Influence of riverine, oceanographic and meteorological forcings on coastal hypoxia in the NW Adriatic, Italy

Long-term series of riverine, oceanographic and meteorological data in the Emilia Romagna Coastal Zone were analyzed with respect to the records of hypoxic events that were collected during the environmental monitoring of ARPA in 1977-2008, in order to highlight seasonal and interannual mechanisms of formation of this phenomenon. On a seasonal scale, hypoxia was found to be largely modulated by the pronounced annual cycle of environmental conditions typical of this area. During winter, its appearance was matched to air and surface seawater temperatures higher than average (difference between monthly medians up to +3.6 °C). In spring and autumn, a greatest importance of large phytoplankton blooms induced by river nutrient loads was observed. In August-October, hypoxia was correlated to stable weather conditions (wind velocity < 2 m s-1, precipitation < 2 dm3 m2 d-1) suggesting a major role played by the persistent stratification of the water column as forcing factor. During all the seasons, wind direction resulted to be a factor enhancing hypoxia when it contributed to the reduction of water flushing along the coast, or to the spreading of hypoxic bottom waters. On an interannual scale, a shift from large, persistent summer hypoxia to short, recurrent, irregular events distributed across all seasons occurred after the end of the 1980s. This change was concomitant to significant decadal trends for air warming (+0.14 C yr-1), wind speed (+0.03 m s-1 yr-1), Po River flow (-0.54 km3 yr-1) and salinity (+0.09 yr-1), oxygen saturation (-0.2 % yr-1), PO4 3- (-0.004 ?mol-P L-1 yr-1) and NH4 + (-0.04 ?mol-N L-1 yr-1) concentrations in surface seawater. These results suggest that ongoing and future climate changes occurring in this region might significantly alter the dynamics of coastal hypoxia.