Assessment Comments
Assessment based on results of (1) ISU lake survey in 2000-01, (2) ISU report on lake phytoplankton communities, (3) results of U.S. EPA / IDNR fish tissue (RAFT) monitoring in 2000 and (4) surveys by IDNR Fisheries Bureau.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (monitored) as "partially supported" primarily due to high levels of non-algal (inorganic) turbidity and secondarily due to aesthetically objectionable blooms of algae. The Class B(LW) aquatic life uses remain assessed (evaluated) as "partially supported" due to high levels of non-algal (inorganic ) turbidity related primarily to sediment re-suspension. Fish consumption uses remain assessed as "fully supported" based on fish contaminant monitoring in 2000. 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), (4) results of IDNR/UHL TMDL monitoring in 2002, and (5) the results of U.S. EPA/IDNR fish tissue (RAFT) monitoring in July 2000.
EXPLANATION: Results from the ISU statewide survey of Iowa lakes suggest that high levels of non algal (inorganic) turbidity, as well as moderately high levels of algal turbidity, may adversely affect the Class A and Class B(LW) uses of Lower Gar 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 76, 66, and 74, respectively. The TSI values for both Secchi depth (74) and chlorophyll-a (66) suggest impairments of designated uses at this lake. According to Carlson (1977), the index values 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 somewhat less than expected, production of suspended algae, and very poor water transparency. According to Carlson (1991), the occurrence of a low chlorophyll-a TSI value relative to those for total phosphorus and secchi depth indicate non-algal particles or color dominate light attenuation. The ISU lake data suggest that non-algal particles do likely limit algal production at Lower Gar 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, Lower Gar Lake had the 18th highest median level of inorganic suspended solids (18 mg/l), thus suggesting that non-algal turbidity limits the production of algae as well as contributes to impairments of both the primary contact recreation and aquatic life uses. Other non-phosphorus limitations to algal production may exist at this lake. For example, the relatively low ratio of total nitrogen to total phosphorus for this lake (10) suggests at least the potential for nitrogen limitation at this lake. Data from Downing et al. (2002) show relatively small populations of zooplankton species (Cladocerans) at this lake that graze on algae. The average summer mass of these zooplankton grazers (15.0 mg/l) in 2000 was the 57th lowest of the 131 lakes sampled, thus suggesting only minor influence of algal grazing on algal production. Thus, the high levels of both non-algal (inorganic) turbidity and (secondarily) algal turbidity at this lake impair the Class A uses due to violation of Iowa’s narrative water quality standard protecting against aesthetically objectionable conditions.
The levels of nuisance algal species (i.e., bluegreen algae) at Lower Gar Lake is elevated relative to most other Iowa lakes and suggests a threat to full support of designated uses. Data from Downing et al. (2002) suggest that bluegreen algae (Cyanophyta) dominate the summertime phytoplankton community of Lower Gar Lake, especially in early and mid-summer. Sampling in 2000 showed the percent wet mass of bluegreens was 100% in the mid-June and mid-July samples and dropped to approximately 60% in the early August sampling. The 2000 average summer mass of bluegreen algae at Lower Gar Lake (25.4 mg/l) was the 38th highest of the 131 lakes sampled. While not extremely high, these levels of bluegreen algae are of concern. The amount of data available for characterizing algal populations at this lake (one season), however, is not sufficient for developing a more accurate assessment of support of these uses. Additional data on plankton have been collected at this lake as part of the ongoing ISU lake survey and will be used to improve the accuracy of future water quality assessments for this lake.
Results of IDNR/UHL ambient chemical/physical water quality monitoring conducted in support of TMDL development in 2002 does not suggest any water quality problems at this lake. None of the 10 samples collected monthly between March and December 2002 violated Class B(LW) state water quality criteria for dissolved oxygen, pH, or ammonia-nitrogen. Information from the IDNR Fisheries Bureau, however, suggests that the Class B(LW) aquatic life uses should remain assessed as "partially supported" due to high levels of (inorganic ) turbidity related primarily to sediment re-suspension at this shallow lake. Fish consumption are assessed as "fully supported" based on results of U.S. EPA / IDNR fish tissue monitoring in July 2000 (see assessment for the 2002 report).
