BIO-OFFSHORE: Grootschalige teelt van zeewieren in combinatie met offshore windparken in de Noordzee

Reith, E.H.; Deurwaarder, E.P.; Hemmes, K.; Curvers, A.P.W.M.; Kamermans, P.; Brandenburg, W.A.; Lettings, G.


This study addresses the technological feasibility of seaweed cultivation in the North Sea in combination with offshore wind parks and harvesting and conversion of seaweed biomass to renewable energy carriers and chemicals. The study also identifies stakeholders and participants for technology development and the ecological and societal conditions to fit in large-scale seaweed cultivation in the marine environment, existing marine infrastructure and functions, and (inter)national regulations and policies for the North Sea. Three seaweed species that are native in the North Sea have been selected for potential cultivation: Ulva sp. (belonging to the green macroalgae), Laminaria sp. (a brown macroalga) and Palmaria sp. (a red macroalga). Current commercial seaweed cultivation systems usually consist of (partly) anchored line structures to which the seaweeds are attached and are generally located on coastal locations. International research shows that cultivation systems in the open sea may become easily damaged by wind and wave action. An experimental ring shaped system has thus far shown the best stability for the conditions in the North Sea. However the production costs are high. Considerable system development is therefore required to enable large-scale, economically attractive cultivation of seaweeds combined with offshore wind parks. The optimal system design is unknown. This study proposes a layered system for seaweed cultivation employing the typical light absorption characteristics of green, brown and red macroalgae respectively, to enable optimal use of the available sunlight and enhance areal productivity. Without addition of nutrients the productivity in the North Sea is estimated at approx. 20 tons dry matter/ha.year. Through layered cultivation and/or addition of nutrients this can potentially be increased to ca. 50 tons dry weight /ha.year. Development of precision nutrient dosage technology is required to prevent eutrophication. Potential negative environmental impacts include: sedimentation of seaweed fragments and other organics with a negative effect on the oxygen budget in the water column, and possible negative impacts on migration of sea mammals including dolphins, porpoises and whales. Seaweed cultivation can also have positive impacts including the uptake of nutrients by the macroalgae (reducing eutrophication) and an enhancement of marine biodiversity, because the seaweeds and the cultivation systems offer substrate for attachment, shelter and feed for molluscs and fish. Indeed, the system could be managed as a nursery for young fish in order to restore fish populations in the North Sea. Integration of seaweed cultivation with other types of aquaculture e.g. cultivation of mussels or fish is a realistic option. The Dutch government target for offshore wind in 2020 is 6.000 MW installed turbine capacity. This will involve a surface area of approx. 1000 km2. The support constructions for the wind turbines can serve as a structural basis for seaweed cultivation systems. Designs must take into account the additional load on the turbine supports due to currents, wind and wave action, and accessibility of the turbines for maintenance vessels. Potential synergistic effects of the combination of offshore wind and seaweed cultivation supporting the profitability of both activities include joint management and maintenance, alternative employment opportunities for fisheries and ecological benefits