Assessment Comments
Assessment based on (1) post-restoration studies by IDNR Fisheries Bureau 1997-98, (2) results of IDNR beach monitoring program 2000-01, (3) ISU lake survey in 2000-01, & (4) ISU report on lake plankton communities.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (evaluated) as "fully supporting / threatened." The Class B(LW) aquatic life uses are assessed (evaluated) as "fully supporting." The Class C (drinking water) uses remain "not assessed." Fish consumption uses remain "not assessed." The sources of data for this assessment include (1) results of the IDNR-UHL beach monitoring program in summers of 2000 and 2001, (2) information from the IDNR Fisheries Bureau, including results of post-restoration monitoring, (3) ISU lake surveys in 2000-01, and (4) 2002 ISU report on lake plankton communities.
EXPLANATION: Results of IDNR beach monitoring in 2002 and 2001 suggest full support of the Class A uses. Levels of indicator bacteria at Lake Ahquabi beach were monitored once per week during the primary contact recreation seasons (May through September) of 2000 and 2001 as part of the IDNR beach monitoring program. According to U.S. EPA guidelines for determining support of primary contact recreation uses (U.S. EPA 1997b, page 3-35), the geometric mean of fecal coliform bacteria level from at least five samples collected over a 30-day period is compared to the water quality standard of 200 fecal organisms/100ml. If a 30-day geometric mean exceeds 200 orgs/100 ml, the primary contact recreation uses are assessed as "not supported." In addition, the U.S. EPA guidelines state that if more than 10% of the total samples taken during any 30-day period has a bacterial density that exceeds 400 fecal coliform organsims/100 ml, the primary contact recreation uses are assessed as "partially supported." Due to the relatively low numbers of samples collected during any 30-day period (N=5), the use of single-sample maximum values to assess beaches is problematic. With less than 10 samples collected during any 30-day period at Iowa beaches, the occurrence of a single level of bacteria above the single-sample maximum value will result in more than 10% violation of the single-sample maximum value and thus suggest impairment of the primary contact recreation uses. The use of less than 10 samples in an assessment based on a critical value of 10% results in large probabilities (approximately 60%) of incorrectly concluding that an impairment exists. For this reason, the single-sample maximum value is not used to assess support of primary contact recreation uses with data from the IDNR beach monitoring program. At Lake Ahquabi beach, none of the 26 thirty-day periods during summers of 2000 and 2001 had geometric means (N = 5 samples per period) greater than 200 orgs/100ml. The maximum 30-day geometric means were 37 orgs/100ml in 2000 and 11 orgs/100 ml in 2001. None of the weekly samples collected during 2000 and 2001 exceeded the U.S. EPA's recommended single-sample maximum value of 400 orgs/100 ml (maximum sample values were 330 orgs/100 ml in 2000 and 40 orgs/100 ml in 2001). These results suggest (1) full support of primary contact recreation uses at this beach and (2) that levels of bacteria at this beach are very low.
Despite the results of IDNR beach monitoring that suggest "full support" of the Class A uses, results from the ISU statewide survey of Iowa lakes suggest that high levels of phosphorus may threaten full support of the Class A uses of Lake Ahquabi. Using the median values from the Iowa State University statewide surveys of Iowa lakes in 2000 and 2001 (approximately six samples), Carlson's trophic state indices for total phosphorus, chlorophyll-a, and secchi depth at Lake Ahquabi are 68, 55, and 56 , respectively. According to Carlson (1977), the index value for total phosphorus places this lake in the upper range between eutrophic and hyper-eutrophic lakes; the index values for chlorophyll-a and secchi depth are in the middle range of eutrophic lakes. These index values suggest relatively high levels of phosphorus but suggest relatively low levels of chlorophyll-a and relatively 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 or zooplankton grazing) limits production of algae. Based on median values from ISU sampling in 2000 and 2001, the ratio of total nitrogen to total phosphorus for Lake Ahquabi is 15.4. This ratio suggest the potential for algal production at this lake to be nitrogen-limited. The levels of inorganic suspended solids at this lake are relatively low and do not suggest the potential for impairing designated uses. The median level of inorganic suspended solids in the 130 lakes sampled for the ISU lake survey in 2000 and 2001 was 5.27 mg/l; the median level at Lake Ahquabi was 2.4 mg/l. Despite the indications of good water quality, the high TSI value for total phosphorus suggests a threat to the full support of the Class A (primary contact) uses through presence of aesthetically objectionable blooms of algae and due to the presence of nuisance algal species (i.e., bluegreen algae). Data from Downing et al. (2002) suggest that bluegreen algae (Cyanophyta) dominate the phytoplankton community of this lake during mid and late summer. Sampling in 2000 showed that, although populations of bluegreen alage were very low in early summer, over 80% of the wet mass of phytoplankton were bluegreen algae in samples collected in late July and mid-August. Although bluegreen algae appear to dominate the phytoplankton community in mid to late summer, the production of chlorophyll is relatively low at this lake, and the presence of bluegreen algae likely does not present a significant water quality impact. The Class B(LW) aquatic life uses remain assessed as "fully supporting" based on review of the previous assessments by the DNR Fisheries Bureau and on results of the Phase II Clean lakes post-project monitoring (see assessments for the 1998 and 2000 reports above). This post project monitoring showed improvements in levels of chlorophyll, total suspended solids, nitrogen, phosphorus and lake transparency. Fish populations of Lake Ahquabi have responded well to improved water quality and lake habitat conditions (see Bonneau 1999). Although results of ISU lake monitoring in 2000 and 2001 suggest threats to full support of the designated beneficial uses of this lake, the amount of data available for characterizing water quality is not sufficient for developing an accurate assessment of support of these uses. Additional data for this lake are being generated as part of the ongoing ISU lake survey; these data will be used to improve the accuracy of future water quality assessments. The Class C (drinking water) uses remain "not assessed" due to lack of water quality information on which to base such an assessment. Fish consumption remain assessed "not assessed" due to the lack of fish contaminant monitoring at this lake.
