Mechanisms of hypoxia frequency changes in the northern Adriatic Sea during the period 1972–2012

Abstract

Hypoxia events frequently occurred in the bottom layer of the northern Adriatic Sea (NAd) from midsummer to mid-autumn, when the water column is highly stratified, with highly variable spatial extent and duration. To determine the mechanisms of changes in hypoxia frequency and their relation to environmental conditions, 40 yr-long time series of dissolved oxygen and of parameters that describe freshwater influence, stratification processes, and circulation patterns were analysed. It was shown that seasonal hypoxic events in the open water areas coincided with the formation of cyclonic or anticyclonic circulation cells, whose stability was estimated by the appearance of the Istrian Coastal Counter Current (ICCC). The oxygenation of bottom waters during the period August-November of the last two decades has increased, whereas a decreasing trend was observed in surface waters. The frequency of hypoxic events at a larger scale in the NAd decreased since 1992, concurrently with reduced ICCC occurrences. However, the frequency of events in the western area, which is under a direct influence of Po River discharges, did not change significantly, although their intensity recently were lower than during the 1970s and 1980s.

Tracing the effects of eutrophication on molluscan communities in sediment cores: outbreaks of an opportunistic species coincide with reduced bioturbation and high frequency of hypoxia in the Adriatic Sea

“…The present-day FML thickness of at least 16 cm and the signs of deeper incomplete mixing at the Po prodelta (with high density of shrimp mounds on the seafloor) can reflect a renewal in bioturbation in the earliest twentyfirst century. This renewal is suggested by the decline in TOC in sediment cores, probably reflecting the lower frequency of hypoxia directly observed since the 1990s in the northern Adriatic Sea (Djakovac et al 2015). At Panzano, stratigraphic changes in stratification cannot be detected due to slower sedimentation rate and because flood events do not reach such magnitude as at the Po prodelta.…”

“…Although there are methodological differences with later surveys, they can provide order-of-magnitude information on C. gibba abundance in the early twentieth century. Minimum monthly concentrations of bottom-water dissolved oxygen for the NW Adriatic for the years 1911-1984 were extracted from the western sector of Justić et al (1987) and for the years 1972-2012 from stations SJ101 and SJ108 of Djakovac et al (2015) (Fig. 2C).…”

Present climate trends and variability in thermohaline properties of the northern Adriatic shelf

“…Overall, both satellite-derived and in situ-derived sea surface temperature trends reach their maximum values in summer (June-July), while they are lowest in October and January-February. Such large temperature trends in the northern Adriatic and in the examined period might be due to (1) much shallower thermocline in the northern Adriatic than in the rest of the Adriatic and Mediterranean (Franco et al, 1992;Artegiani et al, 1997;Lipizer et al, 2014), resulting in a larger accumulation of heat energy near the surface, and (2) a consequence of atmospheric teleconnection processes acting on a larger spatial and temporal scale, like the Atlantic Multidecadal Oscillation (AMO; Knight et al, 2006), which has been found to be responsible for half of the sea surface temperature trend in the Mediterranean (Marullo et al, 2011;Skliris et al, 2012;Macias et al, 2013). Regarding the rest of the Adriatic, the literature implies that sea surface temperature had a negative trend in the coastal eastern Adriatic between 1960 and 1975, while this trend was strongly positive between 1979 and 2015 (0.23-0.32 • C over 10 years; Grbec et al, 2018).…”

“…This indicates a weakening of the stratification south of the Po River delta and implies that these higher-than-average temperature bottom trends might be due to an increase of heat transfer towards the bottom in the area. Temperature trends are lowest on the eastern section of the transect (0.1-0.2 • C over 10 years), where an inflow of waters from the middle Adriatic occurs (Franco et al, 1992;Orlić et al, 1992;Artegiani et al, 1997).…”

“…The BiOS alters salinity in the Adriatic through the advection of the highly saline and ultraoligotrophic LIW from the Levantine Basin, which prevails during the cyclonic BiOS regime, or the advection of less-saline and nutrient-richer Western Mediterranean waters, which occurs with the anticyclonic BiOS regime. These alterations have been documented to affect the middle and southern Adriatic (Gačić et al, 2010;Mihanović et al, 2015), yet they have not proven to affect the shallow northern Adriatic, which is still considered to be mostly affected by local processes, including a substantial freshwater load by rivers (Franco et al, 1992;Artegiani et al, 1997). Recent investigations on some chemical parameters (Dautović et al, 2017) and bivalve growth (Peharda et al, 2018) along the eastern coast of the northern Adriatic have documented a qualitative matching and significant correlations between these variables and the BiOS regimes.…”