Assessment Comments
Assessment is based on results of (1) ISU lake surveys from 2000-04, (2) surveys by IDNR Fisheries Bureau, and (3) ISU reports on lake plankton communities from 2000-05.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (monitored) as "fully supporting". The Class B(LW) aquatic life uses remain assessed (evaluated) as "fully supporting". Nutrient loading to the water column and turbidity remain water quality concerns at this lake. Fish consumption uses are “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: Results from the ISU statewide survey of Iowa lakes suggest “full support” of the Class A and Class B(LW) uses of Center Lake. 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 72, 59, and 58, respectively. According to Carlson (1977), the index value for total phosphorus places this lake in the range of hyper-eutrophic lakes; the index values for chlorophyll-a and secchi depth, however, are in the upper range of eutrophic lakes. These index values suggest (1) very high levels of phosphorus in the water column, (2) relatively low (and less than expected) 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. The ISU lake data suggest that algal production at Center Lake is likely limited by a combination of zooplankton grazing and light limitation due to inorganic turbidity. Based on median values from ISU sampling from 2000 through 2004, the ratio of total nitrogen to total phosphorus for Center Lake is 18. This TN:TP ratio does not suggest a strong possibility that nitrogen limits the production of suspended algae at this lake.
Data from the ISU survey show relatively 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 about 60% of the dry mass of the zooplankton community. Center Lake had the 28th 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 and account for the discrepancy between the TSI values for total phosphorus (72) and chlorophyll-a (59).
The levels of inorganic suspended solids at this lake are relatively low but may contribute somewhat to in-lake turbidity that limits production of suspended algae. 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 Center Lake was 3.7 mg/l.
The relatively low TSI values for chlorophyll-a and secchi depth suggest good water clarity and “full support” of the Class A (primary contact) uses. A comparison of annual TSI values for the 2000-2004 period at this lake suggests a potential improvement in water quality. Annual summer TSI values for Secchi depth have generally decreased (improved) as follows over the five-year period: 2000: 64; 2001: 54; 2002 = 64; 2003 = 51; and 2004 = 54; the Secchi TSI for the current (2006) assessment (58) reflects an improvement over that of the previous (2004) assessment period (62).
The levels of nuisance (=noxious) algal species (i.e., bluegreen algae) at this lake do not suggest an impairment of Class A uses at this lake. While data from the ISU survey from 2000 through 2004 suggest that bluegreen algae (Cyanophyta) comprise the majority (80%) of this lake’s summertime phytoplankton community, sampling from 2000 through 2004 showed that the average per summer sample mass of bluegreen algae at Center Lake was only the 48th highest of the 131 lakes sampled: 17.3 mg/l. This levels is in the lowest two-thirds 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 66th percentile of this distribution (~20 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).
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". Nutrient loading to the water column and moderately high levels of algal and non-algal turbidity remain concerns for support of the Class B(LW) uses of this lake. The ISU lake survey data show no violations of the Class B(LW) criteria for dissolved oxygen in the 15 samples collected during summers of 2000 through 2004. Two of 14 samples (14%), however, exceeded the Class B(LW) criterion for pH (maximum = 9.9; minimum = 8.4 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 Center Lake and do not reflect the input of pollutants into this lake.
Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring at this lake.
