Assessment Comments
Assessment is based on results of (1) ISU lake survey in 2000-01, (2) surveys by IDNR Fisheries Bureau, and (3) 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 remain assessed (evaluated) as "fully supporting / threatened." Fish consumption uses are "not assessed." The sources of data for this assessment include (1) results of the statewide survey of Iowa lakes conducted in 2000 and 2001 by Iowa State University (ISU), (2) information from the IDNR Fisheries Bureau, and (3) information on phytoplankton communities at Iowa lakes in 2000 from Downing et al. (2002). Further monitoring and investigation are needed to better characterize water quality conditions at this lake. EXPLANATION: Results from the ISU statewide survey of Iowa lakes suggest that moderately high levels of algal turbidity may threaten the Class A uses of Hawthorn Lake. Using the median values from this survey in 2000 and 2001 (approximately six samples), Carlson's (1977) trophic state indices for total phosphorus, chlorophyll-a, and secchi depth are 68, 61, and 63, respectively. According to Carlson (1977), all three index values are in the range between range between eutrophic and hyper-eutrophic lakes and suggest relatively high levels of phosphorus, moderately high (and somewhat less than expected) levels of chlorophyll-a (suspended algae), and moderately poor 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) other than phosphorus limits production of algae. Based on median values from ISU sampling in 2000 and 2001, the ratio of total nitrogen to total phosphorus for Hawthorn Lake is 16, thus suggesting that production of suspended algae may be limited by nitrogen. In addition, data from Downing et al. (2002) show relatively large populations of zooplankton species at Hawthorn Lake that graze on algae. Sampling in 2000 showed that Cladoceran taxa (e.g., Daphnia) comprised approximately 70% of the dry mass of the zooplankton community in the late July sample and 60% of the mid-August sample. According to Carlson (1991), the occurrence of a high TSI value for secchi depth relative to that for chlorophyll-a suggests that non-algal turbidity limits algal production. The levels of inorganic suspended solids at this lake are relatively low and do not suggest a strong potential for either contributing to in-lake turbidity or limiting production of suspended algae. 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 Hawthorn Lake was 5.6 mg/l. The moderately high TSI value for total phosphorus and chlorophyll-a suggest potential threats to the Class A (primary contact) uses through presence of aesthetically objectionable blooms of algae. The presence of nuisance algal species (i.e., bluegreen algae), however, does not appear to be a significant problem at this lake. Data from Downing et al. (2002) suggest that bluegreen algae (Cyanophyta) comprise from 30 to 40% of the summertime phytoplankton community of this lake and do not dominate to the degree seen in many other Iowa lakes. Sampling in 2000 showed the percent wet mass of bluegreens ranged from approximately 40% in the late June sampling, to approximately 35% in the late July sampling, and approximately 30% in the mid-August sampling. The eutrophic conditions at this lake, along with information from the IDNR Fisheries Bureau, suggest that the Class B(LW) aquatic life uses should be assessed as "fully supported / threatened" due to moderately high levels of nutrients in the water column; algal turbidity; and siltation. 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. Fish consumption uses are "not assessed." The data upon which the previous assessment was based were collected in 1994; these data are now considered too old (greater than five years) to be useful for characterizing current water quality conditions.
