Sediment Stocks of Carbon, Nitrogen, and Phosphorus in Danish Eelgrass Meadows

Abstract

Seagrass ecosystems provide an array of ecosystem services ranging from habitat provision to erosion control. From a climate change and eutrophication mitigation perspective, the ecosystem services include burial and storage of carbon and nutrients in the sediments. Eelgrass (Zostera marina) is the most abundant seagrass species along the Danish coasts, and while its function as a carbon and nutrient sink has been documented in some areas, the spatial variability of these functions, and the drivers behind them, are not well understood. Here we present the first nationwide study on eelgrass sediment stock of carbon (C stock ), nitrogen (N stock ), and phosphorus (P stock ). Stocks were measured in the top 10 cm of eelgrass meadows spanning semi-enclosed estuaries (inner and outer fjords) to open coasts. Further, we assessed environmental factors (level of exposure, sediment properties, level of eutrophication) from each area to evaluate their relative importance as drivers of the spatial pattern in the respective stocks. We found large spatial variability in sediment stocks, representing 155-4413 g C m −2 , 24-448 g N m −2 , and 7-34 g P m −2 . C stock and N stock were significantly higher in inner fjords compared to outer fjords and open coasts. C stock , N stock , and P stock showed a significantly positive relationship with the silt-clay content in the sediments. Moreover, C stock was also significantly higher in more eutrophied areas with high concentrations of nutrients and chlorophyll a (chl a) in the water column. Conversely, siltclay content was not related to nutrients or chl a, suggesting a spatial dependence of the importance of these factors in driving stock sizes and implying that local differences in sediment properties and eutrophication level should be included when evaluating the storage capacity of carbon, nitrogen, and phosphorus in Danish eelgrass meadows. These insights provide guidance to managers in selecting priority areas for carbon and nutrient storage for climate-and eutrophication mitigation initiatives.

Reduced water motion enhances organic carbon stocks in temperate eelgrass meadows

“…Our results agree with previous field studies examining the role of hydrodynamics on seagrass meadow carbon storage, where greater sediment carbon content was associated with lower wave heights (Samper‐Villarreal et al ), lower wave exposure (Mazarrasa et al ), and lower current velocities (Santos et al ). Carbon stocks have also been shown to be significantly higher in inner fjords compared to outer fjords and open coasts (Kindeberg et al ), which aligns with our finding that more sheltered inner‐coast sites such as Pruth Bay had much higher OC stocks than exposed outer coast sites such as Choked Pass (Table ; Fig. ).…”

“…The low OC stocks and high variability found in our study are not uncommon to other Z. marina meadows located in temperate regions (Hodgson and Spooner ; Jankowska et al ; Röhr et al , ; Kindeberg et al ; Poppe and Rybczyk ; Postlethwaite et al ). These low and variable values, along with a lack of data on meadow extent, present challenges for including seagrasses in national and international blue carbon policies (Hejnowicz et al ; Green et al ; Needelman et al ).…”

“…The scarcity of data on the spatial heterogeneity in carbon stocks and accumulation rates creates challenges for scaling up carbon estimates and incorporating blue carbon into such climate policies (Lavery et al ; Oreska et al ; Mazarrasa et al ). Furthermore, the mechanisms that regulate spatial variability across different scales are only beginning to be understood (Samper‐Villarreal et al ; Kindeberg et al ; Mazarrasa et al ; Santos et al ).…”

Carbon and Nitrogen Concentrations, Stocks, and Isotopic Compositions in Red Sea Seagrass and Mangrove Sediments

“…It is, therefore, more appropriate, for comparative purposes, to compare the top 10 cm stock than to extrapolate to 1 m, beyond the sediment thickness present in many of the meadows sampled here. The thickness we used, however, is the same as in a recent study on C org , N, and phosphorous sediment stocks in Danish seagrass meadows (Kindeberg et al, 2018).…”

High Seasonal Variability in Sediment Carbon Stocks of Cold‐Temperate Seagrass Meadows

“…The seagrass Zostera marina is a common species in the temperate zone of the Northern Hemisphere, creating meadows with a high carbon storage potential where the seagrass areas in the Skagerrak‐Kattegat strait and in the Mediterranean show particularly large carbon stocks, exceeding the stocks found in many terrestrial environments (Mcleod et al, ; Röhr et al, ). There is, however, a large variation in carbon storage efficiency among Z. marina habitats (Dahl, ; Green et al, ; Kindeberg et al, ; Röhr et al, , ), which is mainly related to the location of the meadow, where water depth, salinity, and exposure to hydrodynamic forces are regulating the sedimentary carbon accumulation (Dahl, Deyanova, Gütschow, et al, ; Prentice et al, ; Röhr et al, ). Understanding this variability is a key issue for conservation and protection of these important carbon sinks as some areas are deemed to have a lower accumulation of carbon than others, such as in the Baltic Sea, where Z. marina is mainly found in less sheltered sites compared to the Skagerrak area (Dahl, Deyanova, Gütschow, et al, ; Jankowska et al, ; Röhr et al, ).…”