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) EPA/IDNR fish contaminant monitoring in 2002.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (monitored) as "fully supporting". The Class B(LW) aquatic life uses are assessed (evaluated) as "fully supporting". High levels of nutrients and siltation impacts, however, continue to be concerns at this lake regarding support of the Class B(LW) uses. Fish consumption uses are assessed (monitored) as “fully supported” based on results of fish contaminant monitoring in 2002. 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, and (4) results of U.S. EPA/IDNR fish contaminant monitoring in 2002.
EXPLANATION: Results from the ISU statewide survey of Iowa lakes suggest “full support” of the Class A and Class B(LW) uses of Lake Pahoja. 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 67, 62, and 60, respectively. According to Carlson (1977), the index values for total phosphorus and chlorophyll-a place this lake in the range between eutrophic and hyper-eutrophic lakes; the index value for secchi depth is at the upper boundary of eutrophic lakes. These index values suggest relatively high levels of phosphorus in the water column, somewhat elevated (but less than expected) levels of chlorophyll-a, and relatively good water transparency.
According to Carlson (1991), the occurrence of a high TSI value for total phosphorus with relatively low values for chlorophyll-a and secchi depth indicate that some factor (e.g., nitrogen limitation, zooplankton grazing, or some other factor) other than phosphorus limits production of algae. Based on median values from ISU sampling from 2000 through 2004, the ratio of total nitrogen to total phosphorus for Lake Pahoja is 40; this TN:TP ratio indicates that (1) algal production is not limited by nitrogen availability and (2) an excess of total nitrogen exists at this lake.
The presence of relatively large populations of zooplankton at Lake Pahoja that graze on algae, however, may explain the discrepancy between the TSI value for phosphorus (67) and that for chlorophyll-a (62). Sampling from 2000 through 2005 showed that Cladoceran taxa (e.g., Daphnia) comprised about half 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 (177 mg/l) was the 23rd highest of the 131 lakes sampled.
Levels of inorganic suspended solids at this lake are moderately high and potentially contribute to the non-phosphorus limitations of algal production. 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 for Lake Pahoja was slightly above this median at 5.8 mg/l.
The presence of nuisance (=noxious) algal species (i.e., bluegreen algae) may also present an impairment of the Class A uses at Lake Pahoja. 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 85%. Also, the median per summer sample mass of bluegreen algae at this lake (27 mg/l) was the 95th highest of the 131 lakes sampled. This median is in the worst 25% of the 131 Iowa lakes sampled. The presence of a relatively 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).
Information from the IDNR Fisheries Bureau suggests that the Class B(LW) aquatic life uses should be assessed as "fully supported". Excessive nutrient loading to the water column and siltation, however, remain concerns for support of the aquatic life uses at this lake. 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. Two of 14 samples, however, exceeded the Class B(LW) criterion for pH (maximum = 9.3; minimum = 7.9 pH units). Based on IDNR’s assessment methodology, however, these results do not suggest that significantly more than 10 percent of the samples exceed Iowa’s pH criteria and thus do not suggest an impairment of the Class A and Class B(LW) uses of this lake. These violations likely reflect the excessive primary productivity at Lake Pahoja and do not reflect the input of pollutants into this lake.
Fish consumption uses were assessed (monitored) as “fully supported” based on results of U.S.EPA/IDNR fish contaminant (RAFT) monitoring at Lake Pahoja in 2002. The composite samples of fillets from channel catfish had low levels of contaminants: mercury: 0.036 ppm; total PCBs: <0.09 ppm; and technical chlordane: <0.030 ppm. A predator sample was not collected as part of the 2002 RAFT sample. Typically, samples of both bottom-feeding fish (e.g., common carp) and predator species (e.g., largemouth bass) are collected at RAFT status sites such as the 2002 sampling at Lake Pahoja. Predator species, however, are sometimes difficult to capture, and RAFT status samples occasionally contain only the bottom-feeder sample. 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 Lake Pahoja: 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.
