Assessment Comments
Assessment is based on results of (1) ISU lake survey from 2000-04, (2) surveys by IDNR Fisheries Bureau, (3) ISU reports on lake plankton communities, 2000-05, 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 "partially supporting" primarily due to presence of high levels of non-algal turbidity that reduce water clarity. Monitoring data, however, suggest an improving trend in water clarity at this lake. The Class B(LW) aquatic life uses remain assessed (evaluated) as "fully supporting". Fish consumption uses remain 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 sponsored by IDNR and conducted by Iowa State University (ISU) from 2000 through 2004, (2) surveys by IDNR Fisheries Bureau, (3) information on plankton communities collected at Iowa lakes from 2000 through 2005 as part of the ISU lake survey, and (4) results of IDNR/U.S. EPA fish tissue monitoring in 1995.
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 2004 (approximately 15 samples), Carlson's (1977) trophic state indices for total phosphorus, chlorophyll-a, and secchi depth are 66, 58, and 66, respectively. According to Carlson (1977), the index values for total phosphorus and Secchi depth place this lake in the range between eutrophic and hyper-eutrophic lakes; the index value for chlorophyll-a is in the upper range of eutrophic lakes. These index values suggest high levels of phosphorus in the water column, relatively low (and less than expected) levels of chlorophyll-a, and somewhat poor water transparency. Although the overall TSI value for Secchi depth (66) continues to suggest impairment at this lake due to poor water transparency, a comparison of annual TSI values for the 2000-2004 period at this lake suggests improvement in water quality. Annual summer TSI values for Secchi depth have decreased (improved) as follows over the five-year period: 2000: 73; 2001: 68; 2002 = 68; 2003 = 62; and 2004 = 61; similar improving trends are evident for total phosphorus and chlorophyll-a as well. Annual TSI values for both Secchi depth and chlorophyll-a were well below the impairment trigger of 65 during both 2003 and 2004. If this improving trend continues, this lake would likely not show a turbidity-related impairment for the next (2008) 305(b) assessment cycle.
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 2004 was 5.2 mg/l. Of 131 lakes sampled, Browns Lake had the 20th highest median level of inorganic suspended solids (13.7 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 somewhat poor water transparency at this lake Although levels of ISS have declined somewhat from extreme levels in 2000 and 2001, levels at this lake remain elevated relative to other Iowa lakes. These conditions suggest an suggest impairments to the Class A (primary contact) uses primarily due to presence high levels of inorganic turbidity and poor water clarity 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 2000-04, the ratio of total nitrogen to total phosphorus for this lake is 16; this ratio suggests the possibility that algal production at this lake is limited at times by nitrogen availability.
Data from the ISU survey show moderately large populations of zooplankton species at this lake that graze on algae. Although sampling from 2000 through 2005 showed that Cladoceran taxa (e.g., Daphnia) comprised only about 35% of the dry mass of the zooplankton community, Browns Lake had the 61st highest per summer sample mass of zooplankton (Cladoceran) grazers of the 131 lakes sampled. This level of zooplankton grazers may be able to suppress algal production. 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 aquatic (algal) species (i.e., bluegreen algae) do not appear to be a problem at Browns Lake. While data from the ISU survey from 2000-04 suggest that bluegreen algae (Cyanophyta) comprise a significant portion (approximately 55%) of the summertime phytoplankton community of this lake, the average per sample mass (biovolume) of bluegreen algae in summers of 2000 through 2004 at this lake (10.0 mg/l) was the 53rd lowest of the 131 lakes sampled. These conditions do not suggest any impairments due to presence of nuisance aquatic (e.g., algal) species.
The Class B(LW) aquatic life uses remain assessed (evaluated) as "fully supported" based on previous assessments. The ISU lake survey data also show good chemical water quality at this lake. Results of this monitoring show no violations of the Class A,B(LW) criteria for pH in the 15 samples collected during summers of 2000 through 2004 (maximum = 8.7; minimum = 8.1 pH units). One of 12 samples exceeded the Class B(LW) criterion for dissolved oxygen. Based on IDNR’s assessment methodology, however, these results do not suggest that significantly more than 10 percent of the samples exceed Iowa’s dissolved oxygen criteria and thus do not suggest an impairment of the Class B(LW) uses of Browns Lake.
Fish consumption uses were assessed (evaluated) as “fully supported” based on results of U.S.EPA/IDNR fish contaminant (RAFT) monitoring at Browns Lake in 1995. Because these data are now considered too old (greater than five years) to accurately characterize current water quality conditions, the assessment category is considered “evaluated” (indicating an assessment with relatively lower confidence) as opposed to "monitored" (indicating an assessment with relatively higher confidence). The existence of, or potential for, a fish consumption advisory is the basis for Section 305(b) assessments of support of fish consumption uses in Iowa’s rivers and lakes. Prior to 2006, IDNR used action levels published by the U.S Food and Drug Administration to determine whether consumption advisories should be issued for fish caught as part of recreational fishing in Iowa. In an effort to make Iowa’s consumption more compatible with the various protocols used by adjacent states, the Iowa Department of Public Health, in cooperation with Iowa DNR, developed a risk-based advisory protocol. This protocol went into effect in January 2006 (see http://www.iowadnr.gov/fish/news/consump.html for more information on Iowa’s revised fish consumption advisory protocol). Because the revised (2006) protocol is more restrictive than the previous protocol based on FDA action levels; fish contaminant data that previously suggested “full support” may now suggest either a threat to, or impairment of, fish consumption uses. This scenario, however, does not apply to the fish contaminant data generated from the 1995 RAFT sampling conducted at this lake: levels of all contaminants from this monitoring were below advisory trigger levels, thus suggesting the continued “full support” of fish consumption uses.
