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Multidecadal frequency of bottom hypoxia in the Gulf of Trieste (Northern Adriatic Sea) and recent 2015-2016 events
Kralj, Martina, OGS ...
In the Northern Adriatic episodic late summer % early autumn hypoxic and anoxic events during the 1970s, 1980s and beginning of the 1990s were observed. These events could be ascribed to river-borne ...nutrient discharges coupled with a closed circulation, which favors the entrainment of waters in the northernmost part of the sub-basin. In recent decades, however a marked decrease in the continental discharge of nutrients and in particular of phosphates has caused an oligotrophication of the system. Remarkable variations of nutrient loads discharged by local rivers of the Gulf of Trieste occurred during the last years, from the extreme drought in the mid 2000s to increased freshets in the 2010s, which induced consequent variations of phytoplankton biomass and of the dissolved oxygen in the water column. Physical and chemical data collected in three time-series stations, mostly on a regular monthly basis, from 1983 to 2016 were analyzed in order to detect trends and frequency of occurrence of hypoxia events in bottom waters. Even if preliminary results of the analysis of 30-years data show a tendency toward increasing oxygen concentration in the bottom waters, two marked hypoxia events were recorded during the summers of 2015 and 2016, which have been exceptionally observed in a marginal and relatively shallow area (17m) of the Gulf. In order to evaluate the temporal and spatial extent of these hypoxia events, hydrographic, chemical and biological variables of about 30 sampling stations were analyzed. The area was characterized by extremely high sea water temperature (up to 20.6 °C) and salinity (37.7) at bottom and vertical stabile stratification of the water column, which prevented the mixing of oxygen-rich surface water with oxygen-poor water on the bottom. These conditions were maintained until wind-induced turbulence destabilized the entire water column. The main contribution to oxygen depletion are attributed to benthic and plankton respiration, which exceeded the oxygen production. Moreover, the increasing temperature trend in bottom waters, coupled with the increase in riverine discharges, is possibly favoring the hypoxia events in this shallow coastal ecosystem.
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