Assessment Comments
Assessment is based on results of (1) ISU lake survey from 2000-04, (2) surveys by IDNR Fisheries Bureau, and (3) ISU reports on lake plankton communities from 2000-05.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (monitored) as "fully supporting". The Class B(LW) aquatic life uses remain assessed (evaluated) as "fully supporting". Siltation impacts, however, remain a concern at this lake. Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring at this lake. 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, and (3) information on plankton communities collected at Iowa lakes from 2000 through 2005 as part of the ISU lake survey.
EXPLANATION: Results from the ISU statewide survey of Iowa lakes suggest that the Class A uses at Nelson Park Lake are “fully supported”. 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 60, 53, and 52, respectively. According to Carlson (1977), the index value for total phosphorus is in the upper boundary of eutrophic lakes. The index values for chlorophyll-a and secchi depth are in the lower range of eutrophic lakes. These index values suggest relatively low levels of phosphorus, very low levels of chlorophyll-a, and very good (and somewhat better than expected) water transparency.
According to Carlson (1991), the occurrence of a moderately high TSI value for total phosphorus with relatively lower 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. The ISU lake data do not readily suggest a primary non-phosphorus limitation for this lake. Potentially, a combination of non-algal (inorganic) turbidity and zooplankton grazing may limit algal production at Nelson Park Lake.
Based on median values from ISU sampling from 2000 through 2004, the ratio of total nitrogen to total phosphorus for Nelson Park Lake is 25. This ratio suggests that algal production at this lake is not limited by nitrogen availability.
The presence of moderately large populations of zooplankton at Nelson Park Lake that graze on algae may explain the discrepancy between the TSI value for phosphorus (60) and that for chlorophyll-a (53). 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 (97 mg/l) was the 60th highest of the 131 lakes sampled.
Relative to other Iowa lakes, the levels of inorganic suspended solids at this lake are low and do not likely present a non-phosphorus limitation to algal production 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; the median level at Nelson Park Lake was 3.6 mg/l. This median value was the 43rd lowest of the 131 lakes sampled.
The very low TSI values for chlorophyll-a and Secchi depth suggest a lack of potential threats or impairments of full support of the Class A (primary contact) due to presence of aesthetically objectionable conditions (blooms of algae or high levels of inorganic turbidity).
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 (55%) of this lake’s summertime phytoplankton community, sampling from 2000 through 2004 showed that the median per summer sample mass of bluegreen algae at Nelson Park Lake (5.8 mg/l) was the 30th lowest of the 131 lakes sampled. This level is in the lowest 25% of the 131 Iowa lakes sampled. The presence of a relatively small 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 less than the 75th percentile of this distribution (~29 mg/l) were arbitrarily considered by IDNR staff to not 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 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 as "fully supported". Siltation impacts at this lake, however, remain a concern. Results of ISU monitoring from 2000 through 2004 suggest generally good chemical water quality at this lake. The ISU lake survey data show no violations of the Class B(LW) criteria for dissolved oxygen in the 12 samples collected during summers of 2000 through 2004. Two of 13 samples exceeded the Class A,B(LW) criterion for pH (maximum = 9.1; 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 levels of primary productivity at Nelson Park Lake and do not reflect the input of pollutants into this lake.
Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring at this lake.
