Linking Baltic Sea water VOC concentrations with a summertime phytoplankton bloom

Long-term nutrient loading and warmer, longer summer temperatures have promoted summer cyanobacteria-dominated phytoplankton blooms in the Baltic Sea, shifting the annual chlorophyll maximum toward peak summer. In turn, organic matter production is increasing, altering the carbon cycle by shifting the bioavailable carbon pool to later in the season and towards microbial heterotrophy. These ecosystem changes may have consequential impacts on the production of trace gases, such as volatile organic compounds (VOC). Enhanced stratification and reduced vertical mixing may further regulate VOC water-air exchange. In the coastal zone, significant changes to macroalgae communities have been observed in association with persistent eutrophication. Shifting coastal dynamics, along with increased warming and, consequently, increased decomposition of organic material, will likely impact VOC production. Therefore, the aim of this study is to evaluate the influence of a summertime phytoplankton bloom on the composition and concentrations of VOCs in seawater, and to examine differences between distinct coastal habitats.

Summer sampling was conducted on Utö Island (59º 46'50N, 21º 22'23E; Archipelago Sea), and samples were processed at the Utö Atmospheric and Marine Research Station. Seawater VOCs were collected using the purge and trap method four times across three habitat types along the open coast—open water (250 m off shore; 4.5 m depth), a cove (15 m off shore; 0.5 m depth), and a vegetated beach (on shore; surface). Samples were stored in stainless steel absorbent cartridges and analyzed with Thermal Desorption Gas Chromatography Mass Spectrometry. Phytoplankton community composition and abundance were captured using an Imaging FlowCytobot, complemented by bacterial abundance from flow cytometry and microscopy.

Preliminary results indicate clear temporal variability in open water VOC concentrations. Some compounds such as isoprene were persistently detected throughout the summer whereas other compounds, e.g. toluene and dimethyl disulfide, varied across the season in association with changes in phytoplankton and bacterial abundance. Taxa-specific links between VOCs and phytoplankton composition, as well as the potential influence of abiotic drivers, including dissolved organic matter and vertical mixing, is still under investigation. Further analysis indicates that VOC concentrations are highly dependent on coastal habitat type, with composition and concentration of VOCs from the vegetated beach showing approximately 10-fold higher values as well as a more unique VOC blend, suggesting contributions from macroalgae and sediment processes. In contrast, the cove was highly dominated by bromoform, comprising >50% of the measured proportional VOC signal throughout the summer.