Abstract
Dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) are metabolically important in estuaries. Their availability is influenced by organic matter inputs (internal and external) as well as by internal microbial transformations within the system. In this study, spatial and seasonal dynamics of DOC and DON were evaluated in the Douro River estuary (Portugal). Monthly surveys were performed from October 2005 to December 2006 along a salinity gradient at three different depths. DOC and DON were calculated by subtracting the respective inorganic components (dissolved inorganic carbon, DIC, and dissolved inorganic nitrogen, DIN) from total dissolved carbon (TDC) and total dissolved nitrogen (TDN). In the Douro River estuary, DIC exhibits a linear and positive distribution as a function of salinity, indicating an input of DIC into the estuary from coastal waters. This pattern of distribution along the salinity gradient was also observed for total particulate matter, since a decreasing trend upstream was observed. In contrast, the results showed a general nonconservative behaviour of DOC along the estuarine transect during most months. This typical deviation from conservative mixing was mainly associated with sewage DOC sources into the lower and middle estuary. On average, DON concentrations represented 52–88% of the TDN. Along the transect, relationships between DIN and DON with salinity were generally conservative, decreasing towards the mouth, highlighting the fluvial source of such nitrogen forms; however, occasional DON sewage loads were registered. Estimated global Douro River watershed exports of DOC and DON to coastal waters yielded values of 420 kg C m–2 yr–1 and 125 kg N m–2 yr–1, respectively, which are high when compared with previously predicted levels from global models of DOC and DON export.
Highlights
IntroducciónDissolved organic matter (DOM) is a major component of the organic matter transported to the coastal zone by rivers (Meybeck 1982, Ludwig et al 1996)
We focus on studying the sources and distribution of Dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) along the salinity gradient of the Douro River estuary
Minimum discharge occurred during summer and early fall (June–October), while late fall, winter, and spring were characterized by higher discharges (November–May), with maxima registered on 26 November 2006 corresponding to a flood situation, when hourly river discharge was 7500 m3 s–1
Summary
IntroducciónDissolved organic matter (DOM) is a major component of the organic matter transported to the coastal zone by rivers (Meybeck 1982, Ludwig et al 1996). In addition to controlling ecosystem-level processes, DOM is important from a global and regional biogeochemical perspective, since it constitutes an important pathway for carbon (C) and nitrogen (N) transport from land to sea These compounds are metabolically important in estuaries and their availability is influenced by organic matter inputs (internal and external) and microbial transformations within the estuaries. The potential for utilization or remineralization of organic growth substrates is determined primarily by the substrate C:N ratio (Goldman et al 1987) Another important factor affecting DOM dynamics within an estuarine system is human pressure, for example, through the construction of dams, since they change the timing and quantity of freshwater, sediment, inorganic, and organic matters delivered to estuaries and adjacent coastal zones (Hopkinson and Vallino 1995). The importance of hydrology in retaining these elements has been highlighted in several estuarine ecosystems (Nixon et al 1996)
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