High metabolism and periodic hypoxia associated with drifting macrophyte detritus in the shallow subtidal Baltic Sea

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 O-2 conditions on the seafloor. In this study, we investigate the O-2 dynamics and macrobenthic biodiversity associated with a shallow similar to 2300 m(2) macrophyte detritus field in the northern Baltic Sea. The detritus, primarily Fucus vesiculosus fragments, had a biomass of similar to 1700 g dry weight m(-2), approximately 1.5 times larger than nearby intact F. vesiculosus canopies. A vertical array of O-2 sensors placed within the detritus documented that hypoxia ([O-2] < 63 mu mol 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 O-2 sensors placed above the detritus documented pulses of hypoxic waters originating from within the canopy. These pulses triggered a rapid short-term (similar to 5 min) deterioration of O-2 conditions within the water column. Eddy covariance measurements of O-2 fluxes indicated high metabolic rates, with daily photosynthetic production offsetting 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 O-2 tolerance thresholds were often exceeded.

Automated summary

This study investigates the dynamics of dissolved oxygen (O2) and macrobenthic biodiversity in a shallow macrophyte detritus field in the northern Baltic Sea. The results show that hypoxia is prevalent in the detritus field and negatively affects macrobenthic abundance.