Assessment Comments
Assessment is based on results of (1) ISU lake survey in 2000-01, (2) surveys by IDNR Fisheries Bureau, and (3) ISU report (2000) on lake plankton communities.
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 as (evaluated) as "partially 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). Further monitoring and investigation are needed to better characterize water quality conditions at this lake.
EXPLANATION: Results from the ISU statewide survey of Iowa lakes suggest that high levels of nutrients and non-algal turbidity may affect full support of the Class A and Class B(LW) 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 71, 56, and 59, respectively. According to Carlson (1977), the index value for total phosphorus places this lake in the lower range of hyper-eutrophic lakes; the index values for chlorophyll-a and secchi depth place this lake in the middle and upper range, respectively, of eutrophic lakes. These index values suggest (1) very high levels of phosphorus in the water column, (2) relatively low (and much less than expected) levels of chlorophyll-a, and (3) relatively good (and 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) other than phosphorus 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 14. This TN:TP ratio suggests the possibility that nitrogen limitation suppresses the production of suspended algae at this lake. In addition, data from Downing et al. (2002) show relatively large populations of zooplankton species at this that graze on algae. Sampling in 2000 showed that Cladoceran taxa (e.g., Daphnia) comprised approximately 90% of the dry mass of the zooplankton community in the late July sample and 60% of the early September sample. According to Carlson (1991), the occurrence of a high TSI value for secchi depth relative to that for chlorophyll-a suggests that non-algal turbidity limits algal production. The levels of inorganic suspended solids at this lake are moderately high and suggest (1) possible light limitation that suppresses growth of suspended algae and (2) a potential threat to full support of designated primary contact recreation uses due to high levels of inorganic turbidity. 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 7.8 mg/l. The high TSI values for total phosphorus suggests potential threats to the Class A (primary contact) uses through presence of aesthetically objectionable blooms of algae. In addition, data from Downing et al. (2002) suggest that the presence of nuisance algal species (i.e., bluegreen algae) may also threaten full support of the Class A uses. Sampling in 2000 showed the percent wet mass of bluegreens was nearly 100% in all three summer samplings (late June, mid-July, and early August). However, although bluegreen algae appear to dominate the summertime phytoplankton community, the production of chlorophyll is low at this lake, and the presence of bluegreen algae likely does not present a significant water quality impact.
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 as "partially supported" due primarily to impacts of siltation but also to excessive nutrient loading to the water column, and moderately high levels of non-algal turbidity. 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 a more 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 in this lake.
