Assessment Comments
Assessment is based on results of (1) ISU lake survey from 2000-04, (2) surveys by IDNR Fisheries Bureau, and (3) ISU report on lake plankton communities from 2000-05.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (evaluated) as “fully supported”. The previous (2004) assessment of the Class A uses (evaluated/partially supported) due to high levels of bluegreen algae was in error: subsequent monitoring from 2001 through 2005 has shown that levels of bluegreen algae at Mill Creek Lake are reltaively low and do not suggest impairment of the Class A uses. The Class B(LW) aquatic life uses are assessed (evaluated) as "fully supporting". Moderately high levels of nutrients in the water column, algal turbidity, and siltation in the lake, however, remain water quality concerns. Fish consumption uses remain "not assessed" due to the 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.
EXPLANATION: 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 63, 58, and 58, respectively. According to Carlson (1977), the index value for total phosphorus places this lake in the lower range between eutrophic and hyper-eutrophic lakes. The index values for chlorophyll-a and Secchi depth are in the upper range of eutrophic lakes. These index values suggest relatively low levels of phosphorus in the water column, relatively low (and somewhat less than expected) levels of chlorophyll-a (suspended algae), and relatively good water transparency. A comparison of the TSI values calculated for the 2000-2002 period to the values calculated for the 2002-2004 period does suggest some improvement in water quality at this lake, both in levels of chlorophyll-a (TSI improved from 63 to 58) and in water transparency (Secchi TSI improved from 61 to 58).
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. Results of ISU monitoring suggest that both zooplankton grazing and the somewhat elevated levels of inorganic solids may account for the apparent non-phosphorus limitations at this lake.
Based on median values from ISU sampling from 2000 through 2004, the ratio of total nitrogen to total phosphorus for Mill Creek Lake is 35. This TN:TP ratio does not suggest the potential for nitrogen limitation on algal production at this lake.
The presence of very large populations of zooplankton at Mill Creek Lake that graze on algae, however, may explain the minor discrepancy between the TSI value for phosphorus (63) and that for chlorophyll-a (58). In terms of all Iowa lakes sampled, data from the ISU survey show 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 of this lake. The average per summer sample mass of Cladoceran taxa over the 2000-2005 period (158 mg/l) was the 31st highest of the 131 lakes sampled. This population of zooplankton grazers suggests the potential for this type of non-phosphorus limitation on algal production at Mill Creek Lake.
The levels of inorganic suspended solids at this lake are somewhat elevated but do not suggest the potential for impairing designated uses. These levels are sufficiently high, however, to contribute to in-lake turbidity. 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 Mill Creek Lake was 4.3 mg/l.
In terms of water transparency and algal production (including bluegreen algae), the conditions at this lake suggest relatively good water quality and “full support” of the Class A uses.
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 (66%) of this lake’s summertime phytoplankton community, sampling from 2000 through 2004 showed that the median per summer sample mass of bluegreen algae at Mill Creek Lake (8.9 mg/l) was the 48th lowest of the 131 lakes sampled. This level is in the lowest third 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). This assessment is in contrast to the 2004 assessment when limited sampling for bluegreen algae suggested large populations and a potential impairment of the Class A uses of this lake. Additional monitoring, however, demonstrated that populations of bluegreen algae are relatively low at this lake and to not likely impair the Class A uses.
Information from the IDNR Fisheries Bureau suggests that the Class B(LW) aquatic life uses should be assessed as "fully supported". Moderately high levels of nutrients in the water column, algal turbidity, and siltation, however, remain water quality concerns for support of the Class B(LW) uses at this lake. Results of chemical water quality monitoring conducted as part of the ISU lake survey suggest generally good water quality at Mill Creek Lake. These data show no violations of the Class B(LW) criteria for dissolved oxygen in the 13 samples collected during summers of 2000 through 2004. One of 15 samples exceeded the Class B(LW) criterion for pH (maximum = 9.3; minimum = 8.2 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. This violation likely reflects primary productivity at Mill Creek Lake and does not reflect the input of pollutants into this lake.
Fish consumption uses remain "not assessed" due to the lack of recent fish contaminant monitoring at this lake.
