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" primarily due to presence of high levels of non-algal turbidity. The Class B(LW) aquatic life uses remain assessed (evaluated) as "fully supporting / threatened." Fish consumption uses are "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 from the ISU statewide survey of Iowa lakes suggest that high levels of algal turbidity may adversely affect the Class A and Class B(LW) uses of Browns Lake. Using the median values from this survey in 2000 and 2001 (approximately six samples), Carlson's (1977) trophic state indices for total phosphorus, chlorophyll-a, and secchi depth are 76, 61, and 72, respectively. According to Carlson (1977), the index values for total phosphorus and secchi depth place this lake in the range of hyper-eutrophic lakes; the index value for chlorophyll-a is in the lower range between eutrophic and hyper-eutrophic lakes. These index values suggest excessive levels of phosphorus in the water column, moderately low (and less than expected) levels of chlorophyll-a, and poor water transparency. 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. Of 130 lakes sampled, Browns Lake had the tenth highest median level of inorganic suspended solids (20.8 mg/l), thus suggesting that non-algal turbidity limits the production of algae as well as contributes to impairments of both the primary contact recreation uses. The high levels of total phosphorus also indicate potential impairments to the Class A (primary contact) uses through presence of aesthetically objectionable blooms of algae. Nuisance algal species (e.g., bluegreen algae) do not appear to be a problem at this lake. Data from Downing et al. (2002) suggest that bluegreen algae (Cyanophyta) are a relatively minor component of the early and mid-summer phytoplankton community of Browns Lake, although bluegreen algae showed a significant increase in the late summer sampling. Sampling in 2000 showed the percent wet mass of bluegreens ranged from less than 5% in the mid-June and mid-July samplings, with a large increase in the early August sample up to approximately 90%. Based on median values from ISU sampling in 2000 and 2001, the ratio of total nitrogen to total phosphorus for this lake is 9. This relatively low TN:TP ratio suggest the possibility of nitrogen limitation to algal production. Additional data for this lake are being generated as part of the ongoing ISU lake survey; these data will be used to improve the accuracy of future water quality assessments. The Class B(LW) aquatic life uses remain assessed as "fully supported / threatened" based on previous assessments (see above). Fish consumption uses are "not assessed" due to the lack of recent fish contaminant monitoring at this lake.
