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 remain assessed (monitored) as "not supporting" due to very poor water transparency caused by a combination of algal blooms and high levels of non-algal (inorganic) turbidity. Extremely large populations of bluegreen algae potentially 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 concerns with 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 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 Mariposa Lake was prepared by IDNR and approved by EPA in 2004; thus, this waterbody was placed into IR Category 4a (TMDL approved) for the 2004 assessment/listing cycle. Because all Section 303(d) impairments identified for the 2006 assessment/listing cycle (algal growth and turbidity) are addressed by the TMDL, this waterbody remains in IR Category 4a.
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 and Class B(LW) aquatic life uses are only "partially supported" due to very poor water transparency related to frequent algal blooms and chronically high levels of non-algal (inorganic) turbidity. Using the median values from the Iowa State University statewide surveys of Iowa lakes from 2000 through 2004 (approximately 15 samples), Carlson’s trophic state indices for total phosphorus, chlorophyll-a, and secchi depth at Mariposa Lake are 79, 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, very high, but somewhat less than expected, production of suspended algae, and very poor to extremely 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 due to very poor water transparency that violate Iowa’s narrative water quality criterion to protect against aesthetically objectionable conditions. This poor water transparency is related to frequent blooms of algae and to chronically high levels of non-algal (inorganic) turbidity.
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 5.2 mg/l; the median level at Mariposa Lake was 14.7 mg/l; this was the 28th 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 15. This TN:TP ratio suggests that nitrogen availability may, at times, limit algal production at this lake.
The presence of extremely large populations of zooplankton at Mariposa Lake that graze on algae may also contribute to the discrepancy between the TSI value for phosphorus (79) and that for chlorophyll-a (72). Sampling from 2000 through 2005 showed that Cladoceran taxa (e.g., Daphnia) comprised 77% of the dry mass of the zooplankton community of this lake (second highest percentage composition of the 131 lakes sampled). The average per summer sample mass of Cladoceran taxa over the 2000-2005 period (810 mg/l) was the highest level of all 131 lakes sampled. The extremely large population of zooplankton grazers at Mariposa Lake suggests the potential for limiting algal production at this lake.
The level of bluegreen algae at Mariposa Lake is one of the highest in Iowa and thus suggests an impairment of the Class A designated uses due to presence of this nuisance aquatic species. Summer sampling during the 2000-2004 period as part of the ISU lake survey showed the percent wet mass of the total phytoplankton community in bluegreens was approximately 85%. In addition, Mariposa Lake had the 6th highest median per summer sample mass of bluegreen algae of the 131 lakes sampled: 105 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 are "partially supported" due to excessive nutrient loading to the water column, nuisance blooms of algae, and siltation in the lake. Despite the range of water quality problems at this lake, results of ISU monitoring from 2000 through 2004 suggest generally good chemical water quality at Mariposa Lake. Results of this monitoring show that 2 of the 15 samples collected exceeded the Class B(LW) criteria for dissolved oxygen and that 3 of the 14 samples collected exceeded Iowa’s Class A,B(LW) criteria for pH (maximum = 9.8; minimum = 7.6 pH units). Based on IDNR’s assessment methodology, these results do not suggest violation frequencies are significantly greater than 10% for either parameter and thus do not suggest impairment of either the Class A and Class B(LW) uses of Mariposa Lake. The violations of the pH criterion likely reflect the influence of intense primary productivity at Mariposa Lake and do not reflect the input of pollutants into this lake.
Fish consumption uses remain "not assessed" due to the lack of recent fish contaminant monitoring at this lake.
