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

Large-scale cultivation of the seaweed Gracilaria has important economic and environmental benefits. However, the decomposition of seaweed litter affects water quality in the cultivation area and its adjacent waters. In the present study, laboratory controlled experiments (LCE) and in situ experiments (ISE) were carried out to investigate G. lemaneiformis and G. lichenoides biomass decomposition rates, changes in carbon (C), nitrogen (N) and phosphorus (P) content, and subsequent effects of nutrients release in the surrounding water quality. The results indicated that decomposition of Gracilaria was divided into rapid and slow stages. The amounts of C, N and P decreased, the C/N and C/P first increased and then declined, and N, P were released earlier than C in Gracilaria. The water quality declined in LCE was caused by the dissolved oxygen (DO) concentration decreased and the N, P concentrations increased. The unit cumulative release of TN and TP from G. lemaneiformis were 12.35–14.08 mg g−1 and 7–9.63 mg g−1, respectively, and those from G. lichevoides were 5.37–7.92 mg g−1 and 8.41–10.02 mg g−1, respectively. It is supposed that the decomposition of Gracilaria litter in large-scale and high production cultivation had an unhealthy impact on the N, P concentration of the surrounding water environment, and cause to potential secondary pollution. Therefore, it is supposed the shedding or senescent Gracilaria should be removed timely during the cultivation and harvesting. Harvested seaweeds should be properly treated and a better harvest schemed shall be applied to avoid nutrient release from seaweed decomposition.