Assessment Comments
Assessment is based on results of (1) ISU lake survey in 2000-01, (2) ISU report on lake phytoplankton communities, 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." Fish consumption uses remain "not assessed." (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 in 2000 and 2001 by Iowa State University (ISU), (2) information from the IDNR Fisheries Bureau, and (3) information on phytoplankton 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 uses of Moorehead Lake are "fully supported / threatened." 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 65, 40, and 50, respectively. According to Carlson (1977), the index value for total phosphorus places this lake in the middle range between eutrophic and hyper-eutrophic lakes; the index value for chlorophyll-a is at the boundary between oligotrophic and mesotrophic lakes, and the index value for secchi depth is at the boundary between mesotrophic and eutrophic lakes. These index values suggest that, despite the moderately high levels of phosphorus, the production of suspended algae is extremely low and water transparency is very good for Iowa lakes. These results suggest that this lake does not have impairments to aquatic life uses due to aesthetically objectionable blooms of algae, nuisance aquatic species (e.g., bluegreen algae) or other nutrient-related water quality problems. According to Carlson (1991), the occurrence of a moderately high TSI value for total phosphorus with relatively low values for chlorophyll-a and secchi depth indicate that some factor (e.g., nitrogen limitation or zooplankton grazing) limits production of algae. Based on median values from ISU sampling in 2000 and 2001, the ratio of total nitrogen to total phosphorus for Moorehead Lake is 13.4. This ratio suggests that algal production at this lake is limited, at least in part, 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. Thus, algal production at this lake is also likely limited somewhat by these zooplankton grazing on algae. The levels of inorganic suspended solids at this lake are relatively low and do not suggest the potential for impairing designated uses. The median level of inorganic suspended solids in the 130 lakes sampled for the ISU lake survey in 2000 and 2001 was 5.27 mg/l; the median level at Moorehead Lake was 2.3 mg/l. This median value was the 19th lowest of the 130 lakes sampled. This lake does not appear to have serious problems due to presence of nuisance algal species (i.e., bluegreen algae). Data from Downing et al. (2002) suggest that bluegreen algae (Cyanophyta) tend to dominate the summertime phytoplankton community of this lake, especially in mid to late summer. Sampling in 2000 showed that bluegreen algae comprised approximately 5% of the wet mass of the phytoplankton community in the mid-June sample but increased to 50% and 75% in the mid-July and early August samples, respectively. Although bluegreen algae comprise a significant portion of the summertime phytoplankton community, the production of chlorophyll is extremely low at this lake, and thus the presence of bluegreen algae likely does not present a water quality impact. Although results of ISU lake monitoring in 2000 and 2001 suggest threats to full support of the designated beneficial uses of this lake, the amount of data available for characterizing water quality is not sufficient for developing an accurate assessment of support of these uses. Additional data for this lake are being generated as part of the ongoing ISU lake survey; these data will be used to improve the accuracy of future water quality assessments. 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.
