Assessment Comments
Assessment is based on results of (1) ISU lake survey from 2000-04, (2) ISU reports on lake plankton communities from 2000-05, and (3) information from the IDNR Fisheries Bureau.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (monitored) as "partially supported" due to poor water transparency caused by high levels of inorganic turbidity. The Class B(LW) aquatic life uses are assessed (evaluated) as "partially supported" due to poor water transparency also caused by high levels of inorganic turbidity. 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 relatively high levels of inorganic turbidity impair the Class A uses of Roberts Creek Lake. 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 73, 61, and 67, respectively. According to Carlson (1977), the index value for total phosphorus places this lake in the range of hyper-eutrophic lakes; the index value for chlorophyll-a is in the lower range between eutrophic and hyper-eutrophic lakes, and the index value for secchi depth is in the upper range between eutrophic and hyper-eutrophic lakes. These index values suggest (1) very high levels of phosphorus in the water column, (2) moderately high (but less than expected) levels of chlorophyll-a, and (3) relatively 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 (inorganic) particles do likely limit algal production at Roberts Creek 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, Roberts Creek Lake had the 40th highest median level of inorganic suspended solids (7.6 mg/l), thus suggesting that non-algal turbidity limits the production of algae as well as contributing to in-lake turbidity. Levels of chlorophyll at this lake are only slightly elevated and thus do not likely contribute significantly to this impairment. The IDNR Fisheries Bureau concurs that high levels of inorganic turbidity exist at this lake.
Factors such as nitrogen limitation and grazing of algae by zooplankton can limit algal production in lakes. Data from the ISU lake survey from 2000-2004, however, do not suggest that either of these factors limit algal production a Roberts Creek Lake. Based on median values from ISU sampling from 2000-04, the ratio of total nitrogen to total phosphorus for this lake is 18; this ratio suggests little possibility that algal production at this lake is limited by nitrogen availability. Similarly, zooplankton grazing does not appear likely to limit algal production.
In terms of all Iowa lakes sampled, data from the ISU survey show relatively small populations of zooplankton species at Roberts Creek Lake that graze on algae. Sampling from 2000 through 2005 showed that Cladoceran taxa (e.g., Daphnia) comprised about 30% 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 (39 mg/l) was the 17th lowest of the 131 lakes sampled. These results suggest little if any non-phosphorus limitation due to zooplankton grazing at Roberts Creek Lake. .
These conditions indicate impairments (“partial support”) to the Class A (primary contact) uses due to relatively poor water transparency that violates Iowa’s narrative water quality criterion protecting against aesthetically objectionable conditions. The poor water transparency is caused primarily by relatively high levels of inorganic suspended solids.
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 (approximately 60%) of this lake’s summertime phytoplankton community, sampling from 2000 through 2004 showed that the median per summer sample mass of bluegreen algae at Roberts Creek Lake (4.4 mg/l) was the 21st 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).
Information from the IDNR Fisheries Bureau suggests that the Class B(LW) aquatic life uses should be assessed as "partially supported" due to problems with poor shoreline protection and soil types that contribute to high levels of suspended sediment and poor water transparency. 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 15 samples collected during summers of 2000 through 2004. One of 15 samples exceeded the Class A,B(LW) criterion for pH (maximum = 9.2; minimum = 8.0 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. This violation likely reflects the levels of primary productivity at Roberts Creek Lake and does 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.
