Assessment Comments
Assessment is based on results of (1) ISU lake survey in 2000-02, (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 (monitored) as "not supported" due to presence of aesthetically objectionable blooms of algae. The Class B(LW) aquatic life are assessed (evaluated) as "partially supported" due to blooms of algae and impacts of nutrients and siltation in the lake. Fish consumption remain assessed (monitored) as "fully supported." 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) U.S. EPA/IDNR fish tissue monitoring in 2001.
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 2004 reporting cycle, the level of support of the Class A (primary contact recreation) uses are assessed as "not supported" based on results from the ISU statewide survey of Iowa lakes. 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 80, 71, and 58, respectively, for Arbor Lake. According to Carlson (1977), the index values for total phosphorus and chlorophyll-a place this lake in the range of hyper-eutrophic lakes, and 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) very high levels of chlorophyll-a and suspended algae, and (3) and relatively 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 from 2000 through 2002, the ratio of total nitrogen to total phosphorus for Arbor Lake is 8, thus strongly suggesting that nitrogen availability limits algal production at this lake. In addition, data from Downing et al. (2002) shows a moderately large population of zooplankton grazers at Arbor Lake, with the average summer mass of these (Cladoceran) grazers (29.5 mg/l) ranking 44th highest of the 131 lakes sampled. 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 (71) relative to the TSI for secchi depth (58) 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 131 lakes sampled for the ISU lake survey from 2000 through 2002 was 4.8 mg/l. The median level of inorganic suspended solids at Arbor Lake was 2.7 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) that violate Iowa’s narrative water quality standard protecting against aesthetically objectionable conditions. Data from Downing et al. (2002) 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 average summer mass of bluegreen algae at this lake (3.1 mg/l) was the 29th lowest of the 131 lakes sampled. This low level of bluegreen algae does not suggest either a threat or impairment to support of designated uses.
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 fully supported but threatened due to 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.
