Assessment Comments
Assessment is based on results of (1) IDNR/UHL beach monitoring from 2002 through 2004, (2) ISU lake survey from 2000-04, (2) surveys by IDNR Fisheries Bureau, (3) ISU reports on lake plankton communities from 2000-05, (4) results of EPA/DNR fish tissue monitoring in 2002, and (5) an IDNR investigation of a fish kill in May 2004.
Basis for Assessment
SUMMARY: Results of water quality monitoring over the 2000-2004 period show significant improvements in water quality at North Twin Lake. Despite these improvements, the Class A (primary contact recreation uses) are assessed (evaluted) as "partially supported" due to the large populations of bluegreen algae that continue to occur at this lake. The Class B(LW) aquatic life uses are assessed (evaluated) as "partially supported" due a fish kill in May 2004 attributed to natural causes. Nutrient loading to water column, infrequent algal blooms, and occasional episodes of high inorganic turbidity, however, remain concerns for support of the Class B(LW) uses as this lake. Fish consumption are assessed as “fully supported” based on results of fish contaminant monitoring in 2002. Sources of data for this assessment include (1) the results of the IDNR-UHL beach monitoring program in summers of 2003 and 2004, (2) results of the statewide survey of Iowa lakes sponsored by IDNR and conducted by Iowa State University (ISU) from 2000 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 contaminant (RAFT) monitoring in 2002. Information from the IDNR Fisheries Bureau and results from the ISU statewide survey of Iowa lakes suggest that water quality conditions at North Twin Lake appear to be improving. Additional years of monitoring data are needed to determine the relative permanence of the water quality improvements at this lake that occurred from 2002-2004.
Note: A TMDL for nutrients and algae at North Twin Lake was prepared by IDNR and approved by EPA in 2004; thus, this lake was placed into IR Category 4a (TMDL approved) for the 2004 assessment/listing cycle. Apparently due to events not related to the TMDL (carp migration in 2001), the conditions that previously indicated impairments of the Class A uses at this lake (high levels of algae and turbidity) did not exist during the 2002-2004 assessment period, thus indicating an improving trend in water transparency. Thus, this waterbody was moved from IR Category 4a to IR Category 2b (potentially impaired; need exists for follow-up monitoring) for the 2006 assessment/listing cycle.
EXPLANATION: Results of monitoring for indicator bacteria at both the West Beach and East Beach at North Twin Lake suggest “full support” of the Class A uses. Results of IDNR / UHL monitoring at West Beach in 2003 and 2004 suggest that the Class A (primary contact recreation) uses are "fully supported." Levels of indicator bacteria at West Beach were monitored once per week during the primary contact recreation seasons (May through September) of 2003 (14 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 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 West Beach, the geometric means of all 30 thirty-day periods during the summer recreation seasons of 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 either of the two recreation seasons (2003: 0%; 2004: 0%). According to U.S. EPA guidelines, these results suggest “full support” of the Class A (primary contact recreation) uses.
Results of IDNR / UHL monitoring at North Twin Lake’s East Beach in 2003 (15 samples) and 2004 (23 samples) also suggest that the Class A (primary contact recreation) uses are "fully supported." At East Beach, the geometric means of all 30 thirty-day periods during the summer recreation seasons of 2003 and 2004 were also 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 either of the two recreation seasons (2003: 7%; 2004: 0%). According to U.S. EPA guidelines, these results suggest “full support” of the Class A (primary contact recreation) uses. These results also suggest that levels of indicator bacteria at North Twin Lake are very low.
Results of monitoring conducted by ISU from 2000 through 2004 as part of the statewide survey of Iowa lakes also suggest that the Class A (primary contact) uses at North Twin Lake are "fully supported." 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 70, 64, and 60, respectively. According to Carlson (1977), these index values place this lake in the range of hyper-eutrophic lakes and suggest very high levels of phosphorus in the water column, moderately high production of suspended algae, and relatively good water transparency.
A comparison of the TSI values calculated for the 2000-2002 period to the corresponding values calculated for the 2002-2004 period suggests considerable improvement in water quality at this lake. While the TSI values for total phosphorus improved only slightly from 72 to 70, the TSI values for chlorophyll-a improved from 74 (83 ug/l) to 64 (30 ug/l), and the TSI values for Secchi depth improved from 71 (0.5 m) to 60 (1.0 m). Based on annual TSI values, lake water quality appears to have improved in 2002: prior to 2002, TSI values for chlorophyll-a and Secchi depth were in the upper 60’s and 70’s (indicating “nonsupport”); in 2002 and 2003, these TSI values were in the upper 40’s and 50’s (indicating “full support”, thus suggesting a dramatic shift (improvement) in water quality at this lake. Whether this shift to improved water quality persists remains to be seen. The TSI values for 2004 were somewhat elevated relative to those for 2002 and 2003 (TSI of 61 for Secchi depth and 64 for chlorophyll-a). Additional years of monitoring will be needed to determine the relative permanence of this dramatic improvement in water quality.
According to the IDNR Fisheries Bureau, the primary cause for the improvements in water quality at this lake was the movement of a large number of common carp out of the lake during a high-water event in early May 2001. An approximately 8-inch rainfall on May 1st and 2nd, 2001, caused a rise in the level of North Twin Lake such that some lakeside residents blamed flooded basements on the elevated lake level. In response to these complaints, the local IDNR Fisheries biologist was instructed to open the gates at the North Twin Lake outlet to reduce the lake level. The biologist reported that common carp were extremely numerous near the lake outlet, apparently in response to onset of the spawning season for this species. When the gates were opened, large numbers of common carp moved into South Twin Lake. When the gates were again closed, the carp that had moved to South Twin Lake were prevented from moving back into North Twin Lake.
IDNR Fisheries states that the improvements in lake water quality were rapid and dramatic. The next year (2002), submersed aquatic vegetation (SAV) was observed in the littoral zone of the lake, and fisheries surveys documented an increasing population of bluegill (which, prior to the migration, were not part of the North Twin fishery). In 2003, the SAV increased, and sampling showed thousands of young bluegill and improvement in the crappie population. Over the period 2002-2006, what was once a catfish and crappie fishery has turned into a bluegill, walleye, yellow bass fishery, thus reflecting the shift from a phytoplankton-dominated and turbid system to a SAV/clear-water system. Water clarity has improved dramatically, with some exceptional Secchi depth readings (e.g., in the deepest part of the lake the Secchi disk could be seen resting on the bottom of the lake). IDNR Fisheries cautions that carp are still present in North Twin Lake. And, although numbers are still relatively low, the carp population appears to be increasing. The growth of SAV, however, remains strong (strong to the point that IDNR has received complaints about excessive SAV in the littoral zone of this lake).
Thus, the conditions that previously indicated impairments to the Class A (primary contact) uses (presence of aesthetically objectionable conditions related to blooms of algae and to high levels of inorganic turbidity that lead to poor water transparency) no longer exist at this lake.
Results of ISU monitoring show that algal production at this lake is not strongly limited by nitrogen availability. Based on median values from ISU sampling from 2000 through 2004, the ratio of total nitrogen to total phosphorus for this lake is 19. This TN:TP ratio does not suggest any strong nitrogen limitation to algal production at this lake.
Data on inorganic suspended solids from the ISU survey suggest that this lake is also subject to occasional episodes of 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 North Twin Lake (6.4 mg/l) slightly exceeded this median and was the 51st highest of the 131 lakes, thus suggesting that non-algal turbidity may limit the production of algae at this lake. Corresponding to the improvements seen in TSI values for chlorophyll-a and Secchi depth, the overall median ISS for this lake improved from 9.5 mg/l for the 2000-02 period (11th highest median) to 6.4 mg/l for the 2002-04 period (only the 51st highest of the 131 lakes).
The presence of very large populations of zooplankton at North Twin Lake that graze on algae, however, may explain the discrepancy between the TSI value for total phosphorus (70) and that for chlorophyll-a (64). In terms of all Iowa lakes sampled, data from the ISU survey show large 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 two-thirds 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 (157 mg/l) was the 32nd highest of the 131 lakes sampled. This population of zooplankton grazers suggests the potential for this type of non-phosphorus limitation on algal production at North Twin Lake.
Information from the IDNR Fisheries Bureau suggests that water quality conditions at North Twin Lake have improved in recent years. The data for inorganic suspended solids support this conclusion, with extremely high levels reported in summers of 2000 and 2001 with much lower levels reported in summers of 2002 through 2004. Average summer levels declined from 34 mg/l in 2000 and 17 mg/l in 2001 to 5 mg/l in summer of both 2002 and 2003 and 7 mg/l in summer 2004.
Data from the ISU survey from 2002-2004 suggest that the presence of nuisance aquatic species (i.e., bluegreen algae) may present an impairment of the Class A uses at this lake. Summer sampling during this period showed the percent wet mass of the total phytoplankton community in bluegreens was approximately 80%. In addition, North Twin Lake had the 5th highest median per summer sample mass of bluegreen algae of the 131 lakes sampled: 106 mg/l. This median is in the worst 10 % of the 131 Iowa lakes sampled. The presence of a large population of bluegreen algae at this lake suggests a potential violation of Iowa’s narrative water quality standard protecting against occurrence of nuisance aquatic life. The improvements in chlorophyll levels and water transparency at this lake over the 2000-04 period are, however, not reflected in levels of bluegreen algae at this lake: the highest three levels in the 15 samples collected over the 2000-04 period occurred in 2003 and 2004. 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 90th percentile of this distribution (~59 mg/l) were arbitrarily chosen by IDNR staff to represent the condition of “potential impairment; not supported.” No criteria exist, however, upon which to base a more accurate identification of impairments due to bluegreen algae. Thus, while the , 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, of necessity, remains "evaluated" (indicating an assessment with relatively lower confidence) as opposed to "monitored" (indicating an assessment with relatively higher confidence.
The improving water quality conditions at this lake, along with information from the IDNR Fisheries Bureau, suggest that the assessment of the Class B(LW) aquatic life uses should be changed from “partially supported” to “fully supported”. While this change in assessment was first made for the 2004 reporting cycle, subsequent (2003 and 2004) data confirm that water quality conditions (with the possible exception of blooms of bluegreen algae) have improved at North Twin Lake. Thus, IDNR Fisheries feels that problems with algal blooms and bluegreen algae have lessened in recent several years. Some of this improvement is attributed to a sanitary sewer line installed around parts this lake, thus eliminating the contributions of numerous septic tanks to the nutrient-related problems (e.g., algal blooms and nuisance species of algae) that have impaired the Class A and Class B uses at this lake in the past.
Results of ISU monitoring from 2000 through 2004 suggest generally good chemical water quality at this lake. Results of this monitoring show that only 1 of the 14 samples collected exceeded the Class B(LW) criteria for dissolved oxygen and that only 2 of the 14 samples collected exceeded Iowa’s Class A,B(LW) criteria for pH (maximum = 9.1; minimum = 7.9 pH units). Based on IDNR’s assessment methodology, these results do not suggest violation frequencies 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 North Twin Lake.
Despite indications of "full support" of the Class B(LW) uses based on information from the IDNR Fisheries Bureau and the ISU lakes survey, the occurrence of a fish kill in this lake in May 2004 suggests a potential impairment of these uses. This kill occurred on or before May 10, 2004. The cause of the kill was identified as natural/ spawning stress. An estimated 500 fish were killed; all fish killed were crappie. No estimated value of the fish killed was provided. According to the IDNR investigation, approximately 500 crappies, all about nine inches long, were observed killed at this lake. IDNR staff noted that healthy fish were also observed at the time of the kill; thus indicating little long-term impact to the lake’s fishery. The kill was believed due to post-spawning stress and was not believed to be related to water quality problems. 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 (monitored) as “fully supported” based on results of U.S.EPA/IDNR fish contaminant (RAFT) monitoring at North Twin Lake in 2002. The composite samples of fillets from channel catfish and white crappie had extremely low levels of contaminants, with levels of the primary contaminants (mercury, PCBs, and chlordane) all below their respective levels of detection. Levels of primary contaminants in the composite sample of channel catfish fillets were as follows: mercury: <0.0181 ppm; total PCBs: <0.09 ppm; and technical chlordane: <0.03 ppm. Levels of primary contaminants in the composite sample of white crappie fillets were as follows: mercury: <0.0181 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 2002 RAFT sampling conducted at North Twin Lake: 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.
