Assessment Comments
Assessment is based on results of (1) ISU lake survey in 2000-01, (2) surveys by IDNR Fisheries Bureau, and (3) ISU report on lake phytoplankton communities.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (monitored) as "partially supporting" due to high levels of non-algal turbidity. The Class B(LW) aquatic life uses remain assessed (evaluated) as "partially supporting." The Class C drinking water uses are "not assessed." 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 and 2001 by Iowa State University (ISU), (2) information from the IDNR Fisheries Bureau, and (3) information on phytoplankton 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 non-algal turbidity may affect the Class A and Class B(LW) uses of Orient Lake. Using the median values from the Iowa State University statewide surveys of Iowa lakes in 2000 and 2001 (approximately six samples), Carlsons's trophic state indices for total phosphorus, chlorophyll-a, and secchi depth at Lake Orient are 82, 59, and 70, respectively. 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 of eutrophic lakes, and the index value for secchi depth is in the lower range of hyper-eutrophic lakes. Together, these index values suggest excessive levels of phosphorus in the water column, relatively low (and less than expected) levels of chlorphyll-a, and poor water transparency. The low levels of chlorophyll-a relative to the high concentrations of total phosphorus may be due to (1) high levels of turbidity related to suspended inorganic material in the water column that limit production of algae and/or (2) nitrogen limitation. 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 Orient Lake (33.1 mg/l) was the fourth highest of the 130 lakes, thus strongly suggesting that non-algal turbidity limits the production of algae. In addition, the ratio of total nitrogen to total phosphorus for this lake is relatively low (8) and suggest that the lake is nitrogen-limited. These conditions indicate impairments to the Class A (primary contact) uses through presence of non-algal turbidity due to high levels of inorganic suspended solids. The presence of high levels of phosphorus, however, suggest the potential for aesthetically objectionable blooms of algae. Data from Downing et al. (2002) suggest that bluegreen algae (Cyanophyta) comprise a very small portion (less than 10% wet mass) of the summertime phytoplankton community of Orient Lake; thus, nuisance algal species do not appear to be affecting the beneficial uses of this lake. 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, high levels of non-algal turbidity, and siltation in the lake. Drinking water uses are "not assessed" due to a lack of relevant water quality information. Fish consumption uses remain "not assessed" due to the lack of recent fish contaminant monitoring at this lake.
