Hypoxia and anoxia in the Adriatic Sea

Shallow, coastal and shelf seas are the most threatened parts of the marine ecosystem. One of the key threats is eutrophication, whose symptoms include excessive algal blooms, oxygen deficiency, and mass mortalities of organisms. The sublittoral benthic communities of the Northern Adriatic Sea have suffered repeated large-scale mortalities associated with excessive marine snow development. This makes the Northern Adriatic a case study for eutrophication-related dead zones.
The benthic communities here are composed largely of sessile, epibenthic filter- and suspension-feeding organisms along with a well-developed infauna. They are long-lived and typically aggregated into so-called multi-species clumps or bioherms. Their ecosystem-stabilizing, filter-feeding activity has been overwhelmed by recent low dissolved oxygen events. The condition or status of such macrofauna communities serves as a "memory" of ecosystem collapses.
The Dept. of Marine Biology has been studying these communities for nearly 30 years. Collapses and recolonization have been investigated based on total biomass, percentage contribution of formerly designating species, the growth of selected organisms, and other techniques such as underwater phototransects. Such long-term studies provide information about the composition and function of ecosystems under anthropogenic impact.


Fig. 1 High-biomass aggregation (multi-species clump) consisting of sponges, ascidians and brittlestars at 25 m depth in the Gulf of Trieste.


Fig. 2 Massive mucus aggregates represent an advanced stage in marine snow events.  They eventually sink to the sea floor, further accelera-ting biodiversity loss in the benthic community.


Fig. 3 Dead multi-species clump covered by marine snow. This creates a positive feedback loop of additional mortality.


Fig. 4 Early recolonization is characterized by more rapid-growing, opportunistic forms (tubeworms and the ascidian Ciona). This successional stage bears little resemblance to the typical faunal composition here. This provides information about past collapses and commu-nity resilience.


Fig. 5 Benthic fisheries compound damage due to oxygen crises and marine snow.  Here, a dislodged and crushed bivalve (Pinna sp.) along with its former epigrowth. The current frequency of distur-bances is higher than the required recolonization phase - the Northern Adriatic is a destabilized system.