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Abstract: We investigated the impact of sediment reworking fauna and hydrodynamics on mobilization and transport of organic matter and fine particles in marine sediments. Experiments were conducted in an annular flume using lugworms (Arenicola marina) as model organisms. The impact of lugworms on sediment characteristics and particle transport was followed through time in sediments experimentally enriched with fine particles (<63μm) and organic matter. Parallel experiments were run at low and high water current velocity (11 and 25cms−1) to evaluate the importance of sediment erosion at the sediment–water interface. There was no impact of fauna on sediment composition and particle transport at current velocity below the sediment erosion threshold. At current velocity above the erosion threshold, sediment reworking by lugworms resulted in dramatic particle transport (12kg dry matter m−2) to an adjacent particle trap within 56days. The transported matter was enriched 6–8 times in fine particles and organic matter when compared to the initial sediment. This study suggests that sediment reworking fauna is an important controlling factor for the particle composition of marine sediments. A. marina mediated sediment reworking greatly increases the sediment volume exposed to hydrodynamic forcing at the sediment–water interface, and through sediment resuspension control the content of fine particles and organic matter in the entire reworked sediment layer (>20cm depth). [Copyright &y& Elsevier]

Seagrass global distribution has declined in the last decades due to many causes, and the implementation of recovery programmes as well as the development of new restoration techniques are needed. This work describes the development of an innovative restoration measure to enhance Zostera marina (eelgrass) seed germination and seedling survival in sediments inhabited by lugworms ( Arenicola marina ) and its validation in mesocosm experiments. The technique consists of placing 3 cm thick biodegradable coconut fibre mats (membrane) in the surface sediment to exclude the negative effects of sediment reworking (burial of seeds and destabilization/burial of seedlings). Two different flume mesocosm experiments were setup to test for: i) the effect of membranes on burial of Z. marina seeds; ii) the effect of membranes on survival and growth of Z. marina seedlings. The experiments were run for 8 and 10 weeks, respectively. Results show that the membrane was effectively preventing critical burial of Z. marina seeds as all seed mimics placed on the surface initially were recovered from 0 to 4 cm depth in the plots with membrane, while in the absence of the membrane, all seeds were buried to below the critical depth of 5–6 cm. The membrane also significantly enhanced the survival of Z. marina seedlings. The initial seedling density was in both cases 30/m 2 and the final density was 26.0 ± 3.3/m 2 with membrane versus 8.0 ± 1.6/m 2 without membrane. This new marine restoration measure showed to be effective on the reduction of the physical stress imposed by sediment reworking lugworms on Z. marina recovery, as a membrane keeps seeds at optimal depth for germination and protects seedlings from burial and erosion. In comparison to other measures, this new restoration technique is a low-tech nature-based solution. The results clearly show that this restoration technique can support Z. marina recovery through seeds and seedling protection. In this way, this technique contributes to decrease Z. marina vulnerability and increase its natural recovery potential and stability. [ABSTRACT FROM AUTHOR]

Eelgrass ( Zostera marina L.) depth limit is used as an environmental indicator in Danish coastal waters in the Water Framework Directive (WFD) to evaluate coastal waters and their ecological condition. Even after decades of reduced nutrient loadings the reestablishment of eelgrass has not yet succeeded. The mechanisms hindering/delaying eelgrass recovery were recently identified: 1) lack of sediment anchoring capacity, 2) resuspension created by drifting ephemeral macroalgae, 3) seedling uprooting created by current and wave forces, 4) ballistic stress from attached macroalgae and 5) burial of seeds and seedlings by lugworms. These processes were quantified and introduced to an ecological MIKE 3D model. The developed model was calibrated and validated on two Danish estuaries, Odense Fjord and Roskilde Fjord. Analyses of the simulations were performed on area distribution maps. The parameterized stressors impact has been investigated over a three-year period. The results indicate accumulated effects from multiple stressors weakening the capability of eelgrass to recolonize. Combining all stressors in the model decreased the total area covered by eelgrass 83.72% in Odense Fjord and 80.30% in Roskilde Fjord compared to simulation without stressors. Eelgrass peak biomass declined in both fjords from 33.4 to 4.55 ton C km −2 in Odense Fjord and from 24.42 to 5.58 ton C km −2 in Roskilde Fjord. Combining lugworm burial of seeds and seedlings with resuspension from macroalgae and wave forcing had the second strongest negative impact on eelgrass growth, area reduction of 78.31% and 73.14% in Odense and Roskilde Fjord was seen. Ballistic stress from attached macroalgae also reduced growth drastically. Light conditions, sediment organic content along with shear stress at the sediment surface impact the ability of eelgrass to cope with above mentioned stressors. The spatial resolution of the model setup made it possible to generate maps where eelgrass is exposed to lowest stress, revealing areas for potential eelgrass recovery. The developed eelgrass model is now used as a national tool to predict areas where eelgrass restoration effort may be initiated. [ABSTRACT FROM AUTHOR]

Abstract: Eelgrass (Zostera marina) used to dominate the vegetation in Odense Fjord, Denmark, and covered >17km2 of the shallow fjord in 1983. Decades of excessive nutrient loading has lead to decreased eelgrass distribution, and only ~2km2 is covered at present. The state of low eelgrass coverage has not changed despite significant improvements of water quality in the past >10years, and lugworms, Arenicola marina, have colonized the former eelgrass areas (1–8ind.m−2). It was hypothesized that the lack of eelgrass recovery was due to A. marina, which was investigated by a combined field and laboratory approach. At a study site where eelgrass used to dominate, a seasonal study of lugworm population dynamics and sediment reworking activity was performed. Additionally, density dependent burial of eelgrass seeds and seedlings due to sediment reworking by A. marina was investigated in mesocosm experiments. Our results indicate that A. marina may negatively impact eelgrass recovery, since sediment reworking lead to rapid burial of eelgrass seeds and seedlings; within 1–2months, 95% of seeds and 75% of seedlings were buried below critical depth. Considerations based on empirical modeling suggest that negative impact occur even at low A. marina density (5–10ind.m−2). Therefore the spread of A. marina into former eelgrass areas is critical, since eelgrass recovery may be severely impaired, even when water quality favors eelgrass recolonization. [Copyright &y& Elsevier]