Search

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]

• Links between nutrients and predators can be investigated using ecosystem models. • Nutrient-driven covariance patterns are documented in a coastal area of the Baltic. • Local declines in productivity affected entire benthic food web, including birds. With successful mitigation of eutrophication and the reductions in nutrient concentrations and productivity of coastal waters, the targets set in nature protection legislation in the EU and the United States may no longer be achievable in regions where key ecological functions are coupled to benthic productivity. Yet, due to both the patchiness of invertebrate distribution and the fragmented and non-integrated nature of monitoring data direct coupling between nutrients, productivity and predators has proven difficult to achieve. As a result, assessments of the status of food webs based solely on monitoring data remains an almost impossible task. The aim of this modelling study was to test the application of fine-scale ecosystem models for assessing cost-benefits or food-web consequences of management decisions in relation to water quality of coastal waters. We applied a fine-scale ecosystem model calibrated against measurements in a coastal area in the Baltic Sea to quantify responses in higher trophic levels to changes in eutrophication over a 18-year period, 1990-2007. The resulting spatio-temporal trends reveal a number of characteristic responses and spatial dimensions in coastal food webs. The coupled hydrodynamic, bio-geochemical and waterbird energetics modules indicated nutrient-related changes and fine-scale covariance patterns across all trophic levels. A 50 % decline in bivalve biomass was predicted in a zone characterised by the overall highest biomass of bivalves and highest densities of bivalve-feeding waterbirds. The nutrient-driven local decline in productivity affected the entire food web with a predicted annual mortality of 72,000 Long-tailed Ducks Clangula hyemalis. This model-based study suggests a strong nutrient control of the available food supply to bivalve-feeding birds in coastal areas. Our results also show that high-resolution ecosystem models are required to resolve the heterogeneous distribution of effects. [ABSTRACT FROM AUTHOR]

Odense Fjord (Denmark) has suffered from a drastic decline in eelgrass ( Zostera marina ) coverage during the last decades. In 1983 eelgrass still covered about 25% of the estuary, which in 2005 was reduced to less than 2%. The alarming decline in the past decades initiated preliminary restoration activities, where it was questioned whether the present low eelgrass biomass is able to produce a sustainable seed bank to support natural recovery. Field studies verified that the seed bank was hampered. Laboratory experiments were conducted to determine 1) seed dispersion along the sediment surface and in the water column; 2) settling rates of seeds and flowering shoots; 3) critical current speed for seed movement; 4) floating dynamic of flowering shoots and 5) seed dropping dynamics during transport of rafting shoots. These parameters supported the development of an agent based model (ABM) predicting seed movements in estuaries. The model handled two ways of seed dispersal: 1) seeds dropped in eelgrass beds and transport by hydrodynamic forces along the seabed 2) seeds released by rafting shoots. This setup allowed assessment of both eelgrass seed loss and potential connectivity between beds. Seed losses were divided into direct losses, such as seeds lost on land due to desiccation or external boundary, and indirect losses affecting seedling establishment. The model estimates that app. 92% of the seeds would be retained in the Odense fjord, while only 5.0% of the seeds ended up in areas supporting seedling establishment. Eelgrass seeds were also found in areas with insufficient light, high hydrodynamic pressure, excessive sediment reworking by lugworms or poor anchoring capacity. In addition, the model showed potential bed connectivity via rafting shoots, but also with individual seed movement along the bottom, when beds were not separated by deep areas, such as boating channels. [ABSTRACT FROM AUTHOR]

Highlights: [•] An agent based model was developed to simulate unattached macroalgae in a fjord. [•] Bottom drifting macroalgae generate sediment resuspension and reduce light availability. [•] The reduced light availability at the seabed and the ballistic effect generated by drifting macroalgae negatively affect eelgrass populations. [•] The model simulates the monitored macroalgae distribution. [•] The model simulates the location of hot spots where eelgrass recovery is affected by macroalgae stress. [Copyright &y& Elsevier]