Assessment Comments
Assessment based on (1) results of IDNR beach monitoring program from 2002 through 2004, (2) ISU lake survey from 2000-04, (3) surveys by IDNR Fisheries Bureau, (4) ISU reports on lake plankton communities froom 2000-05, and (4) EPA/DNR fish tissue (RAFT) monitoring in 2004.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (monitored) as "not supporting" due to (1) high levels of indicator bacteria and (2) very poor water transparency related primarily to non-algal turbidity. The Class B(LW) aquatic life uses are assessed (evaluated) as "partially supporting" due to excessive nutrient loading to the water column, high levels of non-algal turbidity, and siltation in the lake. Drinking water uses are "not assessed" due to the lack of informaiton upon which to base an assessment. Fish consumption uses are assessed (monitored) as “fully supported” based on fish contaminant monitoring conducted in 2004. 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) IDNR/UHL beach monitoring from 2002 through 2004, (3) surveys by IDNR Fisheries Bureau, (4) information on plankton communities collected at Iowa lakes from 2000 through 2005 as part of the ISU lake survey, and (5) results of U.S. EPA / IDNR fish tissue monitoring in 2004.
Note: A TMDL for siltation and nutrient at Lake Darling was prepared by IDNR and approved by EPA in 2003. This TMDL addresses impairments at this lake due to turbidity and siltation. Because, however, the Section 303(d) impairment due to indicator bacteria (aka, pathogens) was not addressed in the TMDL, this waterbody is placed in IR Category 5a (impaired; TMDL required) for the 2006 assessment/listing cycle.
EXPLANATION: Results of IDNR beach monitoring at Lake Darling from 2002 through 2004 suggest that the Class A uses are "not supported." Levels of indicator bacteria were monitored once per week during the primary contact recreation seasons (May through September) of 2002 (31 samples), 2003 (29 samples), and 2004 (23 samples) as part of the IDNR beach monitoring program. According to IDNR’s assessment methodology, two conditions need to be met for results of beach monitoring to indicate “full support” of the Class A (primary contact recreation) uses: (1) all five-sample, thirty-day geometric means for the three-year assessment period are less than the state’s geometric mean criterion of 126 E. coli orgs/100 ml and (2) not more than 10 % of the samples during any one recreation season exceeds the state’s single-sample maximum value of 235 E. coli orgs/100 ml. If a 5-sample, 30-day geometric mean exceeds the state criterion of 126 orgs/100 ml during the three-year assessment period, the Class A uses should be assessed as “not supported”. Also, if more than 10% of the samples in any one of the three recreation seasons exceed Iowa’s single-sample maximum value of 235 E. coli orgs/100 ml, the Class A uses should be assessed as “partially supported”. This assessment approach is based on U.S. EPA guidelines (see pgs 3-33 to 3-35of U.S. EPA 1997b).
At Lake Darling beach, the geometric means of 9 thirty-day periods during the summer recreation seasons of 2002, 2003 and 2004 exceeded the Iowa water quality standard of 126 E. coli orgs/100 ml: 7 of 27 geometric means violated in 2002, 1 of 25 geometric means violated in 2003, and 1 of 19 geometric means violated in 2004. Also, the percentage of samples exceeding Iowa’s single-sample maximum criterion (235 E. coli orgs/100 ml) was greater than 10% in all three recreation seasons: 2002: 19%; 2003: 14%; 2004: 22%). According to IDNR’s assessment methodology and U.S. EPA guidelines, these results suggest impairment (nonsupport) of the Class A (primary contact recreation) uses.
Results from the ISU statewide survey of Iowa lakes suggest that the Class A uses are also impaired by high levels of non-algal turbidity. 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 81, 50, and 74, respectively. According to Carlson (1977), the index value for total phosphorus places this lake in the upper range of hyper-eutrophic lakes; the index value for chlorophyll-a is at the boundary between mesotrophic and eutrophic lakes, and the index value for secchi depth is in the range of hyper-eutrophic lakes. These index values suggest extremely high levels of phosphorus in the water column, low (and much less than expected) levels of chlorophyll-a, 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 Lake Darling. 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, Lake Darling had the 16th highest median level of inorganic suspended solids (15.1 mg/l), thus suggesting that non-algal turbidity limits the production of algae and is the primary cause of the very poor water transparency at this lake.
In addition to inorganic turbidity, other non-phosphorus limitations may exist at this lake. Based on the 2000-04 median values from ISU sampling, the ratio of total nitrogen to total phosphorus (TN:TP) at Lake Darling is 17. This ratio is somewhat low and suggests at least the potential for nitrogen availability to limit algal production at this lake.
Populations of zooplankton taxa (Cladocerans) that graze on algae are very large at Lake Darling and thus potentially have an impact on algal production and may explain the discrepancy between the TSI value for phosphorus (81) and that for chlorophyll-a (50). Sampling from 2000 through 2005 showed that, while Cladoceran taxa (e.g., Daphnia) comprised only about 40% 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 (168 mg/l) was the 27th highest of the 131 lakes sampled.
These conditions suggest an suggest impairments to the Class A (primary contact) uses primarily due to presence high levels of inorganic turbidity and very poor water transparency that violate Iowa’s narrative water quality standard protecting against aesthetically objectionable conditions. The IDNR Fisheries Bureau concurs that turbidity-related impairments exist at this lake. The level of chlorophyll-a at Lake Darling is very low (15th lowest of the 131 lakes sampled) and does not appear to contribute to threats or impairments of designated uses at this lakes.
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 70%) of this lake’s summertime phytoplankton community, sampling from 2000 through 2004 showed that the median per summer sample mass of bluegreen algae at Lake Darling (14.2 mg/l) was the 54th highest 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 greater than the 75th percentile of this distribution (~29 mg/l) were arbitrarily considered by IDNR staff to 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).
The hyper-eutrophic conditions at this lake, along with information from the IDNR Fisheries Bureau, suggest that the Class B(LW) aquatic life uses should be assessed (evaluated) as "partially supported" due to excessive nutrient loading to the water column, high levels of non-algal turbidity, and siltation in the lake. The ISU lake survey data, however, suggest relatively good chemical water quality at this lake. Results of ISU monitoring from 2000 through 2004 suggest generally good chemical water quality at this lake. Results of this monitoring show that only 2 of the 14 samples collected violated the Class B(LW) criteria for dissolved oxygen and that only 2 of the 13 samples collected exceeded Iowa’s Class A,B(LW) criteria for pH (maximum = 9.4; minimum = 7.9 pH units). Based on IDNR’s assessment methodology, these results do not suggest violation frequencies that are significantly greater than 10% for either parameter and thus do not suggest impairment of either the Class A and Class B(LW) uses of Lake Darling.
The Class C (drinking water) uses remained "not assessed" due to the lack of water quality information. The only parameter collected as part of the ISU lake survey relevant to support of Class C (drinking water) uses is nitrate. While the results of the ISU survey from 2000-04 show that nitrate levels at this lake are low relative to the drinking water MCL (10 mg/l) (N=15, maximum sample value = 6.9 mg/l; median = 2.9 mg/l), these data are not sufficient for developing a valid assessment of support of the Class C uses.
Fish consumption uses were assessed (monitored) as “fully supported” based on results of U.S.EPA/IDNR fish contaminant (RAFT) monitoring at Lake Darling in 2004. The composite samples of fillets from channel catfish and largemouth bass had low levels of contaminants. Levels of primary contaminants in the composite sample of channel catfish fillets were as follows: mercury: 0.092 ppm; total PCBs: 0.09 ppm; and technical chlordane: 0.049 ppm. Levels of primary contaminants in the composite sample of largemouth bass fillets were as follows: mercury: 0.11 ppm; total PCBs: 0.09 ppm; and technical chlordane: <0.03 ppm. 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 2004 RAFT sampling conducted at Lake Darling: the levels of contaminants do not exceed any of the new (2006) advisory trigger levels, thus indicating no justification for issuance of a consumption advisory for this waterbody.
