Assessment Comments
Assessment is based on results of (1) ISU lake survey from 2000-04, (2) surveys by IDNR Fisheries Bureau (3) ISU reports on lake plankton communities from 2000-05, and (4) results of U.S. EPA/IDNR fish tissue monitoring in 1998.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses remain assessed (monitored) as "partially supported" due to (1) poor water transparency caused by algal blooms and high levels of inorganic turbidity and (2) high levels of pH that frequently exceed the Class A water quality criterion. In addition, the large populations of bluegreen algae potentially represent an additional impairment of the Class A uses as well. The Class B(LW) aquatic life uses are assessed (monitored) as "partially supported" primarily due to frequent violations of the Class B(LW) pH criterion and secondarily to nutrient loading, algal blooms, and re-suspension of sediment. Fish consumption uses are assessed (evaluated) as "fully supported" based on results of fish contaminant monitoring in 1998 and utilization of Iowa’s revised fish consumption advisory protocol. 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, and (4) results of U.S. EPA/IDNR fish contaminant monitoring in 1998. A dredging project at this lake was conducted during the 2002-2004 data collection period used for this assessment. Thus, this project likely affected the data and the water quality assessments based on these data. Where possible, lake monitoring data collected in 2005 and 2006 have been utilized to better determine the impact of the dredging project on water quality at Little Wall Lake.
Note: A TMDL for algae and turbidity at Little Wall Lake was prepared by IDNR and approved by EPA in 2006. Because all Section 303(d) impairments identified for the 2006 assessment/listing cycle (algal growth, pH, and turbidity) are addressed by the TMDL, this waterbody is placed in IR Category 4a (impaired; TMDL approved).
EXPLANATION: Results of monitoring conducted by ISU from 2000 through 2004 as part of the statewide survey of Iowa lakes suggest that the Class A (primary contact) uses are only "partially supported" due to algal blooms and high levels of inorganic 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 72, 68, and 76, respectively, for Little Wall Lake. According to Carlson (1977), these index values place this lake in the range of hyper-eutrophic lakes and suggest (1) very high levels of phosphorus in the water column, (2) relatively high levels of suspended algae, and (3) extremely 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 Little Wall Lake, although zooplankton grazers may limit algal populations as well.
Data on inorganic suspended solids from the ISU survey suggest that this lake is subject to occasional episodes of extremely high levels of non-algal turbidity. 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 of inorganic suspended solids at Little Wall Lake (20.0 mg/l) was the 11th highest of the 131 lakes, thus strongly suggesting that non-algal turbidity limits the production of algae as well as contributes to inorganic turbidity that impairs beneficial uses.
Based on median values from ISU sampling from 2000 through 2002, the ratio of total nitrogen to total phosphorus for this lake is 20. This ratio does not suggest a strong potential for nitrogen availability to limit algal production.
The presence of extremely large populations of zooplankton at this lake that graze on algae, however, may also account for the discrepancy between the TSI value for phosphorus (72) and that for chlorophyll-a (68). Sampling from 2000 through 2005 showed that Cladoceran taxa (e.g., Daphnia) comprised about 65% 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 (264 mg/l) was the 9th highest of the 131 lakes sampled. The large population of zooplankton grazers at Little Wall Lake suggest the potential for limiting algal production at this lake.
These conditions indicate impairments to the Class A (primary contact) due to very poor water transparency that violates Iowa’s narrative water quality criterion protecting against aesthetically objectionable conditions. Based on the ISU monitoring data, this very poor water transparency appears to be caused by a combination of suspended algae and inorganic suspended solids. The IDNR Fisheries Bureau agrees that high levels of suspended sediment increase turbidity and reduce water transparency but that hydraulic dredging scheduled for 2003 should have improved this problem.
Despite the generally poor water quality at this lake, a comparison of the TSI values calculated for the 2000-2002 period to the corresponding values calculated for the 2002-2004 period does suggest some improvement in water quality at this lake. The TSI values for total phosphorus improved from 82 to 72, while the TSI values for chlorophyll-a improved from 71 to 68; the TSI values for Secchi depth improved from 79 to 76. Whether the slight improvement in water transparency seen in recent monitoring data reflects the impact of the dredging project is difficult to determine. Regardless, the monitoring data generated subsequent to the dredging project at Little Wall Lake continue to suggest water quality impairments related to very poor water transparency.
The presence of nuisance (=noxious) algal species (i.e., bluegreen algae) may also present an impairment of the Class A uses at Little Wall Lake. Data from the ISU survey from 2000 through 2004 suggest that bluegreen algae (Cyanophyta) comprise a significant portion of this lake’s summertime phytoplankton community. Summer sampling during this period showed the percent wet mass of the total phytoplankton community in bluegreens (cyanobacteria) was approximately 90%. Also, the median per summer sample mass of bluegreen algae at this lake (41 mg/l) was the 22nd highest of the 131 lakes sampled. This median is in the worst 25% of the 131 Iowa lakes sampled. The presence of a large population of bluegreen algae at this lake suggests the 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 chosen by IDNR staff to represent the condition of “potential impairment: partially supported.” 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 are "partially supported" due to excessive nutrient loading to the water column, nuisance blooms of algae, and re-suspension of sediment. The ISU lake survey data show no violations of the Class B(LW) criteria for dissolved oxygen in the 14 samples collected during summers of 2000 through 2004. Seven of 14 samples, however, exceeded the Class B(LW) criterion for pH (maximum = 9.6; minimum = 6.6 pH units). These violations likely reflect the excessive primary productivity at Little Wall Lake and do not reflect the input of pollutants into this lake.
Fish consumption uses were assessed (evaluated) as “fully supported” based on results of U.S.EPA/IDNR fish contaminant (RAFT) monitoring at Little Wall Lake in 1998 Because these data are now considered too old (greater than five years) to accurately characterize current water quality conditions, the assessment category is considered “evaluated” (indicating an assessment with relatively lower confidence) as opposed to "monitored" (indicating an assessment with relatively higher confidence). The existence of, or potential for, a fish consumption advisory is the basis for Section 305(b) assessments of support of fish consumption uses in Iowa’s rivers and lakes. 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. Although this scenario does not apply to the fish contaminant data generated from the 1998 RAFT sampling conducted at Little Wall Lake, levels of contaminants are sufficiently high for concern and justify follow-up monitoring. The levels of at least one contaminant now exceeds one or more advisory trigger levels, thus suggesting the need for additional (follow-up) monitoring to determine whether a consumption advisory should be issued. The composite samples of fillets from channel catfish and largemouth bass had generally low levels of contaminants. The level of mercury in the sample of largemouth bass fillets (0.21 ppm), however, slightly exceeds the IDNR/IDPH trigger level of 0.20 ppm for a one meal per week consumption advisory.
According to IDNR’s assessment methodology, the single occurrence of contaminant above an advisory trigger level neither warrants issuance of an advisory nor indicates impairment of the fish consumption uses: two consecutive samplings that show contaminant levels are above the trigger level in fillet samples are needed to justify issuance of an advisory. But, this elevated level does indicate a concern and the need to conduct additional monitoring to better define contaminant levels in fish from this river segment. Thus, follow-up monitoring will be conducted in 2007 to better determine (1) levels of mercury in the edible portions (fillets) of fish in this river segment and (2) whether a one-meal-per-week consumption advisory needs to be issued.
