Assessment Comments
Assessment based on results of (1) surveys by IDNR Fisheries Bureau and (2) IDNR beach monitoring program from 2002 through 2004, (3) ISU statewide lake survey from 2000-04, (4) ISU planktons study from 2000-05, and (5) EPA/DNR fish tissue (RAFT) monitoring in 1995.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses remain assessed (monitored) as "partially supporting" due to algal turbidity. The Class B(LW) aquatic life uses are assessed (evaluated) as "fully supporting". Nutrient loading to the water column, siltation, and algal blooms, however, remain concerns for support of the designated aquatic life uses at this lake. The Class C (drinking water) uses remain “not assessed” due to the lack of water quality information upon which to base an assessment. Fish consumption uses remain assessed (evaluated) as “fully supported” based on fish contaminant monitoring in 1995. The sources of data for this assessment include (1) results of the statewide survey of Iowa lakes sponsored by IDNR and conducted by Iowa State University (ISU) from 2000 through 2004, (2) IDNR/UHL beach monitoring from 2000 through 2004, (3) surveys by IDNR Fisheries Bureau, (4) information on plankton communities collected at Iowa lakes from 2000 through 2005 as part of the ISU lake survey, and (5) results of U.S. EPA / IDNR fish tissue monitoring in 1995.
EXPLANATION: Results of IDNR beach monitoring at Green Valley Lake from 2002 through 2004 suggest that the Class A uses are "fully supported." Levels of indicator bacteria were monitored once per week during the primary contact recreation seasons (May through September) of 2002 (30 samples), 2003 (28 samples), and 2004 (16 samples) as part of the IDNR beach monitoring program. According to IDNR’s assessment methodology, two conditions need to be met for results of beach monitoring to indicate “full support” of the Class A (primary contact recreation) uses: (1) all five-sample, thirty-day geometric means for the three-year assessment period are less than the state’s geometric mean criterion of 126 E. coli orgs/100 ml and (2) not more than 10 % of the samples during any one recreation season exceeds the state’s single-sample maximum value of 235 E. coli orgs/100 ml. This assessment approach is based on U.S. EPA guidelines (see pgs 3-33 to 3-35of U.S. EPA 1997b). At Green Valley Lake beach, the geometric means of all 62 thirty-day periods during the summer recreation seasons of 2002, 2003 and 2004 were below the Iowa water quality standard of 126 orgs/100 ml. Also, the percentage of samples exceeding Iowa’s single-sample maximum criterion did not exceed 10% in any of the three recreation seasons (2002: 0%; 2003: 0%; 2004: 6%). According to U.S. EPA guidelines and IDNR’s assessment methodology, these results suggest “full support” of the Class A (primary contact recreation) uses.
Results from the ISU statewide survey of Iowa lakes, however, suggest only “partial support” of the Class A uses of Green Valley Lake. Using the median values from this survey from 2000 through 2004 (approximately 15 samples), Carlson’s (1977) trophic state indices for total phosphorus, chlorophyll-a, and secchi depth are 69, 60, and 59, respectively. According to Carlson (1977), the index value for total phosphorus is in the upper range between eutrophic and hyper-eutrophic lakes, the index value for chlorophyll-a is at upper boundary of eutrophic lakes, and the index value for Secchi depth is in the upper range of eutrophic lakes. These index values—which show improvement over the corresponding TSI values from the previous (2004) assessment of 75, 67, and 63, respectively—suggest relatively high levels of phosphorus the water column, relatively low levels of chlorophyll-a (suspended algae), and relatively good water transparency for Iowa lakes. A comparison of annual TSI values for the 2000-2004 period at Green Valley Lake suggests improvement in water quality. Annual summer TSI values for Secchi depth have generally decreased (improved) as follows over the five-year period: 2000: 72; 2001: 58; 2002 = 59; 2003 = 59; and 2004 = 61; the overall Secchi TSI for the current (2006) assessment (59) reflects an improvement over that of the previous (2004) assessment period (63). Annual TSI values for both Secchi depth and chlorophyll-a at Green Valley Lake were well below the impairment trigger of 65 during both 2003 and 2004. An examination of more recent data, however, suggests continued moderate impairments to the water quality of this lake. Although TSI values for Secchi depth remained below the impairment trigger level in 2005 and 2006, TSI values for chlorophyll-a exceeded this trigger level in both years: 69 in 2005 and 67 in 2006. Based on this more recent information, this lake will remain assessed (monitored) as partially supported due to high levels of algae.
According to Carlson (1991), the occurrence of a high TSI value for total phosphorus with relatively low values for chlorophyll-a and secchi depth indicate that some factor (e.g., nitrogen limitation, zooplankton grazing, or some other factor) other than phosphorus limits production of algae. Results of ISU monitoring from 2000-04 suggest that either large populations of zooplankton grazers or somewhat elevated levels of inorganic suspended solids may limit algal production at this lake.
Based on median values from ISU sampling in 2000-04, the ratio of total nitrogen to total phosphorus for Green Valley Lake is 23, thus suggesting that algal production at this lake is likely limited by the availability of phosphorus..
The presence of very large populations of zooplankton at Green Valley Lake that graze on algae, however, may explain the large discrepancy between the TSI value for phosphorus (69) and those for chlorophyll-a (60) and Secchi depth (59). In terms of all Iowa lakes sampled, data from the ISU survey show large populations of zooplankton species at this lake that graze on algae. Sampling from 2000 through 2005 showed that Cladoceran taxa (e.g., Daphnia) comprised approximately just over half of the dry mass of the zooplankton community of this lake. The average per summer sample mass of Cladoceran taxa over the 2000-2005 period (212 mg/l) was the 20th highest of the 131 lakes sampled. At these high levels, the Cladoceran grazers could be expected to limit algal production.
Relative to other Iowa lakes, the median level of inorganic suspended solids at this lake is in the range from “typical” to “low” and thus inorganic turbidity likely has little impact on limiting algal production at this lake. Of 131 lakes sampled as part of the ISU survey from 2000-04, Green Valley Lake had the 61st lowest median level of inorganic suspended solids (4.5 mg/l).
Thus, although the results of ISU lake monitoring from 2000-2004 suggest some improvements in water quality, more recent (2005 and 2006) data suggest continued water quality concerns at Green Valley Lake. Based on this information, the Class A (primary contact) uses of this lake remain assessed (monitored) as partially supported (impaired) due primarily to algal turbidity.
The levels of nuisance (=noxious) algal species (i.e., bluegreen algae) at this lake do not suggest an impairment of Class A uses. While data from the ISU survey from 2000 through 2004 suggest that bluegreen algae (Cyanophyta) comprise a significant portion (over 60%) of this lake’s summertime phytoplankton community, sampling from 2000 through 2004 showed that the median per summer sample mass of bluegreen algae at Green Valley Lake (10 mg/l) was the 57th lowest of the 131 lakes sampled. This level is in the lowest half of the 131 Iowa lakes sampled. The presence of a relatively small population of bluegreen algae at this lake does not suggest a potential violation of Iowa’s narrative water quality standard protecting against occurrence of nuisance aquatic life. This assessment, however, is based strictly on a distribution of the lake-specific median bluegreen algae values for the 2000-2004 monitoring period. Median levels less than the 75th percentile of this distribution (~29 mg/l) were arbitrarily considered by IDNR staff to not represent an impairment of the Class A uses of Iowa lakes. No criteria exist, however, upon which to base a more accurate identification of impairments due to bluegreen algae. Thus, while the ability to characterize the levels of bluegreen algae at this lake has improved over that of the previous (2004) assessment due to collection of additional data, the assessment category for assessments based on level of bluegreen algae nonetheless remains, of necessity, "evaluated" (indicating an assessment with relatively lower confidence) as opposed to "monitored" (indicating an assessment with relatively higher confidence).
The water quality conditions at this lake, along with information from the IDNR Fisheries Bureau, suggest that the Class B(LW) aquatic life uses should be assessed (evaluated) as "fully supported " Nutrient loading to the water column and algal blooms, however, remain concerns at this lake. The ISU lake survey data show no violations of the Class B(LW) criteria for dissolved oxygen in the 14 samples collected during summers of 2000 through 2004. Two of 15 samples exceeded the Class A,B(LW) criterion for pH (maximum = 9.4; minimum = 7.5 pH units). Based on IDNR’s assessment methodology, however, these results do not suggest that significantly more than 10 percent of the samples exceed Iowa’s pH criteria and thus do not suggest an impairment of the Class A and Class B(LW) uses of this lake. These violations likely reflect the high levels of primary productivity at Green Valley Lake and do not reflect the input of pollutants into this lake.
The Class C (drinking water) uses remain "not assessed" due to a lack of water quality information upon which to base an assessment. The only parameter collected as part of the ISU lake survey relevant to support of Class C (drinking water) uses is nitrate. While the results of the ISU survey from 2000-04 show that nitrate levels are low at this lake (maximum value = 4.7 mg/l; median = 0.8 mg/l), these data are not sufficient for developing a valid assessment of support of the Class C uses.
Fish consumption uses were assessed (evaluated) as “fully supported” based on results of U.S.EPA/IDNR fish contaminant (RAFT) monitoring at Green Valley Lake in 1995. Because these data are now considered too old (greater than five years) to accurately characterize current water quality conditions, the assessment category is considered “evaluated” (indicating an assessment with relatively lower confidence) as opposed to "monitored" (indicating an assessment with relatively higher confidence). The existence of, or potential for, a fish consumption advisory is the basis for Section 305(b) assessments of support of fish consumption uses in Iowa’s rivers and lakes. Prior to 2006, IDNR used action levels published by the U.S Food and Drug Administration to determine whether consumption advisories should be issued for fish caught as part of recreational fishing in Iowa. In an effort to make Iowa’s consumption more compatible with the various protocols used by adjacent states, the Iowa Department of Public Health, in cooperation with Iowa DNR, developed a risk-based advisory protocol. This protocol went into effect in January 2006 (see http://www.iowadnr.gov/fish/news/consump.html for more information on Iowa’s revised fish consumption advisory protocol). Because the revised (2006) protocol is more restrictive than the previous protocol based on FDA action levels; fish contaminant data that previously suggested “full support” may now suggest either a threat to, or impairment of, fish consumption uses. This scenario, however, does not apply to the fish contaminant data generated from the 1995 RAFT sampling conducted at Green Valley Lake: levels of all contaminants from this monitoring were below advisory trigger levels, thus suggesting the continued “full support” of fish consumption uses.
