Assessment Comments
Assessment is based on results of (1) ISU lake survey from 2000-02, (2) surveys by IDNR Fisheries Bureau, and (3) ISU report on lake plankton communities in 2000.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (monitored) as "not supporting" due to reductions in water transparency caused by primarily by high levels of inorganic turbidity (inorganic suspended solids) and secondarily by algal blooms, both of which reduce water transparency. Also, the presence of large populations of bluegreen algae suggests an additional impairment of these uses. The Class B(LW) aquatic life uses remain assessed (evaluated) as "partially supporting" due to excessive nutrient loading to the water column, nuisance blooms of algae, and siltation in the 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 conducted from 2000 through 2002 by Iowa State University (ISU), (2) information from the IDNR Fisheries Bureau, and (3) information on plankton communities at Iowa lakes in 2000 from Downing et al. (2002).
EXPLANATION Results of monitoring conducted by ISU from 2000 through 2002 as part of the statewide survey of Iowa lakes suggest that the Class A (primary contact) uses and Class B(LW) aquatic life uses are impaired. Using the median values from the Iowa State University statewide surveys of Iowa lakes from 2000 through 2002 (approximately nine samples), Carlson’s trophic state indices for total phosphorus, chlorophyll-a, and secchi depth at White Oak Lake are 78, 64, and 73, respectively. According to Carlson (1977), the index values for total phosphorus and Secchi depth place this lake in the range of hyper-eutrophic lakes; the index value for chlorophyll-a is in middle range between eutrophic and hyper-eutrophic lakes. These index values suggest extremely high levels of phosphorus in the water column, somewhat elevated (but less than expected) production of suspended algae, and very 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 particles do likely contribute to limiting algal production at White Oak 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, White Oak Lake had the 60th highest median level of inorganic suspended solids (5.0 mg/l), thus suggesting that non-algal turbidity limits the production of algae as well as contributes to reductions in water transparency. White Oak Lake also had the 24th highest TSI value for chlorophyll-a, thus suggesting that suspended algae also contribute to aesthetically objectionable conditions at White Oak Lake. These conditions suggest an suggest impairment to the Class A (primary contact) uses primarily due to presence high levels of turbidity—caused by a combination of algal and inorganic materials—that violate Iowa’s narrative water quality standard protecting against aesthetically objectionable conditions. The IDNR Fisheries Bureau concurs that turbidity-related impairments exist at this lake.
Apparently, other factors, possibly nitrogen limitation or zooplankton grazing, tend to limit production of algae in this lake. Based on the 2000-2002 data, the TN/TP ratio for this lake is 11. This ratio is relatively low and does suggest that algal production at this lake is limited by nitrogen availability. In addition, data from Downing et al. (2002) show that the zooplankton community of this lake has a large population of species known as algal grazers (Cladocerans). The median summer mass of zooplankton grazers at this lake in 2000 (41.8 mg/l) was the 32nd highest of the 131 lakes sampled, thus suggesting that zooplankton grazers may limit algal production at this lake.
The presence of nuisance algal species (i.e., bluegreen algae) suggests an additional impairment to the designated uses of White Oak Lake. Data from Downing et al. (2002) suggest that bluegreen algae (Cyanophyta), dominate the summertime phytoplankton community of White Oak Lake). Sampling in 2000 showed the percent wet mass of bluegreens ranged from approximately 80% in the late June sampling up to 100% in the mid-August sampling. The summer 2000 average mass of bluegreen algae at this lake (52.9 mg/l) was the 18th highest of the 131 lakes sampled and thus suggests the potential for violating Iowa’s narrative water quality standard protecting against nuisance aquatic life.
The hyper-eutrophic conditions at this lake, along with information from the IDNR Fisheries Bureau, suggest that the Class B(LW) aquatic life uses are "partially supported" due to excessive nutrient loading to the water column, nuisance blooms of algae, and siltation in the lake. Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring at this lake.
