Clear Lake Gnat Monitoring
As part of the Lake County Vector Control District's ongoing surveillance of the Clear Lake Gnat (Chaoborus astictopus) population, we have been monitoring various nuisance pests (midges), associated organisms (plankton), and inshore fish in Clear Lake since 1954. We sample the lake each month for benthic (lake bottom) organisms and plankton. In additon to these "lake checks" we also evaluate water quality parameters: temperature, pH, total hardness, water transparency, and specific conductance. The District has also continued to conduct beach seines (inshore fish sampling) on Clear Lake.
Monitoring is conducted on all three arms of the lake (Upper, Lower, and Oaks). Presently, we sample 14 stations in the Upper arm, 6 stations in the Oaks arm, and 9 stations in the Lower arm. Two 6-in cubed Ekman dredges are used to sample the lake bottom at each of these specified stations. Samples are collected by lowering each Ekman dredge to the lake bottom. Dredges are springloaded with a "messenger" that travels down the rope and triggers the jaws of the dredge to close. The dredges are hauled up and the "mud" samples from each dredge are sieved to remove the sediment, leaving the benthic organisms behind. The organisms collected at each station are identified and counted at our laboratory. We also record top and bottom water temperature and water depth at each station. Additional water quality parameters are only taken once in each arm of the lake.
What's identified in the benthic samples?
Chaoboridae: Clear Lake Gnat (Chaoborus astictopus) immatures; larvae are transparent and nearly invisible in the water; larvae feed on plankton and other small aquatic animals; adults are non-biting.
Chironomidae: Known as chironomids or non-biting midge immatures; these midges are ubiquitous in freshwater worldwide and are often bright red because of hemoglobin content; larvae live in the muddy bottom of Clear Lake (over 50 per square meter) and eat decaying matter; adults are non-biting.
Hirudinea: Leeches; these are small (1-3 cm) leeches with 2 suckers on each end on the bottom of the body; some feed on the blood of fish or turtles, but most are free-living predators; free-living leeches avoid light and generally hide in the detritus at the lake bottom; they feed mostly on insects, molluscs and oligochaetes, or are scavengers, feeding on dead animal matter.
Clear Lake plankton (phytoplankton and zooplankton) are collected via the Vertical Tow method. One sample per arm, per month is typically collected throughout the year. Vertical Tow samples are collected by lowering a conical plankton net to the bottom of the water column and hauling it though a vertical path back to the surface. The net must be hauled slowly to minimize the possibility of positive water pressure developing at the mouth of the net. Positive water pressure will push water out of the path of the net and prevent capture of some of the sample. Because of the potential for this method to decrease plankton capture, vertical tow samples are best regarded as qualitative rather then quantitative. A maximum of 36 plankton samples are collected and analyzed annually through the District's monitoring of Clear Lake. The collected plankton are a minute representative sample of the total population. The absence of a species from these data should not be considered conclusive evidence of its absence from Clear Lake.
For more information on Blue-Green Algae (i.e. Lyngbya) in Clear Lake: Click HERE
The District's inshore fish sampling is routinely done twice a summer. We use a 30 x 6 ft seine net to sample eleven beaches and launch ramps throughout Clear Lake. Three seine hauls are taken at each sampling station and the fish collected are measured (standard length of the first 25 of each species), counted, and returned to the lake.