Assessment Comments
Assessment is based on results of (1) the 1999-2001 "Iowa Voluntary Atrazine Monitoring Program, (2) ISU lake survey from 2000-02, (3) ISU report on lake phytoplankton communities in 2000, and (4) surveys of the DNR Fisheries Bureau.
Basis for Assessment
SUMMARY: The Class B(LW) aquatic life uses remain assessed (evaluated) as "fully supported / threatened." The Class C (drinking water) uses remain assessed (monitored) as "fully supported / threatened." Fish consumption uses remain "not assessed" due to lack of fish contaminant monitoring at this lake. 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 the Iowa Voluntary Atrazine Monitoring Program conducted in 1999, 2000, and 2001 by Syngenta Crop Protection, Inc. (formerly Novartis Crop Protection, Inc.).
EXPLANATION: Using the median values from the ISU lakes survey from 2000 through 2002 (approximately nine samples), Carlson's (1977) trophic state indices for total phosphorus, chlorophyll-a, and secchi depth are 69, 62, and 57, respectively. According to Carlson (1977), the index value for total phosphorus places this lake at the upper range between eutrophic and hyper-eutrophic lakes; the index value for chlorophyll-a is in the lower range between eutrophic and hyper-eutrophic lakes, and the index value for secchi depth is in the upper range of eutrophic lakes. These index values suggest (1) moderately high levels of phosphorus, (2) somewhat elevated levels of chlorophyll-a, and (3) relatively good 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 indicate that some factor (e.g., nitrogen limitation, zooplankton grazing, or some other factor) other than phosphorus limits production of algae. Based on median values from ISU sampling from 2000 through 2002, the ratio of total nitrogen to total phosphorus for Diamond Lake is 14. This ratio does not suggest a strong possibility that algal production at this lake is nitrogen-limited. Downing et al. (2002) show relatively small populations of zooplankton at Diamond Lake as well as small populations of zooplankton (Cladocerans) known to graze on algae. Sampling in 2000 showed that the average mass of Cladocerans (4.9 mg/l) was the 34th lowest of the 131 lakes sampled. This small population of zooplankton grazers suggest little potential for this non-phosphorus limitation on algal production at this lake. 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 Diamond Lake was 1.1 mg/l. This median value is extremely low and was the second lowest of the 131 lakes sampled. The IDNR Fisheries Bureau, however, notes that water clarity problems related to algae and inorganic turbidity have lessened at Diamond Lake following completion of a watershed protection project.
The presence of nuisance algal species (i.e., bluegreen algae) does not appear to be a significant problem at this lake. Data from Downing et al. (2002) suggest that bluegreen algae (Cyanophyta) comprise a relatively small portion of the summertime phytoplankton community of this lake. The 2000 average summer mass of bluegreen algae at this lake (3.1 mg/l) was the 34th lowest of the 131 lakes sampled.
The eutrophic conditions at this lake, along with information from the IDNR Fisheries Bureau, suggest that the Class B(LW) aquatic life uses should remain assessed as "fully supported / threatened."
The assessment of support of the Class C (drinking water) uses (“fully supported / threatened”) was based on the results of the Syngenta "Iowa Voluntary Atrazine Monitoring Program" from 1999 through 2001 (this lake was not included in the Syngenta program in 2002). This monitoring showed that the time-weighted mean levels of atrazine in samples collected from the Montezuma raw water source in calendar years 1999, 2000, and 2001 were below the MCL of 3.0 ug/l. The time-weighted mean of atrazine in 1999 (N=31) was 1.9 ug/l, in 2000 (N=30) was 2.0 ug/l, and in 2001 (N=31) was 1.3 ug/l. Two the 31 samples collected in 1999, and two of the 30 samples collected in 2000, exceeded the MCL of 3.0 ug/l with a maximum value of 5.4 ug/l; none of the 31 samples collected in 2001 exceeded the MCL for atrazine (maximum value = 2.4 ug/l). None of the moving annual averages for atrazine for the years 2000 and 2001 at Diamond Lake exceeded the MCL (maximum average = 2.0 ug/l). Based on DNR's Section 305(b) assessment methodology, if the average contaminant level in source water is less than the MCL, but a level in one or more samples is above the MCL, the Class C (drinking water) uses of the source water should be assessed as "fully supported / threatened." Thus, based on the MCL violations in 1999 and 2000, the Class C uses of Diamond Lake were assessed as "fully supported / threatened." Fish consumption uses were "not assessed due to the lack of fish contaminant monitoring at this lake.
