Spectral and distribution characteristics of colored dissolved organic matter (CDOM) in Sanggou Bay in spring

Colored dissolved organic matter (CDOM) is an important source of carbon in aquatic ecosystems, and the optical properties of dissolved organic matter (DOM) can be used to trace water masses and provide information about the dynamics of the dissolved organic fraction in seawaters. The ultraviolet-visible absorption spectrum was applied to investigate the chemical composition, sources, spatial distribution characteristics and migration and transformation processes of the CDOM in both the water body and interstitial water in Sanggou Bay, a representative semi-enclosed and mariculture bay in Shandong Province, China. The results indicated that, ① the range of absorption coefficient a(355) was 0.23-9.09 /m in Sungo Bay, there are significant differences in the spatial distribution of different water layers, both the surface layer and bottom layer gradually decreased from the nearshore zone to the bivalves & kelp zone, then it rose in the kelp zone and gradually decreased towards the outer sea zone. The a(355) in the kelp zone decreased from surface to bottom and then increased, high-density and large-scale kelp mariculture released a large amount of CDOM into the seawater. The a(355) of sediment interstitial water in each region was 1.3-2.5 times of the sum of the surface, middle and bottom layers, and it was enriched and gradually accumulated in the sediment interstitial water. ② The range of spectral slope (S_275-295) was 0.013-0.036 in Sanggou Bay, the mean values of S_275-295 in outer sea zone and kelp zone were significantly higher than that in other zones, the main components of CDOM are marine organic matter in kelp zone and outer sea zone, and terrigenous organic matter is predominant in the nearshore zone. The content of humic acids in CDOM in the nearshore zone was gradually decreased from the surface layer to the bottom layer, the accumulation of fulvic acid in sediments was high, and kelp zone and outer sea zone showed opposite trend. ③ The range of specific UV absorbance (SUVA₂₅₄)was 4.60-14.10 L/(mg·m) in Sanggou Bay, the SUVA₂₅₄ of the shellfish zone, the bivalves & kelp zone and the kelp zone showed a trend of increasing gradually from the surface to the bottom layer, and the aromaticity of CDOM was gradually increased and reached the maximum in the sediment interstitial water. The SUVA₂₅₄ in the sediment interstitial water of kelp zone and outer sea zone were significantly higher than that in other zones. This study showed that large-scale suspended cultivation activities from kelp zone contributed more refractory dissolved organic matter to the mariculture zone and adjacent seawaters, and it implied that kelp mariculture had a strong carbon sink effect and this sequestration was exported to the offshore waters through the current.