Dionysios E. Raitsos

Plymouth Marine Laboratory, Plymouth, England, United Kingdom

Are you Dionysios E. Raitsos?

Claim your profile

Publications (30)72.81 Total impact

  • Challenger Society for Marine Science Conference 2014, Plymouth, Devon; 09/2014
  • EGU General Assembly 2014, Vienna, Austria; 05/2014
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Changes in phytoplankton dynamics influence marine biogeochemical cycles, climate processes, and food webs, with substantial social and economic consequences. Large-scale estimation of phytoplankton biomass was possible via ocean colour measurements from two remote sensing satellites – the Coastal Zone Colour Scanner (CZCS, 1979–1986) and the Sea-viewing Wide Field-of-view Sensor (SeaWiFS, 1998–2010). Due to the large gap between the two satellite eras and differences in sensor characteristics, comparison of the absolute values retrieved from the two instruments remains challenging. Using a unique in situ ocean colour dataset that spans more than half a century, the two satellite-derived chlorophyll-a (Chl-a) eras are linked to assess concurrent changes in phytoplankton variability and bloom timing over the Northeast Atlantic Ocean and North Sea. Results from this unique re-analysis reflect a clear increasing pattern of Chl-a, a merging of the two seasonal phytoplankton blooms producing a longer growing season and higher seasonal biomass, since the mid-1980s. The broader climate plays a key role in Chl-a variability as the ocean colour anomalies parallel the oscillations of the Northern Hemisphere Temperature (NHT) since 1948.
    Global Change Biology 04/2014; · 8.22 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Red Sea exhibits complex hydrodynamic and biogeochemical dynamics, which vary both in time and space. These dynamics have been explored through the development and application of a 3-D ecosystem model. The simulation system comprises two off-line coupled submodels: the MIT General Circulation Model (MITgcm) and the European Regional Seas Ecosystem Model (ERSEM), both adapted for the Red Sea. The results from an annual simulation under climatological forcing are presented. Simulation results are in good agreement with satellite and in situ data illustrating the role of the physical processes in determining the evolution and variability of the Red Sea ecosystem. The model was able to reproduce the main features of the Red Sea ecosystem functioning, including the exchange with the Gulf of Aden, which is a major driving mechanism for the whole Red Sea ecosystem and the winter overturning taking place in the north. Some model limitations, mainly related to the dynamics of the extended reef system located in the southern part of the Red Sea, which is not currently represented in the model, still need to be addressed.
    Journal of Geophysical Research: Oceans 02/2014; 119(3). · 3.44 Impact Factor
  • Ocean Meeting 2014, Hawai, USA; 02/2014
  • [Show abstract] [Hide abstract]
    ABSTRACT: Using the Food and Agriculture Organization’s (FAO) Mediterranean capture fisheries production dataset in conjunction with global and Mediterranean sea surface temperatures, we investigated trends in fisheries landings and landings per unit of effort of commercially important marine organisms, in relation to temperature oscillations. In addition to the overall warming trend, a temperature shift was detected in the Mediterranean Sea in the late 1990s. Fisheries landings fluctuations were examined for the most abundant commercial species (59 species) and showed significant year-to-year correlations with temperature for nearly 60 % of the cases. From these, the majority (~70 %) were negatively related and showed a reduction of 44 % on average. Increasing trends were found, mainly in the landings of species with short life spans, which seem to have benefited from the increase in water temperature. Τhe effect of oceanic warming is apparent in most species or groups of species sharing ecological (e.g. small and medium pelagic, demersal fish) or taxonomic (e.g. cephalopods, crustaceans) traits. A landings-per-unit-of-effort (LPUE) proxy, using data from the seven Mediterranean European Union member states, also showed significant correlation with temperature fluctuations for six out of the eight species examined, indicating the persistence of temperature influence on landings when the fishing effect is accounted for. The speed of response of marine landings to the warming of the Mediterranean Sea possibly shows both the sensitivity and the vulnerable state of the fish stocks and indicates that climate should be examined together with fisheries as a factor shaping stock fluctuations.
    Climatic Change 01/2014; 122(1-2). · 3.63 Impact Factor
  • Source
    Phytoplankton Functional Types from Space, 1 edited by Shubha Sathyendranath, 01/2014: chapter 5: pages 101-124; IOCCG., ISBN: 1098-6030
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The Red Sea holds one of the most diverse marine ecosystems, primarily due to coral reefs. However, knowledge on large-scale phytoplankton dynamics is limited. Analysis of a 10-year high resolution Chlorophyll-a (Chl-a) dataset, along with remotely-sensed sea surface temperature and wind, provided a detailed description of the spatiotemporal seasonal succession of phytoplankton biomass in the Red Sea. Based on MODIS (Moderate-resolution Imaging Spectroradiometer) data, four distinct Red Sea provinces and seasons are suggested, covering the major patterns of surface phytoplankton production. The Red Sea Chl-a depicts a distinct seasonality with maximum concentrations seen during the winter time (attributed to vertical mixing in the north and wind-induced horizontal intrusion of nutrient-rich water in the south), and minimum concentrations during the summer (associated with strong seasonal stratification). The initiation of the seasonal succession occurs in autumn and lasts until early spring. However, weekly Chl-a seasonal succession data revealed that during the month of June, consistent anti-cyclonic eddies transfer nutrients and/or Chl-a to the open waters of the central Red Sea. This phenomenon occurs during the stratified nutrient depleted season, and thus could provide an important source of nutrients to the open waters. Remotely-sensed synoptic observations highlight that Chl-a does not increase regularly from north to south as previously thought. The Northern part of the Central Red Sea province appears to be the most oligotrophic area (opposed to southern and northern domains). This is likely due to the absence of strong mixing, which is apparent at the northern end of the Red Sea, and low nutrient intrusion in comparison with the southern end. Although the Red Sea is considered an oligotrophic sea, sporadic blooms occur that reach mesotrophic levels. The water temperature and the prevailing winds control the nutrient concentrations within the euphotic zone and enable the horizontal transportation of nutrients.
    PLoS ONE 01/2013; 8(6):e64909. · 3.53 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The North Aegean inter-annual productivity variability was investigated, by means of a coupled hydrodynamic/biogeochemical model simulation, over a multi-year (1985-2001) period covering the Eastern Mediterranean Transient (EMT). The EMT was a historically significant period (1987-1994), characterized by the massive formation of dense waters in the Aegean and their subsequent southward spreading to the Eastern Mediterranean. Our findings suggest that during the dense water formation events, deep nutrient rich water is exported to the South, affecting the N. Aegean nutrient pool. We show that the open sea productivity variability, driven by the deep nutrient pool entrainment to the euphotic zone, is related to both vertical mixing and nutrient fluxes, induced by thermohaline circulation variability. A negative correlation appears between the two processes, as stronger vertical mixing triggers a higher nutrient export. Process oriented experiments adopting maximum and minimum nutrient fluxes to the Southern Aegean revealed a significant impact on the nutrient pool, counterbalancing the effect of different vertical mixing on primary production. The model results presented good correlation with available satellite SST and Chl-a data. The impact of precipitation and BSW inflow variability on the Aegean salinity and dense water formation was investigated, showing a significant contribution by both preconditioning factors.
    Journal of Marine Systems 01/2012; 96-97:72-81. · 2.48 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Pagasitikos Gulf (Greece), presents an interesting area as it depicts strongly non-linear ecosystem character-istics. It is a shallow coastal area where the marine ecosystem picture is strongly influenced by non-linear hydrodynamic interactions and instabilities. In this study, we explore and assess the major influential vari-ables of the surface phytoplankton biomass (Chlorophyll-a). Several different physical and biogeochemical parameters were used (sea surface temperature [SST], mixed layer depth [MLD], salinity, phosphates and nitrates) to identify which variables control or significantly affect the surface Chl-a of Pagasitikos Gulf for the period of 2001–2005. The variables were derived from a coupled hydrodynamic-biogeochemical model and remotely sensed data from SeaWiFS and AVHRR sensor. Generalised Additive Models (GAMs) were used to examine the relationships between Chlorophyll-a and the environmental regime. GAM analysis showed that the combined effects of the variables used, explained 71% of the surface chlorophyll variation. The order of importance of the variables (based on GAM probability) is p = 0.01 (for both phosphate and nitrate), MLD: p = 0.0197, salinity: p = 0.022, and SST: p = 0.046. The results clearly indicated the importance of deep mixing for Pagasitikos Gulf, as the surface phytoplankton blooms appeared to be favoured by cold, nutrient rich, well mixed and higher salinity waters. GAMs indicated that SST plays a significant role having a strong negative relationship with Chl-a, where the highest concentration is reached at 12–15 °C and min-imum at 22–26 °C. Chl-a ceases to increase after 37.9‰ of salinity and 40 m of MLD, while minimum concen-tration is found at 10 m of MLD, with a subsequent increase as the waters become more mixed. Phosphate and nitrates appeared to be of equal importance with Chl-a exhibiting an increase along with the nutrients.
    Journal of Marine Systems 12/2011; · 2.48 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Within the framework of the European INSEA project, an advanced assimilation system has been implemen-ted for the Pagasitikos Gulf ecosystem. The system is based on a multivariate sequential data assimilation scheme that combines satellite ocean sea color (chlorophyll-a) data with the predictions of a three-dimensional coupled physical–biochemical model of the Pagasitikos Gulf ecosystem presented in a compan-ion paper. The hydrodynamics are solved with a very high resolution (1/100°) implementation of the Prince-ton Ocean Model (POM). This model is nested within a coarser resolution model of the Aegean Sea which is part of the Greek POSEIDON forecasting system. The forecast of the Aegean Sea model, itself nested and ini-tialized from a Mediterranean implementation of POM, is also used to periodically re-initalize the Pagatisikos hydrodynamics model using variational initialization techniques. The ecosystem dynamics of Pagasitikos are tackled with a stand-alone implementation of the European Seas Ecosystem Model (ERSEM). The assimila-tion scheme is based on the Singular Evolutive Extended Kalman (SEEK) filter, in which the error statistics are parameterized by means of a suitable set of Empirical Orthogonal Functions (EOFs). The assimilation experiments were performed for year 2003 and additionally for a 9-month period over 2006 during which the physical model was forced with the POSEIDON-ETA 6-hour atmospheric fields. The assim-ilation system is validated by assessing the relevance of the system in fitting the data, the impact of the as-similation on non-observed biochemical processes and the overall quality of the forecasts. Assimilation of either GlobColour in 2003 or SeaWiFS in 2006 chlorophyll-a data enhances the identification of the ecological state of the Pagasitikos Gulf. Results, however, suggest that subsurface ecological observations are needed to improve the controllability of the ecosystem in the deep layers.
    Journal of Marine Systems 12/2011; 94:s102-s117. · 2.48 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The Aegean Sea, due to the complexity and variety of morphological features, bathymetry, hydrological, hydrodynamic fea-tures and climatic characteristics, offers a unique opportunity for monitoring the spreading and/or invasion of alien biota. It is known that the establishment of warm/tropical marine alien species is related mainly to the increase in sea temperature. A significant increase in aliens' number along the southern Aegean (Dodecanese) coasts has been documented in the last twenty years, while their arrival has produced evident changes in coastal ecosystem composition and function. Although the study area acts as a crossroad for tropical alien species introduction in the eastern Mediterranean Sea, a direct attempt to link temp-erature alterations with new alien species arrivals has not been performed. Satellite and in situ derived marine temperature data showed that the study area has gone through a remarkable warming, while the link between global temperatures and regional ones revealed that this warming is not a local phenomenon but part of global climate trends. Examining relationships between new alien species arrivals data since 1929 against global temperatures, it is shown that their introduction rate par-allels the significant increase of marine temperatures. The association of alien introductions with marine water warming, as well as potential effects on ecosystem functioning of selected species is discussed. The Mediterranean Sea, this 'miniature ocean' (Béthoux et al., 1999) although it covers only 0.82% of the whole oceanic surface, hosts about 8% of the known species, showing an exceptionally high biodiversity (Bianchi & Morri, 2000; ESF, 2007). Due to the relatively recent colonization, the Mediterranean Sea seems to be more vulnerable and prone to the arrival of new species, compared to other ecosystems with a more ancient evolutionary history, limited space and higher antagonism between species. In particular, the eastern Mediterranean biocenoses seem to be at an initial stage of maturation, after the rough hydro-geological changes which took place about 9000–7000 years ago between the Aegean and Black Seas, which led to the so-called 'sapropel formation' and to the disappearing of many species. This phenomenon, with a periodicity of about 21,000 years (Rohling & Hilgen, 1991), seems to be partly responsible for the lower biodiversity observed in the eastern Mediterranean, and the consequent availability of ecological niches. The western Mediterranean, on the contrary, was not subjected to so large upsets after the end of the Messinian and appears more mature, hosting biocenoses richer in species. Due to the high diversity of habitats and its geographi-cal position, the Mediterranean is characterized furthermore by a temperate climate and by species occurring both in temperate and in subtropical regions. Global climate change is now 'unequivocal' (IPCC, 2007), while the effect of climate warming can be seen on the biodi-versity of oceanic and coastal areas, sea level, hydrological cir-culation, vertical movements and up-wellings. Each of them could have consequences on the marine ecosystem at various degrees (trophic relations, productivity, reproduction, biodiversity, etc), all inevitably related to social, economic and cultural aspects of the involved areas. Even though organisms have adapted to different climatic conditions during their evolution, the rapidity of currently observed phenomena is alarming (Root et al., 2003). The ability of the marine environ-ment to respond to the climatic changes depends also on other changes induced directly by man (such as overfishing; Daskalov et al., 2007). Following the climatic changes registered during the last 30 years (IPCC, 2007; Nykjaer, 2009), occurrence of non-native marine organisms mostly of tropical and sub-tropical origin has increased in the Mediterranean, the so-called 'Mediterranean tropicalization' (Bianchi, 2007). According to Bianchi (2007), it results from the combination of four factors: Atlantic influx, increase of Indo-Pacific immigrants, introduction of species for economic purposes (aquaculture) and seawater warming, all contributing to the establishment and diffusion of thermophilic species in the whole
    Journal of the Marine Biological Association of the UK 07/2011; · 1.13 Impact Factor
  • Source
    Themis Chronis, Dionysios E. Raitsos, Dimitris Kassis
    [Show abstract] [Hide abstract]
    ABSTRACT: This study highlights an important and previously overlooked summer North Atlantic Oscillation (NAO) influence over the eastern Mediterranean. The featured analysis is based on a synergistic use of reanalysis data, satellite retrievals, and coastal and buoy meteorological observations. The physical mechanisms at play reveal a strong summer NAO involvement on the pressure fields over northern Europe and the Anatolian plateau. Especially during August, the summer NAO modulates the Anatolian low, together with the air temperature, meridional atmospheric circulation, and cloudiness over the eastern Mediterranean. Including the dominant action centers over Greenland and the Arctic, the identified modulations rank among the strongest summer NAO-related signals over the entire Northern Hemisphere.
    Journal of Climate 01/2011; 24:5584–5596.. · 4.36 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Diatoms exist in almost every aquatic regime; they are responsible for ∼20% of global carbon fixation and ∼25% of global primary production, and are regarded as a key food for copepods, which are subsequently consumed by larger predators such as fish and marine mammals. A decreasing abundance and a vulnerability to climatic change in the North Atlantic Ocean have been reported in the literature. In the present work, a data matrix composed of concurrent satellite remote sensing and Continuous Plankton Recorder (CPR) in situ measurements was collated for the same spatial and temporal coverage in the Northeast Atlantic. Artificial neural networks (ANNs) were applied to recognize and learn the complex non-monotonic and non-linear relationships between diatom abundance and spatiotemporal environmental factors. Because of their ability to mimic non-linear systems, ANNs proved far more effective in modelling the diatom distribution in the marine ecosystem. The results of this study reveal that diatoms have a regular seasonal cycle, with their abundance most strongly influenced by sea surface temperature (SST) and light intensity. The models indicate that extreme positive SSTs decrease diatom abundances regardless of other climatic conditions. These results provide information on the ecology of diatoms that may advance our understanding of the potential response of diatoms to climatic change.
    International Journal of Remote Sensing 01/2011; 32(8). · 1.36 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Satellite remote sensing of ocean colour is the only method currently available for synoptically measuring wide-area properties of ocean ecosystems, such as phytoplankton chlorophyll biomass. Recently, a variety of bio-optical and ecological methods have been established that use satellite data to identify and differentiate between either phytoplankton functional types (PFTs) or phytoplankton size classes (PSCs). In this study, several of these techniques were evaluated against in situ observations to determine their ability to detect dominant phytoplankton size classes (micro-, nano-and picoplankton). The techniques are applied to a 10-year ocean-colour data series from the SeaWiFS satellite sensor and compared with in situ data (6504 samples) from a variety of locations in the global ocean. Results show that spectral-response, ecological and abundance-based approaches can all perform with similar accuracy. Detection of microplankton and picoplankton were generally better than detection of nanoplankton. Abundance-based approaches were shown to provide better spatial retrieval of PSCs. Individual model performance varied according to PSC, input satellite data sources and in situ validation data types. Uncertainty in the comparison procedure and data sources was considered. Improved availability of in situ observations would aid ongoing research in this field.
    Remote Sensing of Environment - REMOTE SENS ENVIRON. 01/2011; 115(2).
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: For the first time in Red Sea, we use satellite derived temperature since 1985We report that Red Sea is warming rapidly, with abrupt shift evidence in 1994The warming is not a regional phenomenon, but globally climate change driven
    Geophysical Research Letters 01/2011; 38(14). · 3.98 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Using long-term data of 149 warm alien species since 1924, we show that the introduction of warm and tropical alien species has been exacerbated by the observed warming of the eastern Mediterranean Sea. The phenomenon has accelerated after an abrupt shift in both regional and global temperatures that we detect around 1998, leading to a 150% increase in the annual mean rate of species entry after this date. Abrupt rising temperature since the end of the 1990s has modified the potential thermal habitat available for warm-water species, facilitating their settlement at an unexpectedly rapid rate. The speed of alien species spreading and response to global warming is apparently much faster than temperature increase itself, presenting an important warning for the future of Mediterranean Sea biodiversity. In addition to the sea warming, other factors that enable and enhance biological invasions, such as salinity increase and oceanographic forcing, are also discussed.
    Limnology and Oceanography 03/2010; 55(4):1478-1484. · 3.62 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: 1] Observations from two comprehensive oceanographic surveys on the northwestern Black Sea are analyzed to reveal the hydrological characteristics of the shelf area around the Danube delta in relation to particulate matter dynamics. These processes have broader implications on flows through the adjacent complex strait system. The surveys cover both a period of high (September 2002) and low (September 2004) discharge of the Danube River. The resulting low-salinity waters generally occupied the upper 10–15 m within the 20–30 m thick warm layer that was separated by deeper cooler waters through a sharp thermocline. The buoyant plume was influenced by the discharge conditions and the prevailing wind patterns. The plume structure was identified using in situ bottle data, continuous beam transmission and fluorescence profiles, and satellite remotely sensed data. Particulate matter concentration values correlated with low-salinity riverine waters and their maxima were recorded in the immediate vicinity of the Danube branches, decreasing rapidly toward the open sea. The preferred particle transport pathway in 2002 was along a narrow strip near the coast, with a south-southwest direction, favored by the strong northeasterlies which predominated prior to the cruise. Low-salinity waters and riverine particles in 2004 were characterized by mostly offshore removal, attributed to calmer northwesterlies prior to the survey. Changes in ambient stratification due to wind-driven vertical displacements of the seasonal thermocline appeared to also influence the plume dynamics. The downwelling (upwelling) conditions that prevailed in 2002 (2004) were revealed in both the in situ vertical profile temperature measurements and the satellite-derived sea surface temperature patterns.
    Journal of Geophysical Research Atmospheres 10/2009; 114. · 3.44 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Assessing the skill of biogeochemical models to hindcast past variability is challenging, yet vital in order to assess their ability to predict biogeochemical change. However, the validation of decadal variability is limited by the sparsity of consistent, long-term biological datasets. The Phytoplankton Colour Index (PCI) product from the Continuous Plankton Recorder survey, which has been sampling the North Atlantic since 1948, is an example of such a dataset. Converting the PCI to chlorophyll values using SeaWiFS data allows a direct comparison with model output. Here we validate decadal variability in chlorophyll from the GFDL TOPAZ model. The model demonstrates skill at reproducing interannual variability, but cannot simulate the regime shifts evident in the PCI data. Comparison of the model output, data and climate indices highlights under-represented processes that it may be necessary to include in future biogeochemical models in order to accurately simulate decadal variability in ocean ecosystems.
    Geophysical Research Letters 01/2009; 36:L21601. · 3.98 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: The Northeastern Aegean Sea, a marginal basin with complex topography, is the part of the Mediterranean where coupling with the Black Sea effectively takes place. The relatively oligotrophic Aegean Sea receives low salinity, eutrophic waters of Black Sea origin that substantially contribute to increased biotic production and to reduction in salinity. The latter has implications on the formation of dense waters that eventually influence the Eastern Mediterranean general circulation. The modified Black Sea waters enter through the Dardanelles Strait and account for lateral fluxes that overwhelm all Aegean river sources combined. Their low density characteristics result on a buoyant plume that exhibits intense seasonal and episodic variability, influenced by seasonal basin stratification, atmospheric forcing and changes in outflow volume and properties. A high resolution (1/50 degree) implementation of the Hybrid Coordinate Ocean Model has been nested within the coarser (1/25 degree), data assimilative Mediterranean HYCOM model to study the plume development and evolution. A coupled hydrodynamic-biogeochemical model, also nested within a 1/10 degree Mediterranean Sea outer model (based on the Princeton Ocean Model and the European Regional Seas Ecosystem Model) has been employed for the study of impacts on ecosystem dynamics associated with the Dardanelles outflow. The assessment of transport pathways of the waters of Black Sea origin in the North Aegean employs simulations with high frequency forcing and satellite derived patterns of SST and chlorophyll-a. Data from recent observational surveys and drifter releases in the Dardanelles plume will be also discussed.
    AGU Fall Meeting Abstracts. 11/2008; -1:1209.

Publication Stats

160 Citations
72.81 Total Impact Points

Institutions

  • 2014
    • Plymouth Marine Laboratory
      Plymouth, England, United Kingdom
  • 2013
    • King Abdullah University of Science and Technology
      • Department of Earth Science and Engineering (ErSE)
      Djidda, Makkah, Saudi Arabia
  • 2011
    • Sir Alister Hardy Foundation for Ocean Science
      Plymouth, England, United Kingdom
  • 2009–2011
    • Hellenic Centre for Marine Research
      • Institute of Oceanography
      Anávyssos, Attiki, Greece