The paralic milieux which have been studied and the bibliographical data available underline one fact: there exist species which live and develop only in paralic milieux. Moreover, their distribution is independent of the salinity of the milieu. Examples are numerous, and only a few are mentioned here. Among the most striking is that of Ruppia spiralis. This monocotyledon groups in communities in the Etang d'Urbino (33 ‰), and is often to be found in the Dutch polders where Den Hartog (1971) calculates that they have a salinity tolerence ranging between 1.5 and 23 ‰. On the other hand, R. spiralis communities are also found in the Bahiret el Biban where salt content exceeds 80 ‰ (Guelorget et al., 1982), or at Salin-de-Giraud at 60–80 ‰. It is moreover remarkable that this monocotyledon is never found in the open sea, although it could obviously find salinity conditions there well compatible with its development. Among molluscs, species such as Hydrobia acuta or its homologue Pirenella conica can be cited, or again Cerastoderma glaucum found only in lagunar milieux - whether hypo-or hyperhaline. such examples could be enlarged from protozoa to Tunicata, from blue algae to monocotyledons.
Appendix 2 given a list (incomplete) of “thalassoid” 1 species which are usually found in the Mediterranean lagunar milieux, or at least in those basins or portions of basins most influenced by the sea.
In the zones furthest from the sea, there is a more or less clear-cut transition between original communities which will be different from each other depending on whether the evaporitic environment or milieux with a freshwater tendency are being considered:
- in the evaporitic milieux the macrofauna often consists of only 1 or 2 species (such as Artemia salina), as does the microfauna, either benthic or planktonic (Bacteria, Cyanophyceae, Dunaliella salina-viridis). However, the importance should be mentioned, at least qualitatively, of the meiofauna (Rotifera, Nematodes, etc.) to be found even within the salt deposits (Basson et al., 1977). One should note that these species occur all over the world, and also inhabit the continental evaporitic milieux which, moreover, are likely to contribute to the explosion of typically continental species (insects, amphibians, etc.) during desalination phases (Perthuisot and Jauzein, 1975).
- in the milieux with a freshwater tendency, as the sea becomes more distant, typically continental species appear (insects, Pulmonata, Oligochaeta, etc.), without it being possible to define precisely the transition into a freshwater milieu.
- in the evaporitic milieux the macrofauna often consists of only 1 or 2 species (such as Artemia salina), as does the microflora either benthic or planktonic (Bacteris, Cyanophyceae, Dunaliella salina-viridis). One should however mention the importance at least qualitative of the microfauna (Rotifera, Nematodes, etc.) to be found even within the salt deposits (Basson et. al., 1977). It should be noted that these species occur all over the world and also inhabit the continental evaporitic milieux which, moreover, are likely to contribute to the explosion of typically continental speices (insects, amphibians etc.) during desalination phases (Perthuisot and Jauzein, 1975).
In the milieux with a freshwater tendency, as the sea becomes more distant, typically, continental species, appear (insects, pulmonata, Oligochaeta, etc.) without precise definition being possible of the transition into a freshwater milieu.
There are, however, some rare species capable of passing from evaporitic milieux into freshwater, such as the common eel, but its presence in these extreme milieux seems only transitory. Nevertheless, this shows a remarkable capacity of osmotic regulation. Some Rotifera seem to be totally insensitive to the concentration of the milieu since the same species can be found from 1 ‰ up to 350 ‰ (Ruttner-Kolisko, 1971).
Thus, the mere qualitative examination of the flora and fauna of paralic milieux shows their originality in comparison with the continental and maritime domains, particularly, owing to the presence of species particular to that milieu, i.e., strictly paralic.
A notable property of paralic milieux is their biological zoning particularly in the regions not for from the sea.
This characteristic is obvious for benthic communities which sooner or later integrate the minor fluctuations of the milieu (Guelorget et al., 1983). However, it will be seen that the other links in the trophic chains behave in a comparable way. Moreover, a study of the present-day benthos of paralic milieux is interesting from a geologist's point of view for in the paralic series, very often only the toughest portion of the benthic species remains.
The distribution of species in paralic milieux is generally longitudinal - leaving aside the local anomalies linked to the bathymetry or to the nature of the substratum - and roughly identical whether consider hypohaline or hyperhaline milieux.
(a) Plant communities
The above-mentioned property is obvious with the distribution of macrophytic species. In the Etang de Biguglia for instance, Zostera noltii is confined to the vicinity of the grau, directly under marine influence. Further south, a Ruppia spiralis community has developed, then a Potamogeton pectinatus/Characeas association where Chlorophyceae develop (Cladophora vagabunda, Chaetomorpha linum).
A similar distribution can be observed in the lagoons of the Louros delta (Logarou, Tsoukalio, Rodia) where Z. noltii prevails in the zones close to the sandbars; R. spiralis dominates the central zones, sometimes constituting mixed communities with Chara sp. in the zones further from the sea. The lagunar confines are colonized either by little-developed cyanobacterian fields or by Chlorophyceae (Ulva sp., Enteromorpha sp.).
The bottom of the Bahiret el Biban is carpeted with a vast cymodocea nodosa community, often associated with Caulerpa prolifera. The latter, in the marginal fringes, gives way to a R. spiralis community with fibrous Chlorophyceae (Chaetomorpha linum, Cladophora sp.), which in its turn is superseded by cyanobacterial fields.
In the same way, at Salin-de-Giraud (Fig. 14), the association of R. spiralis with Enteromorpha gr intestinalis which characterizes the first condensers gives way in the more concentrated enclosures, to Lyngbya aestuarii and Microcoleus chonoplastes algal fields, and beyond, to Oscillatoria laetevirens, Phormidium sp., and Spirulins subsalsa (Thomas, 1983).
A very interesting case in that of the Dybsø Fjord where a longitudinal zoning of the benthic macroflora can be noted in spite of the absence of any salinity gradient, which shows that the latter plays no part in the distribution of the macrophytic species in paralic milieux (Fig. 6).
(b) The invertebrate macrofauna
The study of a considerable number of paralic basins shows, whatever the salinity gradient, the following horizontal succession as the distance increases from the communication link with the sea:
- a pelecypod-dominated region, with a few echinoderms, i.e., associations which still have a “stenobionte” (thalassic species) tendency;
- a transitional region still dominated by pelecypods, but from which the echinoderms have disappeared. In this case, the marine influence is too slight to allow an optimal development of species with marine affinities, but still too strong form paralic species to flourish, fully. Local variations can however be introduced in cases of “organic pollution”. Then the number of pelecypods decreases to the advantage of crustaceans and detrivorous annelids. It is in this zone that the “mixed” species which are also present in the sea are found;
- a third region where strictly paralic species abound (Cerastoderma glaucum, Abra ovata, Nereis diversicolor, Gammarus gr locusta, Sphaeroma hookeri, Chironomidae).
A typical fourth region, common in hyperhaline milieux, less clearly so in hypohaline milieux (particularly in cold climates), is characterized by the presence of cyanobacterial fields or structures (algal fields, stromatoliths) mono - or paucispecific, associated with a small number of animal species (Hydrobia acuta, Sphaeroma rugicauda, Pirenella conica , Ammonia beccarii var. tepida).
Beyond these, are the extreme milieux. There, no zoning is obvious, neither in the almost freshwater, milieux, nor in the evaporitic milieux which remain little known. Perhaps the biological zoning continues at the level of the meiofauna but, for instance in a salt marsh, no notable qualitative difference can be observed between the communities in gypsum areas and those of the crystallizers, despite considerable differences in salinity between these two milieux.
As far as the benthic communities (macrofauna) are concerned, the distinctive scales (variety of species, density, biomass, production), can be observed to form gradients from the communication link(s) with the sea, towards the edges of the lagoon. Independently of the salinity field and the value of its gradient, the comparison between the Etang du prévost and the Bahiret el Biban is highly significant in this respect (Fig. 15) (Guelorget and Perthuisot, 1982). Nevertheless, as explained in detail later, in all the cases studied, and according to bibliographical data, the following can be observed progressively from the communication link with the sea:
- a significant decrease in variety of species;
- a progressive increase in the density of the invertebrate macrofauna, followed by a pronounced decrease near the freshwater pole, while the benthic macrofauna disappears completely in the vicinity of the evaporitic pole;
- a progressive decrease in biomass, for the increase in density accompanies a decrease in size (lagunar “dwarfism”);
- a drastic decrease in the overall production (calculated from the malacofauna which predominates in paralic milieux) falling from a maximum situated in the zones directly influenced by the sea (Guelorget et al., 1982);
- a parallel can be drawn with specific scales of the phytoplanktonic and ichthyological communities which present similar patterns.
There are cases where marine organisms reputedly stenohaline, such as echinoderms, live in saline concentrations obviously different from those of the sea.
This is the case in the Gulf of Salwa situated between Saudi Arabia and Qatar peninsula (Basson et. al., 1977), where at least three species of Stelleroides (Astropecten phragmosus, A. polyacanthus and Asterina burtoni “tolerate” salinities in excess of 60 ‰. In the same places live stonebass fish (Epinephelus tauvina), a common gastronomical species of the Persian Gulf, found at over 70 ‰.
Another similar case is the Vonitza Bay, in the Gulf of Amvrakikos, Greece, where a curious association of snakes and sea-urchins (Paracentrotus lividus) have been observed, at salinities which, measured in January and June 1982, do not exceed 5 ‰.
These two examples show that “osmotic regulation” is not a particular problem, even for species considered as not being among the most “stenohaline” (Appendix 1).
Another properly of paralic ecosystems is their extreme stability, when compared to marine or freshwater ecosystem. It is even possible to say that the more paralic an ecosystem is, and therefore situated in an unstable milieu, the more stable it is itself. This is obvious when one considers the margins of paralic basins (e.g., the Bahiret el Biban), which can be subjected, according to the seasons, to considerable variations in the salt content, and change from a generally hyperhaline milieu into a highly desalinated one. In the case of the Bahiret el Biban, pluri-annual floodings do not affect at all the communities in the marginal zones.
“Dystrophic crises” (“malaigues” = “bad waters” which affect the lagoons of the Languedoc coastline should also be mentioned: although they destroy instantly great numbers of individuals - especially those which enter the lagoon to feed - they do not destroy the ecosystem which recovers very quickly as soon as the crisis is over.
“Lagooning”, i.e., the discharge of effluents rich in organic matter into the lagoons, is yet another feature: the only consequence of this is apparently an increase in the production and the output of the ecosystem, whereas, in the open sea, such a process leads to a desertification of the sea bottom. Accordingly, it can be said that pollution of organic origin has little impact upon paralic milieux.
All things considered, this stability of paralic ecosystems is logical, since the species living in these milieux have hardly any regulatory problems - particularly osmotic ones - since they live in condition s (of salinity for example) which are both varied and variable.
The panorama of the paralic milieux outlined above shows that beyond the morphological, geochemical and sedimentological, diversities, these milieux present an undeniable biological unity: an originality and specificity of the communities, an independence of the qualitative and quantitative biological gradients with respect to salinity fields, and stability of their ecosystems. They are not “mixtures” of flora and fauna, some marine and others freshwater, but original communities having their own logic which is different from the marine or inland water logic.
Therefore a paralic milieu exists as an autonomous entity, distinct from the marine and inland water milieux, having its own structure end its own dynamics1.
Further, according to geological and biological data, two sub-systems can be observed:
- the sub-domain closest to the sea (Near paralic) is characterized by a geochemistry little different from that of the sea, by the large quantities of sediment involved in the biogenic phases (carbonates, organic matter), by essentially “thalassoid” communities, and by pronounced biological gradients;
- the sub-domain farthest from the sea (Far paralic) is characterized by a geochemistry radically different from that of the sea with the two poles that are also found in the continental domain: the evaporitic pole and the freshwater pole. The abiogenic sedimentary phases are dominant. The communities are “specialized”, original, and the biological gradients not very pronounced (unless proved to be the contrary by future studies).
The limit between these two domains is approximately immediately beyond the “algal fields”: when leaving the sea, this limit also corresponds to the disappearance of Foraminifera (Zaninetti, 1983).
The above-mentioned affirmations are not intended as dogma, but can be regarded as sufficiently innovative to be used in the following sections as the basic for certain deductions.
