No detectable effects of co-cultivating sugar kelp (Saccharina latissima) and the blue mussel (Mytilus edulis) in a mesocosm experiment

Low-trophic aquaculture species, including seaweeds and shellfish, receive increasing interest. These species can exert reciprocal effects on each other and their surrounding environment through the uptake and recycling of nutrients and the regulation of phytoplankton. This study explores potential benefits or disadvantages of co-cultivating sugar kelp (Saccharina latissima) and the blue mussel (Mytilus edulis), in a mesocosm experiment during the production cycle of kelp (November to April). The experimental setup comprised 4 treatments divided over 24 tanks: either kelp or mussels, both in co-cultivation, and a control with neither. Overall, the results showed no detectable effect of treatment on kelp or mussel biomass growth throughout the experiment, although kelp frond length elongation did show a significant effect of treatment. Filter feeding by mussels was determined by assessing the retention efficiency (RE) of different particle sizes, which showed that RE was high for all particle sizes counted, and in particular for particle sizes between 3 and 20 μm. Although particle counts were generally lower in tanks with mussels, no effect was detected on water turbidity and light conditions. The C:N ratio in kelp tissue was not affected by the presence of mussels, nor by the supplementation of extra nitrogen. On the whole, no statistically detectable differences were found between monoculture and co-cultivation treatments during winter-spring in this flow-through mesocosm experiment.