New paper: What happens to carbon released by seaweeds and seagrasses?

We are very pleased to share a new paper led by Leah Brinch-Iversen, now published in Marine Ecology Progress Series, investigating the release and fate of dissolved organic carbon (DOC) from the brown alga Fucus serratus and the seagrass Zostera marina. The study was carried out together with Ronnie N. Glud (SDU) and Mathias Middelboe (University of Copenhagen).

A key methodological development was the use of flow-through incubation chambers that allowed macrophyte fragments to be followed for 85 days while continuously replacing the surrounding water. Fragments were kept in darkness to simulate export below the photic zone, while their physiological condition was followed using PAM fluorometry. DOC released at different stages of degradation was then exposed to natural microbial communities to determine how much could be consumed and how efficiently microbes converted this carbon into biomass.

Both species continued to release DOC throughout the experiment. Fucus released about 30% more DOC overall, although the proportion of their original carbon released as DOC was similar: approximately 17% for Fucus and 15% for Zostera.

The experiments document how the quality of the released carbon changed as the fragments aged. The bioavailable fraction increased as physiological condition declined, reaching more than 90% around four weeks into the experiment, before shifting strongly towards more recalcitrant DOC during later stages of degradation. Across the experiment as a whole, microbial communities consumed around 40–45% of the DOC, while a substantial fraction resisted microbial degradation and therefore has the potential to contribute to longer-lived marine carbon storage.

The study shows that understanding the carbon role of marine vegetation requires looking beyond how much carbon plants fix while growing. What happens after fragments detach, sink and degrade can determine whether that carbon is rapidly recycled or has the potential to persist in the ocean.

Reference

Brinch-Iversen L, Glud RN, Middelboe M, Attard K (2026). Release and bioavailability of dissolved organic carbon derived from Fucus serratus and Zostera marina. Marine Ecology Progress Series 794, meps15219. Published 17 September 2026. https://doi.org/10.3354/meps15219