We cannot understand changes in any environment without
a basic grasp of the fundamental principles that govern that environment’s
physical, chemical, and biological characteristics. This chapter therefore
introduces in general terms the main processes ongoing in the Mediterranean
basin, as unraveled by oceanographers over especially the last 50 years.
The functioning of the basin can be likened to that of an enormous organism. In fact – as do many ocean basins – the Mediterranean ‘breathes’, taking in oxygen by exchange with the atmosphere at its surface, and it has a distinct ‘circulation system’ that moves the oxygen and other chemical properties around the basin. Combined, these aspects affect the plant and animal life in the basin, while this biological activity in turn exerts its own influences on the distribution of chemical properties such as nutrients and oxygen.
Everywhere, oxygen concentrations in the ocean’s surface
waters are in an active process of exchange, or ‘equilibration’, with oxygen
concentrations in the atmosphere. This equilibration constitutes the only
significant route of oxygen into the oceans. Due to the equilibration,
oxygen levels in surface waters eventually reach what we call ‘saturation’,
meaning that for that water’s particular temperature and salinity, the
concentration of oxygen is fixed – not too high, not too low.
The entire surface layer, which amounts to the upper 150-200 meters or so that is stirred and mixed due to wind and wave action, thus becomes well oxygenated. However, the oxygen is not efficiently passed on to deeper layers. To understand why this is the case, we need to consider the changes with depth in sea-water density (where density is weight per litre).
In simplified terms, this problem can be portrayed by assuming that the ocean is made up of discrete layers of water with different characteristics. The surface layer has a lower density than the deeper layer, which allows the surface layer to ‘float’ on the deeper one. It’s like low-density oil floating on higher-density water, but with a smaller density contrast. The density difference inhibits mixing between the two layers, and so precludes an efficient exchange of properties (eg., oxygen) between them. As a result, there is a very limited transfer into the deeper layer of the oxygen that was taken up into the surface layer via equilibration with the atmosphere.
‘Isolated’ from the atmosphere by the overlying surface layer, the deep layer consequently would be seriously deprived of oxygen. Still, many ocean basins, including the deep Mediterraean today, are quite well oxygenated. This is caused by substantial oxygen transport into the deep sea by localised, and often seasonal, formation of connections between the deeper layer and the atmosphere. The specific locations where such connections develop are described as ‘deep-water formation areas’. They are the vital sites for oxygen transport into the deep sea, much the same as our lungs are for oxygen transport into our bodies.
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So what characteristics of sea water determine its density, what happens in deep water formation areas, and how do these processes operate in the Mediterranean?
The density of sea water depends on two properties: Temperature and Salinity. Density decreases as temperature goes up, and increases as temperature goes down (note 2). Hot air balloons provide a familiar illustration of temperature’s influence on density. As the burner heats the air within the balloon, its density is lowered, causing the balloon to rise. When the burner is then left off, the air in the balloon gradually cools by exchange with the atmophere, its density increases, and the balloon gradually comes down. Besides temperature, salinity also has a major effect on sea-water density. The more dissolved salts per litre – ie., the higher the salinity – the higher the density. At any given temperature, therefore, fresh water has a lower density than sea water. Conversely, sea water that has undergone substantial evaporation will have a higher density than normal sea water, because the evaporation had removed pure water and left the salts behind.
Taking these two crucial influences on density into account, we can predict that high densities in oceanic surface waters should be found in poleward regions, where temperatures are very low. Highest values should be expected in areas where mighty, saline, surface ocean currents from the warm evaporative subtropics penetrate into such regions. In general terms, this is exactly what we see. Because of the low temperatures, polar to subpolar (‘high-latitude’) regions show very high densities, but especially high values occur between Scandinavia and Greenland, where salty waters transported north by the Gulf Stream are subjected to high-latitude cooling.
As surface density values are increased, the contrast between the surface and deeper layers fades. In particular places in the world ocean, during particular seasonal conditions, this leads to the remarkable condition that density becomes equal through the water column from the surface to great depth. Nothing then stops violent mixing throughout the water column and the consequent formation of new deep water. Thus, a connection is established between the deep sea and the surface – where equilibration with the atmosphere takes place – and the formation of new deep water ‘pumps’ fresh oxygen into the deep layers.
For the world ocean, the most important deep-water formation areas are in the Labrador Sea and between Greenland and Scandinavia in the northernmost Atlantic (North Atlantic Deep Water – NADW), and in the Weddell Sea to the east of the Antarctic Peninsula (AntArctic Bottom Water – AABW). Neither of those global deep-water masses can penetrate the Mediterranean, however, because of a shallow sill (284 m) in the Strait of Gibraltar. Why then do observations demonstrate that Mediterranean deep waters are so well oxygenated? Does the basin perhaps have its own regions of deep-water formation? The answer is Yes.
In the northern sector of the western Mediterranean (Gulf of Lions), and in the northern basins of the eastern Mediterranean (Adriatic and Aegean Seas), active deep-water formation takes place. The Gulf of Lions deep water oxygenates the western Mediterranean between the Strait of Gibraltar and the Strait of Sicily, while the Adriatic and Aegean deep waters oxygenate the eastern Mediterranean. There is, however, a complication. Between the surface layers and the deep waters, a distinct intermediate water layer is recognised throughout the Mediterranean between 150/200 m and 600 m depth. In the next section, I will discuss the processes involved in Mediterranean deep-water formation, including the vital role played by the intermediate water.
The deep circulation of the Mediterranean is driven
by a two-stage motor. In essence, surface waters in the Mediterranean have
their origin in the inflow of Atlantic waters through the Strait of Gibraltar.
Flowing eastwards through the Mediterranean, the surface waters are warmed
up by solar irradiation, and evaporation causes a steady increase in salinity.
As the inflowing waters, by now known as ‘Modified Atlantic Water’, arrive
in the easternmost Mediterranean, the salinities are very high indeed –
they reach more than 39 salinity units, compared with about 36 in the original
Atlantic inflow, or with the global ocean’s average salinity of 34.5 units.
This high salinity in the easternmost Mediterranean would cause a high
density, if it were not compensated by very high temperatures, which in
summer reach 26ºC or more.
Figure 2. Highly schematic representation of the two-stage deep ventilation
‘engine’. Inflowing surface water is transformed into intermediate water
by a net salinity increase. The salt in the intermediate layer preconditions
the second stage, whose direct forcing is essentially thermal. The endresult
is the formation of deep water layer that is less saline, but colder, than
the intermediate water. Click on thumbnail for full-sized figure.
Crucially, winter cooling reduces sea surface temperatures in the general easternmost Mediterranean to around 17ºC, while a specific area between Cyprus and Rhodes gets cooled that little bit extra by cold winds from the Anatolian mountains. There, temperatures below 16ºC are reached, closely appoximating those in the original inflow into the basin. The density increase due to high salinity is now no longer compensated by temperature however, and, as a consequence, deep-water formation takes place in the area between Cyprus and Rhodes. This water does not sink all the way to the bottom, but settles between 150/200 and 600 m depth – we call it ‘Levantine Intermediate Water’. As described, the formation of this water mass is essentially driven by a salinity increase. The first stage of the Mediterranean deep circulation motor, therefore, is salt-driven.
The Levantine Intermediate Water is found everywhere in the Mediterranean. Spilling through the Strait of Sicily at depth, it is also distinctly present in the western Mediterranean. With its high salinity, it provides an enormous subsurface supply of salt to the entire basin that is vital for the next stage of the Mediterranean deep circulation motor.
Along the northern margin of the Mediterranean, we observe the coldest winter conditions in places influenced by cold polar/continental air streams. These air flows are channelled over those areas by the mountain ranges around the Mediterranean’s northern margin. In the west, such channelling through the Rhone Valley towards the Gulf of Lions gives rise to the ‘Mistral’ winds. Over the Adriatic Sea, similar conditions cause the ‘Bora’ winds. Again similar conditions occur over the Aegean Sea in winter (‘Vardar’). These cold and dry winds cause strong local cooling of the surface waters, to temperatures of 11-13ºC. The resultant rise in density is kept in check by the fact that salinities are not very high along the relatively wet northern borderlands of the Mediterranean. Still, the cooling effects do suffice to eliminate the density contrast between cool and relatively fresh surface waters, and the underlying warmer but very salty Levantine Intermediate Water. Mixing between those two water masses becomes possible.
Then, something extraordinary happens. Where mixing takes place of two water masses of fundamentally different properties, but similar density, the endproduct will have a higher density than the original components. In our case, the endproducts are known as the Western Mediterranean Deep Water (WMDW) from the Gulf of Lions, and the Eastern Mediterranean Deep Water (EMDW) from the Adriatic and Aegean Seas.
The formation of EMDW and WMDW represents the second stage of the deep circulation motor. The second stage, therefore, is dominantly thermally driven by winter cooling. However, to function ‘properly’, it requires the extra salt supplied by the first stage of the motor – the primarily salt-driven intermediate water formation.
With its two-stage deep-water formation, driven by net evaporation and net cooling, the Mediterranean efficiently converts surface water into subsurface water. A compensating inflow of new surface water occurs from the Atlantic, through the Strait of Gibraltar. Meanwhile, the active deep-water formation ensures that the deep basin remains well ventilated. This process is so vigorous that it only needs about one century to produce enough new deep water to replace the entire volume of Mediterranean deep water. The thus replaced subsurface waters exit from the basin into the Atlantic, through the Strait of Gibraltar. Because of the density differences, the higher-density Mediterranean outflow (temperature ~13ºC, salinity ~38) takes place at depth, underneath the lower-density Atlantic inflow (temperature ~16ºC, salinity ~36).
Generally speaking, plants form the base of the food
web. Landplants use water and mineral nutrients (especially phosphate and
nitrate) from the soil, and carbon from atmospheric CO2, and
combine these compounds to form new biomass (ie., to grow) via a chemical
reaction that requires energy from sunlight. During the process – called
‘photosynthesis’ for ‘combination/ creation with light’ – oxygen is released.
Upon death, the biomass undergoes decomposition, a process that involves
microbes and that can either take place within an animal’s gut or out in
the open (eg., a rotting log). It goes under the technical name ‘respiration’,
and effectively reverses the photosynthetic process: the biomass is broken
down and the energy and nutrients that were used in its formation are released.
Hence, by eating grass, a cow indirectly consumes solar enery. The animal
stores this energy as sugars or fat.
In the oceans, the plants are represented by a rich diversity of phytoplankton – which translates as ‘drifting plants’. These microscopic marine plants do exactly the same as their larger cousins on land. They take up nutrients and carbon from the ambient sea water, and use solar energy in photosysnthesis to form new biomass, and oxygen as a byproduct. For photosynthesis, phytoplankton requires light. In the Mediterranean Sea, light penetrates to a maximum depth of about 120 m. This restricts the phytoplankton to the surface layer.
Phytoplankton forms the basis of the marine food web – it is therefore common practice to refer to phytoplankton organisms as ‘primary producers’. By harnessing solar energy, phytoplankton forms the vital start of energy flow to all animals grazing upon it, or predating and scavenging upon other animals. Marine food webs are highly complex, just like those on land, but in essence everything comes down to the transfer of – originally solar – energy from the plants up though the food web.
Since death and decomposition (respiration) are as common in the sea as on land, most of the nutrients and carbon originally fixed in biomass are eventually recycled back into the water column. In the illuminated surface waters, however, dissolved nutrient concentrations remain low, because any released nutrients get immediately taken up again for photosynthesis. Below the light-penetration limit, in deeper waters, the story is different. There is no photosynthesis at those depths, so that dissolved nutrients are not being taken up again. As a consequence, nutrient concentrations in deep-sea waters increase with time.
Oxygen concentrations in the water column follow the opposite pattern to nutrients. High concentrations are found in the surface layer, due to ongoing equilibration with the atmosphere. Below the surface layer, however, oxygen input is limited to that supplied by the process of deep water formation, while respiration of biomass continues, consuming oxygen. As a result, oxygen levels drop steadily from the (near) saturation values in newly-formed deep waters to progressively lower values in deep waters of increasing age. Oxygen concentrations in the deep sea thus reflect a precarious balance between ventilation by new deep water and oxygen consumption by respiration.
Some nutrients are lost from the oceanic system, by becoming trapped in the continuous process of sedimentation – the formation of sea-floor mud. This loss is offset by delivery of new nutrients to the sea via two pathways.
One route is atmospheric. This route is particularly important for nitrogen, which occurs in several gaseous forms in the atmosphere. Nitrogen gas is especially abundant, constituting almost 80% of the air we breathe. Gaseous nitrogen oxides are also common, and are enhanced by the burning of fossil fuels and by decomposition of sewage. Specialised algae can fix nitrogen from dissolved gases in surface waters, which equilibrate with the atmosphere. Other, normally minor, atmospheric nutrient transports involve dust and aerosols.
The other pathway for nutrients into the oceans, and the dominant one in the case of phosphorus, is via rivers. Where the atmospheric mechanism has fairly uniform influences over very large areas, the riverine flux affects especially the coastal waters. Particularly strong impacts are seen near river mouths. Today, rivers bring very high amounts of dissolved nutrients into the sea, because they drain artificially fertilised lands, and are contaminated with sewage. However, even before this massive anthropogenic pollution, the dissolved nutrient influx via rivers was substantial. Even in a system completely undisturbed by man, therefore, a massive increase in river discharge into an enclosed sea would have ‘fertilised’ that basin, causing enhanced phytoplankton production. The main impact would, however, always be rather limited to the river-fed margins of the basin. For effects further into the open ocean, processes are needed that extensively redistribute the nutrients.
The ocean has an array of magnificent tricks up its sleeve with exactly that potential, and which involve the deep-water nutrient reservoir. Even in a basin like the Mediterranean, with a very active deep-sea ventilation, the intermediate and deep waters accumulate excess nutrients due to respiration. Because the deep reservoir has an enormous volume, even moderately enhanced nutrient levels give it a dramatic long-term ‘fertilisation’ potential (note 3). To unleash this potential, the ‘deep’ nutrients need to be made available for use in photosynthesis. In other words, the deep reservoir needs to be brought into the zone of light penetration – the ‘photic layer’, defined as the depth of 1% light penetration, and reaching as deep as 120 m in the Mediterranean. The result of such a fertilisation process would be a long-term sustained increase in the phytoplankton production. Depending on where the deep reservoir is brought up to shallower depths, the fertilisation may occur anywhere in the basin, not just around the coasts.
In the open ocean, upwelling areas are places where subsurface water is ‘pumped’ all the way to the surface by the prevailing winds. These regions are consequently characterised by enormous production levels that support important fisheries-resources – for example the anchovy fisheries off the coast of Peru. Upwelling is not widely developed in the Mediterranean. A more subtle process, however, does operate in the basin. It involves shallowing but not surfacing of the deep nutrient reservoir. In winter, intensified surface currents ‘pump’ the intermediate water from about 200 m up to about 100 m depth in the Gulf of Lions, causing a fertilisation at the base of the photic layer. Similar ‘subsurface fertilisation’ processes are known from other places in the world ocean. Whenever this happens, a bloom develops of a specific association of phytoplankton, which we call the ‘Deep Chlorophyll Maximum (DCM)’ association.
Based on specialisations regarding a wide variety of parameters, the next level of the food web – the secondary producers –will also reflect specific environmental conditions. To continue our DCM example, those settings are found to sustain a specific, recognisable association of secondary as well as primary producers. The development of such a particular association is driven by complex interplays of parameters such as dietary preferences, living and reproduction depth or temperature preferences, capacity to compete for resources, and reproductive strategies to cope with predation pressure. As a consequence, different DCM settings are never characterised by exactly the same association of secondary producers, but distinct common aspects may be expected.
Of course, specialisation to specific types of ‘feeding grounds’ also takes place higher up in the food web (on ‘higher trophic levels’), but further discussion falls outside the scope of this book. Much of the research into causes and mechanisms of the Mediterranean anoxic events concentrates on phytoplankton and ‘zooplankton’ (ie., animal plankton), and hence to a large extent concerns primary and secondary producers. The study of these microfossils is called ‘micropalaeontology’. The emphasis on micropalaeontology in the Mediterranean studies did not develop by chance, but is based on a number of sound arguments.
First, many of the phytoplankton and zooplankton species form skeletal parts that fossilise well. Second, these skeletal parts have different shapes and sizes for the many different species, which helps us assess how the associations have changed through time. Third, many of the species are highly specialised to specific conditions, facilitating the reconstruction of past oceanic conditions. Fourth, phytoplanton and zooplankton organisms are very small (mostly smaller than 1 mm, but occasionally up to about 5 mm), and they occur in great numbers. This ensures that there virtually always is enough material to perform statistically sound quantitative analyses of species abundance, diversity, dominance, etc. – even when working with small samples from expensive sea-floor coring. Finally, the fossilised skeletal parts of phytoplankton and, especially, zooplankton species are excellently suitable for a variety of chemical analyses that add further vital information to the reconstructions.