There are at present certain basins having poor communication with the sea but which do not have the special biological characteristics for high confinement; this is the case of the Gulf of Amurakikos in Greece. In this respect, it can be noted that the volume of water it contains (8 km3) is high with regard to its surface ares (460 km2). Moreover, it is of recent formation, since the Amurakikos depression has been flooded by sea for a relatively short time, 8 000–10 000 years at the most. It can therefore be assumed that the initial stock of marine water has not yet had time to become sufficiently impoverished, bearing in mind its communication with the open sea. The rarefaction of certain species (according to local fishermen) is however perhaps the indication of the beginning of confinement.
Other examples are even more striking. The Caspian ‘Sea’ (77) 000 km3 and 436 000 km2) has been cut off from the marine domain sine the end of the Tertiary, i.e., for several million years. However, it still contains 60% of thalassoid forms (Zenkevitch, 1957) and therefore has not yet shifted into the continental domain (apparently most of the basin is situated in zones III and IV as defined above), in spite of the great time-lapse since it was cut off from the sea. Moreover, the maps of biomass distribution established by Zenkevitch from 1935 onwards show that the upper layer of the deep southern basins is less confined than the shallow northern basin (Appendix 3).
The Aral “Sea”, whose history is very similar to that of the Caspian Sea but which is much less deep, shows a distinctly higher degree of confinement, with communities for richer in freshwater species (zone VI and beyond).
Therefore, the depth parameter plays an important part in the speed of confinement of a basin in the process of being cut off from the sea; it may also be deduced that the deeper a paralic basin is, the slower its biological response to a diminishing exchange with the sea will be.
It was suggested earlier that confinement influences the communities through an impoverishment of the milieu in certain “vital”elements of marine origin - or, at least, coming essentially from the sea - consumed and immobilized by the living beings themselves, whose activity (with regard to the solar energy available) is at most proportional to the surface area of each basin, whereas the initial reserves of “vital” elements are proportional to the volume of the basin. Thus, in extremely shallow paralic basins, the confinement gradient very quickly reaches its state of equilibrium. In deep basins, on the contrary, the impoverishment of the milieu and the stabilization of the confinement gradient are much slower and if ever they become completely cut off from the sea, a considerable lapse of time is needed for their total “continentalization” to occur.
Mention has already been made of the Caspian which today is an intracontinental basin but which once was in communication with the sea.
This is not the case with some smaller basins - fossil or present-day of North Africa and the Sahara, which contain or have contained in a recent past, typically paralic flora and fauna: for example, the chotts of pre-Saharan Tunisia (Coque, 1962; Levy, 1982) the Pleistocene lake of Shati, in Libya 1, the Fayoun in Egypt, which is the outlet of a branch of the Nile and where a typically paralic fauna thrives 300 km away from the sea (Cerastoderma galucum, Bittium reticulatum, Brachydontes marioni).
Finally, there is the Sebkha Mellala near Ouargla, in Algeria, where Boye et al. (1978) described associations of Cardium, Melania, Hydrobia, Melanopsis, with Foraminifera (Ammonia, Nubeculariidae, Discorbidae) and Oogonia of Characea, in sediments dated between 8 000 and 10 000 years ago.
These were paralic communities which could imply:
- on the one hand, that the continental waters which fed the endoreic basin were re-creating a biochemistry close to that of the paralic domain
- on the other hand, that the milieu had been inseminated by paralic species, and therefore that there was a relationship with the sea, via the usual paralic domain
This second phenomenon is easily explained: perhaps air-borne transportation by paralic birds or by waterspouts, etc.
The first opens up strange horizons, and it is possible to imagine the fortuitous existence of endoreic basins whose catchment area is such as to allow the creation of biochemical conditions compatible with the development of thalassic species.
The authors propose that these endoreic basins with a paralic flora and fauna be termed “telehalic”.
In the preceding sections, it has already been suggested that confinement should correspond to an impoverishment of the milieu in “Vital elements” coming essentially from the sea, and consumed or trapped by the living beings themselves. Among these elements are vitamins and trace elements which include the heavy metals.
In the marine domain, numerous studies have shown that indeed a great many of these elements are “limiting” as far as their concentration and above all their “bio-availability” are concerned. Obviously, the critical values vary according to the species (Provasoli 1963 Belser, 1963). Moreover, it is likely that these “vital elements” include bodies as yet unknown.
Considering only the heavy metals, many studies have shown the accumulation of certain of these in the tissues of benthic animals and, consequently, pro parte in the sediments (Table 6). In this respect, molluscs, especially Lamellibranchia and Gastropoda, and also Polychaeta, seem particularly efficient.
Thus, the zones of the paralic domain where molluscs predominate (zones II, III, IV) constitute a trap for heavy metals, and a screen for the populations of the more confined zones. It is possible to consider that in these zones, it is the species whose requirements in heavy metals are lower, or species which cannot tolerate them in very high concentrations which develop. For example, according to a study on the Nereis diversicolor of the Humber estuary (Jones et al., 1976) it seems that these polychaeta lose their osmoregulatory capacity when the milieu is too rich in Cu. This seems to be true for other paralic species, particularly crustaceans.
It can be supposed that if confinement indeed corresponds to an impoverishment of the waters in “vital elements” its influence is two-fold:
- it sets limits to the extension of species for which there is a minimal threshold, for one or several elements
- it contributes to the development of species for which there is a maximal threshold beyond which one or several elements would act as poisons
In detail, the situation is likely to be more complex and, there may be an interaction (synergy) between several chemical variables of the milieu. However, schematically, the first type of action could apply to mixed species, and the second to strictly paralic species.
Thus, the biological zoning of the paralic milieux would be controlled by all the biohydrochemical gradients of the “vital elements”, each playing a positive, neutral or negative part, according to the species.
In this discussion, the following comments extracted from a study by Amanieu, Ferraris and Guelorget (1978) should be considered:
“From a historical point of view, the “i” ecosystems are presumed to have receded the “s” ecosystems, in order of evolution 1, from a geographical point of view, the “i” strategies are presumed to have been preserved in those regions of the globe where the factors of the milieu show great fluctuations, whereas the “s” strategies are presumed to have development in the sites with a stable environment.”
“We were discussing … the reputation of the lagunar ecosystems of being fragile, juvenile and highly productive. Here we have not broached the subject of productivity which is indeed very high; but then, we have stated why the term juvenile seemed to us inadequate. Lastly, as far as “fragility” is concerned, we consider that it cannot be disassociated from the stability which we have discussed. Indeed, the coastal lagoons are fragile and vulnerable, both to natural disturbances and to the inconsiderate behaviour of man. But, on what time scale and to what extent? Even since Strabon and Pliny has is not been common knowledge that the “malaigues” ruin our lagoons and that excessive fishing depletes them? Thus, for over two thousand years, our fragile lagoons have been in the process of dying. Yet how many “stable” peaks have for ever disappeared in the meantime? All through the cyclic vicissitudes from malaigues to “martegades” (winter freezing-over), through years good and bad, it may be that these everlasting swings of the pendulum, these steps forward hesitating between the worst which never happens and an ever-hoped-for improvement, are not accidents of the ecosystems, an ecosystem into which nearly 100 generations of local fishermen have been integrated, but its very nature. It is true that “Homo economicus” now has the means, with sea-front developments, motorways, stabilized shores and “lagooning”, to stabilize and “valorize” once and for all these great coastal ponds which will soon survive only as a memory.”
