[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. · 3.73 Impact Factor
[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. · 2.66 Impact Factor
[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; · 2.66 Impact Factor
[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. · 2.66 Impact Factor
[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; · 1.14 Impact Factor
[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).
[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
[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
[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 10/2009; 114. · 3.17 Impact Factor
[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.
[show abstract][hide abstract] ABSTRACT: The Black Sea ecosystem experienced severe eutrophication-related degradation during the 1970s and 1980s. However, in recent years the Black Sea has shown some signs of recovery which are often attributed to a reduction in nutrient loading. Here, SeaWiFS chlorophyll a (chl a), a proxy for phytoplankton biomass, is used to investigate spatio-temporal patterns in Black Sea phytoplankton dynamics and to explore the potential role of climate in the Black Sea's recovery. Maps of chl a anomalies, calculated relative to the 8 year mean, emphasize spatial and temporal variability of phytoplankton biomass in the Black Sea, particularly between the riverine-influenced Northwest Shelf and the open Black Sea. Evolution of phytoplankton biomass has shown significant spatial variability of persistence of optimal bloom conditions between three major regions of the Black Sea. With the exception of 2001, chl a has generally decreased during our 8 year time-series. However, the winter of 2000–2001 was anomalously warm with low wind stress, resulting in reduced vertical mixing of the water column and retention of nutrients in the photic zone. These conditions were associated with anomalously high levels of chl a throughout much of the open Black Sea during the following spring and summer. The unusual climatic conditions occurring in 2001 may have triggered a shift in the Black Sea's chl a regime. The long-term significance of this recent shift is still uncertain but illustrates a non-linear response to climate forcing that makes future ecosystem changes in the pelagic Black Sea ecosystem difficult to predict.
Journal of Marine Systems 11/2008; · 2.66 Impact Factor
[show abstract][hide abstract] ABSTRACT: Several environmental/physical variables derived from satellite and in situ data sets were used to understand the variability of coccolithophore abundance in the subarctic North Atlantic. The 7-yr (1997-2004) time-series analysis showed that the combined effects of high solar radiation, shallow mixed layer depth (,20 m), and increased temperatures explained .89% of the coccolithophore variation. The June 1998 bloom, which was associated with high light intensity, unusually high sea-surface temperature, and a very shallow mixed layer, was found to be one of the most extensive (.995,000 km2) blooms ever recorded. There was a pronounced sea-surface temperature shift in the mid-1990s with a peak in 1998, suggesting that exceptionally large blooms are caused by pronounced environmental conditions and the variability of the physical environment strongly affects the spatial extent of these blooms. Consequently, if the physical environment varies, the effects of these blooms on the atmospheric and oceanic environment will vary as well.
Limnology and Oceanography - LIMNOL OCEANOGR. 01/2006; 51(5):2122-2130.
[show abstract][hide abstract] ABSTRACT: Deriving maps of phytoplankton taxa based on remote sensing data using bio-optical properties of phytoplankton alone is challenging. A more holistic approach was developed using artificial neural networks, incorporating ecological and geographical knowledge together with ocean color, bio-optical characteristics, and remotely sensed physical parameters. Results show that the combined remote sensing approach could discriminate four major phytoplankton functional types (diatoms, dinoflagellates, coccolithophores, and silicoflagellates) with an accuracy of more than 70%. Models indicate that the most important information for phytoplankton functional type discrimination is spatio-temporal information and sea surface temperature. This approach can supply data for large-scale maps of predicted phytoplankton functional types, and an example is shown.
[show abstract][hide abstract] ABSTRACT: The Red Sea is a unique marine environment but relatively unexplored. The only available long-term biological dataset at large spatial and temporal scales is remotely-sensed chlorophyll observations (an index of phytoplankton biomass) derived using satellite measurements of ocean colour. Yet such observations have rarely been compared with in situ data in the Red Sea. In this paper, satellite chlorophyll estimates in the Red Sea from the MODIS instrument onboard the Aqua satellite are compared with three recent cruises of in vivo fluorometric chlorophyll measurements taken in October 2008, March 2010 and September to October 2011. The performance of the standard NASA chlorophyll algorithm, and that of a new band-difference algorithm, is found to be comparable with other oligotrophic regions in the global ocean, supporting the use of satellite ocean colour in the Red Sea. However, given the unique environmental conditions of the study area, regional algorithms are likely to fare better and this is demonstrated through a simple adjustment to the band-difference algorithm.
Remote Sensing of Environment 136:218–224. · 5.10 Impact Factor
[show abstract][hide abstract] ABSTRACT: What is the role of outflow properties, strait dynamics and atmospheric forcing in the development of the Dardanelles plume? How does the transport and fate of BSW waters vary in seasonal and inter-annual time scales, how is it modified by the complex topography and how does it impact the North Aegean general circulation? What is the role of BSW on the North Aegean dense water formation?