Assessment Comments
Assessment is based on results of (1) ISU lake survey from 2000-02, (2) surveys by IDNR Fisheries Bureau, and (3) ISU report on lake plankton communities in 2000.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (monitored) as "fully supporting / threatened" due to somewhat elevated levels of chlorophyll (algae). The Class B(LW) aquatic life uses remain assessed (evaluated) as "fully supporting / threatened" due to high levels of nutrients and siltation impacts. Fish consumption uses are assessed as “fully supported” based on results of fish contaminant monitoring in 2002. The 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 monitoring in 2002.
EXPLANATION: Results from the ISU statewide survey of Iowa lakes suggest that high levels of algal turbidity may threaten full support of the Class A and Class B(LW) uses of Lake Pahoja. 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 68, 64, and 59, respectively. According to Carlson (1977), the index values for total phosphorus and chlorophyll-a place this lake in the range between eutrophic and hyper-eutrophic lakes; the index value for secchi depth is in the upper range of eutrophic lakes. These index values suggest moderately high levels of phosphorus in the water column, somewhat elevated (but less than expected) levels of chlorophyll-a, and relatively good 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, zooplankton grazing, or some other factor) other than phosphorus limits production of algae. Based on median values from ISU sampling from 2000 through 2002, the ratio of total nitrogen to total phosphorus for Lake Pahoja is 68; this TN:TP ratio indicates that (1) algal production is not limited by nitrogen availability and (2) an excess of total nitrogen exists at this lake. Data from Downing et al. (2002), however, do show relatively large populations of zooplankton species at this lake that graze on algae. Sampling in 2000 showed that Cladoceran taxa (e.g., Daphnia) comprised greater than 70% of the dry mass of the zooplankton community in the mid-July and early August samples. The 2000 average mass for Cladocerans at Lake Pahoja (49 mg/l) was the 27th highest of the 131 lakes sampled, thus suggesting the potential for zooplankton grazing to limit algal production. Levels of inorganic suspended solids at this lake are moderately high and potentially contribute to the non-phosphorus limitations of algal production. 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 for Lake Pahoja was 5.8 mg/l. Despite the indications of relatively good water quality, the somewhat elevated TSI value for chlorophyll-a suggests threats 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 (=noxious) algal species (i.e., bluegreen algae) at Lake Pahoja may also threaten full support of designated uses. Data from Downing et al. (2002) suggest that bluegreen algae (Cyanophyta) dominate the summertime phytoplankton community of this lake. Sampling in 2000 showed that greater than 95% of the wet mass of the phytoplankton community in the three summer samples was bluegreen algae. The 2000 average summer mass of bluegreen algae at this lake (14 mg/l) was the 49th highest of the 131 lakes sampled; this level suggest that bluegreen algae present an additional threat to full support of the designated uses at this lake.
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 "fully supported / threatened" due to excessive nutrient loading to the water column and siltation in the lake. Fish consumption uses are assessed as “fully supported” based on U.S. EPA/IDNR fish contaminant (RAFT) monitoring in 2002. The composite samples of fillets from channel catfish collected in 2002 had very low levels of contaminants (a predator species was not available at this site). Levels of contaminants in this sample did not approach even one-half the respective FDA action levels or IDNR levels of concern. Levels of primary contaminants in the composite sample of channel catfish fillets were as follows: mercury: 0.036 ppm; total PCBs: <0.09 ppm; and technical chlordane: <0.030 ppm.
