Assessment Comments
Assessment is based on results of (1) ISU lake survey from 2000-04, (2) ISU reports on lake plankton communities from 2000-05, and (3) surveys by IDNR Fisheries Bureau.
Basis for Assessment
SUMMARY: The Class B(LW) aquatic life uses are assessed (monitored) as "fully supporting". In terms of water transparency and nutrient condition, monitoring data suggest excellent water quality at this lake. Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring at this lake. [Note: 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 sponsored by IDNR and conducted by Iowa State University (ISU) from 2000 through 2004, (2) surveys by IDNR Fisheries Bureau, and (3) information on plankton communities collected at Iowa lakes from 2000 through 2005 as part of the ISU lake survey.
EXPLANATION: Results from the ISU statewide survey of Iowa lakes suggest that the Class B(LW) uses of Moorehead Lake are "fully supported". Using the median values from this survey from 2000 through 2004 (approximately 15 samples), Carlson's (1977) trophic state indices for total phosphorus, chlorophyll-a, and secchi depth are 57, 49, and 49, respectively. According to Carlson (1977), the index value for total phosphorus places this lake in the upper range of eutrophic lakes; the index values for chlorophyll-a and Secchi depth are at the upper boundary of mesotrophic lakes. These index values suggest that the production of suspended algae at this lake 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 2004, the ratio of total nitrogen to total phosphorus for Moorehead Lake is 20. This ratio does not suggest a strong possibility that nitrogen availability limits production of algae at this lake.
In terms of all Iowa lakes sampled, data from the ISU survey show relatively small populations of zooplankton species at this lake that graze on algae. Although sampling from 2000 through 2005 showed that Cladoceran taxa (e.g., Daphnia) comprised about 45% of the dry mass of the zooplankton community of this lake, the average per summer sample mass of Cladoceran taxa over the 2000-2005 period (57 mg/l) was the 36th lowest of the 131 lakes sampled. These results suggest little if any non-phosphorus limitation due to zooplankton grazing at Moorehead Lake.
The levels of inorganic suspended solids at this lake are extremely 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 2004 was 5.2 mg/l; the median level at Moorehead Lake was 2.2 mg/l. This median value was the 15th lowest of the 131 lakes sampled.
The levels of nuisance (=noxious) algal species (i.e., bluegreen algae) at this lake do not suggest an impairment of designated uses. While data from the ISU survey from 2000 through 2004 suggest that bluegreen algae (Cyanophyta) comprise a significant portion (50%) of this lake’s summertime phytoplankton community, sampling from 2000 through 2004 showed that the median per summer sample mass of bluegreen algae at Moorehead Lake (2.7 mg/l) was the 15th lowest of the 131 lakes sampled. This level is in the lowest 10% of the 131 Iowa lakes sampled. The presence of a relatively small population of bluegreen algae at this lake does not suggest a potential violation of Iowa’s narrative water quality standard protecting against occurrence of nuisance aquatic life.
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 ". Siltation impacts, however, remain a concern at this lake. Results of ISU monitoring from 2000 through 2004 suggest generally good chemical water quality at this lake. The ISU lake survey data show no violations of the Class B(LW) criteria for dissolved oxygen in the 15 samples collected during summers of 2000 through 2004. One of 15 samples exceeded the Class B(LW) criterion for pH (maximum = 9.2; minimum = 8.2 pH units). Based on IDNR’s assessment methodology, however, these results do not suggest that significantly more than 10 percent of the samples exceed Iowa’s pH criteria and thus do not suggest an impairment of the Class B(LW) uses of this lake. This violation likely reflects primary productivity at Moorehead Lake and does not reflect the input of pollutants into this lake.
Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring at this lake.
