Assessment Comments
Assessment is based on results of (1) ISU lake survey from 2000-02, (2) ISU report on lake plankton communities in 2000, and (3) surveys by IDNR Fisheries Bureau.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (evaluated) as "partially supported" due to blooms of bluegreen algae (=noxious aquatic plants) that potentially violate the Iowa narrative water quality standard protecting against nuisance aquatic life. The Class B(LW) aquatic life uses remain assessed (evaluated) as "fully supporting / threatened" due to potential nutrient impacts and due to siltation. 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 conducted from 2000 through 2002 by Iowa State University (ISU), (2) information from the IDNR Fisheries Bureau, and (3) information on plankton communities at Iowa lakes in 2000 from Downing et al. (2002).
EXPLANATION: Using the median values from the ISU statewide survey of Iowa lakes from 2000 through 2002 (approximately nine samples), Carlson's (1977) trophic state indices for total phosphorus, chlorophyll-a, and secchi depth are 65, 54, and 58, respectively. According to Carlson (1977), the index value for total phosphorus places this lake in the middle range between eutrophic and hyper-eutrophic lakes; the index values for chlorophyll-a and secchi depth are in the middle to upper range, respectively, of eutrophic lakes. These index values suggest moderately high levels of phosphorus, very low levels of chlorophyll-a, and relatively good water transparency. These TSI values do not suggest impairments of the Class A uses designated for Crawford Creek Lake. 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 levels of inorganic suspended solids at this lake may be sufficiently high to attenuate light and reduce algal production; neither nitrogen limitation nor zooplankton grazing appear to limit algal production. Based on median values from ISU sampling from 2000 through 2002, the ratio of total nitrogen to total phosphorus for Crawford Creek Lake is 16. This ratio indicates that algal production at this lake is not strongly nitrogen-limited. The levels of inorganic suspended solids at this lake are somewhat elevated and suggest the potential to attenuate light penetration and thus limit algal production. 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 Crawford Creek Lake was 4.7 mg/l. Data from Downing et al. (2002) show relatively small zooplankton populations at this lake, including a relatively small population of species known to be algal grazers; the median summer mass of zooplankton grazers at this lake in 2000 (6.1 mg/l) was the 40th lowest of the 131 lakes sampled. This small population of algal grazers likely has little affect as a non-phosphorus limitation on algal production.
Data from Downing et al. (2002) suggest that bluegreen algae (Cyanophyta) comprise nearly the entire summer phytoplankton community of Crawford Creek Lake. Sampling in 2000 showed that approximately 40% of the wet mass of the phytoplankton community in early and late summer were bluegreen algae but that bluegreen algae completely dominated in the mid-July sampling. The 2000 average summer mass of bluegreen algae at this lake (330 mg/l) was the third highest of the 131 lakes sampled, thus suggesting a potential impairment due to violation of Iowa’s narrative water quality standard protecting against nuisance aquatic life. The amount of data available for characterizing algal populations at this lake (one season), however, is not sufficient for developing a more accurate assessment of support of these uses. Thus, the assessment category is considered "evaluated" (indicating an assessment with relatively lower confidence) as opposed to "monitored" (indicating an assessment with relatively higher confidence). Additional data on plankton have been collected at this lake as part of the ongoing ISU lake survey and will be used to improve the accuracy of future water quality assessments for this lake.
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 (evaluated) as "fully supported / threatened" due to excessive nutrient loading to the water column and siltation in the lake. Fish consumption uses remain "not assessed" due to lack of fish contaminant monitoring at this lake.
