Phytoplankton Dynamics in the Aegean Sea

The present review constitutes an updated and integrated assessment of phytoplankton spatiotemporal dynamics in the Aegean Sea, based on data compiled from numerous oceanographic surveys of the Hellenic Centre for Marine Research (HCMR), conducted over the last 34 years (1987–2020). Phytoplankton properties, namely chlorophyll a (Chl a) concentrations and primary production (PP) rates, measured as total and as size fractions (0.2–2.0, 2.0–5.0, and >5 μm) in the epipelagic waters (0–200 m) of the Aegean Sea are examined on a seasonal basis within three selected geographic divisions: (a) the North Aegean Sea sub-area (NAS), (b) the Northeast Aegean Sea sub-area (NEAS), and (c) the South Aegean Sea sub-area (SAS). According to the present review, during the productive period (winter-spring) Chl a concentrations and PP rates peak in all sub-areas of the Aegean Sea, with enhanced values reported in NEAS while decreasing toward NAS and SAS. In NEAS-NAS Chl a and PP form prominent maxima within the surface layers resulting from the interplay between the inflow of the nutrient-rich modified Black Sea Waters (BSW) that form a distinct surface layer down to 20–50 m depth in the North Aegean, with winter mixing, introducing nutrients into the euphotic zone from deeper layers. In SAS, vertical profiles appear relatively homogenous down to 100 m, as a result of a well-mixed water column replenished with nutrients from the deeper layers following winter overturn, and of a more extended euphotic zone due to highly transparent waters. During the stratified period (summer-autumn), Chl a and PP levels drop and Chl a vertical profiles converge, with Deep Chlorophyll Maxima (DCM) clearly formed in all the Aegean sub-areas, deepening progressively from NEAS (50–56 m) toward NAS (75–77 m) and SAS (81–92 m). PP profiles, however, follow different vertical patterns, with higher PP values persisting in surface layers (0–50 m) in NEAS-NAS, always related to the BSW inflow, and relatively homogenous PP profiles maintained in SAS, down to 75 m. The established gradient of increasing oligotrophy and/or decreasing productivity from north to south is confirmed, yielding mean annual PP values of 0.51 ± 0.2, 0.33 ± 0.22, and 0.22 ± 0.09 mg C m−3 h−1 for NEAS, NAS, and SAS, respectively, while for Chl a this gradient is relaxed when considering annual means, and becomes evident only during the productive period (mean annual: 0.19 ± 0.09, 0.16 ± 0.07, 0.16 ± 0.04 mg m−3 and productive period: 0.26 ± 0.00, 0.22 ± 0.04, and 0.19 ± 0.03 mg m−3, in NEAS, NAS, and SAS, respectively). The above-mentioned N-S gradient is maintained when water column integrated PP rates are considered; however, when water column integrated Chl a values are considered, the contribution of SAS is enhanced, balancing the respective contribution of NEAS and NAS in winter and outcompeting them by ~1.5–2-fold and by ~1.2–1.5-fold, respectively, in all other seasons. This pattern is related to a more extended euphotic zone in SAS and relevant photo-physiological adaptations of phytoplankton resulting in higher Chl a per cell content rather than in situ growth. Concerning the phytoplankton community structure, picoplankton (0.2–2.0 μm) has the highest contribution to both Chl a and PP, followed by larger size fractions, first >5.0 μm and then 2.0–5.0 μm. However, during winter and spring larger cells considered together ([2.0−5.0] + [>5.0 μm]) outcompete picoplankton in the North Aegean in terms of PP (relative contribution 52–60%), whereas in the South Aegean picoplankton dominates year round (>50%) both Chl a and PP rates. The overall spatiotemporal patterns of phytoplankton properties in the Aegean Sea (Chl a and PP rates) reflect the fundamental differences in hydrographic and environmental forcings between the north and the south basins that give rise to consistent N-S oligotrophy/productivity gradients.