Impact of glufosinate-ammonium and bialaphos on the phytoplankton community of a small eutrophic northern lake

The impact of glufosinate‐ammonium and bialaphos on the phytoplankton community in a shallow eutrophic lake ecosystem was investigated using in situ enclosures. Flow cytometry was used to size phytoplankton cells and sort them as live or dead, depending upon their chlorophyll a autofluorescence intensity. Flow cytometric analyses provided significantly (p < 0.001) higher abundance estimates as compared to conventional microscopic analyses. At the highest treatment levels (10 mg/L), both herbicides caused a significant but transient reduction in live phytoplankton cells (days 3–14), which was particularly apparent in the small (1–2 and 2–3 μm) classes. Transient impacts on phytoplankton live cell abundances were mirrored by depression in dissolved oxygen content in the treated enclosures. At an application rate of 10 mg/L, abundance of smaller phytoplankton in the bialaphos‐treated enclosures recovered more rapidly (14 d) than those in the glufosinate‐ammonium–treated enclosures (49 d). For days of maximal impact, estimated median effect concentrations (EC50) for reduction of phytoplankton abundance ranged from 2.5 to 3.4 mg/L for glufosinate‐ammonium and 3.3 to 8.1 mg/L for bialaphos, whereas estimates of concentration inducing 20% reductions in abundance (EC20; 0.9–1.2 and 1.6–4 mg/L, respectively) more closely approximated the expected environmental concentration (1 mg/L), assuming direct overspray into water bodies of 15‐cm depth.