The Norwegian coastline covers more than 10°in latitude and provides a range in abiotic and biotic conditions for seaweed farming. In this study, we compared the effects of cultivation depth and season on the increase in biomass (frond length and biomass yield), chemical composition (protein, tissue nitrogen, intracellular nitrate and ash content) and biofouling (total cover and species composition) of cultivated Saccharina latissima at nine locations along a latitudinal gradient from 58 to 69°N. The effects of light and temperature on frond length and biofouling were evaluated along with their relevance for selecting optimal cultivation sites. Growth was greater at 1-2 m than at 8-9 m depth and showed large differences among locations, mainly in relation to local salinity levels. Maximum frond lengths varied between 15 and 100 cm, and maximum biomass yields between 0.2 and 14 kg m −2. Timing of maximum frond length and biomass yield varied with latitude, peaking 5 and 8 weeks later in the northern location (69°N) than in the central (63°N) and southern (58°N) locations, respectively. The nitrogen-to-protein conversion factor (averaged across all locations and depths) was 3.8, while protein content varied from 22 to 109 mg g −1 DW, with seasonality and latitude having the largest effect. The onset of biofouling also followed a latitudinal pattern, with a delayed onset in northern locations and at freshwater-influenced sites. The dominant epibiont was the bryozoan Membranipora membranacea. Our results demonstrate the feasibility of S. latissima cultivation along a wide latitudinal gradient in North Atlantic waters and underscore the importance of careful site selection for seaweed aquaculture.

To reach the goal of an industrialised macroalgae industry in Norway and other high-cost countries in the near future, a standardised seedling production method to improve quality control and predictability of cultivated biomass is essential. A total of 11 different treatments for seeding twine or rope with meiospores, gametophytes or juvenile sporophytes from the kelp Saccharina latissima were measured for growth (frond length, frond area, biomass yield and density) and protein content after 80 and 120 days at sea. Meiospore-and gametophyte-seeded twines were pre-cultivated in the hatchery for 14-42 days prior to deployment, while juvenile sporophytes of different ages were seeded on ropes directly on the day of deployment using a commercial binder to attach the seedlings. The results showed that seeding with meiospores pre-cultivated in the hatchery for 42 days (S42) before deployment gave significantly longer fronds (77.0 ± 6.7 cm) and a higher biomass yield (7.2 ± 0.1 kg m −1) at sea compared to other treatments. The poorest growth was measured for the direct-seeded sporophytes pre-cultivated in freefloating cultures for 35 days prior to deployment (D35; 34.4 ± 2.4 cm frond length and 1.6 ± 0.4 kg m −1). Image analysis was used to measure the coverage of the twine substrate before deployment, and a correlation was found between substrate coverage and frond length at sea, indicating that this can be used as a tool for quantity and quality control during the hatchery phase and before deployment. The protein content did not reveal any large differences between the treatments after 120 days of cultivation.

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24measured in-situ, represent an innovative approach to demonstrate that phytoplankton size 48 and stratification influence the nature of particles found in the water column (including 49 aggregates, fecal pellets and grazer abundances). 50