Assessment Comments
Assessment is based on results of (1) ISU lake survey in 2000-04, (2) surveys by IDNR Fisheries Bureau, (3) ISU information on lake plankton communities, 2000-05 and (4) IDNR investigation of a fish kill in July 2005.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (monitored) as "partially supported" due to poor water transparency and due to high levels of pH that violate state water quality criteria. The Class B(LW) aquatic life uses are assessed as (monitored) as "partially supporting" primarily due to high levels of pH that violate state water quality criteria and secondarily due to sediment, nutrients, turbidity, and algae. 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 2004 sponsored by IDNR and conducted by Iowa State University (ISU), (2) information from the IDNR Fisheries Bureau, and (3) information on plankton communities at Iowa lakes from 2000 through 2005 from the IDNR/ISU statewide lake survey.
Note: A TMDL for siltation at Arrowhead Pond was prepared by IDNR and approved by EPA in 2001; thus, this lake was placed into IR Category 4a (TMDL approved) for the 2004 assessment/listing cycle. Not all Section 303(d) impairments identified for the current (2006) assessment/listing cycle (algae and pH), however, are addressed in the TMDL. Thus, this waterbody was moved from IR Category 4a to Category 5a (impaired; TMDL required) for the 2006 assessment/listing cycle.
EXPLANATION: Results from the ISU statewide survey of Iowa lakes suggest that 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 2000-04 (approximately 15 samples), Carlson's (1977) trophic state indices for total phosphorus, chlorophyll-a, and secchi depth are 82, 62, and 66, respectively. According to Carlson (1977), the index value for total phosphorus places this lake in the range of hyper-eutrophic lakes; the index values for chlorophyll-a and secchi depth place this lake in the range between eutrophic and hyper-eutrophic lakes. These index values suggest (1) extremely high levels of phosphorus in the water column, (2) moderately high levels of chlorophyll-a, and (3) poor water transparency. The TSI value for Secchi depth 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 at Arrowhead Pond: (1) nitrogen limitation, (2) zooplankton grazers, and (3) non-algal turbidity. Based on median values from ISU sampling from 2000-04, the ratio of total nitrogen to total phosphorus for Arrowhead Lake is 8. This TN:TP ratio suggests the possibility that nitrogen limitation, at times, suppresses the production of suspended algae at this lake.
In addition, data from the ISU survey show relatively large populations of zooplankton species at this that graze on algae. Sampling from 2000 through 2005 showed that Cladoceran taxa (e.g., Daphnia) comprised approximately 70% of the dry mass of the zooplankton community and that Arrowhead Pond had the nineteenth highest per summer sample level 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 2004 was 5.2 mg/l; the median level at Arrowhead Pond was 7.9 mg/l; this was the 39th highest overall median of the 131 lakes sampled.
Data from the ISU survey from 2002-2004 suggest that the presence of nuisance aquatic species (i.e., bluegreen algae) may also impair support of the Class A uses. Sampling during this time showed that the percent wet mass of bluegreens was nearly 90%. In addition, Arrowhead Pond had the ninth highest median mass of bluegreen algae of the 131 lakes sampled: 84.0 mg/l. This median is in the worst 10 % of the 131 Iowa 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. 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 greater than the 90th percentile of this distribution were arbitrarily considered to represent “potential impairment; not supported.” No other criteria exist, however, upon which to base a more accurate identification of impairments due to bluegreen algae. Thus, while the accuracy of this (2006) assessment 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 "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, information from the IDNR Fisheries Bureau, and the occurrence of a fish kill in July 2005 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. The ISU lake survey data suggest an impairment due to high levels of pH at this lake. Only one violation of the Class B(LW) criteria for dissolved oxygen occurred in the 14 samples collected during summers of 2000 through 2004. Based on IDNR’s assessment methodology, the occurrence of 1 violation in the 14 samples collected at this station does not suggest an impairment of aquatic life uses at this lake. Four of 15 samples, however, violated the Class A,B(LW) criterion for pH (maximum = 9.2; minimum = 8.1 pH units). Based on IDNR’s assessment methodology, these results suggest an impairment of the Class A and Class B(LW) uses of this lake due to high levels of pH that violate state criteria. These violations, however, likely reflect primary productivity at Arrowhead Pond and do not reflect the input of pollutants into this lake.
A fish kill that occurred in this lake on July 26, 2005 also suggests "impairment" of the Class B(LW) uses. The kill was identified as a summerkill due to low levels of dissolved oxygen related to an algal die-off. According to the IDNR investigation, approximately 500 bluegill, 1,500 yellow bass, and 600 channel catfish (including all cage catfish at this lake) were killed. The estimated value of the fish killed was $39,000.
Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring in this lake.
