Degradation of macroalgae across multiple environmental gradientsEcosystem Services

Lydia White¹, Anna Villnäs¹, Anna Vesanen¹, Camilla Gustafsson¹, Roel Lammerant¹, Alf Norkko¹

¹University of Helsinki, Finland

Degradation rates are a key parameter required to estimate export of macroalgal carbon to deeper water. Despite the legacy of experimental work quantifying degradation rates of macroalgae, especially on kelps, most experiments are carried out in shallow environments, and often over short timescales. Moreover, they rarely incorporate environmental gradients (e.g. exposure, depth, light and temperature) important in driving degradation. We conducted a field experiment to study degradation of Fucus vesiculosus in the Tvärminne archipelago in NE Baltic Sea, over the course of 450 days, along a depth gradient from 5 to 40 metres. Degradation rates were slowest at 5 metres, although this varied considerably between sites. Rates were similar at 15 and 40 metres, with almost all algal biomass disappearing from litter bags by day 450. We hypothesise these differences were largely driven by sufficient light availability at 5 metres which allowed algal biomass to photosynthesize and persist in the environment. Litter bags from 5 metres had up to 50% biomass remaining after 450 days, which displayed positive net primary productivity during short mesocosm incubations post-retrieval. Naturally drifting macroalgal detritus retrieved from the sea floor at 40 metres were also photosynthetically viable, with positive net primary productivity rates during mesocosm experiments. We also observed local enrichment of organic matter within the surface layer of sediment beneath litter bags containing degrading Fucus, which increased positively with increasing depth. These results are crucial for parameterising models to estimate how quickly macroalgal carbon is remineralised during transport to deeper water and thus a macroalgal ecosystems’ potential to sequester carbon.