Evidence of climate-driven regime shifts in the Aegean Sea’s demersal
resources: a study spanning six decades
Abstract
Climate change (CC) can alter the configuration of marine ecosystems,
however ecosystem response and resilience to change are usually
case-specific. The effect of CC on the demersal resources of the Aegean
Sea (east Mediterranean Sea) was investigated during the past six
decades applying a combination of multivariate analysis, non-additive
modelling and the Integrated Resilience Assessment (IRA) framework. We
focused on the study of: (i) the biological ‘system’ complex, using
proxies of biomass (landings per unit of capacity) for 12 demersal taxa
and (ii) the environmental ‘stressor’ complex, described by 12 abiotic
variables. Pronounced changes have occurred in both the environmental
and biological system over the studied period. The majority of the
environmental stressors exhibited strikingly increasing trends
(temperature, salinity, primary production indices) with values started
exceeding the global historical means during late 1980s-early 1990s. It
is suggested that the biological system exhibited a discontinuous
response to CC, with two apparently climate-induced regime shifts
occurring in the past 25 years. There is evidence for two fold
bifurcations and four tipping points in the system, forming a folded
stability landscape with three basins of attraction. The shape of the
stability landscape for the Aegean Sea’s biological system suggests that
while the initial state (1966-1991) was rather resilient to CC,
absorbing two environmental step-changes, this was not the case for the
two subsequent ones (intermediate: 1992-2002; recent: 2003-2016). Given
the current trajectory of environmental change, it is highly unlikely
that the biological system will ever return to its pre-1990s state, as
it is entering areas of unprecedented climatic conditions and there is
some evidence that the system may be even shifting towards a new state.
Our approach and findings may be relevant to other marine areas of the
Mediterranean and beyond, undergoing climate-driven regime shifts, and
can assist to their adaptive management.