Green Castle Lake IA 02-IOW-643

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 2002-05.

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 are "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 no impairments to the Class A uses of Green Castle Lake.   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 54, 49, and 45, respectively.   According to Carlson (1977), the index value for total phosphorus is in the middle range of eutrophic lakes, the index values for chlorophyll-a and Secchi depth are in the middle to upper range of mesotrophic lakes.   These index values suggest very low levels of total phosphorus in the water column, extremely low levels of chlorophyll-a (suspended algae), and exceptional water transparency for Iowa lakes.  

According to Carlson (1991), the occurrence of a high TSI value for total phosphorus with relatively lower values for chlorophyll-a and Secchi depth indicate that some factor (e.g., nitrogen limitation, zooplankton grazing, or some other factor) limits production of algae.  

Based on median values from ISU sampling from 2000 through 2002, the ratio of total nitrogen to total phosphorus for Green Castle Lake is 35, thus suggesting that algal production at this lake is limited by phosphorus availability versus nitrogen availability.  

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 (79 mg/l) was the 59th lowest of the 131 lakes sampled.   These results suggest little if any non-phosphorus limitation due to zooplankton grazing at Green Castle Lake.  

The levels of inorganic suspended solids at this lake are extremely low and do not suggest potential threat or impairments to designated uses due to high levels of inorganic turbidity.   Of 131 lakes sampled as part of the ISU survey from 2000 through 2004, Green Castle Lake had the ninth lowest median level of inorganic suspended solids (2.0 mg/l).   Thus, based on results of ISU lake monitoring, these water quality conditions do not suggest potential threats or impairments to the Class A (primary contact) uses through presence of aesthetically objectionable conditions such as blooms of algae and high levels of inorganic turbidity.  

The 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 significant portion (over 60%) of this lake’s summertime phytoplankton community, sampling from 2000 through 2004 showed that the median per summer sample mass of bluegreen algae at Green Castle Lake (2.5 mg/l) was the 13th lowest of the 131 lakes sampled.   This level is in the lowest 10% of the 131 Iowa lakes sampled.   The presence of this very 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.  
Information from the IDNR Fisheries Bureau suggests that the Class B(LW) aquatic life uses should be assessed (evaluated) as "fully supported."  The ISU lake survey data show no violations of the Class B(LW) criteria for dissolved oxygen in the 13 samples collected during summers of 2000 through 2004.   Three of 14 samples exceeded the Class A,B(LW) criterion for pH (maximum = 9.1; minimum = 7.7 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.   These violations likely reflect levels of primary productivity at Green Castle Lake and do 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.