Assessment Comments
Assessment is based on results of (1) ISU lake survey from 2000-04, (2) surveys by IDNR Fisheries Bureau, and (3) ISU reports on lake plankton communities from 2002-05.
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". Nutrients, algae, and siltation, however, remain concerns for support of the Class B(LW) uses at this lake. Fish consumption uses are “not assessed” due to the lack of recent information upon which to base an assessment. 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 statewide lake survey from 2000 through 2004 (approximately 15 samples), Carlson's (1977) trophic state indices for total phosphorus, chlorophyll-a, and secchi depth are 62, 63, and 62, respectively. According to Carlson (1977), all three index values are in the middle to lower range between eutrophic and hyper-eutrophic lakes and suggest somewhat moderately high levels of phosphorus and chlorophyll-a and moderately poor water transparency.
Based on median values from ISU sampling from 2000 through 2004, the ratio of total nitrogen to total phosphorus for Hawthorn Lake is 15, thus suggesting that production of suspended algae may at times be limited by the availability of phosphorus versus nitrogen.
In terms of all Iowa lakes sampled, data from the ISU survey show only moderately large populations of zooplankton species at this lake that graze on algae. Sampling from 2000 through 2005 showed that Cladoceran taxa (e.g., Daphnia) comprised approximately 50% 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 (85.5 mg/l) was the 66th highest of the 131 lakes sampled and roughly equal to the overall median level of Cladocerans for the 2000-2005 period (86 mg/l).
The levels of inorganic suspended solids at this lake are typical for Iowa lakes and thus suggest at least 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 2004 was 5.2 mg/l; the median level at Hawthorn Lake (5.4 mg/l) was approximately equal to this overall median.
The moderately high TSI value for Secchi depth suggests a concernf or support of the Class A (primary contact) uses through presence of aesthetically objectionable conditions due primarily to inorganic turbidity and secondarily to algal blooms. TSI values for Secchi depth and chlorophyll-a, however, are both below the impairment trigger level of 65, thus suggesting the continued “full support” of the Class A uses at Hawthorn Lake.
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 (70%) of this lake’s summertime phytoplankton community, sampling from 2000 through 2004 showed that the median per summer sample mass of bluegreen algae at Hawthorn Lake (14.6 mg/l) was only the 53rd highest of the 131 lakes sampled. This levels is in the lowest two-thirds of the 131 Iowa lakes sampled. The presence of this 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 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". Moderately high levels of nutrients in the water column, algal turbidity, and siltation remain concerns at this lake. The ISU lake survey data show good chemical water quality at this lake. These data show no violations of the Class B(LW) criteria for dissolved oxygen in the 15 samples collected, or for pH in the 13 samples collected, at Hawthorne Lake during summers of 2000 through 2004.
Fish consumption uses are “not assessed” due to the lack of recent information upon which to base a assessment. The most recent assessment was developed for the previous (2004) 305(b) reporting cycle. Fish consumption uses were assessed (evaluated) 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).
