Sensitivity, variability, and recovery of functional and structural endpoints of an aquatic community exposed to herbicides.

A mesocosm study with three photosystem-II inhibitors and an equipotent mixture was performed to address the value of functional and structural endpoints in evaluating the impact of herbicides on aquatic systems. The herbicides atrazine, diuron, and isoproturon were dosed in the ratio of their relative potencies as HC30 for the single substance treatments and as 1/3 HC30 for the mixture treatment to obtain comparable effect concentrations. To investigate the effects of the three herbicides and their mixture on photosynthesis of the whole system, the physical-chemical parameters pH, dissolved oxygen, and conductivity were monitored. To address effects on photosynthesis more specifically, the photosynthetic efficiency of phytoplankton and three submersed macrophytes (Elodea canadensis, Myriophyllum spicatum, and Potamogeton lucens) were investigated applying in vivo chlorophyll fluorescence as an indicator for their activity. As a structural endpoint, the species abundance and community structure of the phytoplankton community was determined. Effects were continuously monitored over a five week period of constant exposure, and during a 3 month post-exposure period. The sensitivity, expressed as maximum effect during constant exposure, was higher for the structural parameters (total and single species abundances and PRC) than for the functional parameters. The mean coefficient of variation (CV) for the physical-chemical parameters was below 10%, for the photosynthesis measurement of the phytoplankton and macrophytes below 10 and 30%, respectively. Structural parameters, however, yielded higher variability with mean CVs for phytoplankton abundance data and single sensitive species reaching up to 96%. Effects on the phytoplankton photosynthesis measured via in vivo chlorophyll fluorescence were constant during the exposure period; whereas macrophytes recovered quickly from photosynthesis inhibition despite constant exposure. Effects on total system photosynthesis, determined via physical-chemical parameters, lasted for a shorter period than for the phytoplankton photosynthesis demonstrating the importance of the macrophytes for total primary production. Thus, the evaluation of effects on communities in model ecosystems such as micro- and mesocosms should not be based on structural endpoints only due to their comparably high inherent variability. Instead, we recommend complementing the risk assessment with data obtained from sensitive functional endpoints addressing the specific mode of action of the respective compound for the most sensitive group of organisms to avoid over-estimation of the recovery potential of the aquatic system.