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 "partially supporting." The Class B(LW) aquatic life uses remain assessed (evaluated) as "partially supporting." Fish consumption uses remain "not assessed." The 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 and Class B(LW) aquatic life uses are only "partially supported." Using the median values from the Iowa State University statewide surveys of Iowa lakes in 2000 and 2001 (approximately six samples), Carlsons's trophic state indices for total phosphorus, chlorophyll-a, and secchi depth at Mariposa Lake are 82, 69, and 74, respectively. According to Carlson (1977), the index value for chlorophyll-a places this lake in the upper range between eutrophic and hyper-eutrophic lakes; these index values suggest excessive levels of phosphorus in the water column, less than expected production of suspended algae, and poor water transparency. These conditions suggest 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 Mariposa Lake. Based on median values from ISU sampling in 2000 and 2001, the ratio of total nitrogen to total phosphorus for this lake is 13. 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 Mariposa Lake (10.8 mg/l) was the 34th 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 siltation in the lake. Fish consumption uses remain "not assessed" due to the lack of recent fish contaminant monitoring at this lake.
