The effect of two triazine herbicides on the productivity of freshwater marsh periphyton

Varying concentrations of the aquatic herbicides simazine and terbutryn were added to in-situ enclosures of marsh water. Colonization of acrylic substrata by periphyton was monitored by measuring chlorophyll a accumulation and carbon assimilation rate. No change in either, relative to an untreated control, was observed at 0.1 mg/l simazine, with increasing inhibition (to approximately 95%) at 1.0 and 5.0 mg/l. Chlorophyll a and carbon assimilation were reduced > 90% with terbutryn concentrations of 0.01 ml/l and higher. Factor analysis indicated that periphytic productivity was correlated with water chemistry, light availability, time and the experimental herbicide treatment, suggesting that herbicidal effects result from a complex interaction of several parameters rather than herbicide concentration alone. Recovery of the communities following decreased herbicide concentrations began within 1 wk, with growth rate equal to or greater than the control. Our results suggest that the long-term impact of a single dosage of these herbicides on a periphyton community may be minimal.