Big Hollow Lake IA 02-ICD-6496

For the 2016 assessment/listing cycle, the Class A1 (primary contact recreation) uses of Big Hollow Lake are assessed (monitored) as "not supported" due to aesthetically objectionable conditions caused by algae blooms and due to frequent violations of the state criterion for pH based on information from the ISU lake survey. Using the median values from these surveys from 2011-2014 (approximately 12 samples), Carlson 's (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 60, 65, and 63 respectively for Big Hollow Lake. According to Carlson (1977) the Secchi depth, chlorophyll a, and total phosphorus values all place Big Hollow Lake in the Eutrophic category. These values suggest high levels of chlorophyll a and suspended algae in the water, moderately poor water transparency, and moderately high levels of phosphorus in the water column. The data show 5 violations of the Class A1 criterion for pH in 12 samples (42%).

The level of inorganic suspended solids was relatively low at Big Hollow Lake, and does not suggest water quality problems due to non-algal turbidity. The median level of inorganic suspended solids in Big Hollow Lake (2.45 mg/L) was ranked 38th among the 138 lakes by the ISU lake survey.

Data from the 2011-2014 ISU lake survey suggest a moderately large population of cyanobacteria exists at Big Hollow Lake. These data show that cyanobacteria comprised 96% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (28.8 mg/L) was ranked 91st of the 138 lakes sampled.

The Class B(LW) (aquatic life) uses are assessed (monitored) as "not supported" due to violations of the Class A1,B(LW) criterion for pH. Information from the IDNR Fisheries Bureau suggest problems at this lake are likely due to the numerous flooded timber areas on this lake as a result of its construction. Decomposition of timber, brush, and other terrestrials plants has released an abundance of nutrients that caused massive algal and duckweed blooms. Additionally decomposition can lead to pH problems. Fisheries biologists have reported that submergent vegetation has slowly begun to establish and expect water quality to improve in the future. Results of the ISU lake survey from 2011-2014 show there were no violations of the criterion for ammonia in 12 samples(0%), no violations of the criterion for dissolved oxygen in 12 samples(0%), and 5 violations of the criterion for pH in 12 samples(42%). Based on IDNR's assessment methodology these violations are significantly greater than 10% of the samples and therefore suggest impairment (not supported/monitored) of the Class B(LW) uses of Big Hollow Lake.

Fish consumption uses were assessed (monitored) as “fully supported” based on results of U.S. EPA/IDNR fish contaminant (RAFT) monitoring at Big Hollow Lake in 2012.    The sample of shoulder muscle from Snapping Turtle had low levels of contaminants.    Levels of primary contaminants in the 2012  sample of Snapping Turtle shoulder muscle was mercury: 0.329 ppm.  in 2012. 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.     The fish contaminant data generated from the 2012 RAFT sampling conducted at this Big Hollow Lake show that the levels of contaminants do not exceed any of the advisory trigger levels, thus indicating no justification for issuance of a consumption advisory for this waterbody.