Assessment Comments
Assessment is based on results of (1) ISU lake survey from 2000-02, (2) surveys by IDNR Fisheries Bureau, (3) ISU report on lake plankton communities in 2002, and (4) results of EPA/DNR fish tissue (RAFT) monitoring in 1994.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (monitored) as "fully supporting / threatened" due to moderately high levels of turbidity due to algae or inorganic suspended solids. The Class B(LW) aquatic life uses remain assessed (evaluated) as "fully supporting / threatened" due to nutrients, algae, and siltation. Fish consumption uses are assessed (evaluated) as “fully supported / threatened” based on result of fish contaminant monitoring in 1994. The sources of data for this assessment include (1) results of the statewide survey of Iowa lakes conducted from 2000 through 2002 by Iowa State University (ISU), (2) information from the IDNR Fisheries Bureau, (3) information on plankton communities at Iowa lakes in 2000 from Downing et al. (2002), and (4) results of U.S. EPA/IDNR fish contaminant monitoring in 1994.
EXPLANATION: Results from the ISU statewide survey of Iowa lakes suggest that moderately high levels of turbidity, due either to algae or inorganic suspended solids, may threaten the Class A uses of Hawthorn Lake. Using the median values from this survey from 2000 through 2002 (approximately nine samples), Carlson's (1977) trophic state indices for total phosphorus, chlorophyll-a, and secchi depth are 64, 63, and 67, respectively. According to Carlson (1977), all three index values are in the range between range between eutrophic and hyper-eutrophic lakes and suggest somewhat elevated levels of phosphorus and chlorophyll-a and moderately poor water transparency. Based on median values from ISU sampling from 2000 through 2002, the ratio of total nitrogen to total phosphorus for Hawthorn Lake is 19, thus suggesting that production of suspended algae is likely limited by the availability of phosphorus versus 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. The summer 2000 average mass of Cladoceran grazers at this lake (33.2 mg/l) was the 37th highest of the 131 lakes sampled. This mass of zooplankton grazers suggest the potential to limit algal production. The levels of inorganic suspended solids at this lake are moderately high and suggest the potential for contributing to in-lake turbidity and limiting production of suspended algae. 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 Hawthorn Lake was 5.2 mg/l. Thus, the moderately high TSI value for Secchi depth suggests a potential threat to the Class A (primary contact) uses through presence of aesthetically objectionable conditions due primarily to inorganic turbidity and secondarily to algal blooms).
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 2000 average summer mass of bluegreen algae (8.7 mg/l) was the 60th lowest of the 131 lakes sampled. This level does not appear to threaten the Class A or Class B uses.
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. Fish consumption uses remain assessed as "fully supported" based on results of the U.S. EPA/IDNR fish tissue (RAFT) monitoring near in 1994 (see assessment developed for the 1998 report). However, because these data are now considered too old (greater than five years) to accurately characterize current water quality conditions, the assessment category is changed from "monitored" (indicating an assessment with relatively high confidence) to "evaluated" (indicating an assessment with relatively lower confidence).
