Continental and marine sources of organic matter and nitrogen for rías of northern Galicia (Spain)

Abstract

Sources of organic matter and nitrogen to the rías of Ortigueira, O Barqueiro and Viveiro (Galicia, N Spain) were traced using stable carbon and nitrogen isotopes. Samples of water, seston, benthic organisms and sediments were collected in summer 2007 and 2008. Instantaneous sources were inferred from the concentrations and isotopic composition of seston, while averaged sources over longer time scales (months) were inferred from stable isotopes in macrophytes (Ascophyllum nodosum and Fucus vesiculosus) and mussels (Mytilus galloprovincialis). Dissolved nitrate, nitrite and ammonium concentrations in surface water were low in the estuarine zone and generally increased towards the river and the open sea in all rías. Nitrite and ammonium values were generally low, suggesting low importance of in situ nitrification and denitrification processes. The concentrations of ammonium, chlorophyll, and heavy nitrogen isotopes in seston were higher only in the main river of Viveiro, suggesting local pollution. Isotopic signatures and high nitrogen concentrations in seston discharged from the wastewater treatment plants indicated incomplete removal of nitrogen. However, taking into account water discharge fluxes and the isotopic composition of all compartments analysed, all rías shared similar biogeochemical and trophic diversity and showed a low impact of anthropogenic sources at both short and long time scales. Dominance of marine waters, low river flow, and small urban population sizes in the watershed all contribute to the maintenance of the relatively pristine status of northern Galician rías.