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 2000-05.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (monitored) as "not supporting" due to very poor water transparency related to high levels of inorganic turbidity and increasingly high levels of suspended algae at this lake. The Class B(LW) aquatic life uses are assessed (evaluated) as "fully supporting". Excessive growth of aquatic macrophytes at this shallow lake, however, remains a concern. Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring at this lake. 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) information on plankton communities collected at Iowa lakes from 2000 through 2005 as part of the ISU lake survey, and (3) information from the IDNR Fisheries Bureau.
Note: A TMDL for turbidity at Spring Lake was prepared by IDNR and approved by EPA in 2006. Because the primary Section 303(d) impairment identified for the 2006 assessment/listing cycle (turbidity) is addressed by the TMDL, this waterbody is moved from IR Category 5a from the 2004 assessment/listing cycle to IR Category 4a (impaired; TMDL approved) for the 2006 cycle.
EXPLANATION: Results from the ISU statewide survey of Iowa lakes suggest that high levels of non-algal turbidity may adversely affect the Class A uses of Spring Lake. 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 64, 62, and 71, respectively. According to Carlson (1977), the index values for total phosphorus is in the middle range between eutrophic and hyper-eutrophic lakes. The index value for chlorophyll-a is in the lower range of eutrophic lakes. The index value for Secchi depth, however, is in the lower range of hyper-eutrophic lakes. These index values suggest somewhat elevated levels of phosphorus and chlorophyll in the water column and very poor water transparency.
According to Carlson (1991), the occurrence of high and similar TSI values for total phosphorus and secchi depth compared to the TSI for chlorophyll-a suggests that non-algal particles dominate light attenuation. Thus, the poorer than expected water transparency may be due to non-algal turbidity. 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 Spring Lake was 8.9 mg/l. This moderately high level of inorganic suspended solids likely accounts for the reductions in water transparency observed at this lake.
Neither nitrogen limitation nor zooplankton grazing appear to limit algal production at Spring Lake. Based on median values from ISU sampling from 2000 through 2004, the ratio of total nitrogen to total phosphorus for this lake is 18, thus suggesting that phosphorus (versus nitrogen) is the nutrient that most often limits production of suspended algae at this lake. In terms of all Iowa lakes sampled, data from the ISU survey show extremely small populations of zooplankton species at this lake that graze on algae. Sampling from 2000 through 2005 showed that Cladoceran taxa (e.g., Daphnia) comprised about 40% of the dry mass of the zooplankton community of this lake; the average per summer sample mass of Cladoceran taxa (24 mg/l) was the 11th lowest level of the 131 lakes sampled. These results suggest little if any non-phosphorus limitation due to zooplankton grazing at Spring Lake.
A comparison of annual TSI values for the 2000-2005 period at Spring Lake suggests a worsening trend in levels of chlorophyll-a. Based on ISU lake survey monitoring data, annual summer TSI values for chlorophyll have steadily worsened over the five-year period: 2000: 51 (approximately 8 ug/l); 2001: 53; 2002 = 56; 2003 = 61; and 2004 = 72 (approximately 68 ug/l). This adverse trend has occurred without corresponding trends in either total phosphorus or Secchi depth. TSI values for total phosphorus have stayed relatively low and constant since 2001 in the high 50s and low 60s. TSIs for Secchi depth have generally been in the low 70s (suggesting impairment) since 2000 but have not worsened over that time.
These water quality conditions suggest that the levels of inorganic suspended solids and, increasingly, suspended algae, reduce water transparency such that an aesthetically objectionable condition exists, thus indicating “nonsupport” of the Class A uses at Spring Lake.
The levels of nuisance (=noxious) algal species (i.e., bluegreen algae) at this lake do not suggest an impairment of Class A uses. Sampling from 2000 through 2004 showed that the median per summer sample mass of bluegreen algae at Spring Lake (4.6 mg/l) was the 22nd lowest of the 131 lakes sampled. This level is in the lowest 25% of the 131 Iowa lakes sampled. The presence of a relatively small 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).
Information from the IDNR Fisheries Bureau suggests that the Class B(LW) aquatic life uses should be assessed as "fully supported". The natural shallowness of this lake leads to problems with excessive growth of aquatic macrophytes (="noxious aquatic plants"); however, this problem does not impair the aquatic life uses of this lake. The ISU lake survey data show no violations of the Class B(LW) criteria for dissolved oxygen or pH in the 14 samples collected during summers of 2000 through 2004.
Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring at this lake.
