River water and nutrient discharges in the Northern Adriatic Sea: Current importance and long term changes

Runoff and nutrient transport by rivers were analysed in the Northern Adriatic continental shelf, in order to evaluate their interannual and multidecal variability, as well as their current contribution to determine freshwater and nutrient budgets in this marine region. During the years 2004–2007, the runoff in the basin (34.1–64.6 km3 yr−1) was highly imbalanced, being 84% of freshwater discharged along the western coast, because of the contributions of Po, Adige and Brenta rivers. In the northern and eastern sections of the coast, freshwater discharge by rivers was less important (10 and 6%, respectively), but not negligible in determining the oceanographic properties at sub-regional scales. The oscillations of the transport of biogenic elements (124–262×103 t N yr−1 for TN, 72–136×103 t N yr−1 for DIN, 4.5–11.1×103t P yr−1 for TP, 2.2–3.5×103 t P yr−1 for PO4 and 104–196×103 t Si yr−1 for SiO2) were strictly dependant to the differences in the annual runoff. A strong excess of N load in comparison to P load characterised all rivers, both in inorganic nutrient (DIN/PO4=37–418) and total (TN/TP=48–208) pools, particularly in the northern and eastern areas of the basin.The annual runoff showed significant oscillations for Po on multidecadal time scale, whereas a general decrease (−33%) was observed for the other N Adriatic rivers as the recent discharges were compared to those before the 1980s. During the dry years 2005–2007, a strong reduction of river water flows and nutrient loads was experienced by the N Adriatic ecosystem with respect to years characterised by medium-high regimes. An increased frequency of similar drought periods, due to ongoing climate changes or to a larger human usage of continental waters, would be easily able to significantly change the biogeochemistry of this basin.

► A strong decrease of minor river loads is taking place in the N Adriatic.
► Po load shows long term oscillations and recurrent periods of drought of 2–3 yr.
► River nutrient transport depends on water load and highly varies on interannual scale.
► Periods of scarce river load affect the biogeochemistry of N Adriatic.
► Decreasing riverine inputs of phosphorus might enhance oligotrophy of N Adriatic.