In order for seaweed aquaculture to be economically sustainable, the operational costs must be balanced by the production generated in each cycle. The depth at which seaweed is cultivated plays a crucial role in determining the amount of light energy that the cultivated biomass receives, which is essential for achieving optimal growth. If the cultivated biomass receives Photosynthetically Active Radiation (PAR) that exceeds the capacity of photosynthetic tissues, a phenomenon known as photoinhibition can occur, resulting in reduced growth rates. Optimizing PAR throughout the cultivation cycle is unlikely to be achieved by selecting a single cultivation depth, due to the seasonal variations in PAR with depth. Therefore, a careful evaluation of the appropriate cultivation depth for each stage of the cycle is necessary to optimise PAR exposure and achieve optimal growth of the cultivated biomass.
This study investigates the affect of cultivation depth adjustments for twine seeded Saccharina latissima and Alaria esculenta as a strategy to optimise growth performance and farm efficiency. For each species, two replicate 50m double- catenary lines were connected together using 9 x 4m seeded lines maintaining growing lines under equal tension at 9 depth intervals (0.1m, 0.5m – 4m in 0.5m intervals). In January 2022 , lines seeded in October 2021 were removed and yield and morphology established. These growing lines were replaced with growing lines seeded at the same time but maintaine d on 50m long lines at a depth of 1.5m. This process was repeated in April and the trial concluded in June. Based on the results from this trial only one depth adjustment (spring equinox -21/03/23) was chosen for the following cultivation cycle ( October 2022- May 2023) . In addition to the double-catenary lines, the affect of depth adjustments was measured across replicate (n=2) 50m longlines maintained at depths of 0.5m (shallow) , 1.5m (deep) and 0.5m then 1.5m (shallow-deep) throughout the cultivation cycle. PAR and temperature were recorded at 15 minute intervals at each depth interval using a newly developed sensor chain. Salinity and nutrients were recorded at monthly intervals throughout the trials.
Results show that cultivation depth had a profound impact on growth (yield), morphology and development and this could be correlated with average PAR at each depth selected. Saccharina latissima and A. esculenta lines maintained at 0.5m depth achieved almost twice the yield by January (2020 and March (2023) when compared to lines maintained a 1.5m optimum previously measured at this site across the entire season. Adjusting cultivation depth from shallow (0.5m) to deep (1.5m) improved yield and overall biomass quality at the point of harvest compared to lines maintained shallow (0.5m) and deep (1.5) . Whilst cultivati on depth choice is likely to be site specific these results validate the utility of making at least one depth adjustment to the supporting buoys of growing systems or across the entire farming infrastructure to improve productivity and biomass quality.