Assessment Comments
Assessment is based on results of (1) ISU lake survey in 2000-02, (2) surveys by IDNR Fisheries Bureau, and (3) ISU report (2000) on lake plankton communities.
Basis for Assessment
For the 2004 report: SUMMARY: The Class A (primary contact recreation) uses are assessed (evaluated) as "partially supported." 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 through 2002 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 nutrients, non-algal turbidity, and presence of nuisance aquatic organisms (bluegreen algae) may affect full support of the Class A and Class B(LW) uses of Arrowhead Pond. Using the median values from this survey from 2000-02 (approximately nine samples), Carlson's (1977) trophic state indices for total phosphorus, chlorophyll-a, and secchi depth are 73, 58, and 54, 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 upper and middle 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. The TSI values for chlorophyll-a and Secchi depth do not suggest impaired conditions. According to Carlson (1991), the occurrence of a high TSI value for total phosphorus with relatively low and similar values for chlorophyll-a and secchi depth indicate that some factor (e.g., nitrogen limitation, zooplankton grazing, or some other factor) other than phosphorus limits production of suspended algae. Results from the ISU survey suggest the following non-phosphorus limitations: (1) nitrogen limitation, (2) zooplankton grazers, and (3) non-algal turbidity. Based on median values from ISU sampling from 2000-02, the ratio of total nitrogen to total phosphorus for Arrowhead Lake is 11. 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 and that Arrowhead Pond had the seventh highest population of zooplankton (Cladoceran) grazers of the 131 lakes sampled. The levels of inorganic suspended solids at this lake are moderately high and suggest possible light limitation that suppresses growth of suspended algae. The overall 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 at Arrowhead Lake was 7.8 mg/l; this was the 36th highest overall median of the 131 lakes sampled. Data from Downing et al. (2002) suggest that the presence of nuisance aquatic species (i.e., bluegreen algae) may also impair 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). In addition, Arrowhead Pond had the seventh highest mass of bluegreen algae of the 131 lakes sampled. The presence of a large population of bluegreen algae at Arrowhead Pond suggests a potential violation of Iowa’s narrative water quality standard protecting against occurrence of nuisance aquatic life. Thus, the Class A (primary contact recreation) uses are assessed (evaluated) as partially supported.
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 primarily to impacts of siltation but also to excessive nutrient loading to the water column, moderately high levels of non-algal turbidity, and presence of large populations of bluegreen algae. Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring in this lake.
