Spatio-temporal variability of the dissolved organic carbon and nitrogen in a coastal area affected by river input: The north eastern shelf of the Gulf of Cádiz (SW Iberian Peninsula)

Four surveys (June and November 2006; February and May 2007) were carried out in the north eastern shelf of the Gulf of Cadiz (southwest Iberian Peninsula) to investigate the dynamics of dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) in a coastal area affected mainly by river input, but also by primary production/respiration, resuspension from the sediments, and mixing. In the period of the present study, DOC values ranged from 42 to 198 μM, while DON ranged from 0 to 20 μM. The seasonal variations showed high mean surface concentrations of DOC (106.2 ± 25.6 μM-C) and DON (8.6 ± 3.1 μM-N) in May, shifting to low DOC (89.1 ± 26.4 μM-C) and DON (4.2 ± 2.8 μM-N) in February. In spring, DOC and DON released by phytoplankton are likely to be the most significant source of organic matter. Low DOC and DON concentrations during winter are probably due to the uptake of DOC and DON by bacteria and to the strong mixing of the water column. The spatial variations showed the highest mean concentrations of dissolved organic matter (DOM) in the lower part of the Guadalquivir Estuary and the lowest mean concentrations of DOM in the bottom water of the oceanic zone. The very high mean DOC and DON concentrations measured off the Guadalquivir Estuary may be due to river input and/or re-suspension of the organic matter from the bottom sediments. The very low mean DOC and DON concentrations measured in the bottom water of the continental shelf may be due to mineralization and a surplus of more refractory organic carbon.