Little Sioux Park Lake IA 06-LSR-1568

Assessment Comments

Assessment is based on results of (1) ISU lake survey from 2000-04, (2) ISU reports on lake plankton communities from 2000-05, 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 (monitored) 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 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:  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 2004 (approximately 15 samples), Carlson's (1977) trophic state indices for total phosphorus, chlorophyll-a, and secchi depth are 46, 44, and 41, respectively.   According to Carlson (1977), the index value for total phosphorus places this lake in the middle range of mesotrophic lakes; the index values for chlorophyll-a and secchi depth place this lake in the middle and lower range, respectively, of mesotrophic lakes.   These index values suggest exceptional water quality for an Iowa lake:  extremely low levels of phosphorus, extremely low algal production, and water transparency that is exceptional for Iowa lakes.   Relative to the 131 lakes sampled from 2000 through 2004 as part of the ISU lake survey, Little Sioux Park Lake had the lowest median level of total phosphorus, the seventh lowest median level of chlorophyll-a, the second best median Secchi depth, the third lowest median level of inorganic suspended solids, and the sixth lowest level of bluegreen algae.   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 general agreement between the TSIs for phosphorus, chlorophyll, and Secchi depth suggests that non-phosphorus limitations—which appear to occur at most Iowa lakes—do not occur at Little Sioux Park Lake.  

In terms of all Iowa lakes sampled, data from the ISU survey show relatively small populations of zooplankton species at this lake that graze on algae.   Although sampling from 2000 through 2005 showed that Cladoceran taxa (e.g., Daphnia) comprised about 50% 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 (55 mg/l) was the 34th lowest of the 131 lakes sampled.   These results suggest little if any non-phosphorus limitation due to zooplankton grazing 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 34.   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 extremely 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 5.2 mg/l; the median level at Little Sioux Lake was 1.6 mg/l.   This median value was the third lowest of the 131 lakes sampled.  

The levels of nuisance (=noxious) algal species (i.e., bluegreen algae) at this lake are extremely low and 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 about 40% of this lake’s summertime phytoplankton community, sampling from 2000 through 2004 showed that the median per summer sample mass of bluegreen algae at Little Sioux Park Lake (0.8 mg/l) was the 6th lowest of the 131 lakes sampled.   This levels is in the lowest 10% of the 131 Iowa lakes sampled.   The presence of an extremely small population of bluegreen algae at this lake does not suggest a potential violation of Iowa’s narrative water quality standard protecting against occurrence of nuisance aquatic life.  

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."  The ISU lake survey data show no violations of the Class B(LW) criteria for dissolved oxygen in the 14 samples collected during summers of 2000 through 2004.   One of 15 samples, however, exceeded the Class B(LW) criterion for pH (maximum = 9.2; minimum = 8.0 pH units).   Based on IDNR’s assessment methodology, however, these results do not suggest that significantly more than 10 percent of the samples exceed Iowa’s pH criteria and thus do not suggest an impairment of the Class A and Class B(LW) uses of this lake.   This violation likely reflects the primary productivity at this lake and does not reflect the input of pollutants into this lake.

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