The catalogue of natural paralic basins is not definite or homogeneous, and varies from one author to another. Moreover, the shape and size of the basin, and those of the channel communicating with the sea, depend on various factors which all belong to a sometimes complex geological history. It is not aimed to give yet another classification but rather to single out the main types.
The “estuary” type of lakes, situated more or less perpendicularly to the coast correspond to portions of valleys (either recent or fossil) flooded by the sea. To this type belong rias, fjords, deep creeks, etc. In some cases the river mouths are cleared by fluviatile currents and/or tides, while in other cases the mouth may be partially or totally obstructed by a barrier of sedimentary or other origin.
The lakes of the “lagunar” (sensu lato) type are generally more isodiametrical (circular in shape), or stretch parallel to the coast. Among them can be mentioned:
“Lagoons” (sensu stricto) which correspond to the isolation of certain portions of the maritime domain by one or several recent soft coastal sounds. There usually remains a communication link with the sea, more or less permanent thanks to one or several “graus” (more or less fixed, sometimes maintained by man). Generally, in this type of basin the water depth is low in relation to the surface area.
In arid countries intermittent stretches of water may alternate with “coastal” sebkhas (Perthuisot, 1975).
Delta areas often have lagoons whose formation is aided by a massive inflow of detrital debris, the pogradation of sedimentary bodies and coastal transit. In this category are situated brackish lagoons (for example in Palavas), delta lagoons, coral lagoons, etc.
“Bahira” (small sea in Arabic), diversely shaped, correspond to land-locked continental depressions of complex and various origin (river, wind, tectonic, etc.) developed or completed during the Quaternary era and flooded by the sea during the Holocene transgression. They communicate with the sea by permanent passages. The water depth varies but may be considerable in relation to the surface area.
To this type belong the Bahiret el Biban (Medhioub, 1979) in the southeast of Tunisia, the Etang de Diana in Corsica, etc.
Recent tectonic movements (subsidence and/or uplift) occurring in the vicinity of the waterline may lead to the isolation of a paralic stretch of water: in this case alone the term “tectonic lagoon” can be used. Examples are few; however one could mention the saltwater lake of Guemsah, Gulf of Suez (Ibrahim et al., 1982).
It is obvious that the above-mentioned types represent only the different extremes of typology. There exist a great number of intermediary types: thus the lagoon of Nador in Morocco or the Etang d'Drbino in Corsica fall between the “lagoon” type and the “bahira” type.
Lastly, there exist artificial stretches of paralic water corresponding to the development of natural paralic zones (salt marshes for example) or harbour structures.
(a) Three standard examples: the Etang du Prevost (France), the Etang d'Urbino (Corsica) and the Bahiret el Biban (Tunisia)
The hydrochemical diversity of the paralic milieux appears when considering only the examples taken around the Mediterranean: the Etang du prévost on the Languedoc coast (Guelorget and Michel, 1976), the Etange d'Urbino in western Corsica (Frisoni et al., 1982) and the Bahiret el Biban in southeast Tunisia (Medhioub, 1979; Medhioub and Perthuisot, 1977, 1981).
These three basins of very different sizes correspond to different morphologies and origins. The Etang du Prévost (3.8 km2, 1 m depth) is a lagoon in the strict sense of the word, i.e., a stretch of water progressively separated from the sea by a sandbar and still communicating with it through a “grau” (communication link). The Bahiret el Biban (230 km2, 6.5 m depth) is a Würmian continental depression flooded by Flandrian transgression; it communicates with the Gulf of Gabes through a series of channels the larger of which is a former cluse (transverse valley). The Etang d'Urbino (7.9 km2, 9 m depth) is a continental depression of a complex origin covered by and then separated from the sea by a sandbar intermittently sectioned by a grau.
If the Etang d'Urbino maintains a salinity close to that of the sea (Fig. 1), the other two basins are very different, one tending towards hypohaline (Fig. 2), the other hyperhaline (Fig. 3). In both cases, there exists a salinity gradient, variable according to the seasons, progressing from the channel communicating with the sea to the continental margins of the basins. The salinity gradient is negative in the Etang du Prévost and positive in the Bahiret el Biban. Naturally these differences in direction and value of the salinity gradients are related to the intensity of the exchange with the sea on the one hand, and on the other, with the total quantity of freshwater received by each basin (direct rainfall, continental inflows as opposed to evaporation). Moreover, the outermost edges are subject to local variations; inflows from tributaries, exchanges with supratidal waters.
(b) Etang de Biguglia (Fig. 4)
This lagoon on the western Corsican plain lies parallel to the sea, from which it is separated by a thin sandbar. The communication link with the sea is situated to the north at the end of a long narrow channel. To the south this lagoon communicates with the mouth of a small coastal river, the Golo, through a system of canals. It has a surface area of 1 450 ha, and a depth of about 1 m. Its average salinity varies between 5 and 26 ‰ however, values below 1 ‰ may be observed in the southern zone which is subject to continental inflows. Nearnormal values are to be found in the northern part of the basin where the influence of the sea is constant (Burelli et al., 1979).
(c) others
The study of other Mediterranean basins, unnecessary to describe here in detail, confirms the generality of salinity gradients in paralic milieux. The following can be mentioned:
- Lagoon of Nador (Mar Chica) (Frisoni et al., 1982)
- Bermuda Triangle (Spain) (Perthuisot et al., 1983)
- Lagoon of the Louros Delta: Tsoukalio, Rodia, Logarou (Greece) (Frisoni et al., 1982)
- Lake Melah (Algeria) (Guelorget et al., in preparation)
Several further examples of paralic milieux, in very different climatic contexts, are discussed below:
(a) Danish fjords
The lagoon of the Dybsø Fjord was described by Muus (1967) in an outstanding study dealing with the entire system of Danish lagoons and estuaries. The Dybsø Fjord is a lagoon opening onto the Baltic Sea in the south of the Danish Sjaelland; it has a surface area of about 1 700 ha and an average depth of 1.50 m. The fjord communicates with the sea through a narrow channel, 3 – 4 m deep, opening to the west onto the lagoon. In spite of several small tributaries, there is no salinity gradient, with values remaining relatively stable around an average of 10 ‰ (Figs. 5 and 6).
The Kysing Fjord is an estuary of 186 ha situated on the east coast of Jutland. This fjord, also studied by Muus (1967), constitutes a basin lying eastwest, with an average depth of 0.60 m; however, along the western shore, there is a channel whose depth in places reaches 2 m.
The channel linking the Kysing Fjord with the sea is narrow and winding with an average depth of around 4.50 m.
The bed of the fjord may be divided into three distinct zones:
- A sandy zone, devoid of vegetation: this zone regularly dries out during low tides;
- Further west, a central sand-mud zone settled by Zostera and Ruppia water plants;
- Finally, at the western extremity of the fjord, a muddy zone where in summer Ulva lactuca covers all that part of the fjord.
The waters of the River Odder Å flow into the Kysing Fjord at its western extremity. This freshwater supply results in a salinity gradient varying from 0 ‰ at the mouth of the river to 24 ‰ at the channel. However, the major part of the fjord has a salinity of around 19–21 ‰ (Fig. 7).
(b) Mangrove swamps in Guadeloupe
The “Manche à Eau” and “Belle Plaine” are two mangrove swamps in Guadeloupe (Gaujous, 1981). These two paralic basins, situated at the end of the “Grand Cul de Sac Marin” which separates Basse Terre and Grande Terre, are where mangroves reach their maximum development (Fig. 8). The “Manche à Eau” has a surface area of 26 ha. Its depth, between 1.50 and 2 m, is uniform for the whole stretch of water. The lagoon bed is bare and consists of a sandy mud extremely rich in organic matter. The lagoon communicates with the sea by a system of branching channels flowing into the Rivière Salée which in its turn flows into the lagoon. The freshwater comes all round the lagoon through diffuse trickling.
This hydrological context results in a salinity gradient averaging between 30 and 25 ‰ (Fig. 9).
The lagoon of “Belle Plaine” has a surface area of 18 ha. It is much more open to the sea than the “Mancha à Eau”; communicating directly with the lagoon through two short channels. Unlike the “Manche à Eau”, the freshwater supplies are concentrated in the “Belle Plaine” channel which drains the mangrove zones and beyond for more than 3 km. This sizable continental water supply flows in at a central point on the eastern shore. Consequently, the wide opening of the lagoon to the sea results in a uniform salinity (30 ‰) almost throughout the basin, except at the very mouth of the “Belle Plaine” channel, where the salinity falls drastically to levels below 10 ‰ (Fig. 9).
The average depth of this lagoon is between 1.50 and 2 m and the sand/mud bed is devoid of vegetation.
(c) A lagoon in the Persian Gulf
The Khour el Aadid with a surface area of around 100 km 2 is situated between the Emirates of Qatar and Abu Dhabi. It is a Flandrian “bahira” progressively reduced by a sandy spit which leaves only a long narrow channel between the Persian Gulf and the first of a succession of three basins. The rarity of rain results in a salinity gradient with levels of salts reaching over 110 g/l in the distal basin which, moreover, is totally covered by a thick layer of Cyanophyceae (Perthuisot and Jauzein, 1978) (Fig. 10).
When paralic basins stretch inland far from the open sea, or when climatic conditions become extreme, the tendencies described above are intensified.
(a) Bocana de Virrila (Fig. 11)
The Bocana de Virrila in Peru is an interesting case, although little known. It is a relict estuary in a particularly arid region, reaching inland over a distance of 20 km with a maximum width of 2 km (morris and Dickey, 1957). The water concentration increases from the mouth up to the continental extremity of the stretch of water and it is possible do define two milieux separated by shallows. In the part closest to the sea, there is a “penesaline” environment, and in the distal part, which is very shallow a “saline” or evaporitic environment where gypsum then halite are precipitated: here, concentrations reach over 400 g/l. Moreover, the lower layers of water are more concentrated than the surface waters, which reveal a certain stratification 1.
At present the Bocana de Virrila seems to be the only paralic stretch of water in direct communication with the sea where halite can precipitate (Perthuisot, 1980).
(b) Example of salt-marsh: Salin-de-Girau (Camargue)
Artificial salt marshes present an organization similar to that of the Bocana de Virrila (Fig. 12) (Perthuisot, 1983) but here, the salinity gradient is artificially established by specialists who adjust the water transfers from one condenser to the other according to weather conditions.
(c) The Baltic Sea
Finally, an example totally opposed to those already studied should be mentioned: that of the Baltic Sea and its annexes (Gulf of Bothnia, Gulf of Finland). Here, freshwater supplies are numerous and very little evaporation takes place: this leads to a concentration gradient showing waters with a salt content of below 1 g/l, and it is possible to pass from the waters of the North Sea, that is, from a marine environment, albeit with a lower-than-average salt content, to an almost freshwater milieu (lacustrian) with no transition (Fig. 13).
The few above-mentioned examples show the diversity of morphological, hydrological and climatic conditions which lead to extreme diversity and variability of salinity scales and geochemical gradients in paralic milieux. However, a general characteristic of these salinity fields seems to emerge: that is their longitudinal organization from the communication link (on the zone of marine supply) towards the continental margins. This organization, particularly obvious in very elongated basins, supplied by the sea at one of their ends, is less schematically obvious in the more isodiametric basins where it assumes more concentric shapes. It should also be noted that, locally, longitudinal gradients may be accompanied by vertical gradients which tend to establish a true stratification of the waters (a phenomenon well known in the Black Sea, for example) with appearance of clines (haline, thermic, etc.).
This wide geochemical diversity - which does not affect salinity alone, but all the mineral components of water - is the most striking aspect of paralic milieux: it has led many authors to propose classifications based on slinities. Their complete description will be found in Petit (1953), Ancona (1954), Aguesse (1957), Petit and Schachter (1959), Segerstråle (1959), Mars (1966), Amanieu (1967), Arnaud and Raimbault (1969), and Marazanof (1972).
The Venice Symposium (1958) adopted a fairly simple classification known as the “Vencie System”, which defines a series of standard waters characterized by an average salinity varying from 0.5 ‰ for limnic waters to over 40 ‰ for hyperhaline waters. (This type of classification is discussed later). Geochemical gradients (essentially salinity, but also other scales such as ionic concentrations, or hydrochemical parameters) depend not only upon the hydrodynamic parameters of each basin, but also local climatic and hydrographic characteristics.
This diversity is apparent by simply considering the difference between estuaries where sedimentation is low and the deltas which are characterized by rapid deposit of large quantities of detrital matter. In the “lagoon” type milieux, sedimentation may vary considerably from one basin to another, according to local conditions: in the Etang du Prevost, for example, the detrital fraction of the sand class predominates along the maritime shore; the opposite occurs in the Bahiret el Biban because of wind-borne deposits on the continental margin. Therefore, their qualitative importance, the nature and the granulometry of the detrital phase, depend upon local of regional transport agents, the importance of the winnowing of the peripheral zones of the basins, the lithological characteristics of the adjacent region, etc.
The biodetrital phase is often considerable, especially in the zones closest to the sea.
The biochemical carbonated phase is strongly modified by bioclimatic conditions; sedimentation of organic matter is also extremely variable according to its origin (autochthonous or allochthonous) and the conditions of its conservation (presence of reducing milieux or brines, etc.).
It is in the extreme paralic milieux that the best examples of sedimentary diversity are seen, since, depending on local and regional physiography either evaporites or on the contrary very coarse detrital debris can be found.
It may even be that both types of sediment lie aide by side in the same paralic environment; for example, the Camargue with its salt marshes, but also its quasi-lacustrian basins.
In general, the sedimentology of paralic milieux is a result of two types of organization which are more or less superimposed:
- a longitudinal organization in parallel with geochemical gradients, in some cases reinforced by the distribution of affluents;
- a concentric organization depending mainly on depth; this too may be reinforced or counteracted by the disposition of the tributaries.
According to local conditions either organization can prevail, but all types of intermediary situations exist between purely longitudinal organization and purely concentric ones (Perthuisot, 1975, 1980; Busson and Perthuisot, 1977).
Thus, to the geochemical diversity of the paralic milieux can be added the extreme variability of the sedimentary deposits they contain: both are example of the variability of climatic hydrological and morphological conditions.
However, it is possible to differentiate schematically two types of paralic milieux or areas:
- In the basins or the portions of basins closed to the sea, the biogenic phases 1 - biodetrital and/or biochemical - prevail; the geochemistry is moderately different from that of sea water.
- In the Portions of basins furthest away from the sea, the abiogenic mineral phases becomes predominant whether one considers the hyperhaline milieux or where evaporites deposit, or the heavily hypohaline quasi freshwater milieux where ferrigenous deposits - associated or not with vegetal debris - constitute practically the totality of the sediment.
