The evaluation of C, N, P release and contribution to the aquatic environment during Sargassum litters biomass decay

Sargassum is an important economic species in China, but its decomposition can impact the water environment in aquaculture areas and adjacent seas. To investigate the effects of Sargassum litters on the water environment, a 48-day in situ experiment (ISE) and a Laboratory-Controlled Experiments (LCE) were conducted. Changes in dissolved oxygen, nitrogen, and phosphorus levels in the water, as well as variations in the carbon (C), nitrogen (N), and phosphorus (P) content, lignin, cellulose, hemicellulose, and mass of Sargassum during decomposition, were assessed. The results indicated that Sargassum decomposition occurred in two phases: rapid and slow. Sargassum decomposed more rapidly and completely under ISE compared to LCE. In ISE, the mass and C, N, and P contents of Sargassum declined, with P being released earlier than C and N. By day 8, the P content had decreased by 40.91 % in S. fusiforme and 31.97 % in S. thunbergii relative to the initial values. The lignin and hemicellulose content of S. fusiforme exhibited minimal changes before and after decomposition under both ISE and LCE (p > 0.05). In S. thunbergii, the cellulose and hemicellulose content remained relatively stable under both ISE and LCE (p > 0.05). In LCE, dissolved oxygen in the water quickly decreased, while nitrogen and phosphorus concentrations increased. The cumulative release of TN and TP per unit mass was 14.759 mg·g⁻¹ and 10.196 mg·g⁻¹ for S. fusiformis, and 12.752 mg·g⁻¹ and 5.693 mg·g⁻¹ for S. thunbergii, respectively. It was inferred that Sargassum decomposition could adversely affect N and P levels in the water environment. In recent years, Sargassum blooms have frequently occurred in the East China Sea. Therefore, it is recommended to appropriately remove dead Sargassum in high-density cultivation areas to prevent N and P pollution from seaweed decomposition.