Enhancing the resilience of Zostera noltei seagrass meadows against Arenicola spp. bio-invasion: A decision-making approach.

Seagrass meadows provide important and valuable ecosystem services. They are affected by several natural and human-induced stressors, but a combination of natural recovery and management actions have recently inverted the worldwide reduction. The main objectives of this study were to provide science-based knowledge on ecology and restoration, framed on environmental-related policies. By coupling the general guidelines with practical experience, obtained from sequential in situ experiments carried out for several months in a show-case study area, this study provides guidelines useful for restoration practitioners. A decision-making approach is proposed to answer the following questions: 1) What is the best Zostera noltei transplanting method? 2) What is the best technique to reduce the bioturbation activity of Arenicola spp.?, 3) Do bioturbation reduction techniques affect the survival rate of Z. noltei transplants?, and finally, 4) What are the key steps to maximize the success of a Z. noltei transplant and increase the species' resilience? Having a Portuguese coastal lagoon as show-case (Mira Channel, Ria de Aveiro), different transplant and restoration methodologies were tested (i.e. metal frames, nails, bamboo sticks, shoots inserted unanchored into the sediment, and intact units of sediment with seagrasses, named as SODs) to assure low environmental impact on donor meadows, high survival rate of transplanted shoots and the recovery of fragmented or lost meadows. Moreover, to potentially reverse a degraded Arenicola spp. colonized seagrass habitat, different types of natural membranes were tested. Results showed that the best transplanting method is the use of SODs as the self-facilitation process of Z. noltei is enhanced, while being the least invasive for the donor population. The use of a natural membrane can significantly decrease the bioturbation stress caused by Arenicola spp., with jute membrane being the best option, given its cost-handling-benefit trade-offs. Enhancing the success of seagrass restoration requires the implementation of effective measures by environmental restoration practitioners. We defined a three-step process to improve the resilience of Z. noltei. This stepwise approach consists on 1) Characterization of the donor population, 2) Identification of the constraints and implementation of measures to prevent them, and 3) Scale-up the restoration plan. The application of this stepwise approach in intertidal coastal and estuarine systems management will, therefore, facilitate the success of Z. noltei restoration plans.

Treatment efficiency of a wet detention pond combined with filters of crushed concrete and sand: a Danish full-scale study of stormwater.

Traditional wet detention ponds and sand filters remove particles efficiently, whereas only a minor part of the dissolved and bioavailable load is removed. To improve the retention of dissolved substances, we tested crushed concrete as a filter material simultaneously with a traditional sand filter placed after an existing wet pond. The particulate fractions (particles, organic matter, phosphorus, and heavy metals) were removed efficiently in the pond and both filter materials, with the concrete filter often being best seen over a year. Dissolved heavy metals (lead (Pb), nickel (Ni), copper (Cu), chromium (Cr), and cadmium (Cd)) were largely retained, though a washout was observed from the pond (Ni and Cu), concrete filter (Cr), and sand filter (Ni) during the first month. The pond only retained total dissolved phosphorus (TDP) during summer. Crushed concrete and sand had a high (>70%) retention of TDP within the first months of operation, but the retention dropped in both filters due to a large oil load into the system (4 kg impermeable ha(-1) in 1 month). The poor retention might to some degree be due to mineralization processes turning particulate phosphorus (PP) into TDP. The massive oil load was retained efficiently (99.3%) in the pond and both filters, clearly illustrating that both filter materials were able to retain either oil or TDP. An additional pilot study showed that at residence times of 1 h, crushed concrete bound 90% TDP whereas sand only bound 22% TDP. Retention of TDP and PP decreased with shorter residence time in both materials, but fastest in sand.

Heavy metal composition in stormwater and retention in ponds dependent on pond age, design and catchment type.

Heavy metals have toxic effects on flora and fauna in the aquatic environments and are of great concern in stormwater. Heavy metal runoff was studied in 37 stormwater ponds in Denmark with varying heavy metal load, catchment type and pond design. The studied metals were Cu, Cr, Cd, Pb, Ni and Zn. The concentrations varied considerably depending on the catchment type, with the highest concentrations coming from industrial areas and the lowest from uncultivated and rural areas. Ponds can effectively remove heavy metals in particulate forms through sedimentation processes, but the dissolved forms are more difficult to retain. The removal efficiency in the ponds varied considerably, with the highest retention of Pb, Ni and Zn due to higher particulate fraction. The retention increased with increased pond volume-to-reduced catchment area ratio. In addition, the pond age affected the efficiency; whereas ponds less than 1-2 years efficiently removed all metals, 30-40-year-old ponds only removed Pb, Ni and Zn, but steeply decreasing over the years. Physical parameters such as pond size, age and sedimentation patterns were found to play a more significant role in the removal compared with chemical parameters such as pH, oxygen and organic matter. Input of metals to the ponds was reflected in the sediment content, but not significantly for all heavy metals probably due to low or varying retention caused by mineralization and re-suspension. The heavy metal concentration in the outlets was reduced to non-toxic levels, except for Cu and Cr at a few study sites.

Crushed concrete as a phosphate binding material: a potential new management tool.

To avoid eutrophication of receiving waters, effective methods to remove P in urban and agricultural runoff are needed. Crushed concrete may be an effective filter material to remove dissolved and particulate P. Five types of crushed concrete were tested in the laboratory to evaluate the retention capacity of dissolved P. All types removed P very effectively (5.1-19.6 g P kg(-1) concrete), while the possible release of bound P varied between 0.4 and 4.6%. The retention rate was positively related to a decreasing concrete grain size due to an increasing surface area for binding. The P retention was also related to a marked increase in pH (up to pH 12), and the highest retention was observed when pH was high. Under these circumstances, column experiments showed outlet P concentrations <0.0075 mg P L(-1). Furthermore, experiments revealed that release of heavy metals is of no importance for the treated water. We demonstrate that crushed concrete can be an effective tool to remove P in urban and agricultural runoff as filter material in sedimentation/infiltration ponds provided that pH in the treated water is neutralized or the water is diluted before outlet to avoid undesired effects caused by the high pH.

Physical stream quality measured by drones and image analysis versus the traditional manual method.

Information on the physical and ecological state of streams along with an overview of the need for maintenance is traditionally a time-consuming manual field task with subsequent limitations in area coverage. Here we propose a novel approach to stream monitoring and management using a low-cost Unmanned Aerial Vehicle (UAV) platform to collect data comparable to that from traditional monitoring schemes. This technology provides high-resolution imagery while being easy to implement at a low cost along with providing data that represent the stream in both fine-scale and at landscape scale. The results show a significant correlation between results obtained by the two methods, with the largest difference in DFI values being 10, but in many cases being <5. The UAV-method is especially strong in supporting geographical measurements of stream width and course along with certain stream parameters such as physical variation, water flow and gravel coverage. The results indicate that UAV mapping of streams is a feasible alternative or support to the traditional mapping of certain open stream types with the possibility of covering more area with the same time-use.

Sand-capping - A large-scale approach to restore organic-enriched estuarine sediments.

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.

The Role of Groundwater for Lake-Water Quality and Quantification of N Seepage.

The heterogeneous nature of both groundwater discharge to a lake (inflow) and nitrate concentrations in groundwater can lead to significant errors in calculations of nutrient loading. Therefore, an integrated approach, combining groundwater flow and transport modelling with observed nitrate and ammonium groundwater concentrations, was used to estimate nitrate loading from a catchment via groundwater to an oligotrophic flow-through lake (Lake Hampen, Denmark). The transport model was calibrated against three vertical nitrate profiles from multi-level wells and 17 shallow wells bordering a crop field near the lake. Nitrate concentrations in groundwater discharging to the lake from the crop field were on average 70 times higher than in groundwater from forested areas. The crop field was responsible for 96% of the total nitrate loading (16.2 t NO3 /year) to the lake even though the field only covered 4.5% of the catchment area. Consequently, a small change in land use in the catchment will have a large effect on the lake nutrient balance and possible lake restoration. The study is the first known attempt to estimate the decrease of nitrate loading via groundwater to a seepage lake when an identified catchment source (a crop field) is removed.

Chemical lake restoration products: sediment stability and phosphorus dynamics.

Laboratory experiments with sediments from three shallow Danish lakes were conducted to evaluate the effects of chemical lake restoration products during resuspension. Phosphorus (P) removal, sediment stability, sediment consolidation and color reduction were studied over time. The investigated products were aluminum (Al), Phoslock (a commercial bentonite product coated with lanthanum) and a combination of Al covered with bentonite (Al/Ben). All treatments effectively reduced the P concentration in the water. However, the treatments containing Al reduced the P concentration immediately after resuspension, whereas Phoslock required several days after resuspension to reduce the P concentration. Especially Phoslock, but also Al/Ben, increased the sediment stability threshold by 265% and 101%, respectively, whereas Al had no stabilizing effect. The fresh Al floc was resuspended 5x easier than untreated sediment. The largest consolidation of the sediment occurred with addition of Phoslock, followed by Al/Ben, while Al alone had no effect. Enhanced consolidation may be of importance for macrophyte colonisation of organic sediment. Phoslock improved the light climate moderately by removing color, whereas Al was very effective in removing color. Ben/Al showed intermediate effects on color reduction. These findings are important when decisions are made on restoration method for a specific lake, which may be more or less wind exposed.