Decomposition and sedimentation of two species of macro algae detritus at the Gouqi Island seaweed beds

The amount and species diversity of fish and other biological resources in island and reef waters are significantly higher than those in the surrounding waters, which is closely related to seaweed beds. As a typical offshore aquatic plant near islands and reefs, more than 90% of its energy enters the food chain in the form of detritus, providing an extremely important energy basis for the conservation and production of biological resources in island and reef waters. In order to explore the decomposition process of macroalgae, the composition and sedimentation characteristics of algal debris, and explore the contribution of macroalgae detritus to the potential scale of ecological functions of the seaweed beds, detritus of Ulva pertusa and Grateloupia livida from the seaweed beds of Gouqi Island was collected by on-site bag hanging and laboratory hydrostatic sedimentation process, and then analyzed in its composition, particle size and chemical characteristics. Results of the present study showed that the detritus density of U. pertusa and G. livida decreased gradually during the decomposition process, and there were no significant differences among different decomposition stages (P>0.05). The content of organic C and organic N in the detritus of G. livida had no significant effect on the change in density (P>0.05), while the content of organic C in the detritus of U. pertusa had no significant effect on its density (P>0.05), whereas the content of organic N had a significant effect on this index (P<0.05). U. pertusa fragmental δ¹³C and δ¹⁵N had a significant effect on its density (P<0.05), and the density of G. livida detritus was only affected by δ¹³C (P<0.05). In the decomposition process, the lignin and cellulose contents of the fragments of the two seaweeds showed a downward trend, and the contents of the two components of the fragments of U. pertusa were higher than those of C. ligulate. The density of both algal detritus was significantly affected by lignin content (P<0.05), while the cellulose content had no significant effect on their density (P>0.05). Particle size is one of the critical factors for the settling speed of the two species of seaweed detritus. The settling speeds of large detritus (>0.830 mm), medium detritus (0.380-0.830 mm) and small detritus (0.180-0.380 mm) of G. livida were 0.016-0.023, 0.008-0.015 and 0.004-0.005 m/s, respectively, and those of U. pertusa were 0.011-0.014, 0.005-0.006 and 0-0.003 m/s, respectively. This study provides useful scientific evidence for studies on the detritus of macroalgae as well as for studies on the ecological radiation range of seaweed beds.