Assessment Comments
Assessment is based on (1) ISU lake survey from 2000-02, (2) ISU report on lake plankton communities in 2000, (3) surveys by the DNR Fisheries Bureau, (4) DNR beach monitoring from 2000-02, and (4) EPA/DNR fish tissue (RAFT) monitoring in 1996.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (monitored) as "not supporting" due to presence of aesthetically objectionable conditions caused by high levels of suspended inorganic materials in the water column and blooms of algae. An additional concern is the presence of moderately large populations of nuisance algal species (i.e., bluegreen algae). Results of IDNR beach monitoring, however, indicate full support of the Class A uses. The Class B(LW) aquatic life uses remain assessed (evaluated) as "fully supporting / threatened" due to algal blooms. Fish consumption uses remain assessed (evaluated) as "fully supported" based on results of fish contaminant monitoring in 1996. The sources of data for this assessment include (1) the results of the IDNR-UHL beach monitoring program in summers 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 tissue monitoring in 1996.
EXPLANATION: Results of IDNR beach monitoring suggest "full support" of the Class A uses. Levels of indicator bacteria at Lake Manawa beach were monitored once per week during the primary contact recreation seasons (May through September) of 2000, 2001, and 2002 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 level 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 Manawa beach, none of the 37 thirty-day periods during summers of 2000, 2001, and 2002 had geometric means (N = 5 samples per period) greater than 200 orgs/100ml. The maximum thirty-day geometric means were 34 orgs/100ml in 2000, 21 orgs/100 ml in 2001, and 50 orgs/100ml in 2002. None of the 61 weekly samples collected from 2000-02 exceeded the U.S. EPA's recommended single-sample maximum value of 400 orgs/100 ml. The single sample maximum values were very low: 160 orgs/100 ml in 2000, 170 orgs/100 ml in 2001, and 120 orgs/100ml in 2002. These results suggest (1) full support of primary contact recreation uses at this beach and (2) that levels of bacteria at this beach are very low.
Although results of IDNR beach monitoring suggest "full support" of the Class A uses, results from the ISU statewide survey of Iowa lakes suggest that high levels of turbidity related primarily high levels of inorganic suspended solids and secondarily to algal blooms may adversely affect the Class A and Class B(LW) uses of Lake Manawa. 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, 65, and 73, respectively. According to Carlson (1977), the index value for total phosphorus is in the lower range of hyper-eutrophic lakes, and the value for chlorophyll-a is in the middle range between eutrophic and hyper-eutrophic lakes; the index value for Secchi depth is in the lower range of hyper-eutrophic lakes. These index values suggest very high levels of phosphorus in the water column, moderately high production of suspended algae, and very poor water transparency. The results of ISU monitoring suggest that the very high levels of inorganic suspended solids and the somewhat elevated levels of chlorophyll likely combine to reduce water transparency and impair designated uses at this lake. 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 Manawa had the 24th highest median level of inorganic suspended solids (12.0 mg/l), thus suggesting that non-algal turbidity limits algal production as well as contributes to reductions in water transparency that impair the primary contact recreation uses. These conditions indicate potential impairments to the Class A (primary contact) uses through presence of aesthetically objectionable conditions caused primarily by very high levels of inorganic suspended solids and secondarily by blooms of algae. The IDNR Fisheries Bureau concurs that this lake is impaired by algal blooms. Algal production at this lake appears to be limited by availability of phosphorus. Based on median values from ISU sampling from 2000 through 2002, the ratio of total nitrogen to total phosphorus for this lake is 18; this ratio does not suggest a significant potential for nitrogen limitation. In addition, data from Downing et al. (2002) show moderately large populations of zooplankton species at Lake Iowa that graze on algae. Sampling in 2000 showed that Cladoceran taxa (e.g., Daphnia) comprised approximately 25% of the dry mass of the zooplankton community in both the late July and early August samples. The 2000 average summer mass of Cladoceran grazers at Lake Manawa (20.6 mg/l) was the 60th highest of the 131 lakes sampled. This moderately large population of zooplankton grazers may limit algal production at this lake.
The presence of nuisance algal species (i.e., bluegreen algae) presents an additional threat to full support of designated uses at this lake. Data from Downing et al. (2002) suggest that bluegreen algae (Cyanophyta) comprise a significant portion of the summertime phytoplankton community of Lake Manawa, especially in the mid and late summer. Sampling in 2000 showed the percent wet mass of bluegreens ranged from approximately 10% in the late June sampling, to approximately 90% in the late July sampling, and approximately 65% in the early August sampling. The 2000 average summer mass of bluegreen algae at this lake (18.2 mg/l) was the 46th highest of the 131 lakes sampled; this level suggests a potential threat to full support of the Class A uses.
Based on information from the DNR Fisheries Bureau and based on the results of ISU monitoring, the Class B(LW) aquatic life uses of this lake are assessed (evaluated) as "fully supported / threatened.” Fish consumption remain assessed (evaluated) as "fully supported" based on results of U.S. EPA / IDNR fish tissue (RAFT) monitoring in 1996 (see results for the 2000 report). 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 low confidence).
