Assessment Comments
Assessment is based on results of (1) ISU statewide survey of Iowa lakes from 2000-04, (2) surveys by the IDNR Fisheries Bureau, (3) ISU reports on plankton communities at Iowa lakes from 2000-05, (4) results of TMDL monitoring conducted by DNR in 2002.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (monitored) as "fully supported/threatened (impaired)" primarily due to increasingly high algal populations and due to increasingly poor water transparency. The Class B(LW) aquatic life uses remain assessed (evaluated) as "partially supporting" due to excessive nutrient loading to the water column, high levels of algal turbidity, and siltation. Fish consumption uses are “not assessed” due to the lack of recent information upon which to base an assessment. The sources of data for this assessment include (1) results of the statewide survey of Iowa lakes sponsored by IDNR and conducted by Iowa State University (ISU) from 2000 through 2004 (with supplemental data from ISU’s 2005 survey), (2) surveys by IDNR Fisheries Bureau, (3) information on plankton communities collected at Iowa lakes from 2000 through 2005 as part of the ISU lake survey, and (4) results of TMDL monitoring conducted by Iowa DNR in 2002.
Note: A TMDL for nutrients and siltation at Lake Meyer was prepared by IDNR and approved by EPA in 2005. Because all Section 303(d) impairments identified for the 2006 assessment/listing cycle (algal growth and turbidity) are potentially addressed by the TMDL, this waterbody is placed in IR Category 4a (impaired; TMDL approved).
EXPLANATION: Results from the ISU statewide survey of Iowa lakes suggest that high levels of algal turbidity impair the Class A uses as well as the Class B(LW) uses of Lake Meyer. 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 67, 61, and 64, respectively. According to Carlson (1977), the index values for total phosphorus and Secchi depth are in the range between eutrophic and hyper-eutrophic lakes. The level of chlorophyll-a is in the lower range between eutrophic and hyper-eutrophic lakes. These index values suggest moderately high levels of phosphorus, moderately high levels of chlorophyll-a, and marginally good water transparency.
According to Carlson (1991), the occurrence of a high TSI value for total phosphorus with relatively low values for chlorophyll-a and secchi depth indicates that some factor (e.g., nitrogen limitation, zooplankton grazing, or some other factor) other than phosphorus limits production of algae. The ISU lake data suggest that algal production at Lake Meyer may be limited by zooplankton grazing; neither nitrogen limitation nor inorganic turbidity appear to limit algal production at this lake.
The presence of moderately large populations of zooplankton at Lake Meyer that graze on algae may explain most of the discrepancy between the TSI value for phosphorus (67) and that for chlorophyll-a (61). In terms of all Iowa lakes sampled, data from the ISU survey show large populations of zooplankton species at this lake that graze on algae. Sampling from 2000 through 2005 showed that Cladoceran taxa (e.g., Daphnia) comprised approximately 55% 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 (118 mg/l) was the 44th highest of the 131 lakes sampled. This population of zooplankton grazers suggests the potential for this type of non-phosphorus limitation on algal production at Lake Meyer.
Based on median values from ISU sampling from 2000 through 2002, the ratio of total nitrogen to total phosphorus for this lake is 24. This TN:TP ratio suggests that algal production at this lake is limited by phosphorus as opposed to nitrogen.
The levels of inorganic suspended solids at this lake are typical for Iowa lakes and do not suggest a strong potential for impairing designated uses. The median level of inorganic suspended solids in the 131 lakes sampled for the ISU lake survey from 2000 through 2004 was 5.2 mg/l; the median level at Lake Meyer was 4.1 mg/l; this was the 54th lowest level of the 131 lakes sampled.
The somewhat elevated TSI values for Secchi depth suggest a threat to full support of the Class A (primary contact) uses through presence of aesthetically objectionable conditions (turbidity). The results of ISU monitoring suggest that increasing levels of chlorophyll-a likely account for this turbidity. Annual summer TSI values for Secchi depth have generally been stable over the five-year period. Annual summer TSI values for chlorophyll-a, however, have generally worsened over the five-year period: 2000: 58 (or approximately 16 ug/l chlorophyll-a); 2001: 55; 2002 = 52; 2003 = 61; and 2004 = 64 (or approximately 30 ug/l chlorophyll-a). The trend in the annual TSI values for chlorophyll-a at Lake Meyer over the 2000-2004 period is approaching the impairment trigger of 65, thus suggesting the potential for impairment conditions in the near future. A review of data from the ISU survey in 2005 suggests that this worsening trend has continued at Lake Meyer: The overall TSI value for chlorophyll-a in 2005 was 83 (approximately 200 ug/l) with the sample TSI values during summer 2005 ranged from 71 to 90. This TSI value for chlorophyll-a in 2005 is solidly in the “non-support” range of TSI values. Also, the overall TSI value for secchi depth for summer 2005 was 68 (also in the "impairment" range). The worsening trends for chlorophyll-a and secchi depth at this lake, as supported by the ISU survey data from summer 2005, suggest that the Class A uses at this lake should be assessed (monitored) as “impaired” for the 2006 cycle. This lake was not sampled as part of the ISU survey in 2006.
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 (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 Meyer (16.4 mg/l) was the 83rd lowest of the 131 lakes sampled. This level is in the lowest two-thirds 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.
The eutrophic conditions at this lake, along with information from the IDNR Fisheries Bureau, suggest that the Class B(LW) aquatic life uses should remain assessed (evaluated) as "partially supported" due to excessive nutrient loading to the water column, high levels of algal turbidity, and siltation in the lake. Despite these impairments of Class B(LW) uses, results of monitoring conducted by IDNR/UHL in support of TMDL development at this lake from March to October 2002 showed no violations of Class B(LW) water quality criteria for dissolved oxygen or ammonia-nitrogen in the 10 samples collected. One of the 10 samples exceeded the Iowa water quality criterion for pH; this violation does not suggest either a threat or impairment to the designated uses of Lake Meyer. The ISU lake survey data also show generally good chemical water quality at this lake. Results of this monitoring show that only 1 of the 12 samples collected exceeded the Class B(LW) criteria for dissolved oxygen and that only 2 of the 13 samples collected exceeded Iowa’s Class A,B(LW) criteria for pH (maximum = 9.4; minimum = 8.1 pH units). Based on IDNR’s assessment methodology, these results do not suggest violation frequencies 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 Meyer.
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 1994 as part of the U.S. EPA/IDNR (RAFT) program. Although results of the 1994 RAFT monitoring at Lake Meyer 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.
