Assessment Comments
Assessment is based on results of (1) ISU lake surveys from 2000-04, (2) ISU reports on lake plankton communities from 2000-05, and (3) surveys by IDNR Fisheries Bureau.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (monitored) as "fully supported". The Class B(LW) aquatic life uses remain assessed (evaluated) as "fully supporting” although concerns exist 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 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 the ISU statewide survey of Iowa lakes from 2000 through 2004 (approximately 15 samples), Carlson's (1977) trophic state indices for total phosphorus, chlorophyll-a, and secchi depth are 64, 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 2004, the ratio of total nitrogen to total phosphorus for Crawford Creek Lake is 17. 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 2004 was 5.2 mg/l; the median level at Crawford Creek Lake was equal to this overall median (5.2 mg/l).
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 51% of the dry mass of the zooplankton community and that Crawford Creek Lake had the 57th lowest per summer sample level of zooplankton (Cladoceran) grazers (76.6 mg/l) of the 131 lakes sampled. This relatively small population of algal grazers likely has little affect as a non-phosphorus limitation on algal production.
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 (52%) of this lake’s summertime phytoplankton community, sampling from 2000 through 2004 showed that the median per summer sample mass of bluegreen algae at Crawford Creek Lake was the 46th lowest of the 131 lakes sampled: 8.7 mg/l. This levels 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). The previous (2004) assessment suggested a potential impairment at this lake due to data only from 2000 that suggested blooms of bluegreen algae (=noxious aquatic plants) may violate the Iowa narrative water quality standard protecting against nuisance aquatic life. The high median mass of bluegreen algae from 2000, however, was due to one sampling event with an extremely high biomass of bluegreen algae (932 mg/l). The highest biomass recorded over the four following years was an order of magnitude less (94.3 mg/l). Thus, additional years of monitoring at Crawford Creek Lake show relatively low levels of bluegreen algae and do not suggest impairments of Class A uses.
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”. Excessive nutrient loading to the water column and siltation in the lake, however, remain concerns. Results of monitoring conducted as part of the ISU lakes survey suggest good chemical water quality at this lake. The ISU lake survey data show no violations of the Class B(LW) criteria for dissolved oxygen in the 10 samples collected, or for pH in the 13 samples collected, during summers of 2000 through 2004.
Fish consumption uses remain "not assessed" due to lack of fish contaminant monitoring at this lake.
