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 supported" due (1) very high levels of algal production (blooms) and (2) extremely poor water transparency that violate Iowa’s narrative water quality standard protecting against aesthetically objectionable conditions. An additional impairment to the Class A uses is due to the extremely high populations of nuisance aquatic life (bluegreen algae) at this lake. The Class B(LW) aquatic life uses remain assessed (evaluated) as "partially supported" due to excessive nutrient loading to the water column, organic enrichment, nuisance blooms of algae, and re-suspension of sediment. The assessment of the Class A and Class B(LW) uses are consistent with the assessments developed for previous Section 305(b) reports (see above). Fish consumption remain "not assessed" due to the 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 algae and turbidity at Tuttle Lake was prepared by IDNR and approved by EPA in 2005. Because the primary Section 303(d) impairments identified for the 2006 assessment/listing cycle (algal growth and turbidity) are 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 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" due to presence of algal blooms and extremely poor water transparency that violate Iowa’s narrative criterion protecting against aesthetically objectionable conditions. Using the median values from this survey from 2000 through 2002 (approximately nine samples), Carlson’s (1977) trophic state indices for total phosphorus, chlorophyll-a, and secchi depth are 82, 71, and 81, respectively, for Tuttle Lake. According to Carlson (1977), these index values place this lake in the range of hyper-eutrophic lakes and suggest (1) extremely high levels phosphorus in the water column, (2) very high, but less than expected, production of suspended algae, and (3) extremely 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 Tuttle 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. The median level of inorganic suspended solids at Tuttle Lake (20 mg/l) was the 12th highest of the 131 lakes. Thus, despite the very high level of algal production at this lake, non-algal (inorganic) turbidity does likely limit the production of algae as well as contributes to the impairment of beneficial uses through reduced water transparency.
The ISU data also suggest that nitrogen availability and zooplankton grazing may limit algal production at Tuttle Lake. Based on median values of ISU lake sampling from 2000 through 2004, the ratio of total nitrogen to total phosphorus (TN/TP) at this lake is 12; 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 explain a portion of the discrepancy between the TSI value for total phosphorus (82) and that for chlorophyll-a (71). Sampling from 2000 through 2005 showed that Cladoceran taxa (e.g., Daphnia) comprised about 45% 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 (332 mg/l) was the seventh highest of the 131 lakes sampled. Thus, these data suggest that populations of Cladocerans may tend to suppress expression, although to a limited degree, the production of chlorophyll as suspended algae.
Thus, the water quality conditions at Tuttle Lake suggest impairments to the Class A (primary contact) uses primarily due to presence of (1) very high levels of algal production (blooms) and (2) extremely poor water transparency that violate Iowa’s narrative water quality standard protecting against aesthetically objectionable conditions. The extremely poor water transparency is caused by a combination of suspended algae and inorganic suspended solids. Levels of either algae or inorganic suspended solids are sufficiently high to independently cause severe reductions in water transparency at this lake.
Data from the ISU survey from 2002-2004 suggest that the presence of nuisance aquatic species (i.e., bluegreen algae) may also present an impairment of the Class A uses at this lake. Summer sampling during this period showed the percent wet mass of the total phytoplankton community in bluegreens was approximately 85%. In addition, Tuttle Lake had the eighth highest median per summer sample mass of bluegreen algae of the 131 lakes sampled: 84.3 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, organic enrichment, nuisance blooms of algae, and re-suspension of sediment. According to the IDNR Fisheries Bureau, the impairments to the aquatic life uses of this natural lake are rather hard to quantify but probably affect some spawning activities of nest building species in varying degrees; e.g., bluegill, bullhead, crappie and largemouth bass. Algal blooms do impact water quality through their contribution to sags in dissolved oxygen and the potential fish mortality that can result.
The ISU lake survey data show fairly good chemical water quality at Tuttle Lake. Results of this monitoring show no violations of the Class B(LW) criteria for dissolved oxygen in the 15 samples collected during summers of 2000 through 2004. Two of 14 samples exceeded the Class B(LW) criterion for pH (maximum = 9.2; minimum = 8.4 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 high levels of primary productivity at Tuttle Lake and do not reflect the input of pollutants into this lake.
Fish consumption remain "not assessed" due to the lack of fish contaminant monitoring at this lake.
