A decade of weekly dissolved organic carbon values in the coastal upwelling of the NW Spain (Atlantic Galician Rías)

• Temporal variability of DOC in five upwelling Galician rías (NW Spain) was studied.
• Surface DOC showed a clear seasonal variability (maximum values: May to October).
• Interannual variability in surface DOC was up to 11% of DOC values.
• Decadal DOC (2002–2011) exhibited positive trends in inner areas of Rías Baixas.

Dissolved organic carbon (DOC) was measured weekly in five coastal upwelling Galician rías (Ría de Vigo, Ría de Pontevedra, Ría de Arousa, Ría de Muros and Ría de Ares–Betanzos) and the first decadal DOC series was examined in these rías. The temporal variability of DOC was assessed by considering biweekly values at 10 oceanographic stations for the decade 2002–2011. Seasonal variability in surface DOC concentrations was observed, with higher values (101 ± 8 μmol L− 1; average ± STD) during the accumulation period (DOCb; May–October) in all inner and outer stations. During the rest of the year (DOCc; November–April) surface DOC concentrations decreased to 88 ± 6 μmol L− 1. The temporal distribution of DOC and temperature were comparable, with a marked seasonal variability, suggesting that the temperature was the main proxy for DOC. The average decadal seasonal increase in surface DOC (ΔDOCs = DOCb − DOCc) was between 7 and 18 μmol L− 1 for all stations. The interannual variability of surface DOC ranged between 0 and 18 μmol L− 1; 0 and 11% of the average DOC of each station. Decadal DOC trends were slightly positive in middle-inner areas of the Rías Baixas (Ría de Vigo, Ría de Pontevedra, Ría de Arousa and Ría de Muros) in agreement with positive temperature trends. The outer area of the Rías Baixas showed positive DOC trends in the stations of Ría de Vigo and Ría de Arousa and negative DOC trends in the stations of Ría de Pontevedra and Ría de Muros, although temperature presented positive trends in these four stations. The Ría de Ares–Betanzos exhibited negative trends in DOC and temperature.

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