Assessment Comments
Assessment is based on results of (1) ISU lake survey from 2000-02, (2) surveys by IDNR Fisheries Bureau, (3) ISU report on lake plankton communities in 2000, and (4) results of EPA/DNR fish tissue monitoring in 2002.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation uses) are assessed (monitored) as "not supported" due to aesthetically objectionable conditions related to algal blooms and high levels of inorganic turbidity. An additional impairment of the primary contact uses is suggested by the very large populations of bluegreen algae (noxious aquatic plant life) at this lake. The Class B(LW) aquatic life uses are assessed (evaluated) as "fully supported / threatened" due to nutrient loading to water column, infrequent (and decreasingly frequent) algal blooms, and occasional episodes of high inorganic turbidity. Fish consumption are assessed as “fully supported” based on results of fish contaminant monitoring in 2002. Sources of data for this assessment include (1) results of the statewide survey of Iowa lakes conducted from 2000 through 2002 by Iowa State University (ISU), (2) information from the IDNR Fisheries Bureau, (3) information on plankton communities at Iowa lakes in 2000 from Downing et al. (2002), and (4) results of U.S. EPA/IDNR fish contaminant (RAFT) monitoring in 2002. Information from the IDNR Fisheries Bureau and results from the ISU statewide survey of Iowa lakes suggest that water quality conditions at North Twin Lake appear to be improving. Additional years of data may show this lake to fully support the designated Class A and/or Class B(LW) uses.
EXPLANATION: Results of monitoring conducted by ISU from 2000 through 2002 as part of the statewide survey of Iowa lakes suggest that the Class A (primary contact) uses are "not supported." 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 72, 74, and 71, respectively, for North Twin Lake. According to Carlson (1977), these index values place this lake in the range of hyper-eutrophic lakes and suggest very high levels of phosphorus in the water column, very high production of suspended algae, and very poor water transparency. These conditions indicate impairments to the Class A (primary contact) uses through presence of aesthetically objectionable conditions related to blooms of algae and to high levels of inorganic turbidity that lead to poor water transparency. Results of ISU monitoring show that algal production at this lake is not strongly limited by nitrogen availability. Based on median values from ISU sampling from 2000 through 2002, the ratio of total nitrogen to total phosphorus for this lake is 20. This TN:TP ratio does not suggest any strong nitrogen limitation to algal production at this lake. Data on inorganic suspended solids from the ISU survey suggest that this lake is also subject to occasional episodes of high levels of non-algal turbidity. 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. The median level of inorganic suspended solids at North Twin Lake (9.5 mg/l) was the eleventh highest of the 131 lakes, thus suggesting that non-algal turbidity may limit the production of algae as well as impair beneficial uses. Data from Downing et al. (2002) show relatively small populations of zooplankton species at North Twin Lake that graze on algae (Cladocerans). The 2000 average summer mass of Cladocerans at this lake (4.0 mg/l) was the 31st lowest of the 131 lakes sampled. Information from the IDNR Fisheries Bureau suggests that water quality conditions at North Twin Lake have improved in recent years. The data for inorganic suspended solids support this conclusion, with extremely high levels reported in summer 2000 and much lower levels reported in summers of 2001 and 2002. Based on this information, turbidity-related impacts to the primary contact and aquatic life uses at this lake will be attributed to both large populations of suspended algae and very high levels of inorganic suspended solids that violate Iowa’s narrative water quality standard protecting against aesthetically objectionable conditions.
Based on results from the ISU plankton studies in 2000, the presence of nuisance algal species (i.e., bluegreen algae) would appear to be a significant water quality problem at this lake. Data from Downing et al. (2002) suggest that bluegreen algae (Cyanophyta), dominated the summertime phytoplankton community of North Twin Lake in 2000, especially in late summer. Greater than 90% of the wet mass of the three phytoplankton samples from this lake was bluegreen algae. The summer 2000 average mass of bluegreen algae (202 mg/l) was the fourth highest of the 131 lakes sampled and thus suggests an impairment of the Class A uses due to the extremely high levels of this nuisance algae at this lake and the apparent violation of Iowa’s narrative water quality standard protecting against presence of nuisance aquatic life. The amount of data available for characterizing algal populations at this lake (one season), however, is not sufficient for developing a more accurate assessment of support of these uses. Additional data on plankton have been collected at this lake as part of the ongoing ISU lake survey and will be used to update this assessment and improve the accuracy of future water quality assessments for this lake.
The improving water quality conditions at this lake, along with information from the IDNR Fisheries Bureau, suggest that the assessment of the Class B(LW) aquatic life uses should be changed from “partially supported” to “fully supported / threatened.” This change in assessment is justified by slight reductions in nutrient loading to the water column, fewer nuisance blooms of algae, and lowered levels of suspended sediment. Thus, IDNR Fisheries feels that problems with algal blooms and bluegreen algae have lessened in recent several years. Some of this improvement is attributed to a sanitary sewer line installed around parts this lake, thus eliminating the contributions of numerous septic tanks to the nutrient-related problems (e.g., algal blooms and nuisance species of algae) that have impaired the Class A and Class B uses at this lake in the past.
Fish consumption uses are assessed as “fully supported” based on results of U.S. EPA/IDNR fish contaminant (RAFT) monitoring in 2002. The composite samples of fillets from channel catfish and white crappie had very low levels of contaminants. Neither sample contained levels of contaminants that approached even one-half the respective FDA ALs or IDNR levels of concern. Levels of primary contaminants in the composite sample of channel catfish fillets were as follows: mercury: <0.0181 ppm; total PCBs: <0.09 ppm; and technical chlordane: <0.03 ppm. Levels of primary contaminants in the composite sample of white crappie fillets were as follows: mercury: <0.0181 ppm; total PCBs: <0.09 ppm; and technical chlordane: <0.03 ppm.
