Extent of the Province
The Mediterranean Sea, Black Sea Province (MEDI) includes the whole of the Mediter- ranean basin, distinguished by the ancients from the “Ocean Stream” lying beyond the Pillars of Hercules at Gibraltar. Included are the marginal seas (Adriatic, Aegean, and Cretan) and also the Black Sea and its marginal Azov Sea. The landlocked seas of Asia (the Caspian and the fast-disappearing Aral) are treated as saline lakes, no longer part of the ocean.
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Continental Shelf Topography and Tidal and Shelf-Edge Fronts
The Mediterranean Sea comprises two deep basins, connected with the Atlantic Ocean across a sill depth of only 290 m at the Straits of Gibraltar. The Black Sea is landlocked except for its connection with the Mediterranean through the Bosphorus; this connection is slender, having a shore-to-shore width of only 725 m at the choke point, and a midchannel sill depth of only 40 m.
The Mediterranean is characterized by rather narrow continental terraces that accom- modate the deltas of a few large rivers: Nile, Po, Rhone, and Ebro; extensive shelf areas occur only in the northern half of the Adriatic, and on the eastern coast of Tunisia where the shelf widens to about 300 km in the Gulf of Gabes. Most of the shelf is rocky or carries quartz sand deposits with a biogenic calcareous component, and mud is restricted to coastal embayments. Some mud occurs, of course, on either side of the Nile delta, and here the edge of the shelf conforms to the line of the out-built delta.
The coasts of the Black Sea are steep-to only in the southern part of the basin. To the north and west, there are extensive shelf areas that are, in general, much muddier than those of the Mediterranean. The peninsula of the Crimea stands on a wide shelf that reaches 200 km wide off Odessa; the edge of this shelf runs east-west, so almost directly across the northern part of the Black Sea. The effects of the great deltaic region, and of the effluents, of the Danube and Dniester on the western coast must be integrated into an understanding of this sea. The Sea of Azov is a very shallow continental shelf embayment, almost completely isolated from the main basin of the Black Sea, to the east of the Crimean peninsula.
In both Mediterranean and Black Seas the principal currents are marginal, adjacent to the coastline or shelf break, so that coastal and shelf-break fronts are a common feature that isolate this flow from the slower gyral and subgyral circulations. Tidal effects, mostly semidiurnal, are small in both the Mediterranean and Black Seas, because of the relatively small size of each basin.
Defining Characteristics of Regional Oceanography
It will be convenient to discuss the Mediterranean and Black Seas separately because of their very different characteristics. The Mediterranean is an evaporative basin con- strained by a shallow sill at the Straits of Gibraltar, whereas in the Black Sea the salt balance is approximately in equilibrium despite its shallow sill. The fast and continuous surface flow from the Black Sea into the Mediterranean through the narrow passages of the Bosphorus and Dardanelles is a unique feature in the circulation of the ocean. The Danube contributes two-thirds of the 304 km3 of freshwater that enters the Black Sea annually. One of the most useful and comprehensive accounts of Black Sea ecology is still that of Caspers, to be found in Hedgepeth (1957); this is particularly useful now as a reference for the near-pristine state of a now highly modified ecosystem.
Mediterranean Sea It is in winter, when the zonal band of the westerlies lies farthest equatorward, that the planetary wind field is most effective in the Mediterranean; wind stress at the surface is strongest and most uniformly distributed from December to March.
For most of the year, although the wind-stress climatology is dominated by northwesterly winds, the dominant effects are local and orographic. In winter there are local irruptions of especially strong northerly winds, cold and dry, that in the western Mediterranean are the adiabatic “mistral” winds channeled down the Rhone valley, while the similar “bora”
is not appreciated by the residents of Venice and Trieste.
In the absence of large-scale geostrophic circulation, the physical oceanography of the Mediterranean is strongly influenced by the local wind fields (Theocariset al., 1998). The circulation of surface water is characterized especially in the eastern basin by a number
Atlantic Westerly Winds Biome 175
Fig. 9.11 Cartoon of the general circulation of the Mediterranean and Black Seas to illustrate the dominance of coast form on the location of both persistent and intermittent eddies and fronts. Comparison with any suitable sea surface chlorophyll image will show the extent to which coastal eddies and dipoles are the locations of chlorophyll enhancement.
of semipermanent gyres resembling in scale the large mesoscale eddies of the open ocean.
However, these are not errant eddies and their location, together with their associated fronts and jet currents, is generally predictable. The general circulation pattern is complex (Fig. 9.11) because of many factors: lateral thermohaline fluxes due to acceleration through narrow straits, flux of freshwater from river flows, topographic effects of the complex continental and insular coastlines, and a Rossby internal deformation radius of 10–15 km. Although we shall not be concerned here with the details of the deep overturning circulation, we should note that deep, cold, dense water-mass formation occurs in the Adriatic Sea and in the Gulf of Lions, forced by the effects of bora and mistral, respectively. The signature at the sea surface of this process is easily enough misinterpreted as the effects of divergent upwelling of cold water.
The Mediterranean Sea comprises two partially isolated basins within each of which a cyclonic surface circulation occurs (see Robinson and Malanotte-Rozzoli, 1993; Minas and Nival, 1988), and the details of coastline alignment impose many smaller, semiper- manent gyres. The narrowness of the Sicilian Channel (140 km) partially isolates the gyral circulations of the eastern and western basins, which Millot (1992) regards as two separate Mediterranean seas. The Tyrrhenian Sea, partially enclosed by Sicily and Sardinia-Corsica, and the Adriatic Sea, behind the narrow (70 km) Strait of Otranto, each have a partially enclosed cyclonic gyral circulation.
The two gyral circulations of the western and eastern Mediterranean are only partially isolated, so there is a general cyclonic flow around the whole basin, with the surface water becoming progressively saline and the return flow progressively deeper. This process preconditions the water that enters the Ligurian Sea and the Gulf of Lions so that mistral wind episodes in winter (that strongly cool the surface water) readily induce deep convection and the formation of Mediterranean Deep Water. It is this mechanism that is responsible for the relative vertical uniformity of Mediterranean water masses. Wind- induced divergent upwelling occurs on the western coast of Sicily, at several places along the eastern coast of the Adriatic and Aegean Seas, and on the western coast of Crete. In
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these places, it is observable as a cold signature in sea surface temperature images but, interestingly, not always in the sea surface chlorophyll images.
Of more ecological interest, perhaps, than the general circulation of the semipermanent gyral system is the more active series of coastal rim currents that proceed cyclonically around the Mediterranean and are especially continuous around the western basin. This flow is forced by topographic steering and perhaps by deflection of motion to the right, or toward the coast, by the positive curl of the wind stress. Although this has long been the accepted explanation for rim currents in both Mediterranean and Black Seas, the actual forcing mechanism may be more complex, according to Sauret al. (1994).
Through the Straits of Gibraltar, Atlantic water enters the Mediterranean at the surface, and this perpetual inflow attracted the attention of the ancient philosophers and was not resolved until surprisingly late—toward the end of the 18th century: this was, with tidal streams, the first problem in oceanography to attract scientific attention. Local fisherfolk, of course, knew from the earliest times that a deep countercurrent was the answer—
although perhaps they did not know that there was a problem to be solved. The Atlantic water passes eastward along the African coast as the density-driven, topographically locked Algerian Current, which continues into the eastern basin through the Sicilian Channel (Millot, 1992) as the Ionian-Atlantic stream. Instability in this coastal flow generates a field of anticyclonic eddies and meanders, perhaps especially rich along the Algerian coast.
These are of order 50–100 km in diameter and drift eastward along the coast at a rate of several kilometers per day. After a lifetime of several months they tend to enlarge and depart progressively from the coast, often trailing a filament of upwelled water with them, originating around their SW flanks. The Western Alboran Sea is occupied by a persistent anticyclonic gyre forced by the orientation and topography of the Straits of Gibraltar. In the eastern part of the Alboran Sea, circulation is more variable (Millot, 1987).
In the eastern Mediterranean basin, the already meandering Ionian-Atlantic stream at the Sicilian Channel becomes a mid-Mediterranean jet that can be traced far into the Levantine Basin passing between flanking cyclonic eddies (Cretan, Rhodian, and West Cyprus) to the north and anticyclonic eddies to the south (Shikmona and Mersa Matruh).
The quasipermanent position of these eddies is determined by topography (POEM Group, 1992). A coastal anticyclonic loop current around the Ligurian Sea between Sicily and Greece creates a series of mostly cyclonic eddies.
Although tidal streams in the Mediterranean are generally weak, the Venturi effect over the shallow sill of the Straits of Messina produces tidal currents that are unusually strong, and violent, local upwelling occurs. This is, of course, the whirlpool of Greek antiquity lying between six-headed, dog-barking Scylla, and Charybdis, who swallowed the sea and vomited it back again thrice daily.
Black Sea With its anoxic interior, the Black Sea is the most extreme case of a meromictic basin in the present-day ocean (for modern reviews, see Murray, 1991, and Özsoy and Ünlüata, 1998). Salinity nowhere exceeds 17‰, and excess precipitation together with runoff from the rivers Danube, Dniester, and Don creates a surface low- salinity layer overlying a halocline at about 100 m that is sufficiently strong to prevent ventilation of the interior of the sea. Below the halocline, the basin lacks oxygen and has high concentrations (increasing with depth) of dissolved H2S. Stratification includes a summer thermocline at 10–30 m, shoaler than the main discontinuity that combines nutricline, pycnocline, and oxycline.
The hydrology of the Black Sea depends strongly on its mass water budget because of its near enclosure. Since precipitation and river discharge exceed evaporation, a fast, permanent flow pours southward through the Bosphorus—making a ferry crossing at Istanbul a memorable affair. Some Mediterranean water (representing <50% of the outgoing flux) enters the Black Sea as a bottom flow along the narrow channel, though
Atlantic Westerly Winds Biome 177
much of this undercurrent water is entrained back into the fast outflowing surface current (Caspers, 1957; Sorokin, 1983). On entering the Black Sea, the remainder spills over the shelf break into deeper water.
The basin-scale circulation is cyclonic and appears as a coastal current that is the analogue of the coastal flow of the Mediterranean. It is more continuous in the Black Sea, however, because of the less complex coastal topography there. From the mouth of the Bosphorus a strong current runs eastward along the Paphlagonian coast, forming the strongest flow of the Rim Current (40–80 km wide) that circles the whole Black Sea at the 200-m depth contour. The Rim Current is associated with two principal cyclonic gyres that occupy the eastern and western basins (divided to the south of the Crimea).
These are constrained by the shelf edge that runs zonally across the basin at the latitude of southern Crimea, so that flow on the northern shallow shelves themselves is more variable. Smaller anticyclonic gyres lie between the main gyres and the coast, trapped by topographic features. During winter, the two-gyre circulation may break down, to be replaced with a single, more elongated cyclonic gyre, and, by the end of winter, very cold shelf water has been formed on the wide shelf regions to the northeast and to the west of the Crimea. This water is progressively advected around the western side of the Black Sea and its effect may be traced even along the southern coast. Especially along the southern and eastern coasts, there is strong mesoscale vorticity in the meandering and filamentous flow, whose features propagate eastward at 10–15 km a day. This field of vorticity widens at Cape Baba.
This circulation pattern explains the topography of the halocline and the oxic/anoxic interface that lies at about 150 m near the centers of circulation of the two main cyclonic gyres and deepens to>200 m around the coastal margins. Only below a small permanent anticyclonic gyre in the southeastern part of the sea does the oxic/anoxic interface deepen away from the coast. Because the chemistry of the oxic/anoxic interface is so intimately connected with biological processes, we shall defer discussion of it to the following section.
Response of the Pelagic Ecosystems
The ecological characteristics of the two seas are sufficiently different that to place them in a single province is largely a matter of convenience. Both, however, were significantly modified during the 20th century, not only from land-based sources of contamination but also by reduced runoff from the major rivers entering the basins. Nitrate values in the mixed layer of the Black Sea have increased significantly in the past 25 years. Also very significant has been the loss of the annual Nile flood, held in recent decades behind the Aswan High Dam, resulting in a very significant modification of the ecology of the eastern Mediterranean. The artificial Lessepsian connection between the Mediterranean basin and the Red Sea is of great significance for taxonomic biogeography because of immigration of Indo-Pacific species through the Suez Canal. This transport, and the introduction of exotic species in ballast water of tankers, was discussed in Chapter 2.
The ecological response of the two seas to seasonal changes environmental forcing is, for all these reasons, quite different as is clearly demonstrated by the seasonal chlorophyll images from the SeaWiFS and MODIS sensors since 1997. The Mediterranean shows a clear seasonal winter–spring bloom, stronger in the western than the eastern basin, and from June until October almost the whole sea is deeply oligotrophic. The Black Sea, on the other hand, appears now to have an almost uniformly high level of algal biomass over deep water—a green field that is relatively invariant seasonally. Even higher biomass is consistently indicated over the northern and western shelf areas, with permanent “hot spots” in the Azov Sea and at the margin of the Danube-Dniester deltaic region.
Mediterranean Sea The seasonal cycle of primary production and consumption resembles that of the subtropical Atlantic. Winter mixing causes nitrate to become
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available in the photic zone and a relatively weak late-winter bloom ensues, which is followed by a long period in which the profile includes a DCM. The western basin consistently supports a more active algal response to wind stress than the relatively olig- otrophic eastern basin. The western winter–spring bloom is patchy, and differs in its distribution rather significantly between years. For instance, in March 1999 there was a strong accumulation of chlorophyll between southern France and Corsica that persisted through the following month. Although the winter–spring bloom in each year tends to accumulate more chlorophyll along the south coast of France than elsewhere, such a strong offshore event did not occur in 1998, 2000, or 2001 and only weakly, to the west of Corsica, in 2002.
It is only in winter months that positive net community production exceeds respiration so that a winter production pulse (mid-January to mid-February) occurs off Southern Spain (Rodriguez et al., 1987), both near-surface and subsurface chlorophyll maxima being dominated by small autotrophic cells. At this season, 80–100% of the biomass passed a 10-mm mesh and 20–60% passed even a 1-m Nuclepore filter. In the Adriatic in summer, Revelante and Gilmartin (1994) found a twofold higher biomass of larger cells in the DCM compared with the rest of the water column, even though picoplankton formed 50% of the total biomass. By late spring, a DCM is established in both the eastern and western Mediterranean basins; typical profiles show that this and the nutricline occurs at the base of the thermocline at 75–80 m, with the topography of the density surface following geostrophic flow. In anticyclonic features, the DCM generally coincides with the nitracline rather than with density surfaces, suggesting that primary production is limited primarily by nutrient supply and only secondarily by light and other factors.
At, or close to, the DCM is the expected layer of abundant zooplankton. All these features deepen through midsummer, at a rate of about 15–20 m a month (Estradaet al., 1993).
Because deep and intermediate water masses are formed within the Mediterranean basin by the modification of surface water, subpycnocline nutrient levels are significantly lower than those in the open ocean, with maximum values in mid-depths of 95M at 250 m in the western basin (Coste et al., 1988). In fact, at the Straits of Gibraltar, the balance between nutrients transported in the incoming and outgoing water masses translates into a net gain of nutrients for the Mediterranean basin. The presence of a discrete Atlantic water body in the Alboran Sea, separated across a density gradient from the shoaler water mass, somewhat complicates observations. Rodriguez (in litt.) suggests that the variable thickness of the Atlantic-Mediterranean water interface controls the thickness of the DCM as well as its maximum chlorophyll concentration. In the eastern basin, Yilmaz (1994) observed the control of DCM formation and maintenance by variance in nutrient levels and irradiance. DCMs were shallower (50 m) and contained more chlorophyll in late winter, and were deeper (100 m) and weaker in summer.
An accessory mechanism for vertical transport of nutrients has been described in the western Mediterranean. Here, diel migrant herbivorous copepods, Centropages typicus, are observed to feed continuously within the DCM by day but to occur near the surface at night in food-poor conditions; this must force an active flux of organic nitrogen up from the DCM into surface water. It has also been shown that individual weather systems may now induce transient blooms by delivery of nitrate and other nutrients in rainfall:
rain collected in the smoggy northern Adriatic contains <80M nitrate and <36M ammonium (Malej, 1997). The response of the phytoplankton to such anthropogenic inputs is a dynamic that has been unduly neglected for—given the widespread turbidity of the lower atmosphere in recent decades—it is surely not restricted to the Adriatic Sea alone?
The majority of the available chlorophyll images, at all seasons, show significant enhancement over the wide continental shelf on the eastern coast of Tunisia. This is strongest just inshore of the small archipelago off Sfax, but in the oligotrophic season it