Temperate marine macrophytes are highly variable sources of Biogenic Volatile Organic Compounds – a comparative study from the Baltic Sea and NE Atlantic

Abstract. Biogenic Volatile Organic Compounds (BVOC), emitted by Earth’s ecosystems, affect several chemical processes in the atmosphere that have profound climate impacts. Despite their climate relevance, global BVOC budget estimations are still highly uncertain, with ocean-derived emissions being particularly poorly constrained. Marine macrophytes (i.e. macroalgae and seagrass) are a large and widespread organismal group whose BVOC emission rates are particularly poorly quantified. In this study, we set out to address this knowledge gap by quantifying ex situ the BVOC emission rates of three temperate macrophytes (Zostera marina, Fucus vesiculosus and Ulva intestinalis) with a Vocus proton-transfer-reaction time-of-flight mass spectrometer (Vocus PTR-TOF). To capture and improve our understanding of the variability of macrophyte BVOC emissions, our quantifications were repeated across two contrasting coastal regions: the northeastern Atlantic (Ireland) and northern Baltic Sea (Finland). The three macrophytes emitted a wide range of BVOCs, with a total of 166 different compounds detected. Although many BVOCs were emitted by all macrophytes, significant differences were observed in the total emission profiles, both between and within species. Interestingly, the seagrass Zostera exhibited significantly higher overall BVOC emission rates per unit biomass than the two macroalgae and showed clearly differing intraspecific emission profiles across the two regions. Regarding individual compounds, dimethyl sulfide (DMS) was emitted at the highest rates, but many other compounds (e.g., sesquiterpenes and C₁₀H₂₁O⁺) also displayed notable emission rates. Although many of the observed BVOCs are commonly investigated compounds (e.g., DMS and terpenoids), our results show that macrophyte BVOC emissions comprise a large number of different compounds, suggesting that future studies would benefit from targeting a wider range of BVOCs than currently considered. Our results highlight macrophytes as highly variable sources of BVOCs, whose better inclusion into marine BVOC budgets should be strived for. However, more robust data are needed, and future research should also focus on investigating the dynamics driving macrophyte BVOC emissions, their variability, and their eventual fate in the environment.