Ecosystem response to human- and climate-induced environmental stress on an anoxic coastal lagoon (Etoliko, Greece) since 1930 AD

To better constrain the effects of anthropogenic
impact on coastal wetlands with respect to
natural variability, we here analyze annually laminated
sediments from Etoliko lagoon (western Greece,
Mediterranean Sea) spanning the last*80 years. Subdecadal-
scale palynomorph (pollen and dinoflagellate
cyst) and seasonal-scale palynomorph (microfacies
and l-XRF) analyses were carried out to investigate
the evolution of the aquatic environment and the
surrounding terrestrial ecosystem. Based on a robust
age model, which was developed using varve counting
and 137Cs dating, our results indicate that land-use
changes have altered the vegetation dynamics and led
to eutrophication of the aquatic environment particularly
from the early 1980s onwards. In agreement with
instrumental data and reports of fish mass mortality
events, our varve composition and high-resolution
element scanning data suggest that the ecosystem has
been under unprecedented pressure since 1990 AD. In
particular, the enhancement of anoxic conditions due
to human-induced eutrophication is linked to high
accumulation rates of organic matter, an increased
presence of bacteria in sediment microfacies, and a
decrease in the Fe/Mn ratio in the sediment. In
addition, a change in varve type from calcite- to
aragonite-dominated in 1983 and a higher Sr concentration
during the 1990s indicate an increasingly saline
aquatic environment. Comparison with meteorological
data suggests that lower precipitation during a
persistent positive North Atlantic Oscillation mode
along with a gradual increase in mean summer
temperature since the 1980s may have enhanced the
saline conditions. These findings demonstrate that
climate change can intensify the human impact on aquatic ecosystems. In conclusion, our analytical
approach provides a valuable tool for evaluating the
degree of degradation of Etoliko lagoon and the
effectiveness of implemented management plans on
the aquatic ecosystem, indicating that the efforts to
restore its water circulation have only weakly contributed
towards an environmental recovery.