Assessment Comments
Assessment is based on results of (1) ISU lake survey from 2000-02, (2) ISU report on lake plankton communities in 2000, and (3) surveys by IDNR Fisheries Bureau.
Basis for Assessment
SUMMARY: The Class B(LW) aquatic life uses remain assessed (evaluated) as "fully supporting / threatened" due to siltation impacts. In terms of water transparency and nutrient condition, however, monitoring data suggest “full support” of these uses. Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring at this lake. (This lake is not designated for Class A (primary contact recreation) uses.) 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 the Class B(LW) uses of Moorehead Lake are "fully supported / threatened." 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, 49, and 49, respectively. According to Carlson (1977), the index value for total phosphorus places this lake in the lower range between eutrophic and hyper-eutrophic lakes; the index values for chlorophyll-a and Secchi depth are at the upper boundary of mesotrophic lakes. These index values suggest that, despite the somewhat elevated levels of phosphorus, the production of suspended algae is extremely low and water transparency is exceptional for Iowa lakes. These results suggest that this lake does not have threats or impairments to the full support of aquatic life uses due to aesthetically objectionable conditions, either in the form of blooms of algae or inorganic turbidity.
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. Potentially, a combination of nitrogen limitation, non-algal (inorganic) turbidity, and zooplankton grazing may limit algal production at Moorehead Lake. Based on median values from ISU sampling from 2000 through 2002, the ratio of total nitrogen to total phosphorus for Moorehead Lake is 16. This ratio, and the tendency for this ratio to drop below 16 (see assessment for the 2002 report), suggests that algal production at this lake may, at times, be limited by nitrogen availability. In addition, data from Downing et al. (2002) show relatively large populations of zooplankton species at Moorehead Lake that graze on algae. Sampling in 2000 showed that Cladoceran taxa (e.g., Daphnia) comprised approximately 55% of the dry mass of the zooplankton community in the both the mid-July and early August samples. The summer 2000 average mass of Cladocerans (14.6 mg/l), however, was the 56th lowest of the 131 lakes; thus, zooplankton grazing likely doesn’t significantly limit algal production at this lake. The levels of inorganic suspended solids at this lake are relatively low and do not suggest a strong potential for impairing designated uses. 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 Moorehead Lake was 2.3 mg/l. This median value was the 19th lowest of the 131 lakes sampled.
This lake does not appear to have serious problems due to presence of nuisance algal species (i.e., bluegreen algae). Although data from Downing et al. (2002) suggest that bluegreen algae (Cyanophyta) tend to dominate the summertime phytoplankton community of this lake, the summer 2000 average summer mass of bluegreen algae at Moorehead Lake (2.3 mg/l) was the 23rd lowest of the 131 lakes sample, thus suggesting little if any impact from bluegreen algae at this lake.
The water quality 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 impacts in the lake. Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring at this lake.
