Individual and combined effects of diuron and light reduction on marine microalgae

Olivia C King, Jason P van de Merwe, Christopher J Brown, Michael St J Warne, Rachael A Smith

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Coastal ecosystems such as those in the Great Barrier Reef (GBR) lagoon, are exposed to stressors in flood plumes including low light (caused by increased turbidity) and agricultural pesticides. Photosystem II (PSII)-inhibiting herbicides are the most frequently detected pesticides in the GBR lagoon, but it is not clear how their toxicity to phototrophic species depends on light availability. This study investigated the individual and combined effects of PSII-inhibiting herbicide, diuron, and reduced light intensity (as a proxy for increased turbidity) on the marine diatom, Phaeodactylum tricornutum. Effective quantum yield (EQY) and cell density were measured to calculate responses relative to the controls over 72-h, in tests with varying stressor intensities. Individually, diuron concentrations (0.1–3 μg l −1) were not high enough to significantly reduce growth (cell density), but led to decreased EQY; while, low light generally led to increased EQY, but only reduced growth at the lowest tested light intensity (5 μmol photons m −2 s −1) after 48-hours. P. tricornutum was less affected by diuron when combined with low light scenarios, with increased EQY (up to 163% of the controls) that was likely due to increased electron transport per photon, despite lesser available photons at this low light intensity. In contrast, growth was completely inhibited relative to the controls when algae were simultaneously exposed to the highest stressor levels (3 μg l −1 diuron and 5 μmol photons m −2 s −1). This study highlights the importance of measuring more than one biological response variable to capture the combined effects of multiple stressors. Management of water quality stressors should consider combined impacts rather than just the impacts of individual stressors alone. Reducing suspended sediment and diuron concentrations in marine waters can decrease harmful effects and bring synergistic benefits to water quality.

Original languageEnglish
Article number113729
Number of pages11
JournalEcotoxicology and Environmental Safety
Early online date3 Jun 2022
Publication statusPublished - Aug 2022

Bibliographical note

© 2022 The Author(s). Published by Elsevier Inc. This is an open access article under the CC BY-NC-ND license (, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited..


Funding Information: Olivia C. King was supported by an Australian Government Research Training Program Stipend Scholarship; Christopher J. Brown was supported by a Discovery Project grant (DP180103124 ) from the Australian Research Council . Publisher Copyright: © 2022 The Authors


  • Light availability
  • Microalgae
  • Coastal ecosystems
  • Multiple stressors
  • Photosystem II herbicides
  • Phytotoxicity


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