Hawthorn Lake IA 03-NSK-862

For the 2016 assessment/listing cycle, the Class A1 (primary contact recreation) uses of Hawthorn Lake are assessed (monitored) as “partially supported" based on information from the ISU lake survey. Using the median values from these surveys from 2011-2014 (approximately 12 samples), Carlson 's (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 62, 64, and 62 respectively for Hawthorn Lake. According to Carlson (1977) the Secchi depth, chlorophyll a, and total phosphorus values all place Hawthorn Lake in the Eutrophic category. 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. The data show one violation of the Class A1 criterion for pH in 12 samples.

These results suggest "full support" of the Class A1 uses, however, Hawthorn Lake was listed as "partially supporting" the Class A1 uses for the 2014 assessment/listing cycle due to algal and non-algal turbidity. Based on DNR'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.The median TSI values for the 2014 assessment and listing cycle for chlorophyll a and Secchi depth were 61and 62, respectively. Based on the metholodgy the impairment for turbidity can be removed due to two cycles of TSI values below 63 for Secchi depth. However, whilemedian-based TSI values for chlorophyll-a must also be 63 or less for two consecutive summer seasons before a lake can be removed from the state’s Section 303(d) list (IR Category 5).Therefore, Hawthorn Lake remains assessed as "partially supported" due to high levels of algae for the 2016 assessment cycle.

The level of inorganic suspended solids was relatively low at Hawthorn Lake, and does not suggest water quality problems due to non-algal turbidity. The median level of inorganic suspended solids in Hawthorn Lake (2.5 mg/L) was ranked 39th among the 138 lakes by the ISU lake survey.

Data from the 2011-2014 ISU lake survey suggest a moderately large population of cyanobacteria exists at Hawthorn Lake. These data show that cyanobacteria comprised 95% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (27.2 mg/L) was ranked 88th of the 138 lakes sampled.

The Class B(LW) (aquatic life) uses are assessed (monitored) as "fully supported." Results of the ISU lake survey from 2011-2014 show there were one violation of the criterion for ammonia in 12 samples(8%), no violations of the criterion for dissolved oxygen in 12 samples(0%), and one violation of the criterion for pH in 12 samples(8%). Based on DNR's assessment methodology these violations are not significantly greater than 10% of the samples and therefore suggest (fully supported/monitored) of the Class B(LW) uses of Hawthorn Lake.

During the 2014 assessment cycle, the Class B(LW) (aquatic life) uses wasassessed (monitored) as “partially supported” based on information from the DNR 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.]. According tothe Lake Restoration Program2015 Report and 2016 Plan,Hawthorn Lake was drawn down in 2010 and watershed and lake restoration projects were completed throughout the assessment period.The DNR designed eight sediment control ponds for watershed improvement on public and privateground. Five of the eight structures on public property are now completed. Additionallylakes fishery has redeveloped quickly following completion of the 2010 restorationproject.Largemouth bass abundance and quality are excellent.Additionally,based on DNR's assessment methodology, median-based TSI values for 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 Secchi depth was65.The median TSI values for the 2014 assessment and listing cycle for Secchi depth as62. Based on the methodologythe impairment for turbidity can be removed due to two cycles of TSI values below 63 for Secchi depth. This suggests that the Class B(LW) (aquatic life) uses assessment of(monitored) and “partially supported” due to the impacts of increasingly poor water transparency, siltation, and an increasingly poor game fishery at this lake should be assessed as(monitored) and "fullysupported”.

Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring in this lake.

Note:A watershed assessment was completed in 2010. Multiple structures were completed since that time and 4 more are currently under construction. Sedimentation in the two main arms has been substantial and up to 20% of the storage capacity has been lost in those sections. The fish community was renovated in 2010.