Vertical distribution of the mesozooplankton community down to 1000 m was investigated in the oligotrophic Aegean Sea during March and September 1997. Total biomass and abundance decreased with depth with a sharp decline <100 m. However, in the northern part, higher biomass values in the 500–1000 m layer than in the 300–500 m layer were observed, due to the abundance of Calanus helgolandicus in deep waters. Species depth preferences and ranges resulted in statistically distinct communities in different zones, as well as in a differentiation along the north–south gradient of oligotrophy in the epipelagic and deep zones. The communities in these zones showed seasonal differences, while in the mesopelagic zone seasonal and horizontal differences were less significant. The discrimination between the north and the south deep communities was mainly due to the dominance of Calanus helgolandicus and Subeucalanus monachus respectively, which had similar seasonal and vertical distribution patterns and were found oversummering in the deep layers. The north–south distinction was related to the gradient of oligotrophy and the hydrological heterogeneity of the Aegean Sea. Spatial community structure and variability between seasons might facilitate an efficient carbon flow in the Aegean Sea under very oligotrophic conditions.

The Mediterranean Sea (MS) represents a complex system that acts as a convergence zone for various biogeographical influences stemming from both temperate and tropical oceanic bodies. Its intricate topography has promoted speciation and adaptation, leading to the development of distinctive and varied marine sites. The MS has a greater total alkalinity than the open ocean, which allows it to absorb a larger amount of human-induced CO₂ per unit of surface area, suggesting an increased threat of acidification. The Eastern Mediterranean (EM) region has been identified as a critical climate change (CC) hotspot; by the end of the 21st century, it is anticipated that heatwaves in the EM will occur more than seven times as often and last more than three times as long. Here, we provide an extensive literature review on the CC-induced impacts and threats on biota throughout EM and adjacent areas, supporting potential mitigation actions. The key elements contributing to the impacts and threats posed by CC in the region are: ocean warming (OW), ocean acidification (OA), and the synergistic effects of OW and OA. Additional factors encompass the combination of: i) OW and marine heatwaves (MHWs), ii) OW and non-indigenous species (NIS), iii) OW and desertification, and iv) OW and water circulation. However, the primary factor causing biodiversity decline, not just in the EM region but throughout the entire MS, seems to be the introduction of NIS, which is further worsened by OW. The primary route through the Suez Canal (SC) and its continuous expansions have sparked worry about the rising propagule pressure. There is a growing consensus that if these environmental risks are not comprehended and mitigated, a significant portion of the Mediterranean ecosystem may face severe threats to its integrity. Ultimately, the initiative of dumping brine waste into the SC, acting as a high salinity barrier that would reduce the transfer of new species carried by the currents, is likely a practical and attractive first step towards mitigation. We suggest that this action should be embraced not only by other countries but also by international environmental organizations and agencies, through a variety of strategies, including financial support. This initiative, along with other measures aimed at alleviating the impacts of invasions on biodiversity, ecosystem services, and health, is an essential next step in the process of mitigation.

HABs pose a threat to coastal ecosystems, the economic sector and human health, and are expanding globally. However, their influence on copepods, a major connector between primary producers and upper trophic levels, remains essentially unknown. Microalgal toxins can eventually control copepod survival and reproduction by deterring grazing and hence reducing food availability. We present several 24-h experiments in which the globally distributed marine copepod, Acartia tonsa, was exposed to different concentrations of the toxic dinoflagellate, Alexandrium minutum, grown under three N:P ratios (4:1, 16:1 and 80:1), with the simultaneous presence of non-toxic food (the dinoflagellate Prorocentrum micans). The different N:P ratios did not affect the toxicity of A. minutum, probably due to the low toxicity of the tested strain. Production of eggs and pellets as well as ingested carbon appeared to be affected by food toxicity. Toxicity levels in A. minutum also had an effect on hatching success and on the toxin excreted in pellets. Overall, A. minutum toxicity affected the reproduction, toxin excretion and, to an extent, the feeding behavior of A. tonsa. This work indicates that even short-term exposure to toxic A. minutum can impact the vital functions of A. tonsa and might ultimately pose serious threats to copepod recruitment and survival. Still, further investigation is required for identifying and understanding, in particular, the long-term effects of harmful microalgae on marine copepods.

Faecal pellet production and content along with egg production of the dominant copepod species Acartia clausi were studied in the Thermaikos Gulf (NW Aegean Sea) during a pre-bloom and a bloom of the toxic dinoflagellate Dinophysis acuminata. Both faecal pellet production (6.8–8.6 ind⁻¹ d⁻¹) and egg production (15.8–47.6 ind⁻¹ d⁻¹) appeared unrelated to the D. acuminata bloom. Less than 11% of the copepod faecal pellets contained one or two D. acuminata cells, almost intact, whereas the other material in the pellets was broken into small pieces or amorphous shapes. The toxin outflux seemed to be insignificant when compared to the mean toxin concentration from the whole D. acuminata population. Finally, the potential grazing impact of A. clausi on D. acuminata during the study period was low.