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The lacking recovery of eelgrass ( Zostera marina ) has been observed in many coastal areas throughout the world. Through a strategic field project we managed to characterize and quantify the impact of new and already known stressors and their thresholds on the recovery process. The stressing mechanisms were 1) Physical stress from wave and current action 2) low sediment anchoring capacity facilitate uprooting of eelgrass seedlings; 3) benthic light intensity 4) ballistic stress from drifting macroalgae are damaging seedlings, 5) too frequent resuspension impoverishing the benthic light climate and dispersing seeds to deeper areas not sufficiently supported with light, 6) lugworms burial of seeds, and uprooting or burial of seedlings. Based on the field and supporting laboratory studies we present a GIS-tool that from data on a suite of stressors are able to predict potential areas for recovery of eelgrass by transplantation actions and seed broadcast. Input data may be field data or model simulation results. Here we have used model results. These input data were reclassified into 5 ranges, according to how much it impacted the eelgrass recovery process: 1) Optimal recovery, 2) Good recovery, 3) Threshold for recovery, 4) Poor recovery and 5) Very poor recovery. Afterward a weighed overlay function was performed, ending up with an accumulated value for stress impact on the eelgrass recovery process in all location in the Danish estuary, Odense Fjord. The GIS-tool is able to calculate and visualize areas of individual and/or multi-stress situations at specific locations. It also managed to identify potential recovery area at the present loading and after a 30% reduction of the external nitrogen loading of the system. Further validation by field activities is needed to verify the precision of the tool. [ABSTRACT FROM AUTHOR]

Decades of eutrophication have increased water turbidity in Danish estuaries and led to light limitation of eelgrass (Zostera marina) growth. Former eelgrass areas are now denuded and consist of organic-rich muddy sediment with frequent resuspension events that maintain a high turbidity state. In addition, low anchoring capacity of eelgrass in the soft organic-rich sediments has contributed to eelgrass loss. When navigation channels in Danish estuaries are dredged, large amounts (~100.000 m3) of sandy sediment are shipped to remote dumping sites. Instead, we suggest that the dredged sand is used to consolidate adjacent muddy areas. We demonstrate in the present laboratory study that capping of fluid muddy sediment with 10 cm of sand is feasible without any vertical mixing and that this marine restoration approach potentially can lower the magnitude and frequency of resuspension events. Erosion of suspended solids change from 5 g m−2 min−1 above muddy sediment as to about 0.2 g m−2 min−1 after sand-capping, implying that the application of sand can improve light conditions. Moreover, since erosion thresholds increase from about 10–12 cm s−1 for mud to 40 cm s−1 for sand-capped mud the anchoring capacity of rooted vegetation is increased. However, the full potential of sand-capping to facilitate restoration of otherwise lost eelgrass habitats requires verification by large-scale field experiments. • The past 40 years of eutrophication have enriched estuarine sediments considerably in organic matter. • Sediments are no longer able to consolidate and resuspension has increased, keeping the water in a more turbid state. • Sand-capping muddy sediments significantly increased the thresholds for sediment erosion. • Sand-capping clearly consolidates muddy sediments, and the average benthic light intensity increases due to less resuspension. [ABSTRACT FROM AUTHOR]

The use of wetlands for processing nutrient- and organic-rich wastewater has proved effective to prevent coastal pollution in developing countries, mainly due to low running costs (low energy consumption and training requirements) and relatively high system filtration efficiency (difference of is flushed in the system minus what is flushed out, multiplied by 100). In this study we present a management model that simulate the fate of area specific organic carbon loadings in constructed mangrove wetlands (CMW). The model was calibrated and validated by measured data obtained from a newly established CMW system in Tanzania. Sewage from a nearby hotel was pumped to a primary stabilization pond with a residence time of 12h. Subsequently, sewage was discharged into 9 unvegetated cells and 9 cells planted with mangrove trees (Avicennia marina). Each group of 9 cells was split into 3 subgroups of 3 cells receiving seawater mixed with 0, 20 and 60% sewage for six months. Raw sewage was assumed to contain 0.5 (150mgBODL−1)person equivalent. The model provided a good simulation of oxygen, carbon and nutrient dynamics in the systems; primary production (by benthic microalgae and phytoplankton); and biomass change of important faunal groups (crabs and gastropods). High sewage loadings and reduced residence time decreased the system filtration efficiency. Thus a discharge of 100L of 1PE (300mgBODL−1)m−2 into the system requires a water residence time of about 60h within the CMW to achieve the Tanzanian emission standards (30mgBODL−1). Higher mangrove fauna biomass and abundance of biogenic structures (i.e. pneumatophores and crab burrows) increased carbon mineralization in the CMW sediment by up to a factor of 2. [Copyright &y& Elsevier]

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]

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 is a key indicator for the water quality in Europe ( WFD , European Union, 2000 ). However, although water quality has been improved in most Danish water bodies, the eelgrass population does not seem to be recovering. In this study, we validate and further develop a GIS tool designed by Flindt et al. (2016) , to evaluate the potential of eelgrass reestablishment in Danish waters. The GIS tool was tested in two large broads of the Limfjorden, Løgstør and Lovns broad (Denmark), where two scenarios are run. The first scenario was set up including modelled data, whereas the second scenario included both monitored and modelled data. All scenarios were validated with monitored data collected over a 5 years period in the two broads. The developed GIS tool highlights areas with eelgrass potential, both for vegetative growth and sexual reproduction, in accordance with those found in situ in the period 2009–2013, in the two investigated broads. A combination of modelled and monitored data was found to be optimal to achieve accurate predictions for eelgrass development in the Limfjorden using this GIS tool. In order to implement the current model or to use this GIS tool in other locations, it is needed to have detailed knowledge of the area in focus, especially on the controlling ecosystem parameters and pressures. This eelgrass GIS tool is been proven to be especially beneficial as site selection tool for marine spatial planning e.g. in relation to the implementation of the WFD and the ICZM directives ( WFD , ICZM ), to help assessing anthropogenic/targeted environmental impacts e.g. assessing mussel fisheries impacts and is as well a powerful tool to optimize monitoring cost efficiency. Finally, the described GIS tool, originally set for Odense fjord (Denmark) by Flindt et al. (2016) , has been validated with data from Limfjorden, corroborating the efficiency of the studied tool in Danish waters. [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]

Highlights: [•] Phosphorous removal was studied in filters with sand and crushed concrete. [•] Set-up of a dynamic model for a combined sedimentation and infiltration pond. [•] We can simulate removal of phosphorous and suspended particulate matter. [•] Model based design of ponds optimize the phosphorous removal efficiency. [•] Dissolved phosphorous is ineffective removed in wet ponds, but what about filters? [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]

Decades of eutrophication have deteriorated marine coastal habitats severely and has led to massive decline of eelgrass along European coastlines and impoverishment of benthic fauna. Although nutrient loadings were reduced and water quality increased decades ago, eutrophication in the past had enriched marine sediments in organic matter to an extent that is still affecting ecosystems today. Organic-rich sediments are readily resuspended, keeping shallow estuaries in a turbid state and benthic fauna communities remain low in density and diversity. Sand-capping of muddy sediments may restore such deteriorated estuaries and is in this study applied for the first time as a large-scale restoration approach. A 10 cm layer of sand was added on 1.0 and 1.4 ha muddy sediments at two locations in Odense Fjord, Denmark. The organic content of the mud at the sites was 8–9% and 3–5% before sand-capping. The sand-cap stabilized the mud without mixing the sand-mud interface, not even after one year. The associated lower resuspension of fine particle improved light conditions in the overlying water by up to 9 and 22% at the two locations. Benthic fauna recruitment improved after sand-capping, leading to a local shift from low to high diversity of the benthic community and increased ecosystem functionality. • Organic-rich sediments are easily resuspended, leading to turbid waters and impoverished benthic flora and fauna communities. • Sand-capping of muddy sediments is in this study applied for the first time as a large-scale restoration approach. • Sand-capping of muddy sediment improves increases the diversity and functionality of benthic fauna communities. [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]

Abstract: In many shallow coastal areas worldwide, several species of opportunistic macroalgae (mainly Chlorophyta) have an excessive growth, as a consequence of eutrophication processes. Therefore, bloom-forming macroalgae become the dominant primary producers within these coastal systems. However, frequently the ecology and the ecophysiology of adult macroalgae have been insufficient to explain their seasonal abundances. Thus, it is essential to understand the factors that regulate the germination and growth of spores of opportunistic green macroalgae. In the present work, we assessed the effects of nutrients (N and P), salinity and light on the germination and growth of Enteromorpha spores. Overall, the results highlight the fact that, such as for adult macroalgae, spore germination and growth are adversely affected by low salinities. Growth of the spores is significantly decreased at 5 psu, while salinities of 20 and, especially of 35 psu, clearly promote the spore growth. Additionally, Enteromorpha spores seem to be particularly sensitive to PO4–P limitation and to NH4–N toxicity, which suggests a higher sensitivity to the variation of external nutrient concentrations than adult macroalgae. The present results contribute to increase the understanding about the factors that control macroalgal growth at its early phases of development. In particular, the results suggest that the growth of spores from opportunistic green macroalgae is strongly salinity-dependent. Consequently, in highly hydrodynamic systems such as most shallow estuaries, salinity variations may play a determinant role in the yearly abundances of green macroalgae, since it controls macroalgal growth from the spores to the adults. [Copyright &y& Elsevier]