Assessment Comments
Assessment is based on results of (1) ISU lake survey from 2000-02, (2) surveys by IDNR Fisheries Bureau, and (3) ISU report on lake plankton communities in 2000.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (monitored) as "not supported" due to very large populations of suspended algae and very high levels of inorganic turbidity caused by re-suspension of sediment that contribute to aesthetically objectionable conditions at this lake. An additional impairment to the Class A uses is due to the very 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 conducted from 2000 through 2002 by Iowa State University (ISU), (2) information from the IDNR Fisheries Bureau, and (3) information on plankton communities at Iowa lakes in 2000 from Downing et al. (2002).
EXPLANATION: Results of monitoring conducted by ISU from 2000 through 2002 as part of the statewide survey of Iowa lakes suggest that the Class A (primary contact) uses are "not supported" due to very large populations of suspended algae and very high levels of inorganic suspended solids that contribute to very poor water transparency at this lake. 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 85, 73, 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 2002 was 4.8 mg/l. The median level of inorganic suspended solids at Tuttle Lake (20 mg/l) was the 20th highest of the 131 lakes. Thus, despite the extremely 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 2002, the ratio of total nitrogen to total phosphorus (TN/TP) at this lake is 9; this TN/TP ratio is low and suggests that algal production may, at times, be limited by the availability of nitrogen. In addition, data from Downing et al. (2002) show large populations of zooplankton species at Tuttle Lake that graze on algae. The summer 2000 average mass of Cladocerans (62 mg/l) was the 23rd 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. These conditions suggest impairments to the Class A (primary contact) uses primarily due to presence of (1) very high levels of algal production (blooms) and (2) very high levels of inorganic turbidity (and the related reductions in water transparency) that violate Iowa’s narrative water quality standard protecting against aesthetically objectionable conditions.
The presence of nuisance (=noxious) algal species (i.e., bluegreen algae) may also contribute to impairment at Tuttle Lake. Data from Downing et al. (2002) suggest that bluegreen algae (Cyanophyta), tend to dominate the summertime phytoplankton community of Tuttle Lake. Sampling in summer 2000 showed that bluegreen algae comprised approximately 85% of the wet mass of the phytoplankton community in the mid-June sample, 65% in the mid-July sample, and 90% in the early August sample. The average mass of bluegreen algae in summer 2000 at this lake (90 mg/l) ranked 11th highest of the 131 lakes sampled. The presence of very large populations of bluegreen algae suggests an impairment of support of the designated uses of this lake due to the potential for violating Iowa’s narrative water quality standard protecting against presence of nuisance aquatic life. Although results of plankton sampling in 2000 suggest that designated uses are impaired, the amount of data available for characterizing algal populations at this lake (one season) is not sufficient for developing a more accurate assessment of support of these uses. Additional data on plankton have been collected at this lake as part of the ongoing ISU lake survey and will be used to improve the accuracy of future water quality assessments.
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. Fish consumption remain "not assessed" due to the lack of fish contaminant monitoring at this lake.
