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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. [ABSTRACT FROM AUTHOR]

Desorption rates for sediment-associated contaminants during resuspension events are investigated for a site on the Palos Verdes shelf, California. This study focuses on p,p′-DDE, the most abundant DDT compound present in an effluent-affected near-surface sediment deposit on the Palos Verdes shelf. Frequency, magnitude, and duration of resuspension events are estimated from bottom boundary layer measurements and a 17-yr record of surface wave conditions. Desorption of p,p′-DDE from sediment in suspension is calculated using a radial diffusion model for porous aggregates. Suspended sediment concentrations and size distributions during resuspension events are found using a shelf sediment-transport model. Calculations are made for a 60-m site where measurements of near-bed flow and sediment properties, including p,p′-DDE concentration and percent organic carbon by size fraction, are available. The results indicate that the time scales of p,p′-DDE desorption from the finest sediment fractions (<20 μm) are typically much shorter than the duration of resuspension events, which average 1.6 days at the study site, whereas desorption time scales are almost always much longer than event duration for the coarser size fractions (>63 μm); the median bed grain size is about 45 μm. Our calculations suggest that 25–50% of the mass of p,p′-DDE in the surface active layer of the bed is lost during a resuspension event; mean active layer depth is 0.2 cm and mean p,p′-DDE concentration in the uppermost 2 cm of the bed at the study site is 11 μg−1. Total annual loss depends on the number of events per year. Wave data indicate an average of 10 resuspension events per year at a depth of 60-m on the Palos Verdes shelf from 1982 to 1999. This leads to an estimated average loss of p,p′-DDE by desorption during wave-driven resuspension of 3–7 μg cm−2 yr−1 for the 60-m site. The range in average loss rates is due to the large range in published values of the partition coefficient for DDE. [Copyright &y& Elsevier]

Long-term (60-yr) predictions of vertical profiles of p,p′-DDE concentrations in contaminated bottom sediments on the Palos Verdes shelf were calculated for three locations along the 60-m isobath using a numerical solution of the one-dimensional advection–diffusion equation. The calculations incorporated the following processes: sediment deposition (or erosion), depth-dependent solid-phase biodiffusive mixing, in situ diagenetic transformation, and loss of p,p′-DDE across the sediment-water interface by two mechanisms (resuspension of sediments by wave action and subsequent loss of p,p′-DDE to the water column by desorption, and desorption from sediments to porewater and subsequent molecular diffusion to the water column). A combination of field measurements, laboratory analyses, and calculations with supporting models was used to set parameters for the model. The model explains significant features observed in measurements made every 2 years from 1981 to 1997 by the County Sanitation Districts of Los Angeles (LACSD). Analyses of available data suggest that two sites northwest of the Whites Point sewage outfalls will remain depositional, even as particulate supply from the sewage-treatment plant and nearby Portuguese Bend Landslide decreases. At these sites, model predictions for 1991–2050 indicate that most of the existing inventory of p,p′-DDE will remain buried and that surface concentrations will gradually decrease. Analyses of data southeast of the outfalls suggest that erosion is likely to occur somewhere on the southeast edge of the existing effluent-affected deposit, and model predictions for such a site showed that erosion and biodiffusion will reintroduce the p,p′-DDE to the upper layer of sediments, with subsequent increases in surface concentrations and loss to the overlying water column. [Copyright &y& Elsevier]

Both submerged and emergent vegetation plays a fundamental role in coastal bays. Vegetation stabilizes the substrate, increasing resilience to storms. Vegetation also traps sediments favoring accretion and therefore counteracting sea level rise. Previous modeling studies on flow-vegetation-sediment interactions have focused on one specific vegetated community, but we lack a general understanding of the synergistic effects of multiple vegetation species. We focus our study on the Virginia Coast Reserve Long Term Ecological Research (LTER) site, USA, where we apply numerical modeling (Delft3D-SWAN) to investigate the independent and synergistic effects of salt marsh vegetation and seagrass. Our numerical results show that salt marshes and seagrass beds reduce the volume of water entering and exiting the shallow coastal bays up to 15% during each tidal cycle. Vegetation also reduces bed shear stress and hence increases sediment deposition in the bay and marshes up to 10% compared to the no-vegetated case. Our study shows the double benefits of seagrass as an ally of salt marsh in promoting bays resilience. On the one hand, seagrass helps the salt marsh to survive during storms by reducing wave energy; on the other hand, seagrass generates more friction in subtidal parts of the bay where salt marsh cannot survive. [ABSTRACT FROM AUTHOR]

In shallow coastal bays where nutrient loading and riverine inputs are low, turbidity, and the consequent light environment are controlled by resuspension of bed sediments due to wind-waves and tidal currents. High sediment resuspension and low light environments can limit benthic primary productivity; however, both currents and waves are affected by the presence of benthic plants such as seagrass. This feedback between the presence of benthic primary producers such as seagrass and the consequent light environment has been predicted to induce bistable dynamics locally. However, these vegetated areas influence a larger area than they footprint, including a barren adjacent downstream area which exhibits reduced shear stresses. Here we explore through modeling how the patchy structure of seagrass meadows on a landscape may affect sediment resuspension and the consequent light environment due to the presence of this sheltered region. Heterogeneous vegetation covers comprising a mosaic of randomly distributed patches were generated to investigate the effect of patch modified hydrodynamics. Actual cover of vegetation on the landscape was used to facilitate comparisons across landscape realizations. Hourly wave and current shear stresses on the landscape along with suspended sediment concentration and light attenuation characteristics were then calculated and spatially averaged to examine how actual cover and mean water depth affect the bulk sediment and light environment. The results indicate that an effective cover, which incorporates the sheltering area, has important controls on the distributions of shear stress, suspended sediment, light environment, and consequent seagrass habitat suitability. Interestingly, an optimal habitat occurs within a depth range where, if actual cover is reduced past some threshold, the bulk light environment would no longer favor seagrass growth. [ABSTRACT FROM AUTHOR]