19 May 2022
19 May 2022
Status: a revised version of this preprint is currently under review for the journal BG.

Drifting macrophyte detritus triggers ‘hidden’ benthic hypoxia

Karl Michael Attard1,2,3, Anna Lyssenko3, and Iván Franco Rodil3,4 Karl Michael Attard et al.
  • 1Department of Biology, University of Southern Denmark, 5230 Odense M, Denmark
  • 2Danish Institute for Advanced Study, University of Southern Denmark, 5230 Odense M, Denmark
  • 3Tvärminne Zoological Station, University of Helsinki, J.A. Palménin tie 260, 10900 Hanko, Finland
  • 4Department of Biology (INMAR), Faculty of Marine and Environmental Sciences, University of Cádiz, Puerto Real, Spain

Abstract. Macrophytes form highly productive habitats that export a substantial proportion of their primary production as particulate organic matter. As the detritus drifts with currents and accumulates in seafloor depressions, it constitutes organic enrichment and can deteriorate O2 conditions on the seafloor. In this study, we investigate the O2 dynamics and macrobenthic biodiversity associated with a shallow ⁓2300 m2 macrophyte detritus field in the northern Baltic Sea. The detritus, primarily Fucus vesiculosus fragments, had a biomass of ⁓1700 g dry weight m-2, approximately 1.5-fold larger than nearby intact F. vesiculosus canopies. A vertical array of O2 sensors placed within the detritus documented that hypoxia ([O2] < 63 µmol L-1) occurred for 23 % of the time and terminated at the onset of wave-driven hydrodynamic mixing. Measurements in five other habitats nearby spanning bare sediments, seagrass, and macroalgae indicate that hypoxic conditions were unique to detritus canopies. Fast-response O2 sensors placed above the detritus documented pulses of hypoxic waters originating from within the canopy. These pulses triggered a rapid short-term (⁓5 min) deterioration of O2 conditions within the water column. Eddy covariance measurements of O2 fluxes indicated that daily photosynthetic production offset up to 81 % of the respiratory demands of the detritus canopy, prolonging its persistence within the coastal zone. The detritus site had a low abundance of crustaceans, bivalves, and polychaetes when compared to other habitats nearby, likely because their low-O2 tolerance thresholds were often exceeded.

Karl Michael Attard et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on bg-2022-119', Dirk Koopmans, 05 Jun 2022
    • AC1: 'Reply on RC1', Karl Attard, 11 Oct 2022
  • RC2: 'Comment on bg-2022-119', Anonymous Referee #2, 08 Aug 2022
    • AC2: 'Reply on RC2', Karl Attard, 11 Oct 2022

Karl Michael Attard et al.

Karl Michael Attard et al.


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Short summary
Aquatic plants produce a large amount of organic matter through photosynthesis that, following seasonal decay or storms, is deposited on the seafloor. In this study, we show that plant detritus can trigger low oxygen conditions (hypoxia) in shallow coastal waters, making conditions challenging for most marine animals. We propose that the occurrence of hypoxia may be underestimated because measurements typically do not consider the region closest to the seafloor, where detritus accumulates.