Drifting macrophyte detritus triggers ‘hidden’ benthic hypoxia

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.