Marked reduction of eutrophication pressure in the northeastern Adriatic in the period 2000–2009

Parameters relevant to the study of eutrophication processes in the northeastern Adriatic (NEA) during the period 2000–2009 were analysed and compared with a previous period (1972–1999). The data used in this study were collected at two stations in the coastal zone up to 13 NM off Rovinj (western Istria). The more recent period was characterised by a strong reduction of the Po River flow rate and different circulation regime with the more frequent inflow of high salinity and oligotrophic water from the central Adriatic. Change in the circulation was evidenced by lower frequencies of the Istrian Coastal Countercurrent (ICCC) as compared to the Eastern Adriatic Current (EAC) after 2000, particularly in spring. As a consequence, a marked increase in surface salinity and decrease in orthophosphate and chlorophyll a concentrations at two stations in the period 2000–2009 occurred. In contrast, during late winter and in spring of the more recent period, dissolved inorganic nitrogen (DIN) concentrations were higher, despite the lower freshwater discharge and increased inflows on nutrient-poorer water. This DIN increase was very probably due to an enhanced reduction of orthophosphate concentration after 2000, limiting further the phytoplankton growth, resulting in a more marked accumulation of unused DIN. The obtained results showed that the eutrophication pressure was markedly reduced in the NEA during 2000–2009 in respect to the previous period.