Abstract:
Agrochemicals are the main pollutants in freshwater ecosystems. Metazachlor and flufenacet are two common
herbicides applied in fall (i.e., August–October) to agricultural fields in Northern Germany. High concentrations
of these herbicides are often found in adjacent aquatic ecosystems. Phytoplankton are one of the highly susceptible
non-targeted aquatic organismal groups for herbicides and effects on phytoplankton may initiate a chain
of consequences in meta communities through trophic interactions. Few studies have focused on responses of the
phytoplankton community for metazachlor and, no studies have focused on flufenacet. We studied the effects of
metazachlor and flufenacet on the phytoplankton community by conducting a microcosm experiment exposing
natural fall phytoplankton communities to environmentally realistic concentrations as 0 (control), 0.5, 5 and 50
μg L 1 of metazachlor and flufenacet treatments over a 4-week period. We measured changes in density,
composition (i.e., in phyla and species level), taxonomic diversity indices, and functional features of phytoplankton
communities as a response to herbicides. A reduction in the density of Chlorophyta species (e.g., Koliella
longiseta, Selenastrum bibraianum) and Cyanobacteria species (e.g., Merismopedia tenuissima and Aphanocapsa
elegans) was observed in herbicide treatments compared to controls. The phytoplankton community shifted towards
a high density of species from Bacillariophyta (e.g., Nitzschia fonticola and Cyclotella meneghiniana), Miozoa
(i.e., Peridinium willei), and Euglenozoa (i.e., Trachelomonas volvocina) in herbicide treatments compared to
controls. Metazachlor and flufenacet showed significant negative effects on taxonomic diversity indices (e.g.,
species richness, the Shannon-Wiener index) and functional features (e.g., functional dispersion and redundancy)
of the phytoplankton communities, with increasing herbicide concentrations. Our study provides insights into
direct, selective, and irrecoverable effects of metazachlor and flufenacet on phytoplankton communities in the
short-term. The comprehensive understanding of these effects of environmentally realistic herbicide concentrations
on aquatic biota is essential for a sustainable management of aquatic ecosystems in agricultural areas.