.01 0.04 0.02 0.90 0.21 39 13 20 2 34 EF1 19 9 0.07 0.01 0.05 0.00 0.46 0.05 18 3 7 1 61 EF2 20 2 0.07 0.01 0.04 0.01 0.48 0.04 14 2 5 1 93 MB 13 3 0.04 0.02 0.02 0.00 0.42 0.03 2 1 3 1 142 ID2 22 1 0.05 0.02 0.03 0.02 0.19 0.01 2 1 2 1 150 ID1 15 2 0.04 0.00 0.02 0.01 0.23 0.05 0 0 2 1 144 IDO 14 3 0.03 0.02 0.00 0.01 0.18 0.03 0 1 3 1 146 Significant differences in bold type. X denotes no data. Water quality parameters (means and standard deviations) 1993. CHLA TOTAL P ORTHO P Nitrate SS TURB SECC ug/L mg/L mg/L mg/L mg/L NTU cm Aug. SPC 21 3 0.06 0.01 0.02 0.01 1.27 0.09 13 1 11 1 X X EF1 50 24 0.05 0.02 0.01 0.01 1.47 0.03 4 2 5 0 86 3 EF2 27 2 0.03 0.01 0.01 0.00 1.48 0.13 5 2 5 0 100 5 MB 20 4 0.04 0.01 0.02 0.01 1.61 0.05 4 1 4 0 126 3 ID2 17 8 0.02 0.01 0.01 0.00 1.61 0.10 2 1 2 1 161 3 ID1 7 2 0.02 0.01 0.00 0.01 1.69 0.03 3 1 2 0 160 2 Sept. SPC 42 4 0.04 0.02 0.00 0.01 0.73 0.04 18 2 14 1 32 1 EF1 30 1 0.03 0.01 0.00 0.00 0.71 0.00 10 2 8 0 66 1 EF2 27 5 0.04 0.01 0.00 0.00 0.82 0.07 5 2 6 0 80 3 MB 13 5 0.54 0.07 0.00 0.00 0.75 0.03 2 1 4 1 149 8 ID2 9 4 0.63 0.11 0.00 0.00 0.66 0.03 2 1 3 0 142 2 ID1 8 2 0.54 0.01 0.00 0.00 1.69 0.08 0 1 2 1 174 4 Oct. SPC 82 13 0.27 0.07 0.02 0.01 0.53 0.03 9 1 14 1 36 1 EF1 41 8 0.27 0.03 0.01 0.01 0.57 0.08 5 3 9 0 55 1 EF2 28 3 0.40 0.19 0.02 0.00 0.74 0.02 3 1 6 1 81 3 MB 15 11 0.27 0.12 0.02 0.00 0.95 0.02 6 2 4 1 130 5 ID2 18 3 0.15 0.02 0.02 0.01 0.79 0.04 2 2 4 1 130 2 ID1 15 5 0.18 0.02 0.00 0.01 1.69 0.02 2 1 2 1 147 2 Significant differences in bold type. X denotes no data. BIBLIOGRAPHY American Public Health Association (APHA). 1985. Standard Methods for the Examination of Water and Wastewater. 16th ed. APHA, Washington, D.C. Archibald, Elaine M. and G. Fred Lee. 1981. Application of the OECD eutrophication modeling approach to Lake Ray Hubbard, Texas. J. A. W. W. A. 73:590-599. Auer, Martin T., Mark S. Kieser, and Raymond P. Canaie. 1986. Identification of critical nutrient levels through field verification of models for phosphorus and phytoplankton growth. Can. J. Fish. Aquat. Sci. 43: 379-387. Bartsch, A. F. 1970. 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