Assessment Comments
Assessment based on (1) results of IDNR beach monitoring program from 2000-02, (2) ISU lake survey from 2000-02, (3) surveys by IDNR Fisheries Bureau, (4) ISU report on lake plankton communities in 2000, and (4) EPA/DNR fish tissue (RAFT) monitoring in 1993.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (monitored) as "not supporting" due to high levels of (1) indicator bacteria and (2) non-algal turbidity. The Class B(LW) aquatic life uses remain assessed (evaluated) as "partially supporting" due to turbidity, nutrients, and siltation. Fish consumption uses are assessed (evaluated) as “fully supported” based on fish contaminant monitoring conducted in 1993. The sources of data for this assessment include (1) results of the IDNR beach monitoring program in summer seasons of 2000, 2001, and 2002, (2) results of the statewide survey of Iowa lakes conducted from 2000 through 2002 by Iowa State University (ISU), (3) information from the IDNR Fisheries Bureau, (4) information on plankton communities at Iowa lakes in 2000 from Downing et al. (2002), and (5) results of U.S. EPA / IDNR fish contaminant (RAFT) monitoring in 1993.
EXPLANATION: Results of IDNR beach monitoring at Lake Darling suggest that the Class A uses are "not supported." Levels of indicator bacteria at Lake Darling beach were monitored once per week during the primary contact recreation seasons (May through September) of 2000-02 as part of the IDNR beach monitoring program. According to U.S. EPA guidelines for determining support of primary contact recreation uses (U.S. EPA 1997b, page 3-35), the geometric mean of fecal coliform bacteria levels from at least five samples collected over a thirty-day period is compared to the water quality standard of 200 fecal organisms/100ml. If a thirty-day geometric mean exceeds 200 orgs/100 ml, the primary contact recreation uses are assessed as "not supported." In addition, the U.S. EPA guidelines state that if more than 10% of the total samples taken during any thirty-day period has a bacterial density that exceeds 400 fecal coliform organsims/100 ml, the primary contact recreation uses are assessed as "partially supported." Due to the relatively low numbers of samples collected during any thirty-day period (N=5), the use of single-sample maximum values to assess beaches is problematic. With less than 10 samples collected during any thirty-day period at Iowa beaches, the occurrence of a single level of bacteria above the single-sample maximum value will result in more than 10% violation of the single-sample maximum value and thus suggest impairment of the primary contact recreation uses. The use of less than 10 samples in an assessment based on a critical value of 10% results in large probabilities (approximately 60%) of incorrectly concluding that an impairment exists. For this reason, the single-sample maximum value is not used to assess support of primary contact recreation uses with data from the IDNR beach monitoring program. At Lake Darling beach, the geometric mean of three of the 13 thirty-day periods during the summer of 2001 exceeded 200 orgs/100 ml; during summer 2002, six of the 25 thirty-day periods exceeded this criterion. All nine geometric means from both years above 200 orgs/100ml are based on monitoring in late spring and early summer (i.e., April to early June). These violations thus likely reflecting the influence of rainfall runoff on bacterial levels. According to IDNR Section 305(b) assessment methodology, these results suggest non-support of the Class A (primary contact recreation) uses. Levels of these indicator bacteria tended to be lower in summer 2000, with a maximum thirty-day geometric mean of 82 orgs/100 ml.
Results from the ISU statewide survey of Iowa lakes suggest that the Class A uses are also impaired by high levels of non-algal turbidity. 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 84, 50, and 74, respectively. According to Carlson (1977), the index value for total phosphorus places this lake in the upper range of hyper-eutrophic lakes; the index value for chlorophyll-a is at the boundary between mesotrophic and eutrophic lakes, and the index value for secchi depth is in the range of hyper-eutrophic lakes. These index values suggest extremely high levels of phosphorus in the water column, low (and much less than expected) levels of chlorophyll-a, and very poor water transparency. According to Carlson (1991), the occurrence of a low chlorophyll-a TSI value relative to those for total phosphorus and secchi depth indicate non-algal particles or color dominate light attenuation. The ISU lake data suggest that non-algal particles do likely limit algal production at Lake Darling. 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. Of 131 lakes sampled, Lake Darling had the 16th highest median level of inorganic suspended solids (17.1 mg/l), thus suggesting that non-algal turbidity limits the production of algae and is the primary cause of the extremely poor water transparency. In addition to inorganic turbidity, other non-phosphorus limitation may exist at this lake. Based on the 2000-02 median values from ISU sampling, the ratio of total nitrogen to total phosphorus (TN:TP) at Lake Darling is 14. This ratio is somewhat low and suggests at least the potential for nitrogen availability to limit algal production at this lake. Levels of zooplankton taxa (Cladocerans) that graze on algae are very low at Lake Darling and thus do not likely have an impact on algal production. These conditions suggest an suggest impairments to the Class A (primary contact) uses primarily due to presence high levels of inorganic turbidity that violate Iowa’s narrative water quality standard protecting against aesthetically objectionable conditions. The IDNR Fisheries Bureau concurs that turbidity-related impairments exist at this lake. The level of chlorophyll-a at Lake Darling is very low (23rd lowest of the 131 lakes sampled) and does not appear to contribute to threats or impairments of designated uses at this lakes.
Populations of bluegreen algae at this lake are moderately high but do not appear to contribute to impairments of either the Class A or Class B uses. Data from Downing et al. (2002) suggest that bluegreen algae (Cyanophyta) tend to dominate the summertime phytoplankton community of Lake Darling, especially in mid to late summer. The 2000 average summer mass of bluegreen algae (10.7 mg/l) was the 72nd lowest of the 131 lakes sampled. This level, while not in the range believed to threaten or impair designated uses, is of concern. The hyper-eutrophic conditions at this lake, along with information from the IDNR Fisheries Bureau, suggest that the Class B(LW) aquatic life uses should be assessed as "partially supported" due to excessive nutrient loading to the water column, high levels of non-algal turbidity, and siltation in the lake. Fish consumption uses are assessed (evaluated) as “fully supporting” based on results of U.S. EPA/IDNR fish contaminant (RAFT) monitoring in 1993. However, because these data are now considered too old (greater than five years) to accurately characterize current water quality conditions, the assessment category is changed from "monitored" (indicating an assessment with relatively high confidence) to "evaluated" (indicating an assessment with relatively lower confidence). See the assessment developed for the 1994 report for more information.
