Short-term effects of herbicides on primary productivity of periphyton in lotic environments.

: Freshwater algae are quite sensitive to herbicides that enter running water ecosystems through direct application, aerial drift, and/or watershed run-off. However, due to a lack of suitable methodologies, few studies examine the effects of such contamination on naturally occurring attached algal communities under field conditions (i. e., exposure regimes using pulsed doses or brief episodes of peak concentrations to simulate surface run-off during storm events). This paper describes a method for determining the acute short-term effects of four herbicides (hexazinone, atrazine, tebuthiuron and metolachlor) on the net primary productivity (NPP) of periphytic algae in the field using a portable bankside incubator; NPP was measured by monitoring changes in oxygen production (mg O2 per m(2)) upper surface of rock substrate per h and mg O2 h per mg chlorophyll using the light-dark technique. All herbicides with photosynthetic inhibition as a mode of action significantly reduced NPP. The lowest observed effect concentrations (LOECs) for the herbicides were 43 μg hexazinone l(-1), 109 μg atrazine l(-1) and 137 μg tebuthiuron l(-1). The no observed effect concentrations (NOECs) for these chemicals were <43 μg hexazinone l(-1), 93 μg atrazine l(-1) and 52 μg tebuthiuron l(-1). Metolachlor did not significantly reduce NPP at the concentrations that were tested (range 19.6-274 μg l(-1)). However, community respiration (which included respiration by invertebrates) was significantly reduced at the highest metolachlor concentration (274 μg l(-1)). Community respiration was not significantly affected by any concentration of the other three herbicides used.