Assessment Comments
Assessment is based on results of (1) ISU lake survey from 2000-02, (2) surveys by IDNR Fisheries Bureau, (3) ISU report on lake plankton communities in 2002, and (4) EPA/DNR fish tissue (RAFT) monitoring in 1995..
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (monitored) as "not 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 assessed (evaluated) as “fully supported” based on fish contaminant monitoring in 1995. 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, (3) information on plankton communities at Iowa lakes in 2000 from Downing et al. (2002), and (4) results of fish contaminant monitoring in 1995 conducted as part of the U.S. EPA / IDNR fish contaminant (RAFT) monitoring program.
EXPLANATION: Results from the ISU statewide survey of Iowa lakes suggest that high levels of inorganic turbidity may adversely affect the Class A and Class B(LW) uses of Browns Lake. Using the median values from this survey from 2000 through 2002 (approximately nine samples), Carlson's (1977) trophic state indices for total phosphorus, chlorophyll-a, and secchi depth are 73, 60, and 72, respectively. According to Carlson (1977), the index values for total phosphorus and secchi depth place this lake in the lower 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 very high levels of phosphorus in the water column, relatively low (and less than expected) levels of chlorophyll-a, and very poor water transparency. According to Carlson (1991), the occurrence of a low chlorophyll-a TSI value relative to those for total phosphorus and secchi depth indicate non-algal particles or color dominate light attenuation. The ISU lake data suggest that non-algal particles do likely limit algal production at Browns Lake. 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. Of 131 lakes sampled, Browns Lake had the 14th highest median level of inorganic suspended solids (18.5 mg/l), thus suggesting that non-algal turbidity limits the production of algae. In addition, the high levels of inorganic suspended solids are likely the primary cause of the very poor water transparency at this lake: the TSI for Secchi depth for the 2000-02 period was 72; this is the 23rd poorest Secchi TSI of the 131 lakes sampled. Although levels of ISS have declined somewhat from extreme levels in 2000, the levels in the three samplings from 2002 were 9, 15, and 19 mg/l. All three of these samples had ISS levels far above the overall 2000-02 median of 4.8 mg/l and suggest that impacts due to inorganic turbidity remain. These conditions suggest an suggest impairments to the Class A (primary contact) uses primarily due to presence high levels of inorganic turbidity that violate Iowa’s narrative water quality standard protecting against aesthetically objectionable conditions. The IDNR Fisheries Bureau concurs that turbidity-related impairments have existed at this lake but that water transparency is improving.
Other factors may limit algal production at Browns Lake. Based on median values from ISU sampling from 2000-02, the ratio of total nitrogen to total phosphorus for this lake is 11; this ratio suggests the possibility that algal production at this lake is limited by nitrogen availability. Data from Downing et al. (2002) show relatively low zooplankton populations at this lake; the median 2000 summer mass of zooplankton (Cladoceran) grazers at Browns Lake (3.5 mg/l) was the 26th lowest of the 131 lakes sampled. The high level of inorganic suspended solids at this lake, however, appears to be the primary non-phosphorus limitation to algal production at this lake.
Nuisance algal species (e.g., bluegreen algae) do not appear to be a significant 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%. The average mass of bluegreen algae in summer 2000 at this lake (11.3 mg/l) was the 56th highest of the 131 lakes sampled. Although somewhat elevated, the mass of bluegreen algae at Brown’s Lake do not appear to either threaten or impair designated uses. 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. Thus, the assessment category is considered "evaluated" (indicating an assessment with relatively lower confidence) as opposed to "monitored" (indicating an assessment with relatively higher confidence). 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.
The Class B(LW) aquatic life uses remain assessed as "fully supported / threatened" based on previous assessments. Fish consumption uses are assessed as “fully supported” based on results of fish contaminant monitoring in 1995 conducted as part of the U.S. EPA / IDNR fish contaminant (RAFT) monitoring program. However, because these data are now considered too old (greater than five years) to accurately characterize current water quality conditions, the assessment category is changed from "monitored" (indicating an assessment with relatively high confidence) to "evaluated" (indicating an assessment with relatively low confidence).
