Assessment Comments
Assessment based on results of (1) ISU lake survey from 2000-04, (2) ISU reports on lake plankton communities from 2000-05, (3) results of IDNR/UHL TMDL monitoring in 2002, (4) results of U.S. EPA / IDNR fish tissue (RAFT) monitoring in 2000 and (5) surveys by IDNR Fisheries Bureau.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses remain assessed (monitored) as "partially supported" due to poor water transparency related primarily 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 sponsored by IDNR and conducted by Iowa State University (ISU) from 2000 through 2004, (2) surveys by IDNR Fisheries Bureau, (3) information on plankton communities collected at Iowa lakes from 2000 through 2005 as part of the ISU lake survey, (4) results of IDNR/UHL TMDL monitoring in 2002, and (5) results of U.S. EPA/IDNR fish contaminant monitoring in 2000.
Note: A TMDL for turbidity at Lower Gar Lake was prepared by IDNR and approved by EPA in 2002; thus, this waterbody was placed into IR Category 4a (TMDL approved) for the 2004 assessment/listing cycle. Because all Section 303(d) impairments identified for the 2006 assessment/listing cycle [turbidity (algal and inorganic)] are addressed by the TMDL, this waterbody remains in IR Category 4a.
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, continue to adversely affect the Class A and Class B(LW) uses of Lower Gar Lake. 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 71, 64, and 72, respectively. The TSI value for Secchi depth (72, approximately 0.4 meters) suggests impairment of designated uses at this lake due to very poor water transparency. 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 middle range between eutrophic and hyper-eutrophic lakes. These index values suggest very 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 2004 was 5.2 mg/l. Of 131 lakes sampled, Lower Gar Lake had the 15th highest median level of inorganic suspended solids (17.5 mg/l), thus suggesting that non-algal turbidity limits the production of algae as well as contributes to the very poor water transparency at this lake and to the 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 (14) suggests at least the potential for occasional nitrogen limitation at this lake.
The presence of extremely large populations of zooplankton at Lower Gar Lake that graze on algae, however, may explain the discrepancy between the TSI value for phosphorus (71) and that for chlorophyll-a (64). Sampling from 2000 through 2005 showed that while Cladoceran taxa (e.g., Daphnia) comprised only about 60% 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 (285 mg/l) was the 8th highest of the 131 lakes sampled. The large population of zooplankton grazers at Lower Gar Lake suggest the potential for limiting algal production at this lake.
These conditions indicate impairments to the Class A uses of Lower Gar Lake through presence of poor water transparency that violates Iowa’s narrative water quality criterion protecting against aesthetically objectionable conditions. Based on the ISU monitoring data, the cause of this poor transparency is primarily high levels of inorganic suspended solids; moderately high levels of suspended algae also likely contribute to this problem.
The levels of nuisance (=noxious) algal species (i.e., bluegreen algae) at this lake do not suggest an impairment of Class A uses. While data from the ISU survey from 2000 through 2004 suggest that bluegreen algae (Cyanophyta) comprise a significant portion (approximately 75%) of this lake’s summertime phytoplankton community, sampling from 2000 through 2004 showed that the median per summer sample mass of bluegreen algae at Lower Gar Lake (20 mg/l) was the 88th lowest of the 131 lakes sampled. This level is in the lowest two-thirds of the 131 Iowa lakes sampled. The presence of a moderately large 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. This assessment, however, is based strictly on a distribution of the lake-specific median bluegreen algae values for the 2000-2004 monitoring period. Median levels less than the 75th percentile of this distribution (~29 mg/l) were arbitrarily considered by IDNR staff to not represent an impairment of the Class A uses of Iowa lakes. No criteria exist, however, upon which to base a more accurate identification of impairments due to bluegreen algae. Thus, while the ability to characterize the levels of bluegreen algae at this lake has improved over that of the previous (2004) assessment due to collection of additional data, the assessment category for assessments based on level of bluegreen algae nonetheless remains, of necessity, "evaluated" (indicating an assessment with relatively lower confidence) as opposed to "monitored" (indicating an assessment with relatively higher confidence).
Results of IDNR/UHL ambient chemical/physical water quality monitoring conducted in support of TMDL development in 2002 does not suggest any chemical 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. Similarly, none of the approximately 15 samples analyzed for the ISU lakes survey during summers of 2000 through 2004 violated the Class B(LW) criteria for dissolved oxygen or pH. 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 uses were assessed (monitored) as “fully supported” based on results of U.S.EPA/IDNR fish contaminant (RAFT) monitoring at Lower Gar Lake in 2000. The composite samples of fillets from both the bottom-feeder and predator species had low levels of contaminants. The existence of, or potential for, a fish consumption advisory is the basis for Section 305(b) assessments of the degree to which Iowa’s lakes and rivers support their fish consumption uses. Prior to 2006, IDNR used action levels published by the U.S Food and Drug Administration to determine whether consumption advisories should be issued for fish caught as part of recreational fishing in Iowa. In an effort to make Iowa’s consumption more compatible with the various protocols used by adjacent states, the Iowa Department of Public Health, in cooperation with Iowa DNR, developed a risk-based advisory protocol. This protocol went into effect in January 2006 (see http://www.iowadnr.gov/fish/news/consump.html for more information on Iowa’s revised fish consumption advisory protocol). Because the revised (2006) protocol is more restrictive than the previous protocol based on FDA action levels; fish contaminant data that previously suggested “full support” may now suggest either a threat to, or impairment of, fish consumption uses. This scenario, however, does not apply to the fish contaminant data generated from the 2000 RAFT sampling conducted at Lower Gar Lake: the levels of contaminants do not exceed any of the new (2006) advisory trigger levels, thus suggesting no justification for issuance of a consumption advisory for this waterbody.
