Assessment Comments
Assessment is based on results of (1) ISU lake survey in 2000-01, (2) surveys by IDNR Fisheries Bureau, and (3) ISU report on lake phytoplankton communities. Note: the original narrative assessment for the 2002 cycle was in error; the corrected narrative was entered in August 2004.
Basis for Assessment
[Note: In developing the original 2002 assessment for Green Castle Lake, the TSI values for total phosphorus and chlorophyll-a from Green Valley Lake (79 and 67, respectively), were inadvertently used. This resulted an erroneous assessment that the Class A uses of Green Castle Lake were impaired (evaluated) due to algal turbidity. The correct TSI values for the 2000-01 period at Green Castle Lake were 59 for total phosphorus and 53 for chlorophyll-a. These corrections, and the corrected assessment, for Green Castle Lake were entered in to ADB+ in August 2004.]
SUMMARY: The Class A (primary contact recreation) uses are assessed (evaluated) as "fully supporting" based on results of lake water quality studies in 2000 and 2001. The Class B(LW) aquatic life uses are assessed (evaluated) as "fully supporting / threatened." Fish consumption uses are "not assessed." The sources of data for this assessment include (1) results of the statewide survey of Iowa lakes conducted in 2000 and 2001 by Iowa State University (ISU) and (2) 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 (1) this lake has above average water quality for Iowa lakes and (2) the lack of threats or impairments to full support of the Class A uses of Green Castle Lake. Using the median values from this survey in 2000 and 2001 (approximately six samples), Carlson’s (1977) trophic state indices for total phosphorus, chlorophyll-a, and secchi depth are 59, 53, and 53, respectively. According to Carlson (1977), the index value for total phosphorus is in the upper range of eutrophic lakes, the index values for chlorophyll-a and Secchi depth are in the lower range of eutrophic lakes. These index values suggest relatively low levels of phosphorus the water column, low levels of chlorophyll-a (suspended algae), and very good water transparency for Iowa lakes. Based on median values from ISU sampling in 2000 and 2001, the ratio of total nitrogen to total phosphorus for Green Castle Lake is 28, This ratio does not suggest any nitrogen limitation on algal production at this lake. Data from Downing et al. (2002), however, show that the zooplankton community of Green Castle Lake has a seasonally large population of species known as algal grazers. Sampling at Green Castle Lake in 2000 showed that Daphnia comprised nearly 100% of the dry mass of the zooplankton community in the late July sample but declined to approximately 20% in the mid-August sample. These large population of zooplankton grazers my limit algal production to some degree and may explain the discrepancy between the TSI for total phosphorus (59) and chlorophyll-a (53). The levels of inorganic suspended solids at this lake are relatively low and do not suggest potential problems with high levels of inorganic turbidity. Of 130 lakes sampled as part of the ISU survey in 2000 and 2001, Green Castle Lake had the 22nd lowest median level of inorganic suspended solids (2.3 mg/l), suggesting the lack of impairments to either the primary contact recreation uses and to the aquatic life uses due to non-algal turbidity. These water quality conditions indicate relatively good water quality and the lack of impairments to the Class A uses of Green Castle Lake. Data from Downing et al. (2002) suggest that bluegreen algae (Cyanophyta) comprise a significant portion of the summertime phytoplankton community of Green Castle Lake, especially in early summer. Sampling in 2000 showed the percent wet mass of bluegreens ranged from approximately 85% in the late June sampling, to approximately 60% in the late July sampling, and down to approximately 35% in the late August sampling. The low levels of chlorophyll-a, however, suggest that populations of bluegreen algae are small and do not represent a water quality impact.
The water quality conditions at this lake suggest that the Class B(LW) aquatic life uses are "fully supporting/threatened." Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring at this lake.
