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 supporting" due to algal blooms and high levels of non-algal (inorganic) turbidity. Extremely large populations of bluegreen algae suggest an additional impairment to the Class A uses at this lake. The Class B(LW) aquatic life uses remain assessed (evaluated) as "partially supporting" due to nutrient loading to the water column, nuisance blooms of algae, and siltation. 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 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 and Class B(LW) aquatic life uses are only "partially supported" due to algal blooms and high levels of non-algal (inorganic) turbidity. Using the median values from the Iowa State University statewide surveys of Iowa lakes from 2000 through 2002 (approximately nine samples), Carlson’s trophic state indices for total phosphorus, chlorophyll-a, and secchi depth at Mariposa Lake are 80, 72, and 75, respectively. According to Carlson (1977), the index values for all three parameters place this lake in the range of hyper-eutrophic lakes. These index values suggest extremely high levels of phosphorus in the water column, somewhat less than expected production of suspended algae, and very poor water transparency. The TSI values for both chlorophyll-a (72) and Secchi depth (75) suggest impairment (non-support) of the designated uses. These conditions suggest impairments to the Class A (primary contact) uses through presence of blooms of algae and high levels of non-algal (inorganic) turbidity that violate Iowa’s narrative water quality standard protecting against aesthetically objectionable conditions.
According to Carlson (1991), the occurrence of a high TSI value for total phosphorus with relatively lower values for chlorophyll-a and secchi depth indicate that some factor (e.g., nitrogen limitation, zooplankton grazing, or some other factor) other than phosphorus limits production of algae. The ISU lake data suggest that non-algal (inorganic) particles likely are the primary factor limiting algal production at Mariposa Lake. The levels of inorganic suspended solids at this lake are very high and suggest the potential for contributing to in-lake turbidity and limiting production of suspended algae. 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 at Mariposa Lake was 12.0 mg/l; this was the 34th highest level of the 131 lakes sampled. Nitrogen limitation may also limit algal production at this lake. Based on median values from ISU sampling from 2000 through 2002, the ratio of total nitrogen to total phosphorus for Mariposa Lake is 13. This TN:TP ratio suggests that nitrogen availability may limit algal production at this lake. Zooplankton grazing, however, likely does not limit algal production. Data from Downing et al. (2002) shows only moderately large populations of zooplankton species at this lake that graze on algae. The 2000 average mass of Cladoceran grazers at this lake (12.6 mg/l) was the 53rd lowest of the 131 lakes sampled.
The level of nuisance algal species (i.e., bluegreen algae) at Mariposa Lake is one of the highest in Iowa and thus suggests an impairment of designated uses. Data from Downing et al. (2002) suggest that bluegreen algae (Cyanophyta), dominate the summertime phytoplankton community of Mariposa Lake. The 2000 summer average mass of bluegreen algae at Mariposa Lake (190.3 mg/l) is the sixth highest of the 131 lakes sampled for the ISU survey, thus strongly suggesting a violation of Iowa’s narrative water quality standard protecting against nuisance aquatic life. The amount of data available for characterizing algal populations at this lake (one season), however, 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 for this lake. Based on this information, turbidity-related impacts due to algal blooms and due inorganic suspended solids suggest impairments of the primary contact and aquatic life uses at this lake.
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.
