Does Spartina invasion affect the carbohydrate assimilation of polychaetes in mangroves? A case study in the Zhangjiang Estuary Mangrove National Nature Reserve

Carbohydrates, which form a large pool of carbon in plant litter, are a major driver of litter dynamics and benthic food webs in Spartina-invaded mangrove ecosystems. However, there are large gaps in knowledge regarding the effects of exotic leaf litter carbohydrates on benthic carbon assimilation. In this study, the dynamics of carbohydrates, including cellulose, lignin and monosaccharides (glucose, mannose, galactose, xylose and arabinose), during the leaf litter decomposition of the exotic Spartina alterniflora (SA) and the native mangrove plant Kandelia obovata (KO) were analyzed and compared. Meanwhile, the differences in the δ¹³C values of cellulose and lignin were used to analyze the utilization of leaf litter carbohydrates by polychaetes after Spartina invasion. Our results showed that the decomposition rate of exotic SA leaf litter was lower than that of KO leaf litter. The contents of cellulose, glucose and galactose in SA leaf litter were higher than those in KO leaf litter (P < 0.05). The decomposition rates of cellulose and monosaccharides (arabinose, xylose, mannose and galactose) in KO leaf litter were faster than those in SA leaf litter (P < 0.05). However, there was no significant difference in the decomposition rates of lignin in the leaf litter between exotic SA leaf litter and native KO leaf litter (P > 0.05). In the native KO plot, the carbohydrates of KO leaf litter contributed to >55% (cellulose, 26.91–28.02%; lignin, 28.05–28.64%) of the organic carbon sources of polychaetes. However, in the exotic SA plot, the carbohydrates of exotic SA leaf litter contributed to >65% (cellulose, 36.83–38.30%; lignin, 28.47–30.52%) of the organic carbon sources of polychaetes. The obtained results suggested that Spartina invasion increased the assimilation of cellulose by polychaetes. The high content and low decomposition rate of cellulose in the carbohydrates of exotic SA leaf litter might be important mechanisms for the carbon assimilation changes in the benthos of Spartina-invaded mangrove ecosystems. Given that Spartina has colonized in the Zhangjiang Estuary Mangrove Reserve, this study provides a theoretical basis for the recovery of benthos after ecological restoration. Furthermore, the mechanisms of carbohydrate assimilation by different benthos of different ecosystems should be comprehensively considered in the future.