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 plankton communities in 2000.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (evaluated) as "fully supporting / threatened" due to moderately high populations of nuisance (noxious) aquatic (algal) life (bluegreen algae). The Class B(LW) aquatic life uses remain assessed (evaluated) as "fully supporting / threatened" due to siltation impacts. 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 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 that moderately high levels bluegreen algae and siltation impacts may threaten full support of the Class A and Class B(LW) uses of Nelson Park 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 61, 56, and 55, respectively. According to Carlson (1977), the index value for total phosphorus is in the lower range between eutrophic and hyper-eutrophic lakes. The index values for chlorophyll-a and secchi depth are in the middle of eutrophic lakes. These index values suggest somewhat elevated levels of phosphorus, very low levels of chlorophyll-a, and relatively good (and somewhat better than expected) water transparency. According to Carlson (1991), the occurrence of a moderately 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) other than phosphorus limits production of algae. The ISU lake data do not readily suggest a primary non-phosphorus limitation for this lake. Potentially, a combination of nitrogen limitation, non-algal (inorganic) turbidity, and zooplankton grazing may limit algal production at Nelson Park Lake. Based on median values from ISU sampling from 2000 through 2002, the ratio of total nitrogen to total phosphorus for Nelson Park Lake is 22. This ratio suggests that algal production at this lake is not limited by nitrogen availability. Data from Downing et al. (2002) show moderately large populations of zooplankton species at Nelson Park Lake that graze on algae. The summer 2000 average mass of Cladocerans (18.7 mg/l), however, was the 67th highest of the 131 lakes; thus, zooplankton grazing may limit algal production at this lake. The levels of inorganic suspended solids at this lake are somewhat elevated and may explain the non-phosphorus limitation observed at this lake. 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 Nelson Park Lake was 3.7 mg/l. This median value was the 52nd lowest of the 131 lakes sampled. The very low TSI values for chlorophyll-a and Secchi depth suggest a lack of potential threats or impairments of full support of the Class A (primary contact) due to presence of aesthetically objectionable conditions (blooms of algae or high levels of inorganic turbidity).
Despite very good water transparency at this lake, levels of nuisance algal species (bluegreen algae) may present a threat to full support of designated uses at Nelson Park Lake. Data from Downing et al. (2002) suggest that bluegreen algae (Cyanophyta) tend to dominate the summertime phytoplankton community of Nelson Park Lake, with approximately 90% of the wet mass in bluegreens in early and late-summer. Sampling in 2000 showed the percent wet mass of bluegreens ranged from approximately 90% in the late June sampling, to approximately 40% in the mid-July sampling, and up to approximately 95% in the late August sampling. The summer 2000 average mass of bluegreen algae (13.3 mg/l) was the 51st highest of the 131 lakes sampled and, based on levels of bluegreen algae in other Iowa lakes, suggests a potential threat to full support of the Class A uses.
The eutrophic conditions at this lake, along with information from the IDNR Fisheries Bureau, suggest that the Class B(LW) aquatic life uses should be assessed as "fully supported / threatened" due to siltation in the lake. Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring at this lake.
