Atlas of phytoplankton phenology indices in selected Eastern Mediterranean marine ecosystems.

Phytoplankton is a fundamental component of marine food webs and play a crucial role in marine ecosystem functioning. The phenology (timing of growth) of these microscopic algae is an important ecological indicator that can be utilized to observe its seasonal dynamics, and assess its response to environmental perturbations. Ocean colour remote sensing is currently the only means of obtaining synoptic estimates of chlorophyll-a (a proxy of phytoplankton biomass) at high temporal and spatial resolution, enabling the calculation of phenology metrics. However, ocean colour observations have acknowledged weaknesses compromising its reliability, while the scarcity of long-term in situ data has impeded the validation of satellite-derived phenology estimates. To address this issue, we compared one of the longest available in situ time series (20 years) of chlorophyll-a concentrations in the Eastern Mediterranean Sea (EMS), along with concurrent remotely-sensed observations. The comparison revealed a marked coherence between the two datasets, indicating the capability of satellite-based measurements in accurately capturing the phytoplankton seasonality and phenology metrics (i.e., timing of initiation, duration, peak and termination) in the studied area. Furthermore, by studying and validating these metrics we constructed a satellite-derived phytoplankton phenology atlas, reporting in detail the seasonal patterns in several sub-regions in coastal and open seas over the EMS. The open waters host higher concentrations from late October to April, with maximum levels recorded during February and lowest during the summer period. The phytoplankton growth over the Northern Aegean Sea appeared to initiate at least a month later than the rest of the EMS (initiating in late November and terminating in late May). The coastal waters and enclosed gulfs (such as Amvrakikos and Maliakos), exhibit a distinct seasonal pattern with consistently higher levels of chlorophyll-a and prolonged growth period compared to the open seas. The proposed phenology atlas represents a useful resource for monitoring phytoplankton growth periods in the EMS, supporting water quality management practices, while enhancing our current comprehension on the relationships between phytoplankton biomass and higher trophic levels (as a food source).

Seawater physics and chemistry along the Med-SHIP transects in the Mediterranean Sea in 2016.

The Mediterranean Sea has been sampled irregularly by research vessels in the past, mostly by national expeditions in regional waters. To monitor the hydrographic, biogeochemical and circulation changes in the Mediterranean Sea, a systematic repeat oceanographic survey programme called Med-SHIP was recommended by the Mediterranean Science Commission (CIESM) in 2011, as part of the Global Ocean Ship-based Hydrographic Investigations Program (GO-SHIP). Med-SHIP consists of zonal and meridional surveys with different frequencies, where comprehensive physical and biogeochemical properties are measured with the highest international standards. The first zonal survey was done in 2011 and repeated in 2018. In addition, a network of meridional (and other key) hydrographic sections were designed: the first cycle of these sections was completed in 2016, with three cruises funded by the EU project EUROFLEETS2. This paper presents the physical and chemical data of the meridional and key transects in the Western and Eastern Mediterranean Sea collected during those cruises.

Ecosystem-level responses to multiple stressors using a time-dynamic food-web model: The case of a re-oligotrophicated coastal embayment (Saronikos Gulf, E Mediterranean).

Multiple stressors may combine in unexpected ways to alter the structure of ecological systems, however, our current ability to evaluate their ecological impact is limited due to the lack of information concerning historic trophic interactions and ecosystem dynamics. Saronikos Gulf is a heavily exploited embayment in the E Mediterranean that has undergone significant ecological alterations during the last 20 years including a shift from long-standing eutrophic to oligotrophic conditions in the mid-2000's. Here we used a historical Ecopath food-web model of Saronikos Gulf (1998-2000) and fitted the time-dynamic module Ecosim to biomass and catch time series for the period 2001-2020. We then projected the model forward in time from 2021 to 2050 under 8 scenarios to simulate ecosystem responses to the individual and combined effect of sea surface temperature increase, primary productivity shifts and fishing effort release. Incorporating trophic interactions, climate warming, fishing and primary production improved model fit, depicting that both fishing and the environment have historically influenced ecosystem dynamics. Retrospective simulations of the model captured historical biomass and catch trends of commercially important stocks and reproduced successfully the marked recovery of marine resources 10 years after re-oligotrophication. In future scenarios increasing temperature had a detrimental impact on most functional groups, increasing and decreasing productivity had a positive and negative effect on all respectively, while fishing reductions principally benefited top predators. Combined stressors produced synergistic or antagonistic effects depending on the direction and magnitude of change of each stressor in isolation while their overall impact seemed to be strongly mediated via food-web interactions. Such holistic approaches advance of our mechanistic understanding of ecosystems enabling us to develop more effective management strategies in the face of a rapidly changing marine environment.

One Step forward: Benthic Pelagic Coupling and Indicators for Environmental Status.

A large data set from the Eastern Mediterranean was analyzed to explore the relationship between seawater column variables and benthic community status. Our results showed a strong quantitative link between the seawater column variables (Chlorophyll a and Eutrophication Index) and various indicators describing benthic diversity and community composition. The percentage of benthic opportunistic species increased significantly in the stations with high trophic status of the seawater column and so did the strength of the coupling between values of seawater column and benthic indicators. The Eutrophication Index threshold level of 0.85, separating the "Bad and Poor" from "Moderate to High" conditions could serve as an acceptable critical value above which there is a readily observable change in benthic community composition.