Assessment Comments
Assessment is based on results of (1) ISU lake survey from 2000-04, (2) ISU reports on lake phytoplankton communities from 2000-05, and (3) surveys by IDNR Fisheries Bureau.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (monitored) as "fully supporting." The Class B(LW) aquatic life uses remain assessed (evaluated) as "fully supporting". Siltation, however, remains a concern at this lake. Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring at this lake. 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) surveys by IDNR Fisheries Bureau, and (3) information on plankton communities collected at Iowa lakes from 2000 through 2005 as part of the ISU lake survey.
EXPLANATION: Using the median values from the ISU lakes 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 52, 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 value for chlorophyll-a is in the upper range of eutrophic lakes, and the index value for Secchi depth is in the lower range of eutrophic lakes. These index values suggest (1) relatively high levels of phosphorus in the water column, (2) low, and much less than expected, production of suspended algae, and (3) very good, and much better than expected, 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 from ISU monitoring suggest non-phosphorus limitation at Hannen Lake is due to nitrogen limitation and to large populations of zooplankton that graze on algae.
Based on median values from ISU sampling from 2000 through 2004, the ratio of total nitrogen to total phosphorus for Hannen Lake is 15. This ratio suggests that algal production at this lake may, in part, be limited by availability of nitrogen.
The levels of inorganic suspended solids at this lake are extremely low and do not suggest the potential for impairing designated uses due to high turbidities. 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 Hannen Lake (2.1 mg/l) was the 10th lowest of the 131 lakes sampled. The levels of ISS seen at Hannen Lake do not suggest the likelihood that inorganic turbidity limits algal production at this lake.
The presence of very large populations of zooplankton at Hannen Lake that graze on algae, however, may explain the large discrepancies between the TSI value for phosphorus (69) and those for chlorophyll-a (60) and (especially) Secchi depth (52). In terms of all Iowa lakes sampled, data from the ISU survey show a very large population of zooplankton species at this lake that graze on algae. Sampling from 2000 through 2005 showed that Cladoceran taxa (e.g., Daphnia) comprised approximately 70% 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 (230 mg/l) was the 15th highest of the 131 lakes sampled. This information suggests that the that the relatively low algal production at this lake is at least partially due to zooplankton grazing.
Despite the high TSI value for total phosphorus, the index values for chlorophyll-a and Secchi depth suggest relatively good water quality and full support of the Class A (primary contact) uses.
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 most (84%) of this lake’s summertime phytoplankton community, sampling from 2000 through 2004 showed that the median per summer sample mass of bluegreen algae at Hannen Lake (20 mg/l) was the 43rd highest of the 131 lakes sampled. This level is in the lowest two-thirds of the 131 Iowa lakes sampled. The presence of a moderately large population of bluegreen algae at this lake does not, however, necessarily 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 (evaluated) as "fully supported". Siltation in the lake, however, remains a water quality concern. Results of ISU monitoring from 2000 through 2004 suggest generally good chemical water quality at this lake although periods of low dissolved oxygen occur. Results of this monitoring show that 3 of the 14samples collected exceeded the Class B(LW) criteria for dissolved oxygen and that 1 of the 15 samples collected exceeded Iowa’s Class A,B(LW) criteria for pH (maximum = 9.2; minimum = 8.0 pH units). Based on IDNR’s assessment methodology, these results do not suggest violation frequencies are significantly greater than 10% for either parameter and thus do not suggest impairment of either the Class A and Class B(LW) uses of Hannen Lake.
Fish consumption uses remain "not assessed " due to the lack of fish contaminant monitoring at this lake.
