Lake Ahquabi IA 04-LDM-1080

Assessment Comments

Assessment based on (1) results of IDNR beach monitoring program from 2002 through 2004, (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."  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 IDNR/UHL beach monitoring from 2002-2004, (2) results of the statewide survey of Iowa lakes sponsored by IDNR and conducted by Iowa State University (ISU) from 2000 through 2004, (3) surveys by IDNR Fisheries Bureau, and (4) information on plankton communities collected at Iowa lakes from 2000 through 2005 as part of the ISU lake survey.  

EXPLANATION:  Results of IDNR beach monitoring at Lake Ahquabi 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 (29 samples), 2003 (29 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 Lake Ahquabi 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:  0%).    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 suggest  that levels of indicator bacteria at this beach are extremely 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 2002 through 2004 (approximately 15 samples), Carlson's trophic state indices for total phosphorus, chlorophyll-a, and secchi depth at Lake Ahquabi are 66, 61, and 48, 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 value for chlorophyll-a is in the lower range between eutrophic and hyper-eutrophic lakes, and the index value for secchi depth is at the upper range of mesotrophic lakes.   These index values suggest moderately to relatively high levels of phosphorus, moderately high level of chlorophyll-a, but extremely 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 15.   This ratio suggests that algal production at this lake is likely limited by phosphorus.  

In terms of all Iowa lakes sampled, data from the ISU survey show very small populations of zooplankton species at this lake that graze on algae.   Sampling from 2000 through 2005 showed that while Cladoceran taxa (e.g., Daphnia) comprised almost 60% of the dry mass of the zooplankton community of this lake, the average per summer sample mass of Cladoceran taxa (47 mg/l) was the 28th lowest level of the 131 lakes sampled.   These results suggest little if any non-phosphorus limitation due to zooplankton grazing at Lake Ahquabi.  

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 2002 through 2005 was 5.2 mg/l; the median level at Lake Ahquabi was 3.0 mg/l.   These results suggest very low levels of inorganic suspended solids and do not suggest turbidity-related threats or impairments at Lake Ahquabi.  

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 75%) 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 Ahquabi (19.3 mg/l) was the 85th lowest of the 131 lakes sampled.   This level is in the lowest two-thirds (but just barely) of the 131 Iowa lakes sampled.   The presence of a moderately high 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 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 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 = 9.2; minimum = 7.9 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 levels of primary productivity at Lake Ahquabi and do not reflect the input of pollutants into this lake.  

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.   Results of the ISU survey from 2000-04 show that nitrate levels are usually very low at this lake (median = 0.1 mg/l; mean = 0.8 mg/l) although levels can, at times, reach higher levels (maximum value for 2000-2004 = 10.0 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.