The Aegean Sea Environment

This chapter is an introduction to the book “The Aegean Sea Environment: The Natural System,” which is the first book of the two-volume issue with the second volume entitled “The Aegean Sea Environment: Anthropogenic Presence and Impact.” In the present volume, we aim to approach the Aegean Sea marine ecosystem as a “water-way” connecting the Black Sea with the Eastern Mediterranean Sea, as a natural system. We discuss specific features and geodynamic evolution of the Black Sea–Mediterranean Sea system. We briefly describe geomorphological characteristics, land–sea and air–sea interaction, water masses, hydrology, chemistry, suspended matter and fluxes, sediments at the bottom, as well as biological dynamic and evolution, and life separately for the Black Sea, Mediterranean Sea, and Aegean Sea systems. The structure of the book is presented at the end of the chapter.

This chapter aims to provide a short, still comprehensive overview of the current knowledge on the bathymetry and geomorphology of the seafloor of Aegean Sea, the seismicity and the kinematics, the tectonic network and active offshore faulting, the Plio-Quaternary and active sedimentary basins. Integrated analysis of the bathymetric, morphological, seismological, kinematic, marine geological-geophysical, and sedimentological information provides the ground for a better understanding of the geological processes that controlled the evolution and led to the formation of the present morphological and geological structure of the Aegean Sea. However, most of the Aegean seafloor is either not surveyed at all or not sufficiently surveyed. Therefore, the hypotheses and thoughts presented hereafter should be considered rather as challenges or working hypotheses for future research.

Largely using monitored data, we estimated water and nutrient fluxes to the Aegean Sea for two time periods, a past period (from 1980 to 1994 regarding discharge and 1980–1994 or broadly 1995–2000 regarding nutrients) and a recent period (between 2012 and 2015 for both, discharge and nutrients). The total water fluxes entering the Aegean were estimated to 481.4 km³/year, of which 62.3% were derived from the Black Sea via the Dardanelles straits, 26.8% by direct precipitation, 10.4% by the Balkan and Asian Minor rivers, and 2.2% by submarine springs. Regarding nutrients, considering only the Balkan rivers (in lack of data from Turkish rivers), the total fluxes of DIN (dissolved inorganic nitrogen) were 268.5 and of P-PO₄ 7.17 kt/year. The main DIN sources were the Black Sea inputs (48.4%) and precipitation (38.4%), whereas for phosphorus the rivers were the main contributors (46%). Considering this result, combined with the anticipated dramatic increase of hydropower production, there are concerns of enhanced future P limitation regarding marine photosynthesis as a result of more efficient phosphorus retention in reservoirs. Since silica is also massively retained within reservoirs, changes in riverine Si:N:P stoichiometry are expected to affect phytoplankton composition. Natural sediment fluxes were estimated to 61.8 Mt/year; however, sediment flux assessment includes large uncertainties. Retention of excessive sediment amounts within reservoirs, combined with a sea level rise, is expected to induce significant land losses in coastal areas. Finally, the Aegean Sea is still under threat regarding organic pollution, while there is a need for toxic pollutants monitoring and control in the circum-Aegean area to safeguard this spectacular environment for future generations.

The coastline of the Aegean Sea has a length of about 13,660 km, which belongs to Greece by 78.6% and to Turkey by 21.4%. The Greek coasts consist of hard rocks (43.0%), soft rocks (47.8%), and non-cohesive sediments (9.3%). Most depositional coasts are located in the northern Aegean, where relatively large rivers form extended deltas. At the same time, numerous small beaches (<1 km long) are situated between the rocky promontories of the eastern and northeastern islands. About 28% of the coastline is subject to erosion, with an average retreat rate of 0.3–0.5 m year⁻¹. As sea levels are expected to rise in the coming years due to climate change, immediate action must be taken to increase the resilience of low-slope beaches.

The meteorological conditions over the Aegean Sea are subjected to strong influences from the large-scale patterns of the general atmospheric circulation, the surrounding continents, the almost enclosed Mediterranean Sea, the complex topography and coastline, as well as the mesoscale and local phenomena imposed on the synoptic-scale atmospheric circulation. This chapter discusses the most important circulation patterns that affect the area determining not only the weather conditions but also the climatic characteristics. The discussion unravels seasonal and spatial characteristics of the weather and climate patterns giving also a particular emphasis on the formation of cyclones (cyclogenesis). The role of upper-air conditions and air-sea interaction is highlighted, especially demonstrating the remarkable patterns of explosive cyclones and Mediterranean tropical-like cyclones (medicanes). This chapter also provides a preliminary analysis of air temperature and precipitation over the Aegean Sea unveiling spatiotemporal variabilities and trends.

The Aegean Sea is an intriguing sub-basin of the Mediterranean, due to (a) its capability to produce large amounts of very dense water, temporarily becoming the major producer of Eastern Mediterranean Bottom Water (recorded during the EMT period) and (b) due to its direct connection with the Black Sea that supplies the Aegean Sea with light, low-salinity waters which contribute buoyancy to the surface layers, and potentially control local convection processes. Thus, a similarity with the North Atlantic rises, in the sense of gradually increased stratification due to the addition of low-salinity surface waters, in an area capable to produce very dense waters, and thus possibly turning the North Aegean to a natural laboratory for studying such intriguing processes. The buoyancy exchanges with the atmosphere over the Aegean basin are partially controlled by the buoyancy gain and the subsequent formation of a shallow surface layer of the modified Black Sea waters. In this chapter, we examine the characteristics and variability of the heat and freshwater and the overall buoyancy fluxes with the exchanges with the atmosphere, focusing on the potential role of the interaction with the Black Sea.

The wind waves and the periodic elevation and fall of the sea surface in coastal areas, i.e. the tides, are the first manifestations of ocean behaviour that mankind ever observed. Thousands of years ago, Aristotle at the Euripus Strait in the city of Halkis and the nearby shoreline tried to understand why the sea was flooding onto the land and then receding away from it revealing the coastline stretches of sea bottom, known to us as the ‘intertidal zone’, that hours ago were flooded with sea water. As ocean science progressed, various aspects of the oceanic environment came into light regarding physical, chemical, biological and geological characteristics of the sea water. However, wind waves and tides are basic oceanic phenomena that exert a lot of influence on issues related to human activities in the ocean and particularly at the coastal areas. This chapter presents elements of our up-to-date knowledge on wind waves and tides in the Aegean Sea.

The role of the atmosphere as an external source of various nutrients and elements of both natural and anthropogenic origin in the Eastern Mediterranean and in particular the Aegean Sea has been investigated by using long-term deposition data collected over a 21-year period at Finokalia, Crete, Greece. Dry deposition was found to be the main mechanism contributing 67% of total insoluble matter flux, 68% of total dissolved inorganic nitrogen (DIN) flux, and 75% of total dissolved inorganic P (DIP) flux. Sediment traps data from 200 sinking particulate matter samples collected at 2 different depths in the seawater column (500 m and 1715 m) in the Cretan Sea (southern Aegean Sea) during a 9-year period (1997–2005) were compared with atmospheric deposition data (simultaneously collected) to assess the role of atmospheric deposition on seawater productivity and mass transfer in the seawater column. Atmospheric deposition was found to play a significant role in seawater productivity of the Aegean Sea by providing essential nutrients and especially N in excess and by facilitating the creation of aggregates between atmospheric dust and biological material and thus influencing their mass transfer to deeper waters by increasing their size and settling velocity.

The Aegean Sea’s hydrological and circulation characteristics are as complex as its geomorphological setting. All major Eastern Mediterranean (EMed) water masses can be found inside the basin, while the Aegean Sea is a producer of dense intermediate and in some cases deep water masses that are exported to the EMed. The role of the Aegean Sea as a dense/deep water source for the EMed has been revised following the Eastern Mediterranean Transient event of the 1990s decade. The local dense water production has been recognized as having a strong effect over the EMed overturning circulation. In this chapter, up-to-date information on the Aegean Sea is summarized, open questions are highlighted, and major future research aspects are proposed.

Based on the reanalysis MEDSEA_REANALYSIS_PHY_006_009 data, the interannual variability of temperature and salinity of the Aegean Sea at depths from 1.5 to 209 m was investigated for 1955–2015. An increase in the average temperature of the Aegean Sea during the study period was found at depth till approximately 100 m. At deeper levels, neutral and negative temperature trends were found. The average growth rate of sea temperature at the surface was approximately 0.01°С per year. Since the surface temperature has increased more strongly and more evenly in space than at depth, this increase may be associated with regional warming. At the same time, the salinity of the Aegean Sea did not change significantly during the study period. Against the background of an average increase in the temperature of the upper layer of the Aegean Sea, its fluctuations were found at various periods, the main of which is approximately 18.6 years. This period can be associated with the lunisolar nutation of the Earth’s axis of rotation, which affects the tides and mixing of waters, in particular, in the Aegean Sea. This is confirmed by the fact that the maximum energies of fluctuations in the salinity of the Aegean Sea on time scales from 1 to 30 years are located exactly on the period of 18.6 years. Teleconnections of temperature and salinity fluctuations in the Aegean Sea for a period of 18.6 years with the Pacific Decadal Oscillation (PDO), which are also significantly affected by the lunisolar nutation of the Earth’s rotation axis, have been found. Another reported period of temperature fluctuations in the Aegean Sea is approximately 11.5 years, and may be associated with an 11-year cycle of solar activity and with the internal dynamics of Eastern Mediterranean. The spectral peaks of the Aegean Sea temperature were found at periods of 8, 4, and 2 years also. They can be associated with the North Atlantic Oscillation (NAO), which has a period of 8 years. The spectral peaks at periods of 4 and 2 years can be 1:2 and 1:4 super-harmonics of the nonlinear effect of NAO on the Aegean Sea. Also, fluctuations in the temperature of the Aegean Sea were found for periods of 4.8, 3.6, and 2.4 years. These periods are characteristic of the El Niño – Southern Oscillation (ENSO), and they may be sub-harmonics of the Chandler wobble of the Earth’s poles over a period of about 1.2 years, which, through pole tides, can affect both El Niño and the Aegean Sea.

A review of the evolution of ¹³⁷Cs activity concentrations in the Aegean Sea is presented almost 34 years after the Chernobyl accident. The data from field measurements are interpreted considering the different water masses present in the Aegean Sea, for better understanding of ¹³⁷Cs spatial and temporal variation. The last 15 years, a lot of effort is given in the study of the vertical advection of ¹³⁷Cs and the related processes in the deep basins of the Aegean Sea. These results provided significant information about the average values along with depth of the velocity and diffusion parameters. The role of ¹³⁷Cs as a circulation and mixing tracer is undergoing a gradual weakening, since the gradient of its activity concentration between the water masses is small. ¹³⁷Cs is proved to be a valuable radiotracer for identifying the Black Sea Water masses since they are still enriched with higher values of ¹³⁷Cs compared to the background values in the Mediterranean Sea.

The concentrations and distribution of nutrients and carbon in the Aegean Sea are strongly influenced by the biogeochemical processes, the complex hydrological and circulation characteristics, as well as by the geographic setting and bottom topography of its sub-basins. Historical data have shown that the distribution of oxygen and nutrients in the water column of the South Aegean Sea was severely affected by the major Eastern Mediterranean Transient (EMT) event. The “nutrient rich-oxygen poor” layer that was established during the EMT in the South Aegean Sea altering the characteristics of this basin, was till recently present in greater depths than previously, though with faded hydrochemical properties. The temporal evolution of the dissolved oxygen concentrations for the period 1997–2011 in the laterally isolated deep layers of the depressions of the North Aegean Sea, revealed long periods of stagnation interrupted by weak dense-water formation events. The signature of these events was also depicted in the synoptic view of the concentrations of dissolved organic carbon, nitrogen, and phosphorus, as well as of the total dissolved inorganic carbon and total alkalinity. In addition, considerable differences of the dissolved organic matter and carbonate system parameters between the North and the South Aegean Sea attributed to the influence of the Black Sea inputs were detected.

An extensive collection of light transmission/beam attenuation coefficient vertical profiles (LT/c_p), particulate matter concentration (PMC) and particulate organic carbon concentration (POC), total particle volume concentration (VC), and particle median diameter (D₅₀) vertical profiles were assembled in order to provide an overview of particulate matter dynamics in the Aegean Sea, as well as a first time assessment of suspended particle size properties. A typical beam c_p vertical profile is composed of a relatively more turbid surface nepheloid layer (SNL) associated with enhanced primary productivity and atmospheric/fluvial inorganic/organic particle inputs, more transparent mid-waters with the sporadic occurrence of intermediate nepheloid layers (INL), and occasionally a more turbid bottom nepheloid layer (BNL) attributed to seabed sediment resuspension. The Aegean Sea is characterized by very low c_p values, apart from river discharge coastal regions of the North Aegean Sea. In a north-south direction, c_p decreases, and the water column becomes progressively more transparent, highlighting the transition from meso/oligo- to ultra-oligotrophic conditions prevailing in the Aegean Sea. There is a clear differentiation of particle abundance between “wet” and “dry” seasons, with the former showing enhanced c_p, PMC, POC, and VC values. Particle size, studied within the range 1.25–250 μm, exhibits relatively small variability, with D₅₀ varying between 81 and 105 μm, with a general increase over depth which is more pronounced in the South Aegean Sea, most likely associated with the variable abundance of Transparent Exopolymer Particles (TEP). Deep basins of the North Aegean Sea host permanent/semi-permanent INLs at mid-depths; their presence is attributed to internal wave activity. Good correlations of c_p:PMC and c_p:POC allow for the conversion of c_p data to PMC and POC values, in support of modeling and remote sensing applications.

The Aegean Sea is a water body located in southeastern Europe and western Asia, between the Greek mainland and the western coast of Turkey. It is connected to the Mediterranean Sea to the south and the Sea of Marmara to the northeast. Its topography is complex and comprised of several alternating deep basins, ridges, and islands. Data on sedimentology (grain size) and inorganic elemental geochemistry have been compiled to present the most up-to-date knowledge of the region and to identify the major processes of sediment provenance and deposition.

The North Aegean Sea receives freshwater and sediment load from several rivers that drain the Balkans, Greece’s mainland, and Asia Minor. Sediment supply, along with bottom topography and local hydrological features are the controlling mechanisms that define the texture and geochemistry of surficial sediments. Fine aluminosilicate sediments occupy prodeltaic areas, parts of the extended continental shelf, and the deep basins. However, in areas where sedimentation rates are low, sediments deposited during the last sea transgression – relict sediments – prevail. The relict sediments are of a sandy texture with high carbonate contents associated with the biogenous component of the sediments. Most of the trace elements examined are associated with lithogenous aluminosilicates but elevated values are observed near industrialized sectors, and the river prodeltas, attributed to contamination from anthropogenic activities.

The main characteristic of the Central Aegean Sea is the shallow Cyclades Plateau which is comprised of many islands. The seabed is generally covered by coarse sandy sediments, rich in biogenous shells and their fragments. As such, major and trace elements are considerably diluted by carbonates, meaning they are substantially lower in content than those of the North Aegean Sea.

Overall, the abundance of elements and their variability in the Aegean Sea is regulated by the relative proportions of terrigenous vs biogenous components, grain size, and water depth. Sediments around the Hellenic Volcanic Arc islands display a different character but this is restricted to very near their sources. The South Aegean Sea is comprised of the Cretan Sea and the Dodecanese Islands. The limited information that is available for the region suggests finer sediment textures of a terrigenous element origin.

The Aegean Sea is a true oceanographic laboratory and rightfully has attracted the attention of the scientific community over the last decades. However, it is striking that large sectors of the Aegean have been largely under-sampled and thus remain unexplored for sedimentological and geochemical studies.