Seagrass meadows are important carbon sinks in the global coastal carbon cycle yet are also among the most rapidly declining marine habitats. Their ability to sequester carbon depends on flow–sediment–vegetation interactions that facilitate net deposition, as well as high rates of primary production. However, the effects of seasonal and episodic variations in seagrass density on net sediment and carbon accumulation have not been well quantified. Understanding these dynamics provides insight into how carbon accumulation in seagrass meadows responds to disturbance events and climate change. Here, we apply a spatially resolved sediment transport model that includes coupling of seagrass effects on flow, waves, and sediment resuspension in a seagrass meadow to quantify seasonal rates of sediment and carbon accumulation in the meadow. Our results show that organic carbon accumulation rates were largely determined by sediment accumulation and that they both changed non‐linearly as a function of seagrass shoot density. While seagrass meadows effectively trapped sediment at meadow edges during spring–summer growth seasons, during winter senescence low‐density meadows (< 160 shoots m−2) were erosional with rates sensitive to density. Small variations in winter densities resulted in large changes in annual sediment and carbon accumulation in the meadow; meadow‐scale (hundreds of square meters) summer seagrass dieback due to marine heatwaves can result in annual erosion and carbon loss. Our findings highlight the strong temporal and spatial variability in sediment accumulation within seagrass meadows and the implications for annual sediment carbon burial rates and the resilience of seagrass carbon stocks under future climate change.
Seagrass meadows are known as hot spots for carbon accumulation, but there is limited field data on the variability of sediment accumulation across time and space. We developed a method to assess spatial and temporal heterogeneity in net sediment accumulation in seagrass meadows using small, inexpensive samplers, allowing for over 200 unique measurements across multiple transects within our study site. Using this method, we assessed sediment accumulation across seagrass meadow edges, and in varying weather conditions. We found the greatest accumulation of sediment 5 m outside of seagrass meadow edges, with sediment accumulation rates averaging just under 100 g m−2day−1, though rates were highly variable. Carbon accumulation from settled sediment was generally greater outside of seagrass meadow edges than within the bed, especially at sites undergoing recent expansion. Measurements made during tropical storms showed both scouring of sediment away from sites, and increased accumulation, depending on site properties as well as individual tropical storm characteristics. In the storm that had a measurable storm surge, scouring of sediment was a more dominant mechanism, whereas deposition dominated in the storm that had high winds but no associated storm surge. Our data demonstrate the necessity of including measurements that characterize both spatial and meteorological variability to develop a more holistic understanding of the movement of sediment and particulate organic carbon associated with seagrass meadows, especially as meadow area becomes increasingly fragmented with human activity and global change.
more » « less- PAR ID:
- 10507944
- Publisher / Repository:
- Springer Science + Business Media
- Date Published:
- Journal Name:
- Estuaries and Coasts
- Volume:
- 47
- Issue:
- 5
- ISSN:
- 1559-2723
- Format(s):
- Medium: X Size: p. 1255-1265
- Size(s):
- p. 1255-1265
- Sponsoring Org:
- National Science Foundation
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Abstract -
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Abstract Seagrass meadows perform an important ecological function as filters for incoming nutrients from surrounding watersheds, especially nitrogen (N). By enhancing N removal processes, including N burial in sediments and denitrification, seagrass meadows improve water quality. With accelerating losses of seagrass meadows worldwide, seagrass restoration plays a key role in reestablishing these coastal ecosystem functions. However, few measurements exist of N burial rates in temperate seagrass meadows and none have been published for restored meadows. In this study, we measured N burial rates in a large (6.9 km2) restored eelgrass (
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Abstract Seagrass growth and senescence exert a strong influence on flow structure and sediment transport processes in coastal environments. However, most previous studies of seasonal seagrass effects either focused on small‐scale field measurements or did not fully resolve the synergistic effects of flow‐wave‐vegetation‐sediment interaction at a meadow scale. In this study, we applied a coupled Delft3D‐FLOW and SWAN model that included effects of seagrass on flow, waves, and sediment resuspension in a shallow coastal bay to quantify seasonal seagrass impacts on bay dynamics. The model was extensively validated using seasonal field hydrodynamic and suspended sediment data within a seagrass meadow and a nearby unvegetated site. Our results show that seagrass meadows significantly attenuated flow (60%) and waves (20%) and reduced suspended sediment concentration (85%) during summer when its density reached a maximum. Probability density distributions of combined wave‐current bed shear stress within the seagrass meadow indicate that significant reductions in sediment resuspension during summer were mainly caused by flow retardation rather than wave attenuation. Although low‐density seagrass in winter resulted in much smaller reductions in flow and waves compared with summer meadows, small changes in winter seagrass density resulted in large differences in the magnitude of attenuation of flow and shear stress. Similarly, while high seagrass densities effectively trapped sediment during summer, small changes in winter density resulted in strong changes in net sediment flux into/out of the meadow. At our study site, low seagrass densities provided significant reductions in wintertime sediment loss compared to losses associated with completely unvegetated conditions.