Assessment Comments
Assessment is based on results of (1) ISU lake survey from 2000-02, (2) surveys by IDNR Fisheries Bureau (3) ISU report on lake plankton communities in 2000, and (4) results of U.S. EPA/IDNR fish tissue (RAFT) monitoring in 2001.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (monitored) as "partially supporting" due to presence aesthetically objectionable conditions caused by non-algal (inorganic) turbidity and blooms of algae. The Class B(LW) aquatic life uses are assessed (evaluated) as "partially supported" due to excess algal production and due to reduced water transparency related to presence of common carp populations and to sedimentation problems in the lake. Fish consumption uses remain assessed (monitored) as "fully supported" based on results of fish contaminant monitoring in 2001. 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, (3) information on plankton communities at Iowa lakes in 2000 from Downing et al. (2002), and (4) results of U.S. EPA / IDNR fish tissue (RAFT) monitoring in 2001.
EXPLANATION: Results from the ISU statewide survey of Iowa lakes suggest that high levels of algal and non-algal (inorganic) turbidity may adversely affect the Class A and Class B(LW) uses of Lake Miami. 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 76, 67, and 71, respectively. According to Carlson (1977), the index values for total phosphorus and secchi depth place this lake in the range of hyper-eutrophic lakes; the index value for chlorophyll-a is in the range between eutrophic and hyper-eutrophic lakes. These index values suggest extremely high levels of phosphorus in the water column, moderately high (but less than expected) production of suspended algae, and very poor water transparency. According to Carlson (1991), the occurrence of a high TSI value for total phosphorus with relatively lower values for chlorophyll-a and secchi depth indicates that some factor (e.g., nitrogen limitation, zooplankton grazing, or some other factor) other than phosphorus limits production of algae. The ISU lake data suggest that algal production at Lake Miami may be limited by non-algal turbidity and by nitrogen limitation; zooplankton grazing does not appear to limit algal production as 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. Of 131 lakes sampled, Lake Miami had the 23rd highest median level of inorganic suspended solids (12.2 mg/l), thus suggesting that non-algal turbidity limits the production of algae as well as contributes to the very poor water transparency that impairs of both the primary contact recreation and aquatic life uses. Based on median values from ISU sampling from 2000 through 2002, the ratio of total nitrogen to total phosphorus for this lake is 11. This ratio does suggest that algal production is limited, at least to some degree, by nitrogen availability. Populations of zooplankton that graze on algae, however, are relatively low at this lake and do not likely limit algal production. The 2000 average mass of these Cladoceran grazers (9.9 mg/l) was the 48th lowest mass of the 131 lakes and thus does not suggest a strong potential to limit algal production. These conditions indicate impairments to the Class A (primary contact) uses through presence of aesthetically objectionable conditions related to (1) high levels of non-algal turbidity (inorganic suspended solids) and (2) blooms of algae.
The presence of nuisance algal species (i.e., bluegreen algae) at this lake does not appear to either threaten or impair designated uses. Data from Downing et al. (2002) suggest that bluegreen algae (Cyanophyta) dominate the summertime phytoplankton community of Lake Miami). Sampling in 2000 showed the percent wet mass of bluegreens ranged from approximately 100% in the late June sampling to approximately 85% in the late July and late August samplings. However, the 2000 average summer mass of bluegreen algae at Lake Miami (8.9 mg/l) is low relative to other Iowa lakes and does not suggest the potential to impact water quality at this lake.
Information from the IDNR Fisheries Bureau suggests that the Class B(LW) aquatic life uses should be assessed as impaired due to excess algal production and due to reduced water transparency related to presence of common carp populations and to sedimentation problems in the lake. Fish consumption uses are assessed (monitored) as "fully supported" based on results of U.S. EPA/IDNR fish tissue (RAFT) monitoring at Lake Miami in 2001 (see assessment for the 2002 report).
