Carbon Storage in Coastal Reed Ecosystems

Abstract. Common reed (Phragmites australis) distribution has increased in coastal ecosystems across the globe, including around the Baltic Sea. Understanding carbon (C) storage in reed beds is critical for developing more accurate blue carbon (BC) budgets and best management practices, yet there currently appears to be a gap in knowledge about C cycling in these ecosystems. Reed beds are typically categorized as salt marsh ecosystems in BC budgets, but preliminary findings indicate reed beds are unique from salt marsh ecosystems and show great potential for C storage. It is, therefore, important to understand C storage in reed beds so that these ecosystems can be taken into consideration while developing BC budgets.

The aim of this spatial study was to quantify how much C is stored in above- and belowground biomass, and sediments in the different zones of reed beds along the Pojo Bay system of the northern Baltic Sea in coastal Finland. We selected 6 reed bed sites to sample along Pojo Bay from the northern-most part of the Bay to the southern-most part opening into the Baltic Sea, covering a range of salinities and wave exposure. In each site, samples were selected randomly within each of the 3 reed bed zones (terrestrial, intermittent, and littoral) and analyzed for sediment parameters (dry bulk density, organic matter content), plant characteristics (stem density), and plant and sediment-bound C content.

Sediment samples were collected down to 1m depth, when possible, with the use of Russian peat borers and a box corer. Dry bulk density (DBD) was variable across all sites, sediment depths, and reed bed zones with highest DBDs measuring 2.55 g cm^-3 (LOI (loss on ignition) = 0.7 % and water = 1.3 %) and the lowest DBD measuring 0.05 g cm^-3 (LOI = 74 % and water = 94.6 %). The results from sediment LOI show higher organic matter content in the upper 30 cm of sediment profiles and a general trend towards higher organic matter content in terrestrial and intermittent zones than littoral zones of reed beds. C content in sediment, above- and belowground biomass was significantly different at the zone level for all sites and, with one exception, was significantly different at the site level for all variables measured (DBD, LOI, sediment C stocks, stem counts, aboveground C stocks, and belowground C stocks). The highest sediment C stocks were typically found in the intermittent zone while the lowest were typically found in the littoral zone across all sites. Average stem counts were variable across sites and reed bed zones with the highest and lowest stem counts being 217 stems m^-2 and 9 stems m^-2, respectively. Aboveground biomass C stock averages were generally highest in the intermittent zones and lowest in the terrestrial and littoral zones. Belowground biomass C stock averages were generally highest in the intermittent zone and lowest in the littoral zone. C storage in reed bed sediments and belowground biomass was higher than C storage in aboveground biomass for all sites.

These findings are significant as they help rectify a gap in knowledge on how much C is stored in reed bed biomass and sediment which is important for management of this rapidly expanding coastal ecosystem type and enables researchers to develop more accurate coastal carbon budgets to combat climate change.