Lake Ahquabi IA 04-LDM-1080

Assessment Comments

Assessment based on (1) post-restoration studies by IDNR Fisheries Bureau 1997-98, (2) results of IDNR beach monitoring program from 2000-02, (3) ISU lake survey from 2000-02, & (4) ISU report on lake plankton communities in 2000.

Basis for Assessment

SUMMARY:  The Class A (primary contact recreation) uses are assessed (monitored) 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, 2001, and 2002, (2) information from the IDNR Fisheries Bureau, including results of post-restoration monitoring, (3) ISU lake surveys from 2000-02, and (4) information on plankton communities at Iowa lakes in 2000 from Downing et al.  (2002).

EXPLANATION:  Results of IDNR beach monitoring in summers of 2000, 2001 and 2002 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 2000-02 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 thirty-day period is compared to the water quality standard of 200 fecal organisms/100ml.   If a thirty-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 thirty-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 thirty-day period (N=5), the use of single-sample maximum values to assess beaches is problematic.   With less than 10 samples collected during any thirty-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 51 thirty-day periods during summers of 2000-02 had geometric means (N = 5 samples per period) greater than 200 orgs/100ml.   The maximum thirty-day geometric means were 37 orgs/100ml in 2000, 11 orgs/100 ml in 2001, and 18 orgs/100 ml in 2002.   None of the weekly samples collected from 2000-02 exceeded the U.S.  EPA's recommended single-sample maximum value of 400 orgs/100 ml.   These results suggest (1) full support of primary contact recreation uses at this beach and (2) that levels of indicator bacteria at this beach are very low.  

Similar to the results of IDNR beach monitoring, results from the ISU lake survey also suggest "full support" of the Class A uses.   Using the median values from the Iowa State University statewide surveys of Iowa lakes from 2000 through 2002 (approximately nine samples), Carlson's trophic state indices for total phosphorus, chlorophyll-a, and secchi depth at Lake Ahquabi are 65, 57, and 57, respectively.   According to Carlson (1977), the index value for total phosphorus places this lake in the middle range between eutrophic and hyper-eutrophic lakes; the index values for chlorophyll-a and secchi depth are in the middle to upper range of eutrophic lakes.   These index values suggest somewhat elevated levels of phosphorus but also 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 lower 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.   Based on median values from ISU sampling from 2000 through 2002, the ratio of total nitrogen to total phosphorus for Lake Ahquabi is 16.   This ratio suggests that algal production at this lake is limited by phosphorus.   Data from Downing et al.  (2002) show relatively large populations of zooplankton species at Lake Ahquabi that graze on algae.   Sampling in 2000 showed that Cladoceran taxa (e.g., Daphnia) comprised approximately 90% of the dry mass of the zooplankton community in both the late July and late August samples.   The 2000 summer average mass of Cladocerans at this lake was 27.1 mg/l; this is the 45th highest of the 131 lakes sampled and suggests that the moderately large populations of these zooplankton may limit algal production.   The levels of inorganic suspended solids at this lake are relatively low and do not suggest the potential for either limiting production of algae or impairing designated uses.   The median level of inorganic suspended solids in the 131 lakes sampled for the ISU lake survey from 2000 through 2002 was 4.8 mg/l; the median level at Lake Ahquabi was 2.7 mg/l.   These results do not suggest turbidity-related threats or impairments at Lake Ahquabi.  

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 algae 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.   The 2000 summer average mass of bluegreen algae at this lake (6.8 mg/l) was the 50th lowest of the 131 lakes sampled; thus, populations of bluegreen algae do not appear to either threaten or impair designated uses of this lake.  

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).   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).  

The Class C (drinking water) uses remain "not assessed" due to lack of water quality information on which to base such 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 2000-02 show that nitrate levels are very low at this lake (maximum value = 0.5 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.