Toxicity of chlorpyrifos adsorbed on clay and humic acid to larval walleye

Agricultural production poses one of the most serious threats to the continued ecological integrity of environmental systems and has been charged as the activity most responsible for loss of fish species in streams. Although all anthropogenic stressors affecting aquatic ecosystems have not been isolated, suspended solids, sediment, and pesticides are believed to be major factors. The objectives of this dissertation were to: (1) evaluate sources of variability in measuring cholinesterase activity, (2) determine the most sensitive life stage of walleye exposed to chlorpyrifos, (3) evaluate the bioavailability of chlorpyrifos adsorbed on humic acid (HA) and Panther Creek (PC) clay to larval walleye (Stizostedion vitreum), and (4) determine the toxicity of Cedar River sediments and water to larval walleye. Water temperature, stress, and the method of euthanasia had no effect on ChE activity of walleye. ChE activity of walleye stored at −80°C for 180 d did not differ from ChE activity of fresh specimens. However, a significant positive relationship was observed between whole body ChE activity and total length of larval walleye, but no significant relationship was found between total length of juvenile walleye and brain ChE activity. These results indicate that ChE inhibition is a reliable and sensitive indicator of OP exposure to walleye and is not affected by environmental factors. Prolarvae (yolk-sac larvae) were the least sensitive larval stage, but there was a significant increase in toxicity of chlorpyrifos from the postlarvae I to postlarvae II stage. The LC50 leveled out in 30 to 90-d-old juvenile walleye. Chlorpyrifos adsorbed on humic acid reduced survival of larval walleye indicating that chlorpyrifos desorbs from HA. Thus, chlorpyrifos-HA complexes represent a potential exposure route for OPs to fish. Also, PC clay alone, without chlorpyrifos, was highly toxic to postlarvae I and postlarvae II walleye, but not prolarvae. Neither Cedar River water nor sediment were more toxic to larval walleye, at any larval stage, than reference water and sediment. These results suggest that Cedar River sediment and water do not pose a serious threat to larval walleye in the Cedar River.