The lessons learned from this project centre around the following aspects:
- artificial substrates are an effective method to increase biomass in harbour basins
- The aggregate of mussels provides an additional water filter capacity which can be beneficial for the ecosystem
- No information is available on the long-term development
- The weight of the pontoonhulas increased with almost 300 kg in less than four months
- The design of pontoonhulas can be optimized by making use of elements such as Mussel Seed Capture Installations
- Polehulas need relatively large rope diameters to be effective
- The artificial structures can be used to dampen wave action within harbour basins
- Artificial substrates, such as polehulas and pontoonhulas, can increase the biomass in a harbour basin considerably. Thirty-four weeks after construction, one polehula yielded on average 8.5 times as much biomass as a pole with no hula around it. A pontoonhula is able to gather a total biomass of some 350 kg after 19 weeks. On both artificial substrates, the mussel Mytilus edulis is the dominant species. On the polehulas, different algae species were also found.
- The collection of all mussels on a single pontoonhula is able to filter the entire water volume of the Scheurhaven basin in 16 days. The additional filtration will contribute to improved light penetration into the water, to contaminant sequestration and a better overall ecological condition of the basin. This is in line with the main objective of the European Water Framework Directive.
- Biomass development and species composition have only been monitored in the first year after construction. This implies that so far no clear indication of their long-term development is available.
- After mussel settlement, the weight of the pontoonhulas increased with almost 300 kg in less than four months. Consequently, the original buoyancy of pontoonhulas was no longer sufficient to keep the structure afloat. For future designs buoyancy should be adapted to the increasing weight.
- The design of pontoonhulas can be optimized by making use of design elements of MZI’s (Knowledge – Mussel Seed Capture Installations), which are developed to capture mussel seed at open sea. These structures are anchored and consist of steel cables and a large number of float levers.
- Polehulas can be improved by using ropes with a larger diameter. In the current design the rope diameter in the polehulas was 0.5 cm. This was rather small, as individual mussels ended up settling on several ropes.
- Floating mussel structures attenuate waves and can be used for this purpose in harbour basins. The amount of wave attenuation depends on the length of the structure and the rope density.
- To realize this pilot, a step by step approach was followed: 1) scope the opportunities and costs for potential eco-design solutions, 2) implement a small-scale pilot to obtain a proof-of-concept, 3) modify the design according to the lessons learned from the pilot and upscale to full scale. This gradual approach has proven helpful in the efforts to achieve an innovative approach that was actually implemented in the field.