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 phytoplankton communities in 2000.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (monitored) as "partially supporting" primarily due to high levels of non-algal turbidity. The Class B(LW) aquatic life uses remain assessed (evaluated) as "partially supporting" due to high levels of nutrients, non-algal turbidity, and siltation impacts. The Class C drinking water uses are "not assessed" due to lack of data upon which to base an assessment. Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring at this lake. 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, and (3) information on plankton 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 from 2000 through 2002 (approximately nine samples), Carlson’s trophic state indices for total phosphorus, chlorophyll-a, and secchi depth at Lake Orient are 85, 63, 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 range between eutrophic and hyper-eutrophic lakes, and the index value for secchi depth is in the lower range of hyper-eutrophic lakes. Together, these index values suggest extremely high levels of phosphorus in the water column, somewhat elevated (and less than expected) levels of chlorophyll-a, and very poor 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. The ISU lake data suggest that algal production at Lake Orient is limited primarily by inorganic turbidity but also (potentially) by nitrogen availability and zooplankton grazing. 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 Orient Lake (13.0 mg/l) was the fourth highest of the 131 lakes, thus strongly suggesting that non-algal turbidity is responsible for light attenuation that limits the production of algae. In addition, the ratio of total nitrogen to total phosphorus for this lake is relatively low (7) and suggest that algal production at the lake is nitrogen-limited. The presence of relatively large populations of zooplankton at Lake Orient that graze on algae may explain a portion of the discrepancy between the TSI value for phosphorus and those for chlorophyll-a and Secchi depth. Downing et al. (2002) show that Cladoceran taxa (e.g., Daphnia) comprised approximately 35% of the dry mass of the zooplankton community in the mid-July sample and 50% of the early August sample. The average 2000-02 summer mass of the Cladoceran grazers at Lake Orient (115 mg/l) was the ninth highest of the 131 lakes sampled, thus suggesting the potential for algal grazing to limit algal production. The very high levels inorganic suspended solids and the resultant non-algal turbidity indicate an impairment to the Class A (primary contact) uses due to violation of Iowa’s narrative water quality standard protecting against aesthetically objectionable conditions. The IDNR Fisheries Bureau concurs that Lake Orient is impaired by non-algal turbidity but believes that algae contribute to this impairment.
Data from Downing et al. (2002) suggest that bluegreen algae (Cyanophyta) comprise a very small portion 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 2000 average summer mass of bluegreen algae at Lake Orient (1.6 mg/l) was the 16th lowest of the 131 lakes sampled.
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 upon which to base an assessment. The only parameter collected as part of the ISU lake survey relevant to support of Class C (drinking water) uses is nitrate. While the results of the ISU survey from 2000-02 show that nitrate levels are low at this lake (maximum value = 5.5 mg/l; median = 1.0 mg/l), these data are not sufficient for developing a valid assessment of support of the Class C uses. Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring at this lake.
