Assessment Comments
Assessment is based on results of (1) ISU lake survey in 2000-01, (2) surveys by IDNR Fisheries Bureau (3) ISU report (2000) on lake phytoplankton communities, and (4) results of U.S. EPA/IDNR fish tissue monitoring in 2001.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation uses) are assessed (evaluated) as "partially supported" due to presence of nuisance blooms of algae. The Class B(LW) aquatic life uses are assessed (evaluated) as "partially supported". Fish consumption are assessed (monitored) as "fully supported." 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, (3) information on phytoplankton communities at Iowa lakes in 2000 from Downing et al. (2002), and (4) U.S. EPA/IDNR fish tissue monitoring in 2001. NOTE: This narrative was revised in May 2004 to be consistent with the correct assessment for Arbor Lake: the degree of support for aquatic life uses was changed from "fully supported / threatened" to "partially supported." EXPLANATION: Previous characterizations of the ability of this lake to support swimming-type uses (e.g., Bachmann et al. (1994), see assessments for previous reports above) are now considered too old (greater than five years) for assessing current water quality conditions. For the 2002 reporting cycle, the level of support of the Class A (primary contact recreation) uses are assessed as "partially supported" based on results from the ISU statewide survey of Iowa lake. Using the median values from this survey in 2000 and 2001 (approximately six samples), Carlsons's (1977) trophic state indices for total phosphorus, chlorophyll-a, and secchi depth are 81, 68, and 57, respectively, for Arbor Lake. 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 is in the upper range between eutrophic and hyper-eutrophic lakes, and the index value for secchi depth is in the upper range of eutrophic lakes. These index values suggest (1) extremely high levels of phosphorus in the water column, (2) moderately high (but less than expected) levels of chlorophyll-a and suspended algae, and (3) and good water transparency for Iowa lakes. 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 Arbor Lake is 8, thus suggesting the possiblity of nitrogen limitation at this lake. In addition, data from Downing et al. (2002) show that the zooplankton community of Arbor Lake has a moderate presence (approximately 15% dry mass) of species known as algal grazers. Carlson (1991) also suggests that a high TSI for chlorophyll-a relative to the value for secchi depth suggsts that the algal community is composed of large particulate algae such as the bluegreen alga Aphanizomenon. Such a phenomenon may explain the high TSI for chlorophyll-a (68) relative to the TSI for secchi depth (57) at this lake. The levels of inorganic suspended solids at this lake are relatively low and do not suggest potential problems with 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 of inorganic suspended solids at Arbor Lake was 2.6 mg/l, thus suggesting that non-algal turbidity contributes relatively little to either limitation on algal production or in-lake turbidity. These conditions indicate impairments to the Class A (primary contact) uses through presence of nuisance blooms of algae and presence of nuisance algal species (i.e., bluegreen algae). Data from Downing et al. (2002), however, suggest that bluegreen algae (Cyanophyta) comprise a relatively small portion of the phytoplankton community of Arbor Lake). Sampling in 2000 showed the percent wet mass of bluegreens ranged from 20% in the late June sampling, to approximately 25% in the late July sampling, and down to approximately 10% in the mid-August sampling. The hyper-eutrophic conditions at this lake, along with information from the IDNR Fisheries Bureau, suggest that the Class B(LW) aquatic life uses are "partially supported" due to excessive nutrient loading to the water column, nuisance blooms of algae, and siltation impacts in the lake. Fish consumption uses are assessed as "fully supported" based on results of U.S. EPA/IDNR fish tissue (RAFT) monitoring in 2001 that showed that levels of all contaminants were below FDA action levels and IDNR levels of concern in the composite fillet samples from channel catfish and largemouth bass.
