Assessment Comments
Assessment is based on (1) results of IDNR beach monitoring from 2000-02, (2) ISU lake survey from 2000-02, (3) ISU report on lake plankton communities in 2000, and (4) surveys by IDNR Fisheries Bureau.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (monitored) as "fully supporting / threatened" due to somewhat reduced water transparency at this lake due (possibly) to suspended inorganic material. Results of IDNR beach monitoring, however, suggest “full support” of the Class A uses. The Class B(LW) aquatic life uses remain assessed (evaluated) as "fully supporting / threatened" due to excessive nutrient loading to the water column, high levels of non-algal turbidity, siltation, and the presence of exotic species (e.g., common carp). Fish consumption uses are "not assessed" due to lack of recent fish contaminant monitoring at this lake. The sources of data for this assessment include (1) results of IDNR beach monitoring from 2000 through 2001, (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, and (4) information on plankton communities at Iowa lakes in 2000 from Downing et al. (2002).
EXPLANATION: Results of IDNR beach monitoring suggest full support of the Class A uses. Levels of indicator bacteria at Lake MacBride beach were monitored once per week during the primary contact recreation seasons (April through October) 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 MacBride Beach, none of the 47 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 20 orgs/100ml in 2000, 18 orgs/100 ml in 2001, and 13 orgs/100ml in 2002. None of the 63 weekly samples collected during 2000, 2001, and 2002 exceeded the U.S. EPA's recommended single-sample maximum value of 400 orgs/100 ml (maximum sample values were 80 orgs/100 ml in 2000, 120 orgs/100 ml in 2001, and 25 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 extremely low.
Despite the results of IDNR beach monitoring that suggest "full support" of the Class A uses, results from the ISU statewide survey of Iowa lakes suggest that the somewhat reduced water transparency at Lake MacBride may threaten full support of the Class A uses. 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 60, 55, and 60, respectively. According to Carlson (1977), the index value for total phosphorus places this lake in the upper range of eutrophic and hyper-eutrophic lakes; the index value for chlorophyll-a is in the middle range of eutrophic lakes, and the index value for secchi depth is in the lower range between eutrophic and hyper-eutrophic lakes. These index values suggest relatively low levels of phosphorus, relatively low (and less than expected) levels of chlorophyll-a, and only marginally good 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 MacBride. The median level of inorganic suspended solids in the 131 lakes sampled for the ISU lake survey in 2000 and 2001 was 4.8 mg/l; the median level at Lake MacBride was 3.8 mg/l. The levels of inorganic suspended solids at this lake are somewhat elevated and thus suggest at least the potential for limiting algal production as well as contributing to in-lake turbidity. Other types of non-phosphorus limitations on algal production (e.g., nitrogen limitation and zooplankton grazing) do not appear to influence algal production at Lake MacBride. Based on median values from ISU sampling in 2000 and 2001, the ratio of total nitrogen to total phosphorus for Lake MacBride is 18. This ratio suggests that algal production at this lake is phosphorus-limited. In addition, levels of zooplankton taxa (Cladocerans) that graze on algae are very low at Lake MacBride and thus do not likely have an impact on algal production. The summer 2000 average mass of Cladocerans at this lake (1.24 mg/l) was the 14th lowest of the 131 lakes sampled.
Populations of nuisance algal species (i.e., bluegreen algae) at Lake MacBride are moderately high and suggest a potential water quality impact. Data from Downing et al. (2002) suggest that bluegreen algae (Cyanophyta) dominate the summertime phytoplankton community of this lake. Sampling in summer 2000 showed that bluegreen algae accounted for from 70 to 95% of the wet mass of the phytoplankton community of this lake. The 2000 summer average mass of bluegreen algae at this lake (13.7 mg/l) is somewhat elevated and suggests a potential threat to the full support of designated uses at this lake.
The 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 "fully supported / threatened" due to excessive nutrient loading to the water column, high levels of non-algal turbidity, siltation in the lake, and the presence of exotic species (e.g., common carp). Common carp often re-suspend sediments and nutrients during feeding and spawning activities, especially in shallow water. Fish consumption uses are "not assessed" due to the lack of recent fish contaminant monitoring at this lake. The most recent fish contaminant monitoring was conducted in 1991 as part of the U.S. EPA/IDNR (RAFT) program. These data are now considered too old (greater than ten years) to characterize current water quality conditions.
