Assessment Comments
Assessment is based on results of (1) ISU lake survey in 2000-01, (2) surveys by IDNR Fisheries Bureau, and (3) 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 remain assessed (evaluated) as "partially supporting." Fish consumption uses are "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), (2) information from the IDNR Fisheries Bureau, and (3) information on phytoplankton communities at Iowa lakes in 2000 from Downing et al. (2002). 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 may threaten full support of the Class A uses of Lake Hendricks. 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 65, 62, and 57, respectively. According to Carlson (1977), the index values for total phosphorus and chlorophyll-a place this lake in the range between eutrophic and hyper-eutrophic lakes; the index value for secchi depth places this lake in the upper range of eutrophic lakes. These index values suggest relatively high levels of phosphorus in the water column, moderately high levels of cholophyll-a (suspended algae); and relatively good (and 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 or zooplankton grazing) other than phosphorus limits production of algae. Based on median values from ISU sampling in 2000 and 2001, the ratio of total nitrogen to total phosphorus for this lake is 66, 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. Also, Carlson (1991) suggests that a lake with a relatively high TSI for chlorophyll-a and a relatively low TSI for secchi depth may have large populations of algae such as Aphanizomenon that form flakes in the water column that continue to allow some light penetration. The levels of inorganic suspended solids at this lake are relatively low and do not suggest the potential for impairing designated uses. 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 Lake Hendricks was 2.1 mg/l. The relatively high TSI values for total phosphorus and chlorophyll-a suggest potential threats to full support of the Class A (primary contact) uses through presence of aesthetically objectionable blooms of algae and presence 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 down to approximately 80% in the mid-September sampling. 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." 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. The previous assessment of the fish consumption uses ("fully supported") was based on sampling conducted in 1994. These data are now considered too old (greater than five years) for characterizing current water quality conditions.
