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. An additional impairment to these uses is suggested by the very large population of bluegreen algae at this lake (=nuisance aquatic life). The Class B(LW) aquatic life uses remain assessed (evaluated) as "partially supporting" due to siltation, excessive nutrient loading to the water column, and high levels of non-algal turbidity. 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) 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: The fish population of this lake was renovated in Fall 2006; a very large population of common carp was eliminated. The IDNR Fisheries Bureau anticipates that, with the removal of the common carp, water clarity of this lake will improve.
EXPLANATION: Results from the ISU statewide survey of Iowa lakes suggest that high levels of non-algal turbidity may adversely affect the Class A and Class B(LW) uses of Thayer 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 76, 62, and 75, respectively. According to Carlson (1977), the index values for total phosphorus and Secchi depth place this lake in the range of hyper-eutrophic lakes; the index value for chlorophyll-a is in the lower range between eutrophic and hyper-eutrophic lakes. These index values suggest extremely high levels of phosphorus in the water column, moderately high (but much less than expected) levels of chlorophyll-a (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 non-algal particles (i.e., inorganic suspended solids) do likely limit algal production at Thayer Lake. 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, Thayer Lake had the 13th highest median level of inorganic suspended solids (18.5 mg/l), thus suggesting that non-algal turbidity likely limits the production of algae as well as contributes to in-lake turbidity that impairs the primary contact recreation uses.
The ISU data also suggest that nitrogen availability and zooplankton grazing may limit algal production at Thayer Lake. Based on median values of ISU lake sampling from 2000 through 2004, the ratio of total nitrogen to total phosphorus (TN/TP) at Thayer Lake is 10; this TN/TP ratio is low and suggests that algal production may, at times, be limited by the availability of nitrogen. In addition, the presence of extremely large populations of zooplankton at this lake that graze on algae may help explain the discrepancy between the TSI value for phosphorus (76) and that for chlorophyll-a (62). Sampling from 2000 through 2005 showed that Cladoceran taxa (e.g., Daphnia) comprised about 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 (373 mg/l) was the fifth highest of the 131 lakes sampled. Thus, these data suggest that populations of these Cladocerans may tend to suppress expression of chlorophyll as suspended algae.
These conditions suggest impairments to the Class A (primary contact) uses primarily due to presence of very poor to extremely poor water transparency that violates Iowa’s narrative water quality criteria protecting against aesthetically objectionable conditions. The very poor water transparency is related primarily to high levels of inorganic suspended solids at Thayer Lake.
The presence of nuisance (=noxious) algal species (i.e., bluegreen algae) may also present an impairment of the Class A uses at Thayer Lake. Data from the ISU survey from 2000 through 2004 suggest that bluegreen algae (Cyanophyta) comprise a significant portion of this lake’s summertime phytoplankton community. Summer sampling during this period showed the percent wet mass of the total phytoplankton community in bluegreens (cyanobacteria) was approximately 75%. Also, the median per summer sample mass of bluegreen algae at this lake (38.6 mg/l) was the 25th highest of the 131 lakes sampled. This median is in the worst 25% of the 131 Iowa lakes sampled. The presence of a large population of bluegreen algae at this lake suggests the 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 75th percentile of this distribution (~29 mg/l) were arbitrarily chosen by IDNR staff to represent the condition of “potential impairment: partially supported.” 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 to hyper-eutrophic conditions at this lake, along with information from the IDNR Fisheries Bureau, suggest that the Class B(LW) aquatic life uses should remain assessed as "partially supported" due to siltation, excessive nutrient loading to the water column, and high levels of non-algal turbidity. The ISU lake survey data show no violations of the Class B(LW) criteria for dissolved oxygen in the 13 samples collected during summers of 2000 through 2004. Two of 14 samples exceeded the Class B(LW) criterion for pH (maximum = 9.2; minimum = 7.8 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 primary productivity at Thayer Lake and do not reflect the input of pollutants into this lake.
Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring at this lake.
