Assessment Comments
Assessment is based on (1) ISU lake survey in 2000-01, (2) ISU report on lake phytoplankton communities, (3) surveys by the DNR Fisheries Bureau, (4) DNR/Parks beach monitoring in 1999, & (4) fish tissue (RAFT) monitoring in 1996.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (monitored) as "partially supporting" due to presence of aesthetically objectionable blooms of algae and presence of nuisance algal species (i.e., bluegreen algae). The Class B(LW) aquatic life uses remain assessed (evaluated) as "fully supporting / threatened." Fish consumption uses remain assessed as "fully supported." The sources of data for this assessment include (1) the results of the IDNR-UHL beach monitoring program in summers of 2000 and 2001, (2) results of the statewide survey of Iowa lakes conducted in 2000 and 2001 by Iowa State University (ISU), (3) information from the IDNR Fisheries Bureau, (4) information on phytoplankton 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 and 2001 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 30-day period is compared to the water quality standard of 200 fecal organisms/100ml. If a 30-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 30-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 30-day period (N=5), the use of single-sample maximum values to assess beaches is problematic. With less than 10 samples collected during any 30-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 12 thirty-day periods during summers of 2000 and 2001 had geometric means (N = 5 samples per period) greater than 200 orgs/100ml. The maximum 30-day geometric means were 34 orgs/100ml in 2000 and 21 orgs/100 ml in 2001. None of the 35 weekly samples collected during 2000 and 2001 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 and 170 orgs/100 ml in 2001. These results suggest (1) full support of primary contact recreation uses at this beach and (2) that levels of bacteria at this beach are 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 algal turbidity may adversely affect the Class A and Class B(LW) uses of Lake Manawa. 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 73, 64, and 79, respectively. According to Carlson (1977), the index values total phosphorus and secchi depth place this lake in the range of hyper-eutrophic lakes; the index value for chlorophyll-a is in the range between eutrophic and hyper-eutrophic lakes. These index values suggest excessive levels of phosphorus in the water column, moderately high, but somewhat less than expected, production of suspended algae, and very poor water transparency. 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. Of 130 lakes sampled, Lake Manawa had the 38th highest median level of inorganic suspended solids (9.3 mg/l), thus suggesting that non-algal turbidity contributes to impairments of both the primary contact recreation and aquatic life uses. These conditions indicate potential impairments to 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) 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. Based on median values from ISU sampling in 2000 and 2001, the ratio of total nitrogen to total phosphorus for this lake is 12.
The Class B(LW) aquatic life uses of this lake are assessed (evaluated) as "fully supported / threatened " based on information from the DNR Fisheries Bureau and based on the hyper-eutrophic conditions at this lake. Fish consumption remain "fully supported" based on results of U.S. EPA / IDNR fish tissue (RAFT) monitoring in 1996 (see results for the 2000 report).
