Assessment Comments
Assessment is based on (1) results of IDNR beach monitoring from 2002-04, (2) ISU lake survey from 2000-04, (3) ISU reports on lake plankton communities from 2000-05, and (4) surveys by IDNR Fisheries Bureau.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (monitored) as "not supported" due to high levels of indicator bacteria at Lake MacBride beach in 2004. This is a new impairment at this lake. Results of ISU lake water quality monitoring, however, suggest good water transparency and “full support” of the Class A uses. The Class B(LW) aquatic life uses are assessed (monitored) as "fully supported". Fish consumption uses remain "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/UHL beach monitoring from 2002-2004, (2) results of the statewide survey of Iowa lakes sponsored by IDNR and conducted by Iowa State University (ISU) from 2000 through 2004, (3) surveys by IDNR Fisheries Bureau, and (4) information on plankton communities collected at Iowa lakes from 2000 through 2005 as part of the ISU lake survey.
Note: A TMDL for nutrients and siltation at Lake MacBride was prepared by IDNR and approved by EPA in 2005. Because, however, the 2006 Section 303(d) impairment due to indicator bacteria (aka, pathogens) at this lake was not addressed in the TMDL, this lake is placed in IR Category 5a (impaired; TMDL required) for the 2006 assessment/listing cycle.
EXPLANATION: Results of IDNR beach monitoring at Lake MacBride from 2002 through 2004 suggest that the Class A uses are "not supported." Levels of indicator bacteria were monitored once per week during the primary contact recreation seasons (May through September) of 2002 (30 samples), 2003 (29 samples), and 2004 (23 samples) as part of the IDNR beach monitoring program. According to IDNR’s assessment methodology, two conditions need to be met for results of beach monitoring to indicate “full support” of the Class A (primary contact recreation) uses: (1) all five-sample, thirty-day geometric means for the three-year assessment period are less than the state’s geometric mean criterion of 126 E. coli orgs/100 ml and (2) not more than 10 % of the samples during any one recreation season exceeds the state’s single-sample maximum value of 235 E. coli orgs/100 ml. If a 5-sample, 30-day geometric mean exceeds the state criterion of 126 orgs/100 ml during the three-year assessment period, the Class A uses should be assessed as “not supported”. Also, if more than 10% of the samples in any one of the three recreation seasons exceed Iowa’s single-sample maximum value of 235 E. coli orgs/100 ml, the Class A uses should be assessed as “partially supported”. This assessment approach is based on U.S. EPA guidelines (see pgs 3-33 to 3-35of U.S. EPA 1997b).
At Lake MacBride beach, the geometric means of 6 of the 26 thirty-day periods during the summer recreation season of 2004 exceeded the Iowa water quality standard of 126 E. coli orgs/100 ml. None of the geometric means exceeded this standard during the recreational seasons of 2002 (26 geometric means) or 2003 (25 geometric means). According to IDNR’s assessment methodology and U.S. EPA guidelines, these results suggest impairment (nonsupport) of the Class A (primary contact recreation) uses.
Despite the results of IDNR beach monitoring that suggest "nonsupport" of the Class A uses, results from the ISU statewide survey of Iowa lakes suggest generally good water transparency and “full support” of the Class A uses. 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 59, 56, and 59, respectively. According to Carlson (1977), the index values for all three parameters are in the middle to upper range of eutrophic lakes. These index values suggest relatively low levels of phosphorus, relatively low (and less than expected) levels of chlorophyll-a, and relatively 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 from 2002 through 2004was 5.2 mg/l; the median level at Lake MacBride was 2.7 mg/l. Relative to other Iowa lakes, the median level of inorganic suspended solids at this lake is very low. Thus, limited potential exists for inorganic turbidity to limit algal production or to contribute 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 from 2000 through 2004, the ratio of total nitrogen to total phosphorus for Lake MacBride is 32. 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 small at Lake MacBride and thus do not likely have an impact on algal production. Sampling from 2000 through 2005 showed that Cladoceran taxa (e.g., Daphnia) comprised approximately one-third 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 (37 mg/l) was the 15th lowest of the 131 lakes sampled. These results suggest little if any non-phosphorus limitation due to zooplankton grazing at Lake MacBride.
The levels of nuisance (=noxious) algal species (i.e., bluegreen algae) at this lake do not suggest an impairment of Class A uses. While data from the ISU survey from 2000 through 2004 suggest that bluegreen algae (Cyanophyta) comprise a significant portion (over 75%) of this lake’s summertime phytoplankton community, sampling from 2000 through 2004 showed that the median per summer sample mass of bluegreen algae at Lake MacBride (12.2 mg/l) was the 67th lowest of the 131 lakes sampled. This level is in the lower half of the 131 Iowa lakes sampled. The presence of a relatively small population of bluegreen algae at this lake does not suggest 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 less than the 75th percentile of this distribution (~29 mg/l) were arbitrarily considered by IDNR staff to not represent an impairment of the Class A uses of Iowa lakes. No criteria exist, however, upon which to base a more accurate identification of impairments due to bluegreen algae. Thus, 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 remains, of necessity, "evaluated" (indicating an assessment with relatively lower confidence) as opposed to "monitored" (indicating an assessment with relatively higher confidence).
Information from the IDNR Fisheries Bureau suggests that the Class B(LW) aquatic life uses should be assessed as "fully supported". 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), however, all remain of concern regarding support of the Class B(LW) uses. Common carp often re-suspend sediments and nutrients during feeding and spawning activities, especially in shallow water.
Results of ISU monitoring from 2000 through 2004 suggest generally good chemical water quality at this lake. Results of this monitoring show that 3 of the 13 samples collected exceeded the Class B(LW) criteria for dissolved oxygen and that 2 of the 14 samples collected exceeded Iowa’s Class A,B(LW) criteria for pH (maximum = 9.2; minimum = 7.8 pH units). Based on IDNR’s assessment methodology, these results do not suggest violation frequencies that are significantly greater than 10% for either parameter and thus do not suggest impairment of either the Class A and Class B(LW) uses of Lake MacBride. The violations of the pH criterion likely reflect the influence of primary productivity and do not reflect the input of pollutants into this lake.
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. Although results of the 1991 RAFT monitoring at Lake MacBride indicated very low levels of contaminants and “full support” of the fish consumption uses, these data are now considered too old (greater than ten years) to characterize current water quality conditions.
