Assessment Comments
Assessment is based on results of (1) ISU lake survey from 2000-02, (2) surveys by IDNR Fisheries Bureau, (3) ISU report on lake plankton communities in 2002, and (4) EPA/DNR fish tissue (RAFT) monitoring in 1994.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (monitored) as "partially supported" due to algal blooms; large populations of nuisance aquatic life (bluegreen algae) are also a concern at this lake. The Class B(LW) aquatic life uses remain assessed (monitored) as "partially supporting" due to continuing concerns with low levels of dissolved oxygen at this lake. Fish consumption uses remain assessed (evaluated) as “fully supported” based on results of U.S. EPA/IDNR fish contaminant monitoring in 1994. 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, (3) information on plankton communities at Iowa lakes in 2000 from Downing et al. (2002), and (4) results of U.S. EPA/IDNR fish contaminant (RAFT) monitoring in 1994. Further monitoring and investigation are needed to better characterize water quality conditions at this lake.
EXPLANATION: Results from the ISU statewide survey of Iowa lakes suggest that high levels of algal turbidity impair the Class A uses of Lake Hendricks. Using the median values from this survey from 2000 through 2002 (approximately nine samples), Carlson's (1977) trophic state indices for total phosphorus, chlorophyll-a, and secchi depth are 71, 66, and 63, respectively. According to Carlson (1977), the index value for total phosphorus is in the lower range of hyper-eutrophic lakes; the index values for chlorophyll-a and Secchi depth place this lake in the range between eutrophic and hyper-eutrophic lakes. These index values suggest very high levels of phosphorus in the water column, moderately high levels of chlorophyll-a (suspended algae); and marginally good (and somewhat better than expected) 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. Results of ISU monitoring suggest that the relatively large population of zooplankton grazers is the most significant non-phosphorus limitation at this lake. Based on median values from ISU sampling from 2000 through 2002, the ratio of total nitrogen to total phosphorus for this lake is 23, thus suggesting that phosphorus is the nutrient that limits production of suspended algae at this lake. Data from Downing et al. (2002), however, do show relatively large populations of zooplankton species at Lake Hendricks that graze on algae. Sampling in 2000 showed that Cladoceran taxa (e.g., Daphnia) comprised approximately 30% of the dry mass of the zooplankton community in the early August sample and increased to approximately 65% of the early September sample. The 2000 average summer mass of zooplankton grazers at Lake Hendricks (33 mg/l) was the 30th highest of the 131 lakes sampled, thus suggesting the potential for these zooplankters to limit algal production. The levels of inorganic suspended solids at this lake are relatively low and do not suggest the potential for either limiting algal production or for contributing to in-lake turbidity. 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 Lake Hendricks was 2.0 mg/l. The moderately high TSI value chlorophyll-a suggests a potential impairment of the Class A (primary contact) uses through presence of blooms of algae that violate the state’s narrative water quality standard protecting against aesthetically objectionable conditions. Staff of the Howard County Conservation Board report that intense algal blooms do occur at this lake.
An additional water quality problem at this lake is presence of large populations of nuisance algal species (i.e., bluegreen algae). Data from Downing et al. (2002) suggest that bluegreen algae (Cyanophyta) tend to dominate the summertime phytoplankton community of this lake, with greater than 80% of the wet mass in bluegreen algae in mid to late summer. Sampling in 2000 showed the percent wet mass of bluegreens ranged from approximately 35% in the early July sampling to approximately 85% in the late July sampling and approximately 80% in the mid-September sampling. The 2000 average summer mass of bluegreen algae at this lake (33 mg/l) was the 30th highest of the 131 lakes sampled. This level suggests a potential impairment of designated uses due to violation of the state’s narrative water quality standard protecting against nuisance aquatic life. Although results of plankton sampling in 2000 suggest impairment of designated uses, the amount of data available for characterizing algal populations at this lake (one season) is not sufficient for developing a more accurate assessment of support of these uses. 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.
The eutrophic conditions at this lake, along with information from the IDNR Fisheries Bureau regarding problems with low levels of dissolved oxygen (see assessment for the 2000 report above) suggest that the Class B(LW) aquatic life uses should remain assessed as "partially supported." Fish consumption uses remain assessed (evaluated) as "fully supported" based on results of the U.S. EPA/IDNR fish tissue (RAFT) monitoring in 1994. However, because these data are now considered too old (greater than five years) to accurately characterize current water quality conditions, the assessment category is considered "evaluated" (indicating an assessment with relatively low confidence) as opposed to "monitored" which indicates an assessment with relatively higher confidence.
