Assessment Comments

Assessment is based on results of (1) ISU lake survey from 2000-04, (2) surveys by IDNR Fisheries Bureau, and (3) ISU reports on lake plankton communities from 2000-05.

Basis for Assessment

SUMMARY:  The Class A (primary contact recreation uses) are assessed (monitored) as "not supported" due to presence of aesthetically objectionable conditions related to poor water transparency related to blooms of algae and high levels of inorganic turbidity.   The presence of moderately large populations of nuisance algal species (i.e., bluegreen algae) constitutes an additional water quality concern at this lake.   The Class B(LW) aquatic life uses are assessed (evaluated) as "partially supported" due to excessive nutrient loading to the water column, nuisance blooms of algae, and impacts from organic enrichment.   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 sponsored by IDNR and conducted by Iowa State University (ISU) from 2000 through 2004, (2) surveys by IDNR Fisheries Bureau, and (3) information on plankton communities collected at Iowa lakes from 2000 through 2005 as part of the ISU lake survey.  

EXPLANATION:  Using the median values from this survey from 2000 through 2004 (approximately 15 samples), Carlson’s (1977) trophic state indices at East Lake Osceola for total phosphorus, chlorophyll-a, and secchi depth are 88, 68, and 73 respectively.   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.   Based on data from the 2000-2004 monitoring period, 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.  

The presence of extremely large populations of zooplankton at East Lake Osceola that graze on algae, however, may explain the large discrepancy between the TSI value for phosphorus (88) and that for chlorophyll-a (68).   In terms of all Iowa lakes sampled, data from the ISU survey show an extremely large population of zooplankton species at this lake that graze on algae.   Sampling from 2000 through 2005 showed that Cladoceran taxa (e.g., Daphnia) comprised approximately 65% of the dry mass of the zooplankton community of this lake.   The average per summer sample mass of Cladoceran taxa over the 2000-2005 period (348 mg/l) was the sixth highest of the 131 lakes sampled.   This large population of zooplankton grazers strongly suggests the potential for this type of non-phosphorus limitation on algal production at East Lake Osceola.  

The median level of inorganic suspended solids in the 131 lakes sampled for the ISU lake survey from 2000 through 2004 was 5.2 mg/l; the median level of inorganic suspended solids at East Lake Osceola (7.2 mg/l) is moderately high, thus suggesting the potential of non-algal turbidity to limit production of algae; this median value was the 54th highest of the 131 lakes sampled.  

Regardless of the non-phosphorus limitations to algal production suggested by the TSI values, the very poor water transparency at the lake, whether due to the high levels of chlorophyll-a  or to the moderately high levels of inorganic suspended solids, continue to suggest a violation of Iowa’s narrative water quality standard protecting against aesthetically objectionable conditions.  

The moderately high levels of nuisance (=noxious) algal species (i.e., bluegreen algae) at this lake do not suggest an impairment of Class A uses.   While data from the ISU survey from 2000 through 2004 suggest that bluegreen algae (Cyanophyta) comprise a large portion (80%) of this lake’s summertime phytoplankton community, sampling from 2000 through 2004 showed that the median per summer sample mass of bluegreen algae at East Lake Osceola (20.9 mg/l) was the 42nd highest of the 131 lakes sampled.   This level is in the lowest two-thirds of the 131 Iowa lakes sampled.   The presence of a moderately large population of bluegreen algae at this lake does not immediately suggest a violation of Iowa’s narrative water quality standard protecting against occurrence of nuisance aquatic life.   This assessment, however, is based strictly on a distribution of the lake-specific median bluegreen algae values for the 2000-2004 monitoring period.   Median levels less than the 75th percentile of this distribution (~29 mg/l) were arbitrarily considered by IDNR staff to not represent an impairment of the Class A uses of Iowa lakes.   No criteria exist, however, upon which to base a more accurate identification of impairments due to bluegreen algae.   Thus, while the ability to characterize the levels of bluegreen algae at this lake has improved over that of the previous (2004) assessment due to collection of additional data, the assessment category for assessments based on level of bluegreen algae nonetheless remains, of necessity, "evaluated" (indicating an assessment with relatively lower confidence) as opposed to "monitored" (indicating an assessment with relatively higher confidence).  

The hyper-eutrophic conditions at this lake, along with information from the IDNR Fisheries Bureau, suggest that the Class B(LW) aquatic life uses should be assessed (evaluated) as "partially supported" due to excessive nutrient loading to the water column, nuisance blooms of algae, and impacts from organic enrichment in the lake.  

Results of ISU monitoring from 2000 through 2004 suggest generally good chemical water quality at this lake.   Results of this monitoring show that only 2 of the 15 samples collected exceeded the Class B(LW) criteria for dissolved oxygen and that only 2 of the 14 samples collected exceeded Iowa’s Class A,B(LW) criteria for pH (maximum = 9.3; minimum = 8.7 pH units).   Based on IDNR’s assessment methodology, these results do not suggest that violation frequencies are significantly greater than 10% for either parameter and thus do not suggest impairment of either the Class A and Class B(LW) uses of East Lake Osceola.   The  violations of the pH criteria likely reflect the influence of primary productivity at East Lake Osceola and do not reflect the input of pollutants into this lake.

Fish consumption remain "not assessed" due to the lack of fish contaminant monitoring at this lake.