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 supported" due to presence of aesthetically objectionable conditions related to blooms of algae and high levels of inorganic turbidity; the presence of nuisance algal species (i.e., bluegreen algae) constitutes a threat to full support of these uses. The Class B(LW) aquatic life uses remain assessed (evaluated) as "partially supported" due to algal blooms, nutrient loading to the water column, and due to siltation. Fish consumption remain "not assessed" due to a lack of fish contaminant monitoring at this lake. 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: 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 88, 68, and 73 respectively, for East Lake. According to Carlson (1977), the index value for total phosphorus places this lake in the upper range of hyper-eutrophic lakes; the value for chlorophyll-a places is in the upper range between eutrophic and hyper-eutrophic lakes, and the index value for secchi depth is in the lower range of hyper-eutrophic lakes. These index values suggest extremely high levels of phosphorus in the water column, moderately high, but somewhat less than expected production of suspended algae, 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 East Lake Osceola is limited—although not to levels below impairment thresholds—by a combination of nitrogen availability, zooplankton grazing, and inorganic turbidity. The relatively low ratio of total nitrogen to total phosphorus (6), suggests that algal production is limited by the availability of nitrogen as opposed to phosphorus. Downing et al. (2002) show relatively large populations of zooplankton at East Lake Osceola as well as large populations of zooplankton species (Cladocerans) known to graze on algae. Sampling in 2000 showed that the average mass of Cladocerans (95.9 mg/l) was the 15th highest of the 131 lakes sampled. This large population of zooplankton grazers suggest at least the potential for this non-phosphorus limitation on 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 of inorganic suspended solids at East Lake Osceola (5.6 mg/l) is moderately high, thus suggesting the potential of non-algal turbidity to limit production of algae; this median value was the 53rd highest of the 131 lakes sampled. Regardless of the non-phosphorus limitations to algal production suggested by the TSI values, the levels of chlorophyll-a and very poor water transparency at this lake continue to suggest blooms of algae and high levels of inorganic turbidity that violate Iowa’s narrative water quality standard protecting against aesthetically objectionable conditions.
The levels of nuisance algal species (i.e., bluegreen algae) are somewhat elevated and thus appear to present a threat to full support of designated uses at this lake. Data from Downing et al. (2002) suggest that bluegreen algae (Cyanophyta) comprise a significant portion of the phytoplankton community. Sampling in 2000 showed the percent wet mass of bluegreens ranged from approximately 75% in the late June sampling, to approximately 90% in the late July sampling, and approximately 89% in the late August sampling. The 2000 average summer mass of bluegreen algae at this lake (12.3 mg/l) was the 54th highest of the 131 lakes sampled, thus suggesting the potential for bluegreen algae to create nuisance conditions and to threaten full 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 "partially supported" due to excessive nutrient loading to the water column, nuisance blooms of algae, and impacts from organic enrichment in the lake. Fish consumption remain "not assessed" due to the lack of fish contaminant monitoring at this lake.
