Assessment Comments
Assessment is based on: (1) results of the statewide survey of Iowa lakes conducted from 2009-2012 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted in 2008 by University Hygienic Laboratory (UHL), and (3) information from the IDNR Fisheries Bureau as summarized by Flammang and Ellison (2008).
Basis for Assessment
SUMMARY: The Class A1 (primary contact recreation) uses are assessed (monitored) as “partially supported” due to aesthetically objectionable conditions caused by algal and non-algal turbidity. The Class B(LW) (aquatic life) uses are assessed (monitored) as “partially supported” due to the impacts of increasingly poor water transparency, siltation, and an increasingly poor game fishery at this lake. Fish consumption uses remain “not assessed” due to a lack of fish contaminant monitoring at this lake. Sources of data for this assessment include (1) results of the statewide survey of Iowa lakes conducted from 2006 through 2009 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted from 2006 through 2008 by University Hygienic Laboratory (UHL), and (3) information from the IDNR Fisheries Bureau as summarized by Flammang and Ellison (2008).
EXPLANATION: For the 2012 reporting cycle, the Class A1 (primary contact recreation) uses for Hawthorn Lake are assessed (monitored) as “partially supported” based on results from the ISU statewide survey of lakes and the UHL ambient lake monitoring program. Using the median values from these surveys from 2009-2012 (approximately 11 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 62, 61, and 64 respectively for Hawthorn Lake. According to Carlson (1977) the Secchi depth, chlorophyll a, and total phosphorus values all place Hawthorn Lake in between the eutrophic and hypereutrophic categories. These values suggest moderately high levels of chlorophyll a and suspended algae in the water, moderately poor water transparency, and moderately high levels of phosphorus in the water column. These results suggest "full support" of the Class A1 uses, however, Hawthorn Lake was listed as "partially supporting" the Class A1 uses for the 2012 assessment/listing cycle due to algal and non-algal turbidity. Based on IDNR's assessment methodology, median-based TSI values for both chlorophyll a and Secchi depth must be 63 or less for two consecutive assessment/listing cycles before a lake can be removed from the state's Section 303(d) list (IR Category 5). The median TSI values for the 2012 assessment and listing cycle for chlorophyll a and Secchi depth were 65 and 65, respectively. Therefore, Hawthorn Lake will remain assessed as "partially supporting" for the 2014 assessment/listing cycle.
The levels of inorganic suspended solids at this lake were relatively low and do not suggest an additional impairment due to non-algal turbidity. The median level of inorganic suspended solids in Hawthorn Lake (2.5 mg/L) and ranked 28th of the 134 lakes sampled by the ISU and UHL programs.
Data from the 2006-2009 ISU and UHL surveys suggest a moderate population of cyanobacteria exists at Hawthorn Lake, which may contribute to the impairments at this lake. These data show that cyanobacteria comprised only 60% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (11.3 mg/L) and ranked 32nd of the 134 lakes sampled.
The Class B(LW) (aquatic life) uses are assessed (monitored) as “partially supported” based on information from the IDNR Fisheries Bureau [Flammang, M. and B. Ellison. 2008. Identification of the fishery and water quality status and needs of Hawthorn Lake, Iowa. Fisheries Bureau, Iowa DNR, 9 p.]. The IDNR Fisheries Bureau states that siltation and shoreline erosion are substantial at this lake, and they have documented worsening water clarity, as measured by mean Secchi depth transparency, from 1988 through 2007. Possible reasons given for this decline include sediment delivered to the lake in sheet/rill and gully erosion in the watershed, shoreline erosion and resultant resuspension of sediment due to wind/wave action, and resuspension of sediments due to presence of common carp. Hawthorn Lake also supports poor quality game fish populations that are continuing to decline due to the presence of gizzard shad and common carp in the lake. Declining angler usage has accompanied the decline in game fish populations. The poor fishery, declining water clarity, and siltation at this lake suggest “partial support” of the Class B(LW) uses of Hawthorn Lake. An analysis of watershed characteristics and water quality characteristics of Iowa impoundments, however, shows that Hawthorn Lake has good potential for improvements in water quality and fish populations (see Flammang and Ellison 2008).
Despite declines in water transparency, results from the ISU and UHL lake surveys suggest relatively good chemical water quality at Hawthorn Lake. The data from these surveys from 2008-2012 show no violations of the Class B(LW) criteria for ammonia in 11 samples, dissolved oxygen in 11 samples, or pH in 11 samples. These results suggest "full support" of the Class B(LW) uses.
NOTE: Hawthorn Lake was drawn down in 2010 and a watershed and lake restoration projects were completed throughout the assessment period. It is expected that water quality will continue to improve in the coming years.
Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring in this lake.
