Assessment Comments
Assessment is based on results of (1) ISU lake surveys from 2000-04, (2) surveys by IDNR Fisheries Bureau, and (3) ISU reports on lake phytoplankton communities from 2000-05.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation uses) are assessed (monitored) as "not supported" due to algal blooms and high levels of inorganic turbidity that violate Iowa’s narrative water quality standard protecting against aesthetically objectionable conditions. The occurrence of blooms of bluegreen algae may present an additional potential impairment to designated uses at this lake. The Class B(LW) aquatic life uses are assessed (evaluated) as "partially supported" due to high levels of nutrients, algal blooms, and high levels of inorganic suspended sediment. Fish consumption 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 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.
Note: A TMDL for organic enrichment at Crystal Lake was prepared by IDNR and approved by EPA in 2002; thus, this waterbody was placed into IR Category 4a (TMDL approved) for the 2004 assessment/listing cycle. Because all Section 303(d) impairments identified for the 2006 assessment/listing cycle (algal growht; turbidity) are addressed by the TMDL, this waterbody remains in IR Category 4a for the 2006 assessment/listing cycle.
EXPLANATION: Results of monitoring conducted by ISU from 2000 through 2004 as part of the statewide survey of Iowa lakes suggest that the Class A (primary contact) uses are "not supported." Using the median values from this survey from 2000 through 2004 (approximately 15 samples), Carlson’s (1977) trophic state indices for total phosphorus, chlorophyll-a, and secchi depth are 83, 69, and 78, respectively, for Crystal Lake. According to Carlson (1977), these index values for total phosphorus and Secchi depth place this lake in the range of hyper-eutrophic lakes while the value for chlorophyll-a is in the upper range between eutrophic and hyper-eutrophic lakes. These values suggest extremely high levels of phosphorus, high levels of suspended algae, and very poor water transparency.
According to Carlson (1991), the occurrence of a low chlorophyll-a TSI value relative to those for total phosphorus and secchi depth indicate non-algal particles or color dominate light attenuation. The ISU lake data suggest that, while non-algal particles do likely limit algal production at Crystal Lake, nitrogen limitation and zooplankton grazing may also limit algal production. 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. Of 131 lakes sampled, Crystal Lake had the 10th highest median level of inorganic suspended solids (20.4 mg/l), thus suggesting that non-algal turbidity limits the production of algae and is primary cause of the extremely poor water transparency at this lake. Based on median values from ISU sampling from 2000 through 2004, the ratio of total nitrogen to total phosphorus for this lake is 11. This ratio suggests at least the possibility that algal production at this lake is at times limited by nitrogen availability. Despite the high level of inorganic suspended solids at Crystal Lake, the level of chlorophyll-a is very high (12th highest of the 131 lakes sampled) and is alone sufficient to cause transparency-related impairments at this lakes.
Data from the ISU survey also show relatively large populations of zooplankton species at Crystal Lake that graze on algae. Sampling from 2000 through 2005 showed that Cladoceran taxa (e.g., Daphnia) comprised approximately 59% of the dry mass of the zooplankton; this lake had the 10th highest per summer sample median mass of Cladoceran grazers (247.1 mg/l) of any of the 131 lakes sampled. Thus, both nitrogen limitation and zooplankton grazing may contribute to the lower than expected production of suspended algae at this lake.
The extremely high levels of phosphorus and the very high levels of primary productivity at this lake likely overwhelm any limitations by zooplankton grazers on algal production. These conditions indicate impairments to the Class A (primary contact) uses through presence of blooms of algae and very poor water transparency due to algal and inorganic turbidity that violate Iowa’s narrative water quality standard protecting against aesthetically objectionable conditions. The IDNR Fisheries Bureau concurs that turbidity-related impairments due to algae and suspended solids exist at this lake. Although the overall (2000-04) TSI values for chlorophyll-a (69) and Secchi depth (78) continue to suggest moderate to severe impairment (non-support) at this lake due to poor water transparency, a comparison of annual TSI values for the 2000-2004 period at this lake does suggest some improvement in water quality. Annual summer TSI values for Secchi depth have decreased (improved) as follows over the five-year period: 2000: 86; 2001: 81; 2002 = 81; 2003 = 71; and 2004 = 67. Additional years of data will be needed to determine whether this improving trend continues.
Data from the ISU survey from 2002-2004 suggest that the presence of nuisance aquatic species (i.e., bluegreen algae) may present an impairment of the Class A uses at this lake. Summer sampling during this period showed the percent wet mass of bluegreens was 92% of the phytoplankton community. In addition, Crystal Lake had the fourth highest median per summer sample mass of bluegreen algae of the 131 lakes sampled: 139.8 mg/l. This median is in the worst 10 % of the 131 Iowa lakes sampled. The presence of a large population of bluegreen algae at this lake suggests 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 greater than the 90th percentile of this distribution (~59 mg/l) were arbitrarily chosen by IDNR staff to represent the condition of “potential impairment; not supported.” No criteria exist, however, upon which to base a more accurate identification of impairments due to bluegreen algae. Thus, while the , 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, of necessity, remains "evaluated" (indicating an assessment with relatively lower confidence) as opposed to "monitored" (indicating an assessment with relatively higher confidence).
The hyper-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 (evaluated) as "partially supported" due to excessive nutrient loading to the water column, nuisance blooms of algae, and re-suspension of sediment. Results of chemical water quality monitoring conducted as part of the ISU lake survey, however, suggest at least moderately good chemical water quality at this lake. Results of this monitoring do not show violations of Class B(LW) criteria for dissolved oxygen: no violations of the Class B(LW) criteria for dissolved oxygen occurred in the 14 samples collected at Crystal Lake during summers of 2000 through 2004. Three of 15 samples exceeded the Class B(LW) criterion for pH (maximum = 9.9; minimum = 7.2 pH units). Based on IDNR’s assessment methodology, however, these results do not suggest that significantly more than 10 percent of the samples exceed Iowa’s pH criteria and thus do not suggest an impairment of the Class A and Class B(LW) uses of this lake. These violations likely reflect the high levels of primary productivity at Crystal Lake and do not reflect the input of pollutants into this lake.
Fish consumption remain "not assessed" due to the lack of recent fish contaminant monitoring at this lake.
