Assessment Comments
Assessment is based on results of (1) ISU lake survey in 2000-01 and (2) ISU report on lake plankton communities.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (evaluated) as "fully supporting / threatened." The Class B(LW) aquatic life uses are assessed (evaluated) as "fully supporting / threatened." Fish consumption uses remain "not assessed." The sources of data for this assessment include (1) results of the statewide survey of Iowa lakes conducted in 2000 and 2001 by Iowa State University (ISU) and (2) information on phytoplankton communities at Iowa lakes in 2000 from Downing et al. (2002). EXPLANATION: Results from the ISU statewide survey of Iowa lakes suggest that high levels of phosphorus and non-algal turbidity may threaten full support of the Class A and Class B uses of Roberts Creek Lake. Using the median values from this survey in 2000 and 2001 (approximately six samples), Carlson's (1977) trophic state indices for total phosphorus, chlorophyll-a, and secchi depth are 76, 56, and 62, respectively. According to Carlson (1977), the index value for total phosphorus places this lake in the range of hyper-eutrophic lakes; the index value for chlorophyll-a is in the middle range of eutrophic lakes, and the index value to secchi depth is in the lower range of eutrophic lakes. These index values suggest (1) extremely high levels of phosphorus in the water column, (2) relatively low (and much less than expected) levels of chlorophyll-a, and (3) moderately poor 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 or zooplankton grazing) limits production of algae (based on median values from ISU sampling in 2000 and 2001, the ratio of total nitrogen to total phosphorus for Roberts Creek Lake is 14). This TN:TP ratio suggests the possibility that nitrogen limitation suppresses the production of suspended algae at this lake. Data from Downing et al. (2002), however, show relatively small populations of zooplankton species at Roberts Creek Lake, thus suggesting that algal grazing is not a major influence in the relatively low levels of chlorophyll-a at this lake. According to Carlson (1991), the occurrence of a high TSI value for secchi depth relative to that for chlorophyll-a suggests that non-algal turbidity limits algal production. The levels of inorganic suspended solids at this lake are moderately high and suggest the potential for contributing to in-lake turbidity and limiting algal production. The median level of inorganic suspended solids in the 130 lakes sampled for the ISU lake survey in 2000 and 2001 was 5.27 mg/l; the median level at Roberts Creek Lake was 6.8 mg/l. Despite the indications of good water quality, the very high TSI value for total phosphorus, and the moderately high value for secchi depth suggest threats to the full support of the Class A (primary contact) and Class B (aquatic life) uses through presence of aesthetically objectionable blooms of algae and/or non-algal turbidity. 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 (20% or less by mass weight) of the summertime phytoplankton community. Although results of ISU lake monitoring in 2000 and 2001 suggest threats to full support of the designated beneficial uses of this lake, the amount of data available for characterizing water quality is not sufficient for developing an accurate assessment of support of these uses. Additional data for this lake are being generated as part of the ongoing ISU lake survey; these data will be used to improve the accuracy of future water quality assessments. Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring at this lake.
