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 "partially supported" due to (1) algal blooms and (2) frequent violations of Iowa's pH criteria. Large populations of nuisance aquatic life (bluegreen algae) are also a concern at this lake. The Class B(LW) aquatic life uses are assessed (monitored) as "partially supporting" due to continuing concerns with low levels of dissolved oxygen at this lake but primarily due to unusually high levels of pH and frequent violations of Iowa's pH criteria. Fish consumption are “not assessed” due to the lack of recent information upon which to base an assessment. 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. 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 2004 (approximately 15 samples), Carlson's (1977) trophic state indices for total phosphorus, chlorophyll-a, and secchi depth are 71, 65, 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 do not suggest an obvious non-phosphorus limitation to algal production at this lake: neither nitrogen limitation, zooplankton grazing of algae, nor high levels of inorganic suspended solids would appear to limit production of algae 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 41, thus suggesting that phosphorus is the nutrient that limits production of suspended algae at this lake.
In terms of all Iowa lakes sampled, data from the ISU survey show relatively small populations of zooplankton species at this lake that graze on algae. Sampling from 2000 through 2005 showed that Cladoceran taxa (e.g., Daphnia) comprised about 30% 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 (49 mg/l) was the 30th lowest of the 131 lakes sampled. These results suggest little if any non-phosphorus limitation due to zooplankton grazing at Lake Hendricks that would explain the discrepancy between the TSI for total phosphorus (71) and that for chlorophyll-a (65).
The levels of inorganic suspended solids at this lake are very 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 2004 was 5.2 mg/l; the median level at Lake Hendricks was 2.4 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 sometimes occur at this lake. Thus, the Class A uses are assessed (monitored) as “partially supported”.
Data from the ISU survey from 2002-2004 suggest that the presence of nuisance aquatic species (i.e., bluegreen algae) may also present an impairment of the Class A uses at this Lake Hendricks. Summer sampling during this period showed the percent wet mass of the total phytoplankton community in bluegreens was approximately 75%. In addition, Lake Hendricks had the 22nd highest median per summer sample mass of bluegreen algae (64 mg/l) of the 131 lakes sampled. This median is in the worst 10 % of the 131 Iowa lakes sampled. The presence of a large population of bluegreen algae at this lake suggests 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 greater than the 90th percentile of this distribution (~59 mg/l) were arbitrarily chosen by IDNR staff to represent the condition of “potential impairment; not supported.” No criteria exist, however, upon which to base a more accurate identification of impairments due to bluegreen algae. Thus, while the , 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, of necessity, remains "evaluated" (indicating an assessment with relatively lower confidence) as opposed to "monitored" (indicating an assessment with relatively higher confidence).
The eutrophic conditions at this lake, along with updated (2006) 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." The ISU lake survey data also suggest impairment of the Class B(LW) uses at Lake Hendricks, primarily due to very high levels of pH. Only 1 violation of the Class B(LW) criteria for dissolved oxygen occurred in the 14 samples collected during summers of 2000 through 2004. Based on IDNR’s assessment methodology, the occurrence of this single violation does not suggest an impairment of aquatic life uses at this lake. Eleven of the 15 samples collected, however, violated the Class A,B(LW) criterion for pH (maximum = 10.4; minimum = 8.1 pH units). Based on IDNR’s assessment methodology, these results suggest that significantly more than 10% of the samples exceed Iowa’s pH criteria and thus suggest an impairment of the Class A and Class B(LW) uses of this lake. While levels of pH in Iowa lakes occasionally exceed the upper Class B(LW) pH criterion of 9.0 units due to expected shifts in the carbonate equilibrium as primary productivity utilizes the available carbon dioxide, the frequency and magnitude of pH violations at this lake suggest special circumstances that cannot be readily explained given that levels of primary productivity (as suggested by the levels of chlorophyll-a) are only moderately high at Lake Hendricks. The seven highest violations were as follows: 9.4, 9.4, 9.6, 9.7, 9.8, 10.0, and 10.4 pH units. All of these seven values are in the upper 95% of pH values from the 1,963 samples analyzed for pH as part of the ISU lake survey from 2000 through 2004.
Fish consumption uses are “not assessed” due to the lack of recent information upon which to base an assessment. The most recent fish contaminant monitoring at this lake was conducted in 1994 as part of the U.S. EPA/IDNR fish tissue (RAFT) monitoring program. Although results of the 1994 RAFT monitoring indicated very low levels of contaminants and “full support” of the fish consumption uses, these data are now considered too old (greater than 10 years) to characterize current water quality conditions
