Assessment Comments
Assessment is based on results of (1) ISU lake survey from 2000-02, (2) ISU report on lake plankton communities, 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 high levels of inorganic turbidity that reduce water transparency. The Class B(LW) aquatic life uses are assessed (evaluated) as "partially supported" due to poor water transparency caused by high levels of suspended sediment in the water column. 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 conducted from 2000 through 2002 by Iowa State University (ISU), (2) information on plankton communities at Iowa lakes in 2000 from Downing et al. (2002), and (3) information from the IDNR Fisheries Bureau.
EXPLANATION: Results from the ISU statewide survey of Iowa lakes suggest that high levels of inorganic turbidity impair the Class A uses of Roberts Creek Lake. 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 73, 60, 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 upper range of 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) relatively low (and less than expected) levels of chlorophyll-a, and (3) moderately 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 2002 was 4.8 mg/l. Of 131 lakes sampled, Roberts Creek Lake had the 49th highest median level of inorganic suspended solids (7.0 mg/l), thus suggesting that non-algal turbidity limits the production of algae as well as contributing to in-lake turbidity. These conditions suggest an impairment to the Class A (primary contact) uses primarily due to presence high levels of inorganic turbidity that violate Iowa’s narrative water quality standard protecting against aesthetically objectionable conditions. 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 other than inorganic turbidity may limit algal production at Roberts Creek Lake. Based on median values from ISU sampling from 2000-02, the ratio of total nitrogen to total phosphorus for this lake is 14; this ratio suggests the possibility that algal production at this lake may, at times, be limited by nitrogen availability. Zooplankton grazing, however, does not appear likely to limit algal production. Although data from Downing et al. (2002) suggest relatively large zooplankton populations at this lake, the percentage of species known as algal grazers (Cladocerans) is relatively small. The average summer mass of zooplankton grazers at this lake in 2000 (2.2 mg/l) was the 19th lowest of the 131 lakes sampled.
The presence of nuisance algal species (i.e., bluegreen algae) does not appear to be a significant problem at this lake. Data from Downing et al. (2002) suggest that bluegreen algae (Cyanophyta) comprise a relatively small portion (20% or less by mass weight) of the summertime phytoplankton community. The summer 2000 average mass of bluegreen algae at this lake (0.3 mg/l) was the fifth lowest of the 131 lakes sampled. This level of bluegreen algae is very low relative to other Iowa lakes and does not suggest either threats or impairments to designated uses of this lake.
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 type that contribute to high levels of suspended sediment and poor water transparency. Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring at this lake.
