Recent evolution of river discharges in the Gulf of Trieste and their potential response to climate changes and anthropogenic pressure

Freshwater and nutrient discharges by rivers were analysed in the northeastern Adriatic continental shelf from 1998 to 2008, in order to assess their role in the biogeochemistry of this coastal zone, as well as their potential future changes due to the effects of climate variability and anthropogenic pressure. River water loads (up to 6.05 km³ yr⁻¹) and transport of nutrients (up to 13,200 t N yr⁻¹ for TN, 86 t P yr⁻¹ for TP and 12,400 t Si yr⁻¹ for Si(OH)₄) were high from 1998 to 2002, but they decreased by over 70% during the driest years 2003, 2005, 2006 and 2007. The precipitation in the surrounding continental region (average of 1371 mm yr⁻¹) is the major forcing that regulates the runoff in this area, with peaks in early spring and autumn that are associated respectively to negative phases of Mediterranean Oscillation index and positive phases of West Mediterranean Oscillation index. This finding, together with the minor contribution of snowmelt in early spring (up to 3.2·10⁻⁵ kg m⁻² s⁻¹), indicates that the dynamics of the natural water cycle still overtake in this coastal zone the effects of anthropogenic usage of continental waters. During the last four decades, the northeastern Adriatic continental shelf has been subjected to an increasing pressure due to a high river transport of nitrogen and, currently, only a deep phosphorus deficiency in total (TN/TP = 49–405) and inorganic (DIN/PO₄ = 37–418) river nutrient pools prevents its severe eutrophication. By contrast, the decrease of river loads of nutrients from 2003 to 2007 indicates that recurrent water crisis might significantly lower the trophic level in this coastal zone in the future. In this perspective, other allochthonous sources of nutrients, like sewage loads, atmospheric deposition and benthic fluxes might become more important for their balance, with possible implications on the structure of this ecosystem.