Search

Abstract Ocean warming and acidification, decreases in dissolved oxygen concentrations, and changes in primary production are causing an unprecedented global redistribution of marine life. The identification of underlying ecological processes underpinning marine species turnover, particularly the prevalence of increases of warm-water species or declines of cold-water species, has been recently debated in the context of ocean warming. Here, we track changes in the mean thermal affinity of marine communities across European seas by calculating the Community Temperature Index for 65 biodiversity time series collected over four decades and containing 1,817 species from different communities (zooplankton, coastal benthos, pelagic and demersal invertebrates and fish). We show that most communities and sites have clearly responded to ongoing ocean warming via abundance increases of warm-water species (tropicalization, 54%) and decreases of cold-water species (deborealization, 18%). Tropicalization dominated Atlantic sites compared to semi-enclosed basins such as the Mediterranean and Baltic Seas, probably due to physical barrier constraints to connectivity and species colonization. Semi-enclosed basins appeared to be particularly vulnerable to ocean warming, experiencing the fastest rates of warming and biodiversity loss through deborealization.

Abstract 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.

Barely any data exist on metal speciation in the marine surface microlayer (SML), a rather complex environment, the study of which contributes to enhancement of knowledge on metal speciation and its effect to the ocean. Metal speciation is significant since life requires a wide variety of trace metals that are essential for the growth of the organisms. Ligand concentrations (L) of copper (Cu), zinc (Zn), and cadmium (Cd) were studied by anodic stripping voltammetry (ASV) in the SML of coastal areas within the Aegean Sea in the Eastern Mediterranean. Complexing capacities in the SML in decreasing order ranged for Cu from 29 to 201 nM (median 101 nM), for Zn 24–149 nM (45 nM), and for Cd 1.0–1.5 nM (1.4 nM). Average enrichment factors (EFs) of SML samples compared to subsurface ones (SSW) were calculated equal to 0.9 ± 0.8, 1.2 ± 0.5, and 1.7 ± 1.6 for LZn, LCu, and LCd, respectively. In five out of the six total paired samples of SML and SSW, lower concentrations of LZn were measured in the SML, which is not the case for LCu and LCd. Due to elevated dissolved Zn concentrations in the SML, its complexation is incomplete, contrary to those of Cu and Cd, which are fully complexed. These trace metals are essential nutrients for biological functions, hence any differences on their concentration and chemical speciation may directly influence the distribution of phytoplankton species in the upper water column and neuston. EFs of SML relatively to subsurface water ranged in average between 1.2 and 2.4 for total organic carbon (TOC), chlorophyll a (Chla), and plankton, being generally >2 for most of the amino acids detected, demonstrating a relative enrichment of the SML in organic matter. A significant correlation was found between Cu ligands and dinoflagellates

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.

According to the methodological standards established by Marine Strategy Framework Directive, the assessment for the pelagic habitat under the Biodiversity Descriptor should be carried out at the regional or sub-regional level. In the case of Mediterranean Sea, the sub-regional assessment seems optimal to take into account biogeographic differences in species composition and functional characteristics. Previous research has shown that phytoplankton diversity indicators are efficient for reliable environmental assessments, although more effort has been recommended to test these indicators on a wide spatial scale to cover wider gradients of natural and anthropogenic pressures. In this work, a set of eight diversity indices was tested against the pressure levels within a common data set of the structure and abundance of phytoplankton communities from the Adriatic, Ionian and Aegean Seas. Expert knowledge was used to define four categories of impacts that take into account partial pressures, such as point and non-point pollution, industry, ports and fisheries. At the level of the common data set, most of the diversity, evenness and dominance indices could only distinguish between the highest level of impact and the rest of impact categories. These indices maintained the distinction between two levels of subsequently dichotomised impacts (no to low impact vs. high impact) across latitudinal and longitudinal gradients. On average, the indices were less sensitive to impacts in the northernmost and westernmost areas than in the southernmost and easternmost areas, although they still showed a significant response. The results also suggest that phytoplankton communities become more uniform and less dominated by a single taxon as sampling depth increases at sites with low impact, while evenness and dominance at impacted sites remain similar at all depths. In order to establish meaningful definitions of good environmental status and targets for pelagic habitats in the Mediterranean Sea, it is necessary to establish spatially specific thresholds by additional examination of indices of good performance.

The effects of atmospheric deposition on plankton community structure were examined during a mesocosm experiment using water from the Cretan Sea (Eastern Mediterranean), an area with a high frequency of atmospheric aerosol deposition events. The experiment was carried out under spring-summer conditions (May 2012). The main objective was to study the changes induced from a single deposition event, on the autotrophic and heterotrophic surface microbial populations, from viruses to zooplankton. To this end, the effects of Saharan dust addition were compared to the effects of mixed aerosol deposition on the plankton community over 9 days. The effects of the dust addition seemed to propagate throughout the food-web, with changes observed in nearly all of the measured parameters up to copepods. The dust input stimulated increased productivity, both bacterial and primary. Picoplankton, both autotrophic and heterotrophic capitalized on the changes in nutrient availability and microzooplankton abundance also increased due to increased availability of prey. Five days after the simulated deposition, copepods also responded, with an increase in egg production. The results suggest that nutrients were transported up the food web through autotrophs, which were favored by the Nitrogen supplied through both treatments. Although, the effects of individual events are generally short lived, increased deposition frequency and magnitude of events is expected in the area, due to predicted reduction in rainfall and increase in temperature, which can lead to more persistent changes in plankton community structure. Here we demonstrate how a single dust deposition event leads to enhancement of phytoplankton and microzooplankton and can eventually, through copepods, transport more nutrients up the food web in the Eastern Mediterranean Sea.