Assessment Comments
Assessment is based on results of (1) ISU lake survey in 2000-01, (2) ISU report (2000) on lake phytoplankton communities, and (3) surveys by IDNR Fisheries Bureau.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (evaluated) as "fully supporting / threatened." The Class B(LW) aquatic life uses remain assessed (evaluated) as "fully supporting." Fish consumption uses remain "not assessed." The sources of data for this assessment include (1) results of the statewide survey of Iowa lakes conducted in 2000 and 2001 by Iowa State University (ISU), (2) information from the IDNR Fisheries Bureau, and (3) information on plankton communities at Iowa lakes in 2000 from Downing et al. (2002). EXPLANATION: Results from the ISU statewide survey of Iowa lakes suggest that high levels of phosphorus may threaten full support of the Class A uses of Arrowhead Lake. 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, 54, and 49, respectively. According to Carlson (1977), the index value for total phosphorus places this lake in the range of hyper-eutrophic lakes; the index value for chlorophyll-a, however, places this lake in the middle range of eutrophic lakes; and the TSI value for secchi depth falls in the upper range of mesotrophic lakes. These index values suggest (1) very high levels of phosphorus, (2) low, and much less than expected, production of suspended algae, and (3) exception, and much better than expected, 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 indicate that some factor (e.g., nitrogen limitation or zooplankton grazing) limits production of algae. Based on median values from ISU sampling in 2000 and 2001, the ratio of total nitrogen to total phosphorus for Arrowhead Lake is 10.5. This ratio suggest that algal production at this lake is, in part, limited by nitrogen availability. In addition, data from Downing et al. (2002) show moderately large populations of zooplankton species at Arrowhead Lake that graze on algae. Sampling in 2000 showed that Cladoceran taxa (e.g., Daphnia) comprised approximately 30% of the dry mass of the zooplankton community in the mid-July sample and approximately 35% of the early August sample. The presence of this moderately large population of zooplankton suggests the potential for zooplankton grazing to limit algal production. The levels of inorganic suspended solids at this lake are very low and do not suggest the potential for impairing designated uses due to high turbidities. 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; the median level at Arrowhead Lake was 1.3 mg/l: this median value is the fourth lowest of the 130 lakes sampled. The very high TSI value for total phosphorus suggests a threat to the full support of the Class A (primary contact) uses through the potential presence of aesthetically objectionable blooms of algae. The presence of nuisance algal species (i.e., bluegreen algae), however, does not appear to be a significant problem at this lake. Data from Downing et al. (2002) suggest that bluegreen algae (Cyanophyta) comprise a relatively small portion of the summertime phytoplankton community of this lake. The percent wet mass of bluegreen algae in 2000 ranged from between less than 15% in early summer, up to nearly 40% in midsummer, and down to less than 10% by late summer. The eutrophic conditions at this lake (based on the TSI value for chlorophyll-a), along with information from the IDNR Fisheries Bureau, suggest that the Class B(LW) aquatic life uses should be remain assessed as "fully supported." Although results of ISU lake monitoring in 2000 and 2001 suggest threats to full support of the designated beneficial uses of this lake, the amount of data available for characterizing water quality is not sufficient for developing an accurate assessment of support of these uses. Additional data for this lake are being generated as part of the ongoing ISU lake survey; these data will be used to improve the accuracy of future water quality assessments. Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring at this lake.
