Spatial variability of organic matter in marine sediments from the Gulf of Cadiz

This work analyses the spatial variability of organic matter (phytopigments, proteins and carbohydrates) in surface sediments from the Gulf of Cadiz (SW Iberian Peninsula). Sediment samples were taken in summer 2019 during the oceanographic cruise “INPULSE-19” at different stations with depths ranging from nearly 500 m to 1000 m depth. The distribution of the different organic matter fractions showed a high variability with marked differences between stations. The spatial distribution of phytopigments (PHYTOPIG) in the upper sediment layer (0-1 cm below the seafloor) evidenced high degree of accumulation at two close stations, where the higher PHYTOPIG concentrations were found, while PHYTOPIG concentrations were one other of magnitude lower at other sampling stations. Protein and carbohydrate concentrations in the water-soluble fractions (PROT_WS and CHO_WS) from the upper 0-1 cm sediment layer were correlated with each other, although they exhibited some differences in their spatial distribution pattern. Moreover, PROT_WS in the 0-1 cm sediment layer was highly correlated to PHYTOPIG, suggesting a similar origin for both fractions, while the correlation with CHO_WS was weaker. Alkaline extractable proteins and carbohydrates (PROT_ALK andCHO_ALK) were one order of magnitude higher than the concentrations in their respective water-soluble fractions. PROT_ALK andCHO_ALK in the 0-1 cm sediment layer were highly correlated with each other, showing a quite similar distribution pattern. The PROT_WS/CHO_WS ratio, an index of organic matter lability, ranged between 0.3 and 2.1 in the upper sediment layer, while the PROT_ALK/CHO_ALK ratio ranged between 1.6 and 3.6 The observed variability in these ratios indicates differences in the lability of sedimentary organic matter at different stations, which might affect the bioavailability of organic matter in the sediments.