In der Adria kommt es im Sommer und Herbst durch die Wasserschichtung immer wieder zu sauerstoffarmen Phasen, sogenannten Hypoxie-Ereignissen. Eine gute Nachricht: In den letzten 40 Jahren sind die sauerstoffarmen Phasen seltener geworden, wie Djakovac et al. 2015 berichteten:
Mechanisms of hypoxia frequency changes in the northern Adriatic Sea during the period 1972–2012
Hypoxia events frequently occurred in the bottom layer of the northern Adriatic Sea (NAd) from mid-summer 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.
Tomasovych et al. 2017 erweiterten nun mithilfe von Untersuchungen an Muscheln den Betrachtungszeitraum in der Adria auf die vergangenen 500 Jahre. Die Forscher fanden, dass es seit dem 18. Jahrhundert immer wieder zu Hypoxie-Phasen kam, die eng an die Wassertemperatur gekoppelt waren. Insofern sind die modernen sauerstoffarmen Phasen lediglich eine Fortsetzung der bekannten Reihe. Abstract:
Stratigraphic unmixing reveals repeated hypoxia events over the past 500 yr in the northern Adriatic Sea
In the northern Adriatic Sea and in most semi-enclosed coastal regions worldwide, hypoxia induced by eutrophication in the late 20th century caused major die-offs of coastal marine organisms. However, ecosystem responses to hypoxia over longer centennial scales are unclear because the duration of direct observations is limited to a few decades and/or the temporal resolution of sedimentary archives is compromised by slow sedimentation and bioturbation. To assess whether perturbations of ecosystems by hypoxia recurred over centuries in the northern Adriatic Sea, we evaluate the timing and forcing of past hypoxia events based on the production history of the opportunistic, hypoxia-tolerant bivalve Corbula gibba, using 210Pb data, radiocarbon dating, amino acid racemization, and distribution of foraminifers in sediment cores that capture the past 500 yr in the Gulf of Trieste. Unmixing the stratigraphic record on the basis of 311 shells of C. gibba, we show that the reconstructed fluctuations in abundance do not correlate with abundances in the raw stratigraphic record. We find that production of C. gibba has undergone major decadal-scale fluctuations since the 18th century, with outbreaks corresponding to density of more than 1000 individuals per square meter. These outbreaks represent long-term phenomena in the northern Adriatic ecosystem rather than novel states characteristic of the 20th century eutrophication. They positively correlate with centennial-scale fluctuations in sea-surface temperature, indicating that the hypoxia events were coupled with water-column stratification rather than with nutrient enrichment.
