Assessment Comments
Assessment is based on results of (1) ISU lake survey 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 are assessed (evaluated) as "fully supporting". Nutrients and siltation, however, remain concerns at this lake. 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 survey from 2000 through 2002 (approximately nine samples), Carlson's (1977) trophic state indices for total phosphorus, chlorophyll-a, and secchi depth are 66, 62, and 64, respectively. According to Carlson (1977), the index values for all three parameters are in the range between eutrophic and hyper-eutrophic lakes. These index values suggest relatively high levels of phosphorus, moderately high levels of chlorophyll-a, and marginally 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 general agreement between the TSIs for phosphorus, chlorophyll, and Secchi depth suggests that non-phosphorus limitations—which appear to occur at most Iowa lakes—do not occur at Otter Creek Lake.
Based on median values from ISU sampling from 2000 through 2004, the ratio of total nitrogen to total phosphorus for Otter Creek Lake is 19. This ratio does not suggest a strong possibility that algal production at this lake is limited by nitrogen availability.
the presence of moderately large populations of zooplankton at Otter Creek Lake that graze on algae may explain the minor discrepancy between the TSI value for phosphorus (66) and that for chlorophyll-a (62). Sampling from 2000 through 2005 showed that Cladoceran taxa (e.g., Daphnia) comprised about 45% 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 (103 mg/l) was the 57th highest of the 131 lakes sampled.
The levels of inorganic suspended solids at this lake are relatively low and do not suggest the potential for either limiting algal production or impairing designated uses. 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 Otter Creek Lake was 3.6 mg/l; this is the 40th lowest median value of the 131 lakes sampled.
Despite the moderately high levels of phosphorus, the TSI values for chlorophyll-a and secchi depth suggest relatively good water quality at Otter Creek Lake. The slight reduction in water transparency and the upward trend in annual TSI values for chlorophyll-a, however, are concerns. Additional years of monitoring data will help determine whether water transparency declines to the point that the Class A uses are impaired.
The levels of nuisance (=noxious) algal species (i.e., bluegreen algae) at this lake do not immediately 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 (70%) of this lake’s summertime phytoplankton community, sampling from 2000 through 2004 showed that the median per summer sample mass of bluegreen algae at Otter Creek Lake (22 mg/l) was the 40th highest of the 131 lakes sampled. This level is in the lowest two-thirds of the 131 Iowa lakes sampled. The presence of a moderately large population of bluegreen algae at this lake does not immediately suggest a violation of Iowa’s narrative water quality standard protecting against occurrence of nuisance aquatic life. The levels of bluegreen algae are, however, sufficiently high to warrant concern. This assessment 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).
Information from the IDNR Fisheries Bureau suggests that the Class B(LW) aquatic life uses should be assessed as "fully supported". Potential impacts from siltation and nutrient loading to the water column, however, remain concerns at this lake. The ISU lake survey data show generally good chemical water quality at this lake. Only one violation of the Class B(LW) criteria for dissolved oxygen occurred in the 14 samples collected (7%) during summers of 2000 through 2004. According to U.S. EPA guidelines (U.S. EPA 1997b, page 3-17), however, a violation frequency of less than 10 % for conventional parameters such as dissolved oxygen nonetheless suggests "full support" of aquatic life uses. Thus, the percentage of violations of the dissolved oxygen criterion at this station does not suggest an impairment of aquatic life uses at this lake. None of the 15 samples, however, violated the Class A,B(LW) criterion for pH (maximum = 8.8; minimum = 7.9 pH units).
Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring at this lake.
