Assessment Comments
Assessment is based on (1) ISU lake survey from 2000-04, (2) ISU report on lake plankton communities from 2000-05, (3) surveys by the DNR Fisheries Bureau, (4) DNR beach monitoring from 2002-04, (4) EPA/DNR fish tissue (RAFT) monitoring in 1996, and (5) an IDNR investigation of a fish kill in 2004.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (monitored) as "not supporting" due to presence of aesthetically objectionable conditions caused by high levels of suspended inorganic materials in the water column and by blooms of algae. An additional concern is the presence of moderately large populations of nuisance algal species (i.e., bluegreen algae). Results of IDNR beach monitoring, however, indicate low levels of indicator bacteria and "full support" of the Class A uses. The Class B(LW) aquatic life uses remain assessed (evaluated) as "fully supporting". Algal blooms, however, remain a concern at this lake. Fish consumption uses remain assessed (evaluated) as "fully supported" based on results of fish contaminant monitoring in 1996. The sources of data for this assessment include (1) the results of the IDNR-UHL beach monitoring program in summers of 2002, 2003, and 2004, (2) results of the statewide survey of Iowa lakes conducted from 2000 through 2004 by Iowa State University (ISU), (3) information from the IDNR Fisheries Bureau, (4) information on plankton communities collected from 2000 through 2005 for the ISU lakes survey, (5) results of U.S. EPA / IDNR fish tissue monitoring in 1996, and (6) results of an IDNR investigation of a fish kill in 2004. Note: In the absence of the impairment of Class A uses (IR Category 5a (=303(d) list)), this waterbody would be placed into Category 2b (at least one use assessed as "fully supported" with at least one other use "evaluated" as "impaired").
EXPLANATION: Results of IDNR beach monitoring at Lake Manawa from 2002 through 2004 suggest that the Class A uses are "fully supported." Levels of indicator bacteria were monitored once per week during the primary contact recreation seasons (May through September) of 2002 (30 samples), 2003 (29 samples), and 2004 (16 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. This assessment approach is based on U.S. EPA guidelines (see pgs 3-33 to 3-35of U.S. EPA 1997b).
At Lake Manawa beach, the geometric means of all 63 thirty-day periods during the summer recreation seasons of 2002, 2003 and 2004 were below the Iowa water quality standard of 126 orgs/100 ml. Also, the percentage of samples exceeding Iowa’s single-sample maximum criterion did not exceed 10% in any of the three recreation seasons (2002: 0%; 2003: 3%; 2004: 6%). According to IDNR’s assessment methodology, these results suggest “full support” of the Class A (primary contact recreation) uses.
Although results of IDNR beach monitoring continue to suggest "full support" of the Class A uses, results from the ISU statewide survey of Iowa lakes suggest that very high levels of turbidity related primarily high levels of inorganic suspended solids and secondarily to algal blooms adversely affect the Class A uses of Lake Manawa. 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 75, respectively. According to Carlson (1977), the index value for total phosphorus is in the lower range of hyper-eutrophic lakes, and the value for chlorophyll-a is in the middle range between eutrophic and hyper-eutrophic lakes. The index value for Secchi depth is in the range of hyper-eutrophic lakes. These index values suggest very high levels of phosphorus in the water column, moderately high production of suspended algae, and very poor water transparency. The results of ISU monitoring suggest that the very high levels of inorganic suspended solids and the somewhat elevated levels of chlorophyll likely combine to reduce water transparency and impair designated uses at this 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, Lake Manawa had the 24th highest median level of inorganic suspended solids (12.3 mg/l), thus suggesting that non-algal turbidity limits algal production as well as contributes to reductions in water transparency that impair the primary contact recreation uses.
These conditions suggest impairments to the Class A (primary contact) uses through presence of aesthetically objectionable conditions (very poor water transparency) caused primarily by very high levels of inorganic suspended solids and secondarily by blooms of algae.
The IDNR Fisheries Bureau concurs that this lake is impaired by algal blooms. Algal production at this lake appears to be limited by availability of phosphorus. Based on median values from ISU sampling from 2000 through 2004, the ratio of total nitrogen to total phosphorus for this lake is 13; this ratio does not suggest a significant potential for nitrogen limitation.
In terms of all Iowa lakes sampled, data from the ISU survey show relatively small populations of zooplankton species at this lake that graze on algae. Sampling from 2000 through 2005 showed that Cladoceran taxa (e.g., Daphnia) comprised approximately one-third 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 (66 mg/l) was the 25th lowest of the 131 lakes sampled. These results suggest little if any non-phosphorus limitation due to zooplankton grazing at Lake Manawa.
The presence of nuisance (=noxious) algal species (i.e., bluegreen algae) may also present an impairment of the Class A uses at Lake Manawa. 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 75%. Also, the median per summer sample mass of bluegreen algae at this lake (35.6 mg/l) was the 26th 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).
Based on information from the DNR Fisheries Bureau and based on the results of ISU monitoring, the Class B(LW) aquatic life uses of this lake are assessed (evaluated) as "fully supported.” The ISU lake survey data show generally good chemical water quality at Lake Manawa. None of the 14 samples collected during the 2000-2004 period violated Class B(LW) criteria for dissolved oxygen. Only one of 15 samples violated of the Class B(LW) criteria for pH (7%) during summers of 2000 through 2004 (maximum = 9.4; minimum = 8.1 pH units). Based on IDNR’s assessment methodology, these results do not suggest that violation frequency for pH is significantly greater than 10% and thus does not suggest impairment of either the Class A and Class B(LW) uses of Lake Manawa. This violation likely reflects the influence of primary productivity and does not reflect the input of pollutants into this lake.
Despite indications of "full support" based on information from the IDNR Fisheries Bureau and from the ISU lake survey, the occurrence of a fish kill in this lake in May 2004 suggests that the Class B(LW) aquatic life uses should be assessed (evaluated) as "partially supported". This kill occurred on or before May 10, 2004. The cause of the kill was identified as disease (bacterial). An estimated 50 fish were killed; no estimated value of the fish killed was provided. The species killed were not identified. According to IDNR’s assessment/listing methodology, the occurrence of a single pollutant-caused fish kill, or a fish kill of unknown origin, on a waterbody or waterbody reach during the most recent assessment period (2002-2005) indicates a severe stress to the aquatic community and suggests that the aquatic life uses should be assessed as “impaired”. If a cause of the kill was not identified during the IDNR investigation, or if the kill was attributed to non-pollutant causes (e.g., winterkill), the assessment type will be considered “evaluated.” Such assessments, although suitable for Section 305(b) reporting, lack the degree of confidence to support addition to the state Section 303(d) list of impaired waters (IR Category 5). Waterbodies affected by such fish kills will be placed in IR subcategories 2b or 3b and will be added to the state list of waters in need of further investigation.
Fish consumption uses were assessed (evaluated) as “fully supported” based on results of U.S.EPA/IDNR fish contaminant (RAFT) monitoring at Lake Manawa in 1996. 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. This scenario, however, does not apply to the fish contaminant data generated from the 1996 RAFT sampling conducted at Lake Manawa: levels of all contaminants from this monitoring were below advisory trigger levels, thus suggesting the continued “full support” of fish consumption uses.
