New paper: When drifting seaweed accumulates on the seafloor

Highly productive coastal vegetation does not keep all of the organic matter it produces. Seaweeds and seagrasses regularly break away, drift with currents and can eventually accumulate in large deposits on the seafloor. In a paper published in Biogeosciences, Karl Attard, Anna Lyssenko and Iván Rodil investigated what happens within one of these natural detritus accumulations in the northern Baltic Sea.

The study focused on a roughly 2300 m² deposit dominated by fragments of the brown seaweed Fucus vesiculosus near Tvärminne Zoological Station in Finland. We combined vertical arrays of oxygen sensors, aquatic eddy covariance measurements of ecosystem metabolism, hydrodynamic observations and surveys of benthic fauna.

The detritus formed a surprisingly active ecosystem. Photosynthesis by the accumulated material could offset as much as 81% of its respiratory oxygen demand, but respiration remained greater than production overall. During periods of weak water movement, oxygen became depleted within the lower canopy, with hypoxic conditions occurring for around 23% of the time. Increased water movement rapidly ventilated the deposit but could also release short pulses of oxygen-poor water into the surrounding water column.

The detritus habitat also supported fewer species and lower abundances of several crustaceans, bivalves and polychaetes than nearby vegetated habitats. The study illustrates another side of coastal primary production: exported plant material can continue photosynthesising and cycling carbon after it leaves its original habitat, while its eventual decomposition can create highly dynamic and locally stressful oxygen conditions.

Reference

Attard KM, Lyssenko A, Rodil IF (2023). High metabolism and periodic hypoxia associated with drifting macrophyte detritus in the shallow subtidal Baltic Sea. Biogeosciences 20: 1713–1724. doi: 10.5194/bg-20-1713-2023. Article