Little Sioux Park Lake IA 06-LSR-1568

Assessment Comments

Assessment is based on results of (1) ISU lake survey from 2000-02, (2) ISU report on lake phytoplankton communities in 2000, and (3) surveys by IDNR Fisheries Bureau.

Basis for Assessment

SUMMARY:  The Class A (primary contact recreation) uses are assessed (monitored) as "fully supporting."  The Class B(LW) aquatic life uses remain assessed (evaluated) as "fully supporting."  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 the Class A uses of Little Sioux Lake are "fully supported."  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 53, 44, and 44, respectively.   According to Carlson (1977), the index value for total phosphorus places this lake in the middle range of eutrophic lakes; the index values for chlorophyll-a and secchi depth place this lake in the middle range of mesotrophic lakes.   These index values suggest very low levels of phosphorus, extremely low algal production, and water transparency that is exceptional for Iowa lakes.   These results suggest that this lake does not have impairments due to aesthetically objectionable conditions due either to blooms of algae or to high levels of inorganic turbidity.   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 does not suggest an obvious explanation for the divergence between the TSIs for total phosphorus and those for chlorophyll-a and Secchi depth.   The moderately large population of zooplankton that graze on algae (Cladocerans) may explain the apparent non-phosphorus limitation at this lake.   The 2000 average summer mass of Cladocerans (20.6 mg/l) was the 59th highest of the 131 lakes sampled.   Other non-phosphorus limitations (e.g., inorganic turbidity and nitrogen limitation) do not appear to affect algal production at Little Sioux Park Lake.   Based on median values from ISU sampling from 2000 through 2002, the ratio of total nitrogen to total phosphorus for Little Sioux Lake is 26.   This ratio suggests little potential that algal production at this lake is limited by nitrogen availability.   The levels of inorganic suspended solids at this lake are very low and do not suggest the potential for causing inorganic turbidity that would limit 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 at Little Sioux Lake was 1.6 mg/l.   This median value was the second lowest of the 131 lakes sampled.  

This lake does not appear to have problems due to presence of nuisance (noxious) algal species (i.e., bluegreen algae).   Data from Downing et al.  (2002) suggest that bluegreen algae (Cyanophyta) comprise a relatively small portion of the summertime phytoplankton community of this lake, with the exception of late summer.   Sampling in 2000 showed that bluegreen algae comprise 15% or less of the wet mass of phytoplankton in the mid-June and early July samples and increased to comprise approximately 60% of the wet mass of phytoplankton in the early August sample.   The 2000 average summer mass of bluegreen algae at this lake (6.7 mg/l) was the 48th lowest of the 131 lakes sampled.  

The water quality conditions at this lake, along with information from the IDNR Fisheries Bureau, suggest that the Class B(LW) aquatic life uses should remain assessed as "fully supported."  Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring at this lake.