Assessment Comments
Assessment is based on results of (1) ISU lake survey from 2000-02, (2) surveys by IDNR Fisheries Bureau, and (3) ISU report on lake phytoplankton communities in 2002.
Basis for Assessment
[2004] 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 / threatened." 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 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 no impairments to the Class A uses of Green Castle Lake. 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 57, 50, and 52, respectively. [Note: In developing the 2002 assessment for this 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 due to algal turbidity. The correct TSI values for the 2000-01 period were 59 for total phosphorus and 53 for chlorophyll-a.] 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 total phosphorus in the water column, extremely low levels of chlorophyll-a (suspended algae), and very good 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. The respective TSI values for Green Castle Lake, however, are similar and suggest only a minor non-phosphorus limitation.
Based on median values from ISU sampling from 2000 through 2002, the ratio of total nitrogen to total phosphorus for Green Castle Lake is 28, thus suggesting that algal production at this lake is limited by phosphorus availability versus nitrogen availability.
In addition, data from Downing et al. (2002) show that the zooplankton community of Green Castle Lake has a relatively small population of species known as algal grazers (Cladocerans). Although sampling at this lake in 2000 showed that Daphnia comprised from 20% up to nearly 100% of the dry mass of the zooplankton community, the average wet mass of both the total zooplankton community and the Cladoceran portion of that community were very low relative to other Iowa lakes: the average mass of the zooplankton community (12 mg/l) was the 19th lowest, and average mass of the Cladoceran community (2.4 mg/l) was the 21st lowest, of the 131 lakes sampled thus suggesting little potential for zooplankton grazers to limit algal production.
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 2002, Green Castle Lake had the 24th lowest median level of inorganic suspended solids (2.6 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.
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 2000 average summer mass of bluegreen algae (8.7 mg/l), however, was 61st lowest of the 131 lakes sampled. Thus, although bluegreen algae appear to dominate the summertime phytoplankton community, the somewhat elevated mass of bluegreen algae are not believed to present either threats or impairments to the designated uses of this lake.
The eutrophic conditions at this lake, along with information from the IDNR Fisheries Bureau, suggest that the Class B(LW) aquatic life uses are assessed as "fully supported / threatened." Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring at this lake.
