A decade of weekly dissolved organic carbon values in the coastal upwelling of the NW Spain (Atlantic Galician Rías)
Introduction
Dissolved organic carbon (DOC) is the major form of organic carbon in aquatic environments, playing a key role in global biogeochemical cycles (Carlson, 2002). DOC produced within the system (autochthonous DOC) originated from phytoplankton exudation, cell lyses and grazing (Nagata, 2000). Allochthonous DOC primarily originates from plant and soil materials (Cauwet, 2002). DOC fractions have been characterized according to their reactivity (Hansell, 2013). Labile DOC, with a turnover-time of hours to days is preferentially used by bacterioplankton, although it can be used by primary producers, and does not accumulate. Recalcitrant DOC can be subclassified in: i) semi-labile and semi-refractory DOC fractions, which can accumulate in the surface ocean and cycle in the order of months to decades; they account for most of DOC that is exported from the euphotic zone (Lorenzoni et al., 2013) and; ii) refractory and ultra-refractory DOC fractions, the largest and least reactive pool of DOC in the ocean, with assumed cycling over millennial time scales (Hansell, 2013).
Coastal marine ecosystems are one of the most productive and biogeochemical active zones of the biosphere (Gatuso et al., 1998). Coastal processes are intensified in eastern boundary upwelling systems when along-shore equatorward wind drives the surface layer offshore, inducing the rise of deep, cool and inorganic nutrient-rich waters into the coastal photic layer (Walsh, 1991, Wollast, 1998). The Galician coast, 42–44°N, is at the northern limit of the NW Africa upwelling system, which extends from 10°N to 44°N. In this area, coastal winds are favorable to the upwelling of deep, cool inorganic nutrient-rich and also organic-poor waters into the coastal photic layer from March–April to September–October. The downwelling of warm, inorganic-poor and organic-rich shelf surface coastal waters takes place during the rest of the year (Wooster et al., 1976, Fraga, 1981). Several inlets of varying sizes make up the Galician coast. These inlets, called rías, are freely connected to the adjacent shelf. The largest and deepest rías, four large coastal indentations of more than 2.5 km3 (Rías Baixas: Ría de Vigo, Ría de Pontevedra, Ría de Arousa and Ría de Muros) and a medium sized ría of 0.75 km3 (Ría de Ares–Betanzos), occupy the NW area of the Galician coast (Atlantic Galician Rías; Fig. 1). The hydrodynamics of these rías, essentially dictated by coastal winds, are characterized by a reinforced positive circulation during upwelling periods and a depressed positive or even negative circulation during downwelling pulses. These rías circulate ‘like’ partially mixed estuaries but they are forced by shelf wind-stress (as coastal upwelling systems) not by continental runoff (as estuaries). Middle and outer parts of the rías behave as an extension of the adjacent shelf. Only the most inner parts of these rías (< 15 m) can be considered ‘like’ a partially stratified estuary where fresh and marine waters mix gradually (Álvarez-Salgado et al., 2000, Villegas-Ríos et al., 2011, Piedracoba et al., 2014). The average low flow (between 3 and 50 m3 s− 1; Otero et al., 2010) of unpolluted rivers of the rías with an average nutrient concentration of 17 ± 3 μmol L− 1 dissolved inorganic nitrogen (DIN) and 80 ± 20 μmo L− 1 DOC (Gago et al., 2005), and the average salinity in the inner stations of rías (> 30.0; this study), could indicate that most of the nutrients measured there are of marine origin (≥ 70% of total DIN; Pérez et al., 2000, Villegas-Ríos et al., 2011). The calculated average flushing times of the rías (between 1 and 2 weeks, for Ría de Ares–Betanzos and Rías Baixas, respectively; Álvarez-Salgado et al., 2000, Villegas-Ríos et al., 2011) are generally short enough to prevent significant accumulation of inorganic and organic nutrients. However, some of the shallow areas (depth < 20 m) can be converted into ‘potentially eutrophic waters’ (Ferreira et al., 2011) during stratification periods and increased water residence times. In Europe, actions against eutrophication brought about conventions and legislation (e.g. Oslo and Paris Regional Sea Convention-OSPAR- in the Atlantic; OSPAR, 2002) which were followed by two main Directives: i) the Water Framework Directive (WFD; European Commission, 2000) and ii) the Marine Strategy Framework Directive (MSFD; European Commission, 2008). The physico-chemical indicators used for eutrophication and trophic status of marine water assessments include DIN and dissolved inorganic phosphorous (DIP). Additional indicators, such as total nitrogen (TN), total phosphorous (TP), DOC and silica are included in the OSPAR Eutrophication Monitoring Program (OSPAR, 2005), in MSFD (European Commission, 2008; Table 1, Table 2, Appendix III) and in most of the assessment frameworks (Table 2.4; OSPAR, 2012).
Despite the growing awareness of the importance of organic substances as nutrients in the last decades, relatively few studies have addressed their dynamic and temporal variability in time and/or space. Often, only inorganic nutrients have been measured in monitoring efforts. The Instituto Tecnológico de Control del Medio Marino de Galicia (INTECMAR) oceanographic program, is one of the few long-term monitoring projects that consistently measured inorganic nutrients (nitrate, nitrite, ammonium, phosphate, silicate), DOC, and TP and TN on a weekly basis together with CTD data in the five main rías of Galicia (NW Spain).
Recent studies of trends in the NW Spain upwelling area have pointed out a multidecadal decrease in the intensity of upwelling (e.g. Lemos and Sansó, 2006), an increase in water residence time in the rías and a warming of coastal and shelf waters (Pérez et al., 2010) and changes in plankton biomass and composition (Bode et al., 2009, Pérez et al., 2010). How these changes affect DOC pool is not known. Previous studies of DOC in these rías related DOC with temperature during an annual cycle in the Ría de Vigo (Doval et al., 1997, Álvarez-Salgado et al., 1999, Nieto-Cid et al., 2004), and also with DIN, ammonium and phosphate during an average upwelling and downwelling period in the five main rías of Galicia (Doval et al., 2013). The objective of our work was to study the temporal variability of DOC for the five main rías of Galicia, between the years 2002 and 2011. Seasonal DOC accumulation was assessed and the first decadal DOC trends were investigated in all rías. Intra and interannual variability of DOC was also compared.
Section snippets
Study area and sampling program
The four large Rías Baixas (Ría de Vigo, Ría de Pontevedra, Ría de Arousa and Ría de Muros) and a medium-size central ría (Ares–Betanzos) in the coastal upwelling of the NW Spain were sampled weekly to monitor their oceanographic conditions. The position of each station is shown in Fig. 1, coded as V for Ría de Vigo, P for Ría de Pontevedra, A for Ría de Arousa, M for Ría de Muros and L for Ría de Ares–Betanzos. The inorganic nutrient dataset (nitrate, nitrite, ammonium, phosphate and silicate)
Average DOC levels
The study of DOC, salinity, and temperature between 2002 and 2011 is summarized in Table 1 for the 10 selected stations (inner stations: V3, P3, A3, M2 and L3; outer stations: V5, P4, A0, M5 and L1).
The inner stations of rías showed higher concentration of surface DOC (99.7 ± 5 μmol L− 1) and lower salinity (33.7 ± 0.6) than the outer stations (89.3 ± 2.5 μmol L− 1 and 34.8 ± 0.3, respectively). Except Ría de Arousa, inner stations presented higher surface temperature (15.2 ± 0.3 °C) than the outer stations
Annual cycle and accumulation of DOC
Coastal upwelling areas, where the shelf wind regime supports a strong exchange with the adjacent ocean waters (Walsh, 1991), are characterized by an intensification of biogeochemical processes (especially primary production (PP), and vertical and horizontal export). The morphology and orientation of the upwelling-dominated Galician rías influences the fate of new and regenerated nutrients, efficiently consumed to increase PP (e.g. Bode et al., 1996, Bode et al., 2011), which result in biogenic
Conclusions
The first decadal DOC trends (2002–2011) were investigated in five coastal upwelling-dominated Galician rías (NW Spain): Rías Baixas (Ría de Vigo, Ría de Pontevedra, Ría de Arousa and Ría de Muros) and Ría de Ares–Betanzos. Results from the temporal variability of DOC can be extended to other coastal areas.
The temporal distribution of DOC and temperature were comparable, with a marked seasonal variability, suggesting that the temperature was the main proxy for DOC. This 10-year dataset of
Acknowledgements
The authors thank the technical assistance of J. Mouriño and L. Abal for oceanographic database maintenance of INTECMAR and the crew of the R/V J.M. Navaz and R/V Lura. The valuable suggestions and comments by two anonymous reviewers and E. Peltzer are gratefully acknowledged.
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