In the following, the environmental impacts, which were identified in Table 1, are assessed for their relevance and significance for seaweed cultivation in the Faroe Islands and potential mitigation measures are suggested. Present assessments are based on small to medium sized (20–150 ha) cultivation farms, and an upscaling will call for a large-scale specific assessment including cascading, and potential compensating, effects as well as cumulative impacts.
3.1 Light attenuation
The on-grow of seaweed species in the cultivation farm at sea, requires that the seaweeds are positioned in the top metres of the water column where light conditions are optimal. However, this may result in light attenuation (shading of light) below the farm towards the seafloor. At shallow water sites it may reduce the light available for benthic autotrophic organisms adapted to the natural conditions (Hancke et al., 2018; Campbell et al., 2019; Visch et al., 2020; Armoskaite et al., 2021; Norderhaug et al., 2021). Visch et al. (2020) found from in situ light logger measurements underneath a Saccharina latissima cultivation site and at a control site, that the irradiance was significantly reduced up to 40% beneath the cultivated kelp biomass a week before harvest. No effects from shading from a medium sized kelp farm (18 ha) was reported on an eelgrass meadow beneath the farm (Walls et al., 2017), even though impacts from reduction in light may be expected on light sensitive species such as eelgrass (Nielsen et al., 2002). Visch et al. (2020) suggest that it may be due to a relative short time period of light reduction, only when the kelp biomass is peaking before harvest.
The impact from reduction in light conditions on the seafloor communities may also depend on if, e.g., the seafloor substratum is suitable for autotrophic organisms at all or is situated at a depth outside the photic zone, as well as if the light attenuation rate is naturally high. However, to apply the precautionary principle as a mitigation measure, it is in general recommended to avoid placing the cultivation farm above other light dependent benthic communities (Armoskaite et al., 2021).
The Faroese shore is quite steep and the maximal depth for growth of the naturally occurring kelp species is reached at 20–30 m (Bruntse, Lein and Nielsen, 1999). In general, as the straits and fjords are characterised by these steep slopes, reaching maximum depths of 40–100 m, but with a flat muddy base as well as the more offshore seafloor substratum (central shelf) consists of a < 1 mm grain size sediment, and depths ≥ 100 m (Erenbjerg et al., 2020; ICES, 2023), a large area is thus not suitable for benthic autotrophs. As such, the total area of natural kelp forests is restricted by substrate availability and estimated to 275 km2 in the Faroe Islands (Kvile et al., 2022). Thus, to minimise the risk for light reduction impacts in the Faroe Islands from seaweed cultivation, cultivation areas close to shore and depths < (20) 30 m for medium sized farms should be avoided. According to Chapter 1.1, and the description of present size of seaweed farming in the Faroe Islands, the sea surface areas used are around or below medium size (150 ha). Monitoring of light conditions beneath Faroese seaweed cultivation farms as well as establishing baseline data on the local depth distribution of seaweed may give information causing the need to change or adjust the mitigation recommendations (see Chapter 1).
3.2 Uptake of nutrients and carbon
For growth of autotrophic organisms, besides light, sufficient concentrations of nutrients and inorganic carbon is necessary. Thus, introducing a growing kelp biomass to a natural habitat that take up nutrients may impact the natural competition of autotrophic organisms (phytoplankton) in the water phase (Hancke et al., 2018; Campbell et al., 2019; Visch et al., 2020; Armoskaite et al., 2021; Norderhaug et al., 2021; ICES, 2023).
However, it has been found that the concentration of dissolved inorganic nutrients is not negatively affected by seaweed cultivation farms (Visch et al. (2020), and references herein), as such, if the background concentration of nutrients is large, the amount taken up by cultivated kelp may be neglectable. However, if ambient nutrient concentrations are low (which they often are in coastal regions in summer) and nutrients is a limiting source, farmed kelp might take up significant amounts of nutrients, but Hancke et al. (2021) found that phytoplankton outcompete seaweed in the efficiency for nutrient uptake and thus it is unlikely that seaweed will pose a threat for phytoplankton nutrient uptake even when concentrations are low and availability is limited. In general, at small to medium scale cultivation activities, negative environmental effects from nitrogen depletion are not expected (Campbell et al., 2019).
For the Faroe Islands, the coastal areas can roughly be divided in two, when it comes to the ecological state of nutrients; the mixed shelf water and the stratified fjords with estuarine circulation. In the mixed shelf water, the tidal currents are strong and the water masses are vertically mixed from surface to bottom. This implies that there is seldom nitrogen depletion and that effluents from anthropogenic activity are quickly dispersed over wide areas. In the fjords, nitrate depletion is regularly occurring in the upper water masses during the growth season, but the stratification is so weak that there is frequent upwelling of nutrients. Thus, the annual microalgae production in Faroese fjords is 2–3 times higher than in neighbouring regions due to the frequent nutrient upwelling (ICES, 2023). Therefore, considering that phytoplankton may be the strongest competitor for nutrients (Hancke et al., 2021), it is not expected that the installation of small to medium sized seaweed cultivation farms in the Faroe Islands will cause nutrient depletion that may impact phytoplankton production negatively. As a potential mitigation measure, areas with periodically establishment of a stratified water column, limiting nutrients in the upper water masses, may be avoided, also due to potential limiting seaweed biomass growth and yield. However, establishment of a local baseline for nutrient fluctuations may give information for adjusting the placement of the cultivation farms in space and depth.
Regarding uptake of CO2, it is not expected to have any detrimental effects in and around the cultivation sites (Campbell et al., 2019). However, in the Arctic where long summer photoperiods create optimal conditions for marine vegetated habitats’ photosynthesis, a sustained up-regulation of pH can be observed (Krause-Jensen et al., 2016). This effect is not considered to be of potential negative impact; however, the findings may suggest potential CO2 limitation of photosynthesis in such dense communities during summer (Krause-Jensen et al., 2016). In the Faroe Islands (see Chapter 1.1), it may have no implications for the cultivated seaweed biomass yield, which is harvested before summer due to avoidance of biofouling. However, for biomass harvested during the period from April to October, data on pH in and outside the cultivated seaweed biomass may give information on potential CO2 limitation for biomass growth and yield, and, as such, may lead to adjustments in harvest timing.
3.3 Hydrography changes
In-sea cultivation systems may influence the water velocity, the tidal drag and turbulence within and around (beneath) the seaweed cultivation farm, and thus affect the pelagic and benthic organisms and communities by, e.g., oxygen conditions as well as nutrients and food supply (Campbell et al., 2019; Armoskaite et al., 2021). If these parameters are critical, careful considerations must be applied when selecting the cultivation site. However, in the Faroe Islands, the effect from the cultivation farm on hydrography is overall assessed as insignificant on the pelagic ecosystem, although local effects may be observed. In the Faroe Islands there are strong tidal currents in most straits, which, to a lesser degree though, also influence the water exchange in the fjords, as well as many areas are exposed to ocean swells (ICES, 2023). As such, data for water velocities inside and outside the farm, effects on sediment transportation, i.e., sedimentation rates within and outside the farm (up- and downstream) as well as data on potential changes in sediment deposition locations is recommended (Armoskaite et al., 2021).
3.4 Mother plant harvest
To follow established protocols and practices using spores for seeding lines for cultivation of kelp species (and also other species), mother plants must be collected from nature in vicinity to the farm site. The plants may be harvested from kelp forests, which otherwise are creating habitats and sustaining communities of flora and fauna (e.g., (Norderhaug et al., 2021; Bekkby et al., 2023)). The number of specimens needed for spore production may be relatively low, though, depending on the scale of activities, as 5–10 kg mother plant can produce spores for 1000 m of seeding lines (See Chapter 1.1). However, the need for mother plants from nature can be reduced by establishing hibernating gametophytes, which can be sprayed onto the seeding lines (Kraan and Guiry, 2000; Mols-Mortensen et al., 2007). In addition, sporogenesis in cultivated plants can be induced by blocking the transport of sporulation inhibitors in the lamina of the kelp species (Pang and Lüning, 2004) (see also Chapter 1.1).
There is little local knowledge on the importance of seaweed as nursing areas for commercial fish stocks in the Faroe Islands (á Norði et al., 2023), although there is an ongoing study on Faroese kelp forest as nursery ground for cod and pollock in Kaldbaksfjørð, which may serve as an indicator of the importance of kelp forest and provide an understanding for the baseline.
3.5 Noise
Boat transport and activity due to initiation of on-grow, maintenance and harvest of the seaweed cultivation farm, may increase disturbance and underwater noise (Campbell et al., 2019; Armoskaite et al., 2021).
Disturbance from activities may have an above-water impact on especially breeding birds and haul-out areas of, e.g., seals, whereas underwater noise may scare off marine mammals from their migration routes and feeding grounds or mask their communication (Christensen et al., 2015; Campbell et al., 2019).
The impact from small boats and vessels are likely small in connection with small and medium scale cultivation farms, and thus it is unlikely to cause significant impact on birds and marine mammals, as long the presence and migration is taken into account (Campbell et al., 2019). Thus, baseline information on presence, seasonality and migration of relevant species should be collected, to avoid sites of or adjust activity timing with important breeding, feeding and migratory activities. In case of large-scale seaweed cultivation activities, cumulative impacts with other industrial activities must be considered.
There is relatively good information on the presence of breeding seabirds and marine mammals in the Faroe Islands (ICES, 2023). Mitigating measures are avoiding activities in seasons when especially very site-specific breeding birds are present as well as avoiding activities close to haul-out areas, migration routes and feeding grounds. Local baseline information on breeding seabirds and marine mammals should be collected and taken into account when selecting areas for seaweed cultivation farms. A noise level and impact assessment should be performed for larger projects (Armoskaite et al., 2021).
3.6 Emissions
From energy consumption using fossil fuels, emission of greenhouse gases (carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O)), other air pollutants such as carbon monoxide (CO), nitrogen oxides (NOx), particulate matter (PM), volatile organic compounds (VOCs) and benzene (C6H6) are emitted into the environment and impacting air quality. Further, sulphur compounds (SOx) can be emitted, depending on the sulphur content of the fuels used.
It is not expected that the emissions from seaweed cultivation activities will have a significant impact on the Faroe Islands total emission of, e.g., CO2, which is relatively high due to a large fishing fleet with a high dependence on fuel oils. The Faroese fishing fleet accounts for around half (48% and 42% in 2021 and 2022 respectively) of the Faroe Islands' CO2 emissions (Hansen, 2023; Nielsen et al., 2023).
However, in general, emissions from burning of fuel for power can be reduced by the use of electricity from renewable sources, such as hydro, wind or solar power, e.g., for laboratory activities, which may be established locally (see Chapter 1.1).
To reduce, e.g., emission of sulphur from shipping, the maximal sulphur content of ships' fuel oil allowed was reduced to 0.5% in the IMO 2020 regulation prescribed in the MARPOL Convention. This is a global requirement and relevant for the Faroe Islands, since the limit of 0.1 % in the Sulphur Emission Control Areas (SECAs) from 2015 was only partially incorporated in the Faroe Islands in 2018. However, smaller boats are not included in the IMO regulation, but they usually use marine gas oil (MGO), which is a compliant fuel.
3.7 Discharges and pollution
Discharges may be wastewater from hatchery, process water from seeding in hatchery or discharges from boats in operation.
In general, it is assessed that normal wastewater from toilets and washing (black and grey) is treated as part of the municipality wastewater treatment or handling, and is not considered of particular concern within the environmental assessment of seaweed cultivation activities in the Faroe Islands.
Further, process water from the seeding and sporeling development tanks is as such not considered of concern if discharged to sea. The process water will be of salinities within the natural range and although, if nutrients are added to the sporeling development tanks, and may have a higher content of nutrients than ambient waters, the expected limited amounts of discharged water can be diluted considering the water exchange in the Faroe Islands’ waters and natural nutrient levels (see Chapter 3.2). However, in large-scale amounts of process water discharge, the salinity and nutrient concentrations in the process water for discharge should be measured and assessed if concentrations may impact the recipient water body, and thus if mitigation measures will be necessary, e.g., water treatment before discharge. Where national regulation is in place, this should be followed. In small scale production, and if a continuous seawater flow system is used (see Chapter 1.1), the impact is assessed as insignificant.
However, in sporeling development tanks without a continuous seawater flow system, and where nutrients are added, germanium dioxide (GeO2) may be added to prevent growth of diatoms competing for light and nutrients (see Chapter 1.1). GeO2 is toxic to diatoms because it disrupts silica deposition, but has also been shown to inhibit kelp growth in higher concentrations in one study (1 millilitre of the stock solution (4.47 mg ml-1) per litre corresponding to 4470 µg l-1) (Shea and Chopin, 2007). Addition of low concentrations of GeO2 (0.02 ml of the stock solution (4.47 mg ml-1) per litre corresponding to 89.4 µg l-1) was shown to block diatom growth. There is no regulation or established threshold concentrations of GeO2 within the EU water frame directive (WFD, Directive 2000/60/EC), and according to the toxicity test presented in the safety data sheet of GeO2 (as Germanium (IV) oxide), the acute toxicity concentration for aquatic environment given for ErC50 (50 % reduction in growth rate) for algae is 206.7 ug l-1, which is twice as high as the concentration recommended to prevent diatom growth in kelp cultivation systems (Shea and Chopin, 2007). However, discharging process water containing GeO2 may be of concern regarding impacts on the natural primary production by diatoms in the recipient water body if on a larger scale. Thus, in such a case, the amount of GeO2 discharged needs to be monitored or calculated with models for water exchange, and thus the dilution effect, in the recipient water body for assessing a potential environmental impact. However, the use of GeO2 in cultivation systems in the Faroe Islands is not common due to the use of continuous seawater flow systems. Such systems also prevent competing growth of diatoms due to constant natural nutrient levels (see Chapter 1.1).
As Campbell et al. (2019) state, debris and discarded or lost components may contribute to existing environmental pollution such as plastics. Especially, in case of large-scale cultivation, proper maintenance and responsible management must ensure that loss of deployed cultivation gear to the environment is avoided.
A study, also mentioned by Norderhaug et al. (2021), showed that a large-scale cultivation system for Pyropia yezoensis in China, was a source for a relatively high amount of microplastics in the environment (Feng et al., 2020). There are rising concerns for microplastics in the environment of the Faroe Islands (Bråte et al., 2020; Collard et al., 2022), and Norderhaug et al. (2021) recommend for Norway that technical solutions and monitoring are considered to mitigate addition of more microplastics to the environment as more knowledge may be obtained on seaweed cultivation systems as source for microplastics.
Pollution by heavy metal is not expected as such toxic materials are not used in seaweed cultivation in the Faroe Islands (see Chapter 1.1). However, within the finfish aquaculture in the Faroe Islands, background as well as threshold values are established (ICES, 2023).
3.8 DOM / POM contribution to the ecosystem and potential changes in seabed oxygen concentrations
From the farmed seaweed there may be a loss of organic material (Dissolved Organic Material, DOM, and Particulate Organic Material, POM) during the on-growth period as well as during harvest (Fieler et al., 2021; Hancke et al., 2021). Contribution of organic material for feed and decomposition may affect the benthic fauna communities and oxygen concentration in the seabed. As such, the contribution of POM may have a positive impact increasing the feed, on the other hand higher oxygen demand may lead to hypoxia and stress on infauna at the seafloor.
It has been shown for kelp farms in Norway that the loss of organic material from kelp farms is about 5% of the harvested biomass during the first months of the growth season and increases to 8–13% of the harvested biomass before harvest during summer (Fieler et al., 2021). If the biomass is not harvested the loss increases to more than 50% in late summer (August). The distance within which the POM may settle depends highly on the degree of exposure driven by currents and/or wind, and thus it is shown that 90% is settled within 4 to 28 km to the kelp farm anticipated to produce a kelp biomass of 100 t ha-1, the 4 km being at a relatively sheltered location (Hancke et al., 2021; Broch, Hancke and Ellingsen, 2022).