Assessment Comments
Assessment is based on results of (1) ISU lake survey in 2000-01, (2) surveys by IDNR Fisheries Bureau, and (3) ISU report on lake phytoplankton communities.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation uses) are assessed (monitored) as "not supported." The Class B(LW) aquatic life uses remain assessed (evaluated) as "partially supported." Fish consumption remain "not assessed." Sources of data for this assessment include (1) results of the statewide survey of Iowa lakes conducted in 2000 and 2001 by Iowa State University (ISU), (2) information from the IDNR Fisheries Bureau, and (3) information on phytoplankton communities at Iowa lakes in 2000 from Downing et al. (2002). EXPLANATION: Results of monitoring conducted by ISU in 2000 and 2001 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 in 2000 and 2001 (approximately six samples), Carlsons's (1977) trophic state indices for total phosphorus, chlorophyll-a, and secchi depth are 78, 77, and 73, respectively, for North Twin Lake. According to Carlson (1977), these index values place this lake in the range of hyper-eutrophic lakes and suggest excessive phosphorus loading to the water column, excessive production of suspended algae, and poor water transparency. These conditions indicate impairments to the Class A (primary contact) uses through presence of aesthetically objectionable blooms of algae and presence of nuisance algal species (i.e., bluegreen algae). Data from Downing et al. (2002) suggest that bluegreen algae (Cyanophyta), dominate the summertime phytoplankton community of North Twin Lake, especially in late summer). Sampling in summer 2000 showed that greater than 90% of the wet mass of the three phytoplankton samples from this lake was bluegreen algae. Based on median values from ISU sampling in 2000 and 2001, the ratio of total nitrogen to total phosphorus for this lake is 18. Data on inorganic suspended solids from the ISU survey suggest that this lake is also subject to occasional episodes of high levels of non-algal turbidity. The median level of inorganic suspended solids in the 130 lakes sampled for the ISU lake survey in 2000 and 2001 was 5.27 mg/l. The median level of inorganic suspended solids at North Twin Lake (20.0 mg/l) was the eleventh highest of the 130 lakes, thus suggesting that non-algal turbidity may limit the production of algae as well as impair beneficial uses. Based on this information, turbidity-related impacts to the primary contact and aquatic life uses at this lake will be attributed to both suspended algae and re-suspended inorganic material. The hyper-eutrophic conditions at this lake, along with information from the IDNR Fisheries Bureau, suggest that the Class B(LW) aquatic life uses are "partially supported" due to excessive nutrient loading to the water column, nuisance blooms of algae, and re-suspension of sediment. According the IDNR Fisheries Bureau, a sanitary sewer line has been installed around parts this lake, thus eliminating a the contributions of numerious septic tanks to the nutrient-related problems (e.g., algal blooms and nuisance species of algae) that have impaired the Class A and Class B uses at this lake. IDNR Fisheries feels that conditions have improved and that problems with algal blooms and bluegreen algae have improved in recent several years. Fish consumption remain "not assessed" due to the lack of recent fish contaminant monitoring at this lake.
