The influence of hydrodynamic exposure on carbon storage and nutrient retention in eelgrass (Zostera marina L.) meadows on the Swedish Skagerrak coast

Abstract

Cold-temperate seagrass (Zostera marina) meadows provide several important ecosystem services, including trapping and storage of sedimentary organic carbon and nutrients. However, seagrass meadows are rapidly decreasing worldwide and there is a pressing need for protective management of the meadows and the organic matter sinks they create. Their carbon and nutrient storage potential must be properly evaluated, both at present situation and under future climate change impacts. In this study, we assessed the effect of wave exposure on sedimentary carbon and nitrogen accumulation using existing data from 53 Z. marina meadows at the Swedish west coast. We found that meadows with higher hydrodynamic exposure had larger absolute organic carbon and nitrogen stocks (at 0-25 cm depth). This can be explained by a hydrodynamically induced sediment compaction in more exposed sites, resulting in increased sediment density and higher accumulation (per unit volume) of sedimentary organic carbon and nitrogen. With higher sediment density, the erosion threshold is assumed to increase, and as climate change-induced storms are predicted to be more common, we suggest that wave exposed meadows can be more resilient toward storms and might therefore be even more important as carbon-and nutrient sinks in the future. Carbon sequestration and nutrient retention are highly important ecosystem services provided by seagrass ecosystems 1-4. The natural carbon and nutrient sinks these meadows provide are suggested to contribute to the mitigation of the acute threats to human wellbeing posed by climate change and increased anthropogenic pressure on the coastal zone 5,6. Therefore, there is an urgent need to increase protection of environments with high capacity for long-term carbon storage and nutrient filtering 7,8 , where environmental managers also need to consider future effects of climate change. Seagrass meadows have been highlighted as efficient carbon and nutrient sinks 9-12 , but this ecosystem function is not constant and varies significantly among regions and species 13-15. The ability of seagrass meadows to sequester and store organic matter is linked to their high primary production, the slow decomposition in marine sediment and their efficient trapping of allochthonous particles from surrounding habitats 16,17. The contribution of allochthonously derived organic matter to the accumulation of sedimentary organic matter can be substantial 17,18 , as seagrasses trap organic particles by reducing the flow of the water within the canopy 19-21 causing suspended organic matter to settle within the meadow and increasing the sedimentation rate 22. Hydrodynamics will thus largely determine sedimentation processes in seagrass-dominated environments 23 and it has been shown that low exposure to wind and wave action tends to favour a build-up of

Dynamics and fate of blue carbon in a mangrove–seagrass seascape: influence of landscape configuration and land-use change

“…Sediment density seems also to have a significant role for C org levels in seagrass habitats, as shown by the models in this study and by earlier studies elsewhere (e.g. Avnimelech et al 2001;Dahl et al 2016bDahl et al , 2020aRöhr et al 2018). High C org content is generally associated with low sediment density since organic material has a lower weight compared to minerogenic particles (Winterwerp and van Kesteren 2004) and a higher water content (Avnimelech et al 2001).…”

The sediment carbon stocks of intertidal seagrass meadows in Scotland

“…Three sedimentary variables (mud content, sediment density, and degree of sediment sorting), and two environmental variables (water depth and salinity) explained over 62% of this variation in the study by Röhr et al (2018). Earlier studies in other regions with Z. marina meadows (Dahl et al, 2016;Dahl et al, 2020;Miyajima et al, 2015;Röhr et al, 2016) or other species of seagrass (Macreadie et al, 2013Serrano et al, 2016 have also indicated that sediment characteristics, specifically the sediment grain size distribution and sediment density, appear to be the most important predictors for seagrass carbon stocks. However, the seagrass organic carbon stocks here were not related to the sediment dry bulk density (top 5cm or 10cm).…”

Effects of seagrass overgrazing on sediment erosion and carbon sink capacity: Current understanding and future priorities

“…Grain size distribution might also change due to overgrazing, as smaller particles are more volatile than those of larger sizes (Rose et al 1999). Hydrodynamic exposure level is highly influencing the sedimentation process and is an important factor affecting the carbon storage potential in seagrass meadows (Samper‐Villarreal et al 2016; Prentice et al 2019; Santos et al 2019; Dahl et al 2020). The attenuation of the water flow by the seagrass canopy, which allows the sediment and sedimentary carbon to accumulate (Gacia and Duarte 2001), will be disrupted during an overgrazing event as the shoot biomass is being lost (James et al 2020).…”