Assessment Comments
Assessment is based on results of (1) ISU lake surveys from 2000-04, (2) surveys by IDNR Fisheries Bureau, (3) ISU reports on lake plankton communities, 2000-05, (4) results of U.S. EPA/IDNR fish tissue (RAFT) monitoring in 1994, and (5) a fish kill investigation in June 2005.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (monitored) as "fully supporting". The Class B(LW) aquatic life uses are assessed (monitored) as "partially supporting" due to a fish kill in June 2005. Nutrient loading to the water column remains a water quailty concern at this lake. Fish consumption uses are assessed (evaluated) as "fully supported" based on results of fish contaminant monitoring in 1994. 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 1994.
EXPLANATION: Using the median values from this survey from 2002 through 2004 (approximately 15 samples), Carlson's (1977) trophic state indices for total phosphorus, chlorophyll-a, and secchi depth are 71, 58, and 53, respectively. According to Carlson (1977), the index value for total phosphorus places this lake in the lower range of hyper-eutrophic lakes; the index values for chlorophyll-a and Secchi depth, however, place this lake in the range of eutrophic lakes. These index values suggest (1) very high levels of phosphorus, (2) relatively low (and less than expected) production of suspended algae, and (3) very good (and better than expected) water transparency. According to Carlson (1991), the occurrence of a high TSI value for total phosphorus with relatively low values for chlorophyll-a and secchi depth indicates 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 algal production at Briggs Woods Lake may be limited by zooplankton grazing.
Data from the ISU survey show moderately large populations of zooplankton species at this lake that graze on algae. Sampling from 2000 through 2005 showed that Cladoceran taxa (e.g., Daphnia) comprised approximately 40% of the dry mass of the zooplankton community and that Briggs Woods Lake had the 34th 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.
Neither nitrogen availability nor inorganic turbidity, however, appear to limit algal production. Based on median values from ISU sampling from 2000-04, the ratio of total nitrogen to total phosphorus for Briggs Woods Lake is 127; this extremely high ratio reflects the extremely high concentration of total nitrogen at this lake. This very high TN:TP ratio reflects the often exceptionally high levels of total nitrogen in impoundments of the Des Moines Lobe region of north-central Iowa. For example, the 2000-04 summer median total nitrogen at this lake (12.8 mg/l) was the third highest of the 131 lakes sampled. The levels of inorganic suspended solids at this lake are very low and do not suggest the potential for impairing designated uses. 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; the median level at Briggs Woods Lake was 2.2 mg/l; this was the 12th lowest level of the 131 lakes sampled.
Nuisance aquatic (algal) species (i.e., bluegreen algae) do not appear to be a problem at Briggs Woods Lake. While data from the ISU survey from 2000-04 suggest that bluegreen algae (Cyanophyta) comprise a significant portion (approximately 50%) 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 (7.5 mg/l) was the 39th lowest of the 131 lakes sampled. These conditions do not suggest any impairments due to presence of nuisance aquatic (e.g., algal) species. These results showing relatively small populations of bluegreen algae at Briggs Woods Lake stand in contrast to the assessment of bluegreen algae developed for the previous (2004) Section 305(b) reporting cycle. The 2004 assessments stated that levels of bluegreen algae at this lake in 2000 (Downing et al. 2002) suggested that bluegreen algae (Cyanophyta) comprised a relatively large portion (approximately 75%) of the summertime phytoplankton community of this lake. Also, the average mass of bluegreen algae in summer 2000 at this lake (22 mg/l) was the 41st highest of the 131 lakes sampled. The decrease in the levels of bluegreen algae from 2000, however, is demonstrated in the sample results of zooplankton monitoring for the years 2002 through 2004.
The Class B(LW) uses are assessed (monitored) as "partially supported" based on results of a fish kill investigation in June 2005. Information from the IDNR Fisheries Bureau and from the ISU statewide lake survey, however, suggests good water quality and full support of the aquatic life uses of this lake. Information from the IDNR Fisheries Bureau suggests that the Class B(LW) aquatic life uses should be assessed as "fully supported", although nutrient loading to the water column remains a concern. The ISU lake survey data show no violations of the Class B(LW) criteria for dissolved oxygen in the 14 samples collected during summers of 2000 through 2004. Three of 15 samples exceeded the Class A,B(LW) criterion for pH (maximum = 9.5; minimum = 7.9 pH units). Based on IDNR’s assessment methodology, however, these results do not suggest that significantly more than 10 percent of the samples exceed Iowa’s pH criteria and thus do not suggest an impairment of the Class A or Class B(LW) uses of this lake. These violations likely reflect the high levels of primary productivity at Briggs Woods Lake and do not reflect the input of pollutants into this lake.
Despite these indications of "full support" of the Class B(LW) uses, a fish kill occurred on June 1, 2005 and was attributed to low levels of dissolved oxygen related to the excessive growth of submergent aquatic vegetation in the lake. An estimated 1,000 bluegill of a variety of sizes were killed. Because the kill affected immature as well as adult fish, the kill was attributed to an oxygen sag rather than spawning stress. This was only a partial kill: many live fish were observed and anglers were catching fish during the IDNR investigation. According to IDNR’s assessment/listing methodology, the occurrence of a single pollutant-caused fish kill, or a fish kill of unknown origin, on a waterbody or waterbody reach during the most recent assessment period (2002-2005) indicates a severe stress to the aquatic community and suggests that the aquatic life uses should be assessed as “impaired”. If a cause of the kill is identified, and the cause is either known, or suspected, to be a “pollutant”, the assessment type is considered “monitored” and the affected waterbody is a candidate for Section 303(d) listing.
Fish consumption uses were assessed (evaluated) as “fully supported” based on results of U.S.EPA/IDNR fish contaminant (RAFT) monitoring at Briggs Woods Lake in 1994. 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 1994 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.
