Assessment Comments
Assessment based on (1) reports by IDNR Fisheries Bureau, (2) results of the IDNR beach monitoring program from 2002-04, (3) ISU lake survey from 2000-04, and (4) ISU reports on lake plankton communities from 2000-05.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (monitored) as "fully supporting ". The Class B(LW) aquatic life uses are assessed (evaluated) as fully supporting". Moderate impacts from nutrients in the water column and minor siltation impacts, however, remain concerns at this lake for support of the Class B(LW) uses. The Class C (drinking water) uses remain "not assessed" due to lack of monitoring information upon which to base an assessment. Fish consumption uses remain "not assessed" due to a lack of fish contaminant monitoring at this lake. The sources of data for this assessment include (1) the results of the IDNR-UHL beach monitoring program in summers of 2002, 2003, and 2004, (2) results of the statewide survey of Iowa lakes conducted from 2000 through 2004 by Iowa State University (ISU), (3) information from the IDNR Fisheries Bureau, and (4) information on plankton communities collected from 2000 through 2005 for the ISU lakes survey.
EXPLANATION: Results of IDNR beach monitoring at Lake Wapello 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 (31 samples), 2003 (29 samples), and 2004 (23 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 Lake Wapello beach, the geometric means of all 71 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 (the maximum 30-day geometric mean was 34 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: 7%; 2004: 4%). According to U.S. EPA guidelines and IDNR’s assessment methodology, these results suggest “full support” of the Class A (primary contact recreation) uses. These results also indicate extremely low levels of bacteria at this lake.
The results of IDNR beach monitoring that suggest "full support" of the Class A uses are generally consistent with results from the ISU statewide survey of Iowa lakes that show exceptional water transparency and low levels of nuisance algal species. 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 59, 52, and 50, respectively. According to Carlson (1977), the index value for total phosphorus is in the upper range of eutrophic lakes, while the index values for chlorophyll-a and Secchi depth are in the lower range of eutrophic lakes. These index values suggest relatively low levels of phosphorus in the water column, very low (and less than expected) levels of chlorophyll-a, and very good water transparency.
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 suggest that nitrogen availability may limit algal production while zooplankton grazing and inorganic turbidity do not.
The ISU lake data does suggest that availability of nitrogen may limit algal production, at times, at Lake Wapello. Based on median values from ISU sampling from 2000 through 2002, the ratio of total nitrogen to total phosphorus for Lake Wapello is 19. This TN:TP ratio suggests little possibility that nitrogen limitation suppresses the production of suspended algae at this lake.
In terms of all Iowa lakes sampled, data from the ISU survey show relatively small populations of zooplankton species at this lake that graze on algae. Although sampling from 2000 through 2005 showed that Cladoceran taxa (e.g., Daphnia) comprised nearly 75% 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 (58 mg/l) was the 39th lowest of the 131 lakes sampled. These results suggest little if any non-phosphorus limitation due to zooplankton grazing at Lake Wapello.
The levels of inorganic suspended solids at this lake are very low and also do not suggest the potential for impairing designated uses or limiting algal production. The median level of inorganic suspended solids in the 131 lakes sampled for the ISU lake survey from 2000 through 2004 was 5.2 mg/l; the median level at Lake Wapello was 2.2 mg/l.
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 (approximately 65%) of this lake’s summertime phytoplankton community, sampling from 2000 through 2004 showed that the median per summer sample mass of bluegreen algae at Lake Wapello (7 mg/l) was the 36th lowest of the 131 lakes sampled. This level is in the lowest 25% 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).
Information from the IDNR Fisheries Bureau suggests that the Class B(LW) aquatic life uses should be assessed as "fully supported". Moderately high nutrient loading to the water column and minor impacts from siltation, however, remain water quality concerns at this lake. The ISU lake survey data show no violations of the Class B(LW) criteria for dissolved oxygen in the 15 samples collected during summers of 2000 through 2004. One of 13 samples, however, exceeded the Class B(LW) criterion for pH (maximum = 9.4; minimum = 7.8 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. This violation likely reflects the excessive primary productivity at Lake Wapello and does not reflect the input of pollutants into this lake.
The Class C (drinking water) uses remain "not assessed" due to a lack of relevant water quality information for this lake 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 extremely low at this lake (maximum value = 0.4 mg/l; median = 0.1 mg/l), these data are not sufficient for developing a valid assessment of support of the Class C uses.
Fish consumption remain assessed "not assessed" due to the lack of fish contaminant monitoring at this lake.
