Riverine transport of terrestrial organic matter to the North Catalan margin, NW Mediterranean Sea

• We investigate the terrestrial organic matter discharged to the North Catalan margin.
• River hydrology affects the quantity and quality of discharged organic matter.
• Water reservoirs act as efficient traps for terrestrial organic matter.
• Human activities have altered δ15N of suspended organic matter.
• Terrestrial organic matter in the study area is not a relevant source of organic carbon to the deep sea.

Rivers are the primary pathway for organic matter transport from the terrestrial to the marine environment and, thus, river fluxes are critical in regulating the quantity of terrestrial organic matter that reaches the coastal ecosystems. Hydrodynamic processes typical of the coastal zone can lead to the transport of terrestrial organic matter across the continental shelf and beyond. Such organic matter can eventually reach the deep margin and basin ecosystems. Riverine inputs of organic matter to the sea can be a significant food source to marine ecosystems contributing to carbon cycling in these ecosystems. In order to assess the marine carbon cycle it is essential to know the biogeochemical characteristics and temporal dynamics of the fluvial organic matter input discharged by rivers to the coastal zone. In this study we present a one and a half year long (November 2008 to May 2010) assessment on organic carbon (OC) and nitrogen (N) inputs from the three main rivers discharging into the North Catalan margin (Tordera, Ter and Fluvià, from south to north). Furthermore, we investigate the characteristics of the particulate organic matter discharged by these rivers by means of stable isotopic (δ13C and δ15N) and grain size analyses.We found that the hydrological regime of the rivers is a relevant factor in regulating the quantity and mediating the quality of organic matter inputs to the North Catalan margin. Overall, the three main rivers discharging into the study area deliver 1266 and 159 tonnes of terrestrial OC and N per year, respectively, to the coastal zone. Most of the OC and N load is transported during floods, which indicates that the Mediterranean climate of the area, with a strong seasonal contrast in precipitation, determines the timing of the main inputs of OC and N to the sea. Therefore, the annual OC and N load experiences a high temporal variability associated to the number and magnitude of floods with in each hydrological year. In addition, we found that water reservoirs along the rivers act as traps for terrestrial organic matter, reducing its delivery and ultimate burial into marine sediments. River hydrology also affects the quality of organic matter that reaches the coastal zone (both in terms of C and N) by shifting the relative weight of the various sources of terrestrial organic matter. During low river discharge (i.e., in summer and early autumn) the main contributor to the organic matter pool is mostly associated with freshwater primary producers, whereas with relatively high water flows (i.e., in winter and spring) the main contributor is associated with erosion and release of soil organic matter. Furthermore, the impact of waste water treatment plants into the studied rivers results in the alteration of the isotopic signal of suspended N. The three studied rivers play a major role in transporting terrestrial organic matter to the North Catalan margin, but the fraction that is exported to the deep margin by high-energy episodic hydrodynamic events, such as large coastal storms, has a minor importance.