Assessment Comments
Assessment is based on results of (1) the Iowa Voluntary Atrazine Monitoring Program from 2003-04, (2) ISU lake surveys from 2000-04, (3) ISU reports on lake phytoplankton communities from 2000-05, and (4) surveys of the DNR Fisheries Bureau.
Basis for Assessment
SUMMARY: The Class B(LW) aquatic life uses are assessed (monitored) as "fully supported." The Class C (drinking water) uses are assessed (monitored) as "fully supported." 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 sponsored by IDNR and conducted by Iowa State University (ISU) from 2000 through 2004, (2) surveys by IDNR Fisheries Bureau, and (3) information on plankton communities collected at Iowa lakes from 2000 through 2005 as part of the ISU lake survey, and (4) results of the Iowa Voluntary Atrazine Monitoring Program conducted in 2003 and 2004 by Syngenta Crop Protection, Inc. (formerly Novartis Crop Protection, Inc.).
EXPLANATION: Using the median values from the ISU lakes survey from 2000 through 2004 (approximately 15 samples), Carlson's (1977) trophic state indices for total phosphorus, chlorophyll-a, and secchi depth are 69, 62, and 60, 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 at the upper boundary 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 2004, the ratio of total nitrogen to total phosphorus for Diamond Lake is 15. This ratio does not suggest a strong possibility that algal production at this lake is nitrogen-limited.
Data from the ISU survey show relatively small populations of zooplankton species at this that graze on algae. Sampling from 2000 through 2005 showed that Cladoceran taxa (e.g., Daphnia) comprised about 45% of the dry mass of the zooplankton community and that Diamond Lake had the 35th lowest median summer sample level of zooplankton (Cladoceran) grazers (55.6 mg/l) of the 131 lakes sampled. This relatively small population of zooplankton grazers suggests little potential zooplankton grazers to limit 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 2004 was 5.2 mg/l; the median level at Diamond Lake was 2.7 mg/l. This median value is very low for Iowa lakes and was the 22nd lowest of the 131 lakes sampled. According to the IDNR Fisheries Bureau, water clarity problems related to algae and inorganic turbidity have lessened at Diamond Lake following completion of a watershed protection project.
The levels of nuisance (=noxious) algal species (i.e., bluegreen algae) at this lake do not suggest an impairment of Class A uses. While data from the ISU survey from 2000 through 2004 suggest that bluegreen algae (Cyanophyta) comprise a significant portion (nearly 60%) of this lake’s summertime phytoplankton community, sampling from 2000 through 2004 showed that the median per summer sample mass of bluegreen algae at Diamond Lake was the 32nd lowest of the 131 lakes sampled: 6.0 mg/l. This level is in the lowest 25% of the 131 Iowa lakes sampled. The presence of a relatively small population of bluegreen algae at this lake does not suggest a potential violation of Iowa’s narrative water quality standard protecting against occurrence of nuisance aquatic life. This assessment, however, is based strictly on a distribution of the lake-specific median bluegreen algae values for the 2000-2004 monitoring period. Median levels less than the 75th percentile of this distribution (~29 mg/l) were arbitrarily considered by IDNR staff to not represent an impairment of the Class A uses of Iowa lakes. No criteria exist, however, upon which to base a more accurate identification of impairments due to bluegreen algae. Thus, while the ability to characterize the levels of bluegreen algae at this lake has improved over that of the previous (2004) assessment due to collection of additional data, the assessment category for assessments based on level of bluegreen algae nonetheless remains, of necessity, "evaluated" (indicating an assessment with relatively lower confidence) as opposed to "monitored" (indicating an assessment with relatively higher confidence).
Information from the IDNR Fisheries Bureau suggest that the Class B(LW) aquatic life uses should be assessed (evaluated) as "fully supported". The ISU lake survey data show generally good chemical water quality at Diamond Lake. Only one violation of the Class B(LW) criteria for dissolved oxygen occurred in the 15 samples collected during summers of 2000 through 2004. Two of 13 samples (15%), however, violated the Class A,B(LW) criterion for pH (maximum = 9.1; minimum = 8.1 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 and Class B(LW) uses of this lake. These violations likely reflect the high levels of primary productivity at Diamond Lake and do not reflect the input of pollutants into this lake.
The assessment of support of the Class C (drinking water) uses (“fully supported”) was based primarily on the results of the Syngenta "Iowa Voluntary Atrazine Monitoring Program" from 2003 through 2004. This monitoring showed that the time-weighted mean levels of atrazine in samples collected from the Montezuma raw water source in calendar years 2003 and 2004 were well-below the MCL of 3.0 ug/l. The mean and median atrazine level over this two-year period (N=60) was 0.8 ug/l. The maximum value for this two-year period was 1.3 ug/l. None of the 27 moving annual averages for atrazine for the years 2003 and 2004 at Diamond Lake exceeded the MCL (maximum average = 0.8 ug/l). Based on DNR's Section 305(b) assessment methodology, if the average contaminant levels in source water is less than the MCL, the Class C (drinking water) uses of the source water should be assessed as "fully supported." The only parameter collected as part of the ISU lake survey relevant to support of Class C (drinking water) uses is nitrate. The results of the ISU survey from 2000-04 show that nitrate levels are very low at this lake (N = 15; maximum value = 1.7 mg/l; median = 0.1 mg/l) and are well-below the MCL of 10 mg/l.
Fish consumption uses were "not assessed due to the lack of fish contaminant monitoring at this lake.
