Dynamics of Two Picophytoplankton Groups in Mediterranean Sea: Analysis of the Deep Chlorophyll Maximum by a Stochastic Advection-Reaction-Diffusion Model

A stochastic advection-reaction-diffusion model with terms of multiplicative white Gaussian noise, valid for weakly mixed waters, is studied to obtain the vertical stationary spatial distributions of two groups of picophytoplankton, i.e., picoeukaryotes and Prochlorococcus, which account about for 60% of total chlorophyll on average in Mediterranean Sea. By numerically solving the equations of the model, we analyze the one-dimensional spatio-temporal dynamics of the total picophytoplankton biomass and nutrient concentration along the water column at different depths. In particular, we integrate the equations over a time interval long enough, obtaining the steady spatial distributions for the cell concentrations of the two picophytoplankton groups. The results are converted into chlorophyll a and divinil chlorophyll a concentrations and compared with experimental data collected in two different sites of the Sicily Channel (southern Mediterranean Sea). The comparison shows that real distributions are well reproduced by theoretical profiles. Specifically, position, shape and magnitude of the theoretical deep chlorophyll maximum exhibit a good agreement with the experimental values.     

Stochastic models for phytoplankton dynamics in Mediterranean Sea

In this paper, we review some results obtained from three one-dimensional stochastic models, which were used to analyze picophytoplankton dynamics in two sites of the Mediterranean Sea. Firstly, we present a stochastic advection–reaction–diffusion model to describe the vertical spatial distribution of picoeukaryotes in a site of the Sicily Channel. The second model, which is an extended version of the first one, is used to obtain the vertical stationary profiles of two groups of picophytoplankton, i.e. Pelagophytes and Prochlorococcus, in the same marine site as in the previous case. Here, we include intraspecific competition of picophytoplanktonic groups for limiting factors, i.e. light intensity and nutrient concentration. Finally, we analyze the spatio-temporal behaviour of five picophytoplankton populations in a site of the Tyrrhenian Sea by using a reaction–diffusion–taxis model. The study is performed, taking into account the seasonal changes of environmental variables, obtained starting from experimental findings. The multiplicative noise source, present in all three models, mimics the random fluctuations of temperature and velocity field. The vertical profiles of chlorophyll concentration obtained from the stochastic models show a good agreement with experimental data sampled in the two marine sites considered. The results could be useful to devise a new class of models based on a stochastic approach and able to predict future changes in biomass primary production.     

Picophytoplankton, nanophytoplankton, heterotrohpic bacteria and viruses in the Changjiang Estuary and adjacent coastal waters

Flow cytometry (FCM) was used to examine the abundances anddistributions of different picophytoplankton groups (i.e. Synechococcus,Prochlorococcus and picoeukaryotes), nanophytoplankton, heterotrophicbacteria and viruses were examined in the Changjiang Estuary,China and adjacent coastal waters during autumn 2004. Watertemperature and light availability were found to be criticalfactors for picophytoplankton growth. Positive correlationswere found between picophytoplankton, heterotrophic bacteriaand viruses, and a seaward-increasing trend in the V-I (thegroup yielding high green fluorescence according to FCM) populationwithin viruses was detected. The importance of nanophytoplanktonis progressively usurped by picophytoplankton with increasingdistance offshore. Picoeukaryotes are the most successful groupamong picophytoplankton in near-shore eutrophic waters, whereasProchlorococcus surpasses other groups within the pico- andnanophytoplankton community in offshore oligotrophic regionsof the East China Sea Shelf.     

The Temporal Dynamics of Coastal Phytoplankton and Bacterioplankton in the Eastern Mediterranean Sea

This study considers variability in phytoplankton and heterotrophic bacterial abundances and production rates, in one of the most oligotrophic marine regions in the world–the Levantine Basin. The temporal dynamics of these planktonic groups were studied in the coastal waters of the southeastern Mediterranean Sea approximately every two weeks for a total of two years. Heterotrophic bacteria were abundant mostly during late summer and midwinter, and were positively correlated with bacterial production and with N₂ fixation. Based on size fractionating, picophytoplankton was abundant during the summer, whereas nano-microphytoplankton predominated during the winter and early spring, which were also evident in the size-fractionated primary production rates. Autotrophic abundance and production correlated negatively with temperature, but did not correlate with inorganic nutrients. Furthermore, a comparison of our results with results from the open Levantine Basin demonstrates that autotrophic and heterotrophic production, as well as N₂ fixation rates, are considerably higher in the coastal habitat than in the open sea, while nutrient levels or cell abundance are not different. These findings have important ecological implications for food web dynamics and for biological carbon sequestration in this understudied region.     

Understanding the Causes of Recent Warming of Mediterranean Waters. How Much Could Be Attributed to Climate Change?

During the past two decades, Mediterranean waters have been warming at a rather high rate resulting in scientific and social concern. This warming trend is observed in satellite data, field data and model simulations, and affects both surface and deep waters throughout the Mediterranean basin. However, the warming rate is regionally different and seems to change with time, which has led to the question of what causes underlie the observed trends. Here, we analyze available satellite information on sea surface temperature (SST) from the last 25 years using spectral techniques and find that more than half of the warming tendency during this period is due to a non-linear, wave-like tendency. Using a state of the art hydrodynamic model, we perform a hindcast simulation and obtain the simulated SST evolution of the Mediterranean basin for the last 52 years. These SST results show a clear sinusoidal tendency that follows the Atlantic Multidecadal Oscillation (AMO) during the simulation period. Our results reveal that 58% of recent warming in Mediterranean waters could be attributed to this AMO-like oscillation, being anthropogenic-induced climate change only responsible for 42% of total trend. The observed acceleration of water warming during the 1990s therefore appears to be caused by a superimposition of anthropogenic-induced warming with the positive phase of the AMO, while the recent slowdown of this tendency is likely due to a shift in the AMO phase. It has been proposed that this change in the AMO phase will mask the effect of global warming in the forthcoming decades, and our results indicate that the same could also be applicable to the Mediterranean Sea. Henceforth, natural multidecadal temperature oscillations should be taken into account to avoid underestimation of the anthropogenic-induced warming of the Mediterranean basin in the future.     

Modelling the presence of anchovy Engraulis encrasicolus in the Aegean Sea during early summer, based on satellite environmental data

Acoustic and satellite environmental data as well as bathymetry data were used to model the presence of anchovy, Engraulis encrasicolus during early summer in the northern Aegean Sea (Eastern Mediterranean). Generalized Additive Models (GAMs) were used for modelling and subsequently applied in a predictive mode to identify those areas in the Greek Seas and the entire Mediterranean basin that could support species’ presence. Model results were evaluated with the estimation of Receiver Operating Characteristic (ROC)-plots as well as qualitatively, based on (a) acoustic data from concurrent studies in certain areas of the northern Aegean Sea that were not included in the estimation of the GAM model and (b) historical acoustic data from the central Aegean and Ionian Seas. Mapping the estimated environmental conditions in the Mediterranean basin indicated areas that generally agree with the known distribution grounds of anchovy, such as the straits of Sicily and coastal waters of Tunisia, areas in the Tyrrhenian Sea, the Adriatic Sea, the Gulf of Lions and the Catalan Sea. Guest editor: V. D. Valavanis Essential Fish Habitat Mapping in the Mediterranean     

Metagenomes of the picoalga Bathycoccus from the Chile coastal upwelling

Among small photosynthetic eukaryotes that play a key role in oceanic food webs, picoplanktonic Mamiellophyceae such as Bathycoccus, Micromonas, and Ostreococcus are particularly important in coastal regions. By using a combination of cell sorting by flow cytometry, whole genome amplification (WGA), and 454 pyrosequencing, we obtained metagenomic data for two natural picophytoplankton populations from the coastal upwelling waters off central Chile. About 60% of the reads of each sample could be mapped to the genome of Bathycoccus strain from the Mediterranean Sea (RCC1105), representing a total of 9 Mbp (sample T142) and 13 Mbp (sample T149) of non-redundant Bathycoccus genome sequences. WGA did not amplify all regions uniformly, resulting in unequal coverage along a given chromosome and between chromosomes. The identity at the DNA level between the metagenomes and the cultured genome was very high (96.3% identical bases for the three larger chromosomes over a 360 kbp alignment). At least two to three different genotypes seemed to be present in each natural sample based on read mapping to Bathycoccus RCC1105 genome.     

Is it possible to calibrate the pollution level of the region of Algiers (Mediterranean Sea) by exploiting marine macrophytes?

In the absence of water-quality data, a biological indicator is the only way to estimate the pollution level. Samples of macrophytes from exposed shallow rocky substrata of the region of Algiers (Algeria, Mediterranean Sea) were collected in supposedly (in the absence of available pollution data) polluted and pristine waters. These samples were compared to a set of samples spanning a known pollution gradient found near Marseilles (France) and to some samples from a variety of other Mediterranean localities. All samples were collected in similar conditions. The diversity point (i.e., the number of species per sample) was not greater at Cherchell (control site) than at the three sites in the Bay of Algiers. Analysis of the dataset was successful in ranking the Algerian sites, but failed to calibrate the pollution level of the Algerian sites by inserting Algerian samples within the pollution gradient of Marseilles. In contrast, regional characteristics of the macrophyte communities appear to be largely prevalent. This means that water-quality biological indicators and indices based upon marine macrophytes, at least for the open waters and exposed shallow Mediterranean habitats studied here, could be reliable within a given region, but may require validation and/or adjustment, perhaps considerable, for other regions.     

Saharan Dust Deposition May Affect Phytoplankton Growth in the Mediterranean Sea at Ecological Time Scales

The surface waters of the Mediterranean Sea are extremely poor in the nutrients necessary for plankton growth. At the same time, the Mediterranean Sea borders with the largest and most active desert areas in the world and the atmosphere over the basin is subject to frequent injections of mineral dust particles. We describe statistical correlations between dust deposition over the Mediterranean Sea and surface chlorophyll concentrations at ecological time scales. Aerosol deposition of Saharan origin may explain 1 to 10% (average 5%) of seasonally detrended chlorophyll variability in the low nutrient-low chlorophyll Mediterranean. Most of the statistically significant correlations are positive with main effects in spring over the Eastern and Central Mediterranean, conforming to a view of dust events fueling needed nutrients to the planktonic community. Some areas show negative effects of dust deposition on chlorophyll, coinciding with regions under a large influence of aerosols from European origin. The influence of dust deposition on chlorophyll dynamics may become larger in future scenarios of increased aridity and shallowing of the mixed layer.     

Plastic Accumulation in the Mediterranean Sea

Concentrations of floating plastic were measured throughout the Mediterranean Sea to assess whether this basin can be regarded as a great accumulation region of plastic debris. We found that the average density of plastic (1 item per 4 m²), as well as its frequency of occurrence (100% of the sites sampled), are comparable to the accumulation zones described for the five subtropical ocean gyres. Plastic debris in the Mediterranean surface waters was dominated by millimeter-sized fragments, but showed a higher proportion of large plastic objects than that present in oceanic gyres, reflecting the closer connection with pollution sources. The accumulation of floating plastic in the Mediterranean Sea (between 1,000 and 3,000 tons) is likely related to the high human pressure together with the hydrodynamics of this semi-enclosed basin, with outflow mainly occurring through a deep water layer. Given the biological richness and concentration of economic activities in the Mediterranean Sea, the affects of plastic pollution on marine and human life are expected to be particularly frequent in this plastic accumulation region.     

Westernmost record of the diamondback puffer,Lagocephalus guentheri (Tetraodontiformes: Tetraodontidae) in the Mediterranean Sea: First record from Greek waters

On 5th November 2017, a puffer fish specimen was caught in the Aegean Sea. Morphological and molecular analysis identified it as Lagocephalus guentheri. This first record from Greek waters represents the westernmost observation of the species in the Mediterranean Sea. The expansion and establishment of puffer fish species in the basin are discussed, with particular emphasis on the importance of the additional use of molecular analysis for increasing the efficiency in understanding species spread and dynamics.     

Genetic isolation of a now extinct population of bottlenose dolphins (Tursiops truncatus)

A number of dolphin species, though highly mobile, show genetic structure among parapatric and sometimes sympatric populations. However, little is known about the temporal patterns of population structure for these species. Here, we apply Bayesian inference and data from ancient DNA to assess the structure and dynamics of bottlenose dolphin (Tursiops truncatus) populations in the coastal waters of the UK. We show that regional population structure in UK waters is consistent with earlier studies suggesting local habitat dependence for this species in the Mediterranean Sea and North Atlantic. One genetically differentiated UK population went extinct at least 100 years ago and has not been replaced. The data indicate that this was a local extinction, and not a case of historical range shift or contraction. One possible interpretation is a declining metapopulation and conservation need for this species in the UK.     

Genetic analysis of housekeeping genes reveals a deep-sea ecotype of Alteromonas macleodii in the Mediterranean Sea

The genetic diversity of 19 strains belonging to Alteromonas macleodii isolated from different geographic areas (Pacific and Indian Ocean, and different parts of the Mediterranean Sea) and at different depths (from the surface down to 3500 m) has been studied. Fragments of the 16S rRNA gene, the internal transcribed spacer (ITS) between 16S and 23S rDNA genes, the gyrB and the rpoB genes, have been sequenced for each strain. Amplified fragment length polymorphisms were used to characterize similarity at the level of the whole genome. Most of the diversity reflected the existence of a cluster of strains isolated from deep Mediterranean waters and two isolates from the Black Sea. Particularly the isolates from the deep sites were consistently different from all the others indicating the existence of a specific ecotype adapted to these conditions. Amplification of gyrB gene and ITS directly from DNA retrieved from deep Mediterreanean waters and one Atlantic sample showed that presence of this deep-sea ecotype is widespread and is not a product of culture bias. On the other hand, strains isolated from surface tropical waters showed a remarkable level of resemblance to the first isolate of this species obtained from Hawaii in 1972. The results indicate the existence of both lineages of global distribution and ecotypes adapted to specific conditions such as deep or more diluted (the Black Sea) waters.     

Comparison of prokaryotic diversity at offshore oceanic locations reveals a different microbiota in the Mediterranean Sea

The bacterial and archaeal assemblages at two offshore sites located in polar (Greenland Sea; depth: 50 and 2000 m) and Mediterranean (Ionian Sea; depth 50 and 3000 m) waters were studied by PCR amplification and sequencing of the last 450-500 bp of the 16S rRNA gene. A total of 1621 sequences, together with alignable 16S rRNA gene fragments from the Sargasso Sea metagenome database, were analysed to ascertain variations associated with geographical location and depth. The Ionian 50 m sample appeared to be the most diverse and also had remarkable differences in terms of the prokaryotic groups retrieved; surprisingly, however, many similarities were found at the level of large-scale diversity between the Sargasso database fragments and the Greenland 50 m sample. Most sequences with more than 97% sequence similarity, a value often taken as indicative of species delimitation, were only found at a single location/depth; nevertheless, a few examples of cosmopolitan sequences were found in all samples. Depth was also an important factor and, although both deep-water samples had overall similarities, there were important differences that could be due to the warmer waters at depth of the Mediterranean Sea.     

Growth and grazing mortality rates of Prochlorococcus, Synechococcus and eukaryotic picophytoplankton in a bay of the Uwa Sea, Japan

Growth and grazing mortality rates of Synechococcus, Prochlorococcusand eukaryotic picophytoplankton were determined using dilutionexperiments in a bay of the Uwa Sea, Japan. Seasonal changesin the vertical distribution of each picophytoplankter wereinterpreted based on previous physico-chemical measurementsand potential ecophysiological differences. Significant relationshipsbetween dilution and picophytoplankton growth rate were foundonly during the stratified period, suggesting that food linkagebetween the picophytoplankton and grazers was active duringthe stratified period. We found an almost 1:1 balance betweengrowth and grazing mortality rates of Prochlorococcus and Synechococcusin the presence of both nanoflagellates and ciliates, suggestingthat the abundance of the cyanobacteria is in a quasi-steady-statesystem with grazing losses compensated by growth. The relationshipbetween growth and grazing mortality rate of eukaryotic picophytoplanktonwas unclear. We discuss the dynamics of each picophytoplankterabundance with special reference to the characteristics of themicrobial food web in the bay, together with two physical eventsresponsible for matter cycling within the bay.     

Contribution of Crenarchaeota and Euryarchaeota to the prokaryotic plankton in the coastal northwestern Black Sea

The NW Black Sea is an area with peculiar hydrological, hydrochemicaland biological features (river plume dynamics, coastal upwelling,intense eutrophication, blooms algae). We determined the abundanceand distribution of the two major groups of Archaea (Crenarchaeotaand Euryarchaeota) in three contrasting sites of the NW BlackSea from May to August 2004 to estimate the contribution ofnon-extremophilic Archaea to the coastal prokaryotic plankton.Using catalyzed reporter deposition-fluorescence in situ hybridization,Euryarchaeota were found to be particularly prominent (11–22%of total prokaryotic plankton) in the low-salinity waters duringlate-spring and summer, whereas Crenarchaeota contributed only5% to the prokaryotic community in these waters. The abundanceof Crenarchaeota was highest in nutrient- and particle-richwaters and lowest in offshore Black Sea surface waters. Theseresults confirm recent studies on the distribution of planktonicArchaea with Euryarchaeota dominating the archaeal communityin surface waters. Statistical analyses indicated significantcorrelations between crenarchaeotal abundance, ammonia and nitrateconcentrations at each sites. Our data show that the archaealplankton constitute an important and dynamic component of theprokaryotic plankton in the coastal oxygenated waters of theNW Black Sea and might play a significant role in the nitrogencycle.     

THE NORTHEASTERN ALBORAN SEA, AN IMPORTANT BREEDING AND FEEDING GROUND FOR THE LONG-FINNED PILOT WHALE (GLOBICEPHALA MELAS) IN THE MEDITERRANEAN SEA

Little is known about the long-finned pilot whale's population size, structure, distribution, and dynamics in the Western Mediterranean basin. The research region covered since 1992 in southeast Spain, at the edge of the Alboran Sea, is considered an important oceanographic transition zone between the Mediterranean and the Atlantic Ocean. The research ship Toftevaag carried out surveys covering a total of 10,173 nmi (18,840 km) from April to September each year, 1992-1997, all years pooled. Effort for ten-by-ten-mile quadrants was stratified by depth and sea state to ascertain encounter rates. Tracking was used together with photo-identification of animals to analyze home range of groups. Behavior was recorded ad libitum , and underwater video taping was used to analyze specific behavior patterns. One hundred and nine sightings of pilot whales were made. The average group size was 41.4 ± 58.4, ranging from 1 to 350. The average depth at encounters was 848.7 ± 281.2 m ranging from 300 to 1,800 m. Compatison of results for encounter rate and group size with those for other Mediterranean regions, together with site fidelity shown by photo-identification and observations of reproductive behavior, reflect the importance of the Alboran Sea to the species in the Mediterranean.     

Spatial patterns and GIS habitat modelling of fish in two French Mediterranean coastal areas

The spring and summer distribution of adults and larval fish stages of Sparids and Scorpaenids was studied in two sites in the western Mediterranean. Fish adults and larvae of those two taxa were identified and sorted according to their life stages in order to study their distribution and develop suitable habitat maps. Study areas were located on French coastal waters, the “Côte Bleue” Marine Park (CBMP) has an east-west orientation with substratum dominated by Posidonia beds and the Marine Reserve of Cerbère-Banyuls (MRCB) runs north–south and is dominated by rocky substrates. Generalised Additive Models (GAM) combined with Geographic Information System (GIS), were used to model the suitable habitats for fish larvae and adults. During spring months, waters exhibit low Sea Surface Temperatures (SST), low Photosynthetically Active Radiation (PAR), high values of Sea Surface Chlorophyll-a concentration (Chl-a), and mostly negative values of Sea Level Anomaly (SLA) indicating anticyclonic eddies. During summer months, waters have higher values of SST, lower values of Chl-a and positive values of SLA indicating cyclonic eddies. The results revealed different environmental responses in the distribution of fish adults and larvae in the CBMP and MRCB. Suitable habitats for adult were mainly dependent on the substrate types (Posidonia meadows and sand) and they were found close to the coast, whereas fish larvae were dependent on environmental cues (Chl-a, SLA, SST) with a sparse spatial distribution.     

PYCNOIB: Biodiversity and Biogeography of Iberian Pycnogonids

Biodiversity and biogeographic studies comparing the distribution patterns of benthic marine organisms across the Iberian Atlantic and Mediterranean waters are scarce. The Pycnogonida (sea spiders) are a clear example of both endemicity and diversity, and are considered a key taxon to study and monitor biogeographic and biodiversity patterns. This is the first review that compiles data about abundance and diversity of Iberian pycnogonids and examines their biogeographic patterns and bathymetric constraints using GIS tools. A total of 17762 pycnogonid records from 343 localities were analyzed and were found to contain 65 species, 21 genera and 12 families. Achelia echinata and Ammothella longipes (family Acheliidae) were the most abundant comprising ~80% of the total records. The Acheliidae is also the most speciose in Iberian waters with 15 species. In contrast, the family Nymphonidae has 7 species but is significantly less abundant (<1% of the total records) than Acheliidae. Species accumulation curves indicate that further sampling would increase the number of Iberian species records. Current sampling effort suggests that the pycnogonid fauna of the Mediterranean region may be richer than that of the Atlantic. The Strait of Gibraltar and the Alboran Sea are recognized as species-rich areas that act as buffer zones between the Atlantic and Mediterranean boundaries. The deep waters surrounding the Iberian Peninsula are poorly surveyed, with only 15% of the sampling sites located below 1000 m. Further deep-water sampling is needed mainly on the Iberian Mediterranean side.     

The use of DGGE analyses to explore eastern Mediterranean and Red Sea marine picophytoplankton assemblages

In vast areas of the oceans, most marine photosynthetic production is performed by cells smaller than 2-3 microm (picoplankton). Here, we report on denaturing gradient gel electrophoresis (DGGE) analyses of naturally occurring marine oxygenic picophytoplankton using the conserved photosynthetic psbA gene. The psbA gene proved to be a good indicator for picophytoplankton presence and was shown to work with DGGE. The DGGE results show the distribution of photosynthetic marine groups belonging to cyanobacteria and the eukaryotic prasinophytes (green algae) in the Red and eastern Mediterranean Seas in the seasons examined. The present study demonstrates the value of DGGE as a tool for rapid analyses of natural marine communities of picophytoplankton.