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.
Highlights
Carbon sequestration and nutrient retention are highly important ecosystem services provided by seagrass ecosystems[1,2,3,4]
When correlating the percent (% DW) carbon and nitrogen of the surface sediment (0–5 cm) with the hydrodynamic exposure for all sites (n = 53), we found that there were inverse relationships between wave exposure and both surface sedimentary organic carbon (SOC) (d.f. = 51, F = 43.11, p < 0.001) and nitrogen content (SON) (d.f. = 51, F = 38.43, p < 0.001) (R2 = 0.42–0.45) (Fig. 1)
Seagrass morphology and the associated function of water attenuation can be affected by the hydrodynamic conditions; there were no correlations between aboveground biomass or shoot density and wave exposure (Fig. S2a,b)
Summary
Carbon sequestration and nutrient retention are highly important ecosystem services provided by seagrass ecosystems[1,2,3,4]. Due to its large distribution range and difference in environmental conditions, Z. marina meadows show a large variability in carbon-14,15,30,31 and nitrogen s torage[32], with exceptionally high carbon stocks at the Swedish west coast[14,15] This variability in storage capacity has been linked to sediment properties, with meadows having a high proportion of fine grain-sized particles and degree of sorting (i.e. the uniformity of sediment grain sizes), and low sediment density being related to higher carbon and/or nutrient stocks[14,15,31,33,34]. We assessed the relationship between grain size properties (i.e. mud content and degree of sorting) and hydrodynamics as these sediment characteristics have previously been linked to high sedimentary organic carbon content in Z. marina meadows[14,15,31,34]
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