The following chapter is a short summary of the main results from the Norwegian project “Kelp industrial production: Potential impacts on coastal ecosystems” (
KELPPRO, 2017–2020) that was carried out in Norway, funded by the Research Council of Norway. The project aimed to establish a knowledge base for sustainable management of large-scale kelp cultivation, and the following chapter is based on the concluding project report “Environmental impacts of kelp cultivation and recommendations for a management strategy” from 2021 (Hancke
et al., 2021).
The background for the project was a numerical modelling study that demonstrated a large potential for kelp cultivation along the Norwegian coast and offshore, with harvesting potentials ranging from 70 to 200 tonnes per hectare (Broch et al., 2019). The study concluded that Norway’s long coastline with clean and nutrient-rich water provides ideal conditions for cultivation of kelp, although conditions vary at scale and with distance to the coast. Areas with stable and good nutrient supply, often at a good distance to the coast, stood out as particularly suitable and productive locations.
To ensure a long-term and profitable industry, kelp cultivation must be developed in a sustainable way with an understanding of potential impacts on the marine environment. Thus, KELPPRO investigated how cultivation of kelp possibly could impact the environmental conditions and marine life on the seafloor and in the pelagic, and it assessed whether kelp farms play a role in spreading alien or endangered species.
Detailed studies of Norwegian kelp farms showed that under normal operational conditions there was a loss of biomass from cultivation farm to the environment, in the scale of 8 to 13% of the actually harvested biomass per year. If the harvest is delayed until late summer, the loss of biomass from the farm to the ocean interior could be >50% of the harvested biomass (Fieler et al., 2021). This biomass originated from kelp eroded from the cultivation farm and is subsequently transported with the water currents while sinking towards the bottom. A case study using numerical modelling (which was informed from laboratory experiments) showed that >90% of the released kelp biomass ended up at the seafloor within 4 km of the kelp farm (Broch, Hancke and Ellingsen, 2022).
At scales of current Norwegian kelp farms (< 100 ha), the effects on the seafloor of released kelp biomass were minimal, and no significant effects on biodiversity or ecological function were documented during normal operations. In contrast, field experiments with deposition of a large amount of kelp (>8 kg fresh kelp per square metre), corresponding to “worst-case” scenarios such as loss of kelp lines, showed a significant worsening of bottom oxygen conditions, reduction in animal diversity, and increased production of toxic sulphide (Hancke et al., 2021). Degradation of deposited kelp was fast, and about 50% of the biomass had disappeared after 3 weeks, and more than 90% after 3 months, indicating reversible impacts on seafloor ecosystems. Laboratory studies showed that the decomposition time was dependent on temperature and bottom oxygen and was longer for winged kelp (Alaria esculenta) than for sugar kelp (Saccharina latissima) (Boldreel et al., 2023). The amount of kelp that constitutes the 'tipping point' from a bioresource for the food web to an ecosystem threat was not possible to quantify in this project.
In open water habitats, the impact of kelp cultivation is closely linked to the competition for nutrients. Field measurements showed that nutrient concentrations in the water around kelp farms, as well as the nutrient status of the phytoplankton, were unaffected by the cultivated kelp. Calculations showed that this applies regardless of the size of the kelp farm. A short-term reduction in light availability under kelp farms (as when phytoplankton is drifting through) will not influence phytoplankton growth. The net discharge of nutrients from kelp plants is negative (kelps take up nutrients during growth), and kelp cultivation might contribute positively to nutrient reduction where concentrations are too high (eutrophic conditions).
Biodiversity studies demonstrated that kelp farms can function as artificial habitats and establish ecosystems but with fewer species and individuals than natural kelp forests (Bekkby et al., 2023). The location of farms will likely play a role in the spread of species and in the impacts on biodiversity. The established principle of not moving kelp plants between ecoregions was supported since different population genetic structures for sugar kelp were identified across ecoregions.
Although, with the above findings, the KELPPRO project found that there are still significant knowledge gaps on the environmental effects of kelp cultivation, including the significance of season, latitude, and location. If kelp cultivation develops into a large-scale industry, the KELPPRO project recommends that further studies of environmental effects on the seafloor, spread of species and of genetic material are performed to ensure a sustainable industry. The project’s recommendations for baseline studies and monitoring according to scale of kelp production in Norway are summarised in Table 2.