Effects of a pulsed treatment with the herbicide afalon (active ingredientlinuron) on macrophyte-dominated mesocosms. I. Responses of ecosystem metabolism

Experimental drainage ditches used as mesocosms were treated with linuron t hree times, at 4-week intervals, at nominal concentrations of 0, 0.5, 5, 15 , and 50 mu g/L After each treatment, the mesocosms were kept static for 1 week. They were then flushed with noncontaminated surface water, with a res idence time of 5 d, until the next treatment. Structural and functional mea surements were made. A companion paper discusses the structural aspects. Di ssolved oxygen concentrations and pH were measured continuously at 15-min i ntervals. Both variables declined during the static periods, and a lowest-o bserved-effect concentration (LOEC) of 5 mu g/L was noted for both response variables. All mesocosms recovered during the flushing periods. Diurnal ox ygen curves were used to calculate ecosystem gross primary production (GPP) and ecosystem oxygen consumption. Ecosystem oxygen consumption was not aff ected by the linuron treatments, but GPP had an EC50 of 21 mu g/L during th e static period after the first treatment. The EC50 for GPP increased to 36 mu g/L after the second treatment and to 80 mu g/L after the third treatme nt. This increase in EC50 is interpreted as an adaptation of the ecosystem, the mechanism of which could not be identified. The LOEC was 15 mu g/L for all treatments and did not change over time. Between treatments, GPP recov ered to the level of the controls. The LOEC for the long-term GPP was 50 mu g/L.