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 2000, and (4) results of U.S. EPA/IDNR fish tissue (RAFT) monitoring in 1994.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (evaluated) as "fully supporting / threatened" due to the presence of nuisance (=noxious) aquatic (algal) life (bluegreen algae). The Class B(LW) aquatic life uses are assessed (evaluated) as "fully supporting / threatened" due to nutrient loading to the water column. 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 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 IDNR/U.S. EPA fish tissue monitoring in 1994.
EXPLANATION: 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 66, 53, and 50, respectively. According to Carlson (1977), the index value for total phosphorus places this lake in the middle range between eutrophic and hyper-eutrophic lakes; the index values for chlorophyll-a and Secchi depth, however, place this lake in the lower range of eutrophic lakes. These index values suggest (1) moderately high levels of phosphorus, (2) 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 Downing et al. (2002) show that the zooplankton community of this lake has a moderately large population (approximately 40% dry mass) of species known as algal grazers; the median summer mass of zooplankton grazers at this lake in 2000 (18 mg/l) was the 69th highest of the 131 lakes sampled. Neither nitrogen availability nor inorganic turbidity, however, appear to limit algal production. Based on median values from ISU sampling from 2000-02, the ratio of total nitrogen to total phosphorus for Briggs Woods Lake is 187; this extremely high ratio reflects the extremely high concentration of total nitrogen at this lake: the 2000-02 summer median total nitrogen at this lake (13.4 mg/l) was the second 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 2002 was 4.8 mg/l; the median level at Briggs Woods Lake was 2.2 mg/l; this was the 13th lowest level of the 131 lakes sampled.
Data from Downing et al. (2002) suggest that bluegreen algae (Cyanophyta) dominate the summertime phytoplankton community of this lake (greater than 75% on a wet mass basis). The average mass of bluegreen algae in summer 2000 at this lake (22 mg/l) ranked 41st highest of the 131 lakes sampled. The presence of a moderately large population of bluegreen algae suggests a threat to support of the designated uses of this lake due to the potential for violating Iowa’s narrative water quality standard protecting against presence of nuisance aquatic life. Although results of plankton sampling in 2000 suggest that designated uses are threatened by nuisance aquatic (e.g., algal) life, the amount of data available for characterizing algal populations at this lake (one season) 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.
The eutrophic conditions at this lake, along with information from the IDNR Fisheries Bureau, suggest that the Class B(LW) aquatic life uses should be assessed as "fully supported / threatened" due to nutrient loading to the water column. Fish consumption uses remain assessed (evaluated) as "fully supported" based on results of the U.S. EPA/IDNR fish tissue (RAFT) monitoring in 1994 (see assessment developed for the 1996 report). 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).
