Arrowhead Lake IA 04-RAC-1136

Assessment Comments

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

Basis for Assessment

For the 2004 report:  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."  The sources of data for this assessment include (1) results of the statewide survey of Iowa lakes conducted in 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 levels of phosphorus are low at Arrowhead Lake and do not threaten full support of the Class A uses.   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 51, 52, and 49, respectively.   According to Carlson (1977), the index values for all three parameters are near the boundary between mesotrophic and eutrophic waters.   These index values suggest (1)very low levels of phosphorus for an Iowa lake, (2) low production of suspended algae, and (3) exceptional water transparency.   Based on median values from ISU sampling in from 2000 through 2002, the ratio of total nitrogen to total phosphorus for Arrowhead Lake is 34.   This ratio suggest that algal production at this lake is likely limited by the availability of phosphorus.   The levels of inorganic suspended solids at this lake are very low and do not suggest the potential for impairing designated uses due to high turbidities.   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 Arrowhead Lake was 1.6 mg/l:  this median value is the sixth lowest of the 131 lakes sampled.   The presence of nuisance algal species (i.e., bluegreen algae) does not appear to be a significant problem at this lake.   Data from Downing et al.  (2002) suggest that bluegreen algae (Cyanophyta) comprise a relatively small portion of the summertime phytoplankton community of this lake.   The percent wet mass of bluegreen algae in 2000 ranged from between less than 15% in early summer, up to nearly 40% in midsummer, and down to less than 10% by late summer.   The average wet mass of bluegreens ranked the 34th lowest of the 131 lakes sampled.  

The water quality conditions at this lake (based on the TSI values for chlorophyll-a and Secchi depth), along with information from the IDNR Fisheries Bureau, suggest that the Class B(LW) aquatic life uses should be assessed as "fully supported."  Fish consumption uses remain “not assessed” due to the lack of fish contaminant monitoring at this lake.