|Volume of solution||d° Balmé||Density||Concentration|
|Normal sea water||1000||3,6||1,0257||37,1||36||20,5||0,58|
|saturation in gypsum||240||14||1,107||154||140||85,6||2,41|
|Saturation in halite (rock salt)||110||25,6||1,216||323||266||186||5,25|
(P) indicates strictly paralic species
Chaetomorpha linum (P)
C. vagabunda (P)
Ulva lactuca (P)
Gracilaria verrucosa (P)
Neogoniolithon notarisii (P?)
Althenia filiformis (P)
Potamogeton pectinatus (P)
Ruppia spiralis (P)
1. Soft bottom
Cereoratulus marginatus (P)
Mercierrella enigmatica (P)
Nereis diversicolor (P)
Akera bullata (P)
Bittium reticulatum (P)
Hydrobia acuta (P)
Hydrobia ventrosa (P)
Nassarius (Hinia) mamillata
Natica (Neverita) josephina (P)
Pirenella conica (P)
Urosalpinx (Ocinebra) edwardsii
Abra ovata (P)
Brachydontes marioni (P)
Cerastoderma glacum (P)
Cerastoderma exiguum (P)
Corbula (Varicorbula) gibba
Corbula (Lentidium) mediterranea
M. corallina var. stultorum
V. gallina var. radiata
Carcinus mediterraneus (P)
Corophium insidiosum (P)
Cymodoce truncata (P?)
Gammarus aequicauda (P)
Gammarus insensibilis (P)
Idothea balthica basteri
Microbeutopus grayllotalpa (P)
Sphaeroma hookeri (P)
Dipterous insects (larvae)
2. Hard bottom
Mercierella enigmatica (P)
Conopeum seurati (P)
Cerasioderma glaucum (P)
Balanus amphitrite amphitrite (P)
Corophium insidiosum (P)
Gammarus aequicauda (P)
Gammarus insensibilis (P)
Microdeutopus gryllotalpa (P)
Idothea balthica (P?)
Sphaeroma hookeri (P)
1. SALINITY TOLERANCE, EURYHALINITY, STENOHALINITY
In ecology “tolerance” of species with regard to salinity is often mentioned, and species are defined as “stenohaline”, supposedly tolerating only very minor fluctuations in salinity, as opposed to species defined as “euryhaline”; capable of adapting to much more variable saline concentrations.
As far as thalassoid species are concerned, these distinctions must be totally re-examined since, for example, sea urchins (Paracentrotus lividus which are generally considered to be particularly stenohaline, are to be found in “abnormally” low salinity conditions (Vonitza Bay). Moreover, even for euryhaline species, the questions remains as to whether a salinity limit really exists. There has been a recent study on brine pools situated in a sebkha of the Gulf of Suez fed by subterranean marine inflows: these pools contains an associations of Cyanophyceae grazed upon by Pirenella conica and small fish (Aphanius?) which, when distributed, hurry to concealment beneath salt rafts. The salinity concentration is near saturation (350 g/l). Thus the ranges of salinity indicated by the authors for any thalassoid species result above all from their personal experience.
Perhaps the terms “stenohaline” and “euryhaline” are applicable to continental species, but the observation of endoreic basins passing from almost freshwaters to evaporitic brines casts a certain doubt on this.
In the authors' opinion, these terms are erroneously precise, and should be abandoned.
2. CONFINED MILIEUX VS REDUCING MILIEUX
According to the definition of confinement as used here, a confined environment is not synonymous with a reducing environment in spite of the anoxia (lack of oxygen) implied by the terms confinement - from a very anthropomorphic point of view.
However, in the paralic domain, confinement often leads to the reducing character of the milieu, notably in the region of the bed, for different reasons:
- Lateral variations of salinity and therefore of density are liable to induce the stratification of successive layers of water (one above the other), which contributes to the anoxia of the lower layer. This phenomenon occurs particularly in hyperhaline milieux and/or when a salt “wedge” (coin salé) develops where freshwater and seawater are in contact (estuaries, delta channels, mangroves).
- The generally slow hydrodynamic quality of paralic basins increases this phenomenon, for it tends to stabilize bodies of water; this is obviously not the case in estuaries where salt zones move with the tide and the flow of the river. Estuaries are rarely anoxic milieux.
- The temperature and salinity of the waters limit oxygen solubility, thus lessening the effects of photosynthesis. Paralic milieux in hot and/or arid climates are more affected by anoxia than elsewhere.
- Intense biological production, which may be implemented by continental inflows, results in considerable sedimentation of organic matter. The saprophytic microorganisms which consume it contribute to the anoxia of the milieu by their respiration and possibly by the production of reduced compounds (H2S, CH4, NH4).
In general, reducing milieux are more frequent in the Far paralic and in environments with evaporitic tendencies.
3. LAGUNAR DWARFISM AND GIGANTISM
Paralic species and the forms of mixed species near their limit of confinement are generally small compared either with neighbouring thalassic species, or with the forms of mixed species in slightly confined zones. “Lagunar dwarfism” is often referred to.
A possible reason for this property in species undergoing severe confinement and often “abnormal” salinity levels, might appear to be the energy expended in “contending” with this situation. This anthropomorphic explanation is not satisfactory, for thalassic species living in conditions of “abnormal” salinity are not particularly small in size. It seems rather that the effort to reproduce in paralic populations (cenotic strategy type; according to Brandin et al., 1976) take place to the detriment of growth and thus of individual size: this could be one of the biological manifestations of deficiency connected with confinement. Moreover, and especially for mixed species near their limit of confinement, infantile mortality may contribute to the rarity of well developed adult forms (lagunar infantilism).
Finally, lagunar gigantism is occasionally referred to: this concerns, only the mixed species in relatively little-confined zones where there is no problem of deficiency and where the abundance of food contributes to faster growth than at sea. Examples are oysters, and especially mussels, often much larger in lagoons than their equivalent in a marine environment, or again sea-bream (Sparus aurata) which for example in a lagoon north of Tunis reach a weight of over 1 kg in two years, while the sea bream remaining in the Bay of Tunis weigh only a little over 200 g at the same age (Chauvet, 1979).
4. MIXED SPECIES
“Mixed” species here refers to those which may be found both in the marine and paralic domains, without going into detail as regards classification.
These species enter the paralic milieu in two different ways:
- Passive establishment of all species of phytoplankton (holoplanktonic species) and of certain beneath species whose metroplanktonic larvae are brought in, along with the former, by the action of currents and tides.
- Active establishment of ichthyofauna (migrant) species which penetrate the paralic environment for feeding or reproduction purposes.
Thus the word “mixed” applied to thalassoid species found in paralic milieux does not have exactly the same meaning when applied to different biological aspects, in particular depending on the mobility of the different species concerned.
5. ABIOTIC PARAMETERS
Abiotic parameters are all the non-biological parameters which participate in the functioning of the ecosystem. As far as paralic milieux are concerned, and taking into account their morphological characteristics (bathydmetry, surface area), direct contacts with the numerous adjacent ecosystems) the most significant parameters appear to be:
- Hydrological, parameters (ionic concentrations, temperature, pH, levels of dissolved oxygen, turbidity, content of nutritive salts).
- Sedimentological parameters (granulometry, biodetrital phases, pelite content, mineralogy of the carbonated matrix, content of organic matter).
5.1 HYDROLOGICAL PARAMETERS
(a) As far as ionic concentrations of the major elements are concentrated (salinity values of different ion ratios), it has been amply proved that they have no influence whatever on the organization and biological structure of paralic ecosystems. Nevertheless, if local climatic and hydrographic data are taken into consideration, their study often permits an approach to the hydrological system.
(b) Temperature is obviously an important ecological factor, especially in these generally very shallow environments. It influences particularly the biological cycles of the majority of the species involved: nutritional activity (filtration rate, for example). sexual maturity (period and numbers attracted) 1.
1 The maintenance at adequate temperatures by artificial means of species which in their natural habitat are exposed to marked seasonal temperature variations, permits an early gonadial maturation and the production in hatcheries of several successive layings in one year.
(c) The turbidity of paralic milieux is dependent upon phytopanktonic activity (green waters, and red waters, in salt production condensers) and upon the concentration in the water of mineral particles in suspension as a result of the disturbance of the milieu. Whatever its origin, high turbidity is a habitual characteristic of paralic milieux. There are, however, basins where it remains low (lagoons in arid climates): the biological structure of the ecosystem is not modified because of this.
(d) The pH and the level of dissolved oxygen are measurements which are very complex to determine, in which salinity, temperature, water disturbance and various biological considerations (chlorophyll synthesis, respiration, bacterial reduction, etc.) intervene. Except in extreme examples - in particular evaporitic zones - or in a environments subject to dystrophic crises in summer, the values of these parameters remain within a range such that they have little or no influence on the biological organization of the ecosystem.
(e) The importance of nutritive salts (phosphate and nitrogen compounds) is emphasized, particularly in basins receiving continental inflows. Even if there is no doubt that these salts affect the activity of chlorophyll organisms, they have only a minor influence on the specific qualitative composition of the plant populations, since these are identical both in paralic basins receiving continental inflows and in those which do not.
In the natural paralic domain, one can assume that the notion of “limiting factor” is illusory, otherwise fundamental qualitative and quantitative differences in plant populations would be observed in the different basins: the nutritive elements necessary for the proliferation of plant species are always present in sufficient quantities in natural paralic basins. On the other hand, it is likely that many hydrochemical parameters (levels of trace elements, of discrete organic compounds, etc.) intervene in the organization of paralic ecosystems. Knowledge of these parameters still remains imperfect both in the laboratory and in the field; furthermore all the chemical substances which intervene in confinement and their respective roles are still unknown.
5.2 SEDIMENTOLOGICAL PARAMETERS
(a) In the majority of cases, the detritic, elements of the paralic sediments are fine of very fine (sands, pelites, clays). Added to this is the extent of the carbonated biochemical and organic phase which is always extremely fine. In general, mean granulometry decreases significantly from zones under marine influence towards the continental shores, moreover the winnowing of the edges tends to concentrate the finer particles in the lowest zones. In the paralic milieu, it is however possible to find granulometric anomalies sometimes linked with coarse continental inflows, but most often due to the biodetrial phase (Foraminifera sands, accumulation of test debris, Lithothamnion facies, Hydrobiidae levels, Cardium muds, etc.). Thus the majority of lateral variation in the granulometry of paralic sediments is in relation with the field of confinement, notably via the biodetrital phase.
(b) Organic enrichment is a fundamental characteristic of paralic basins except when the system of current prevents this, particularly in the case of estuaries. It is the result of trapping organic matter of marine and/or continental origin, and the intrinsic production of the milieu (it is possible to measure up to 60% of dry organic matter in paralic sediments).
The amount of organic matter in the superficial sediment is often a good indication of confinement. Moreover, local accumulation of organic matter for diverse reasons (hydraulic bubble, intensive aquaculture) results in a notable transformation of the benthic populations and. in particular, in the development of detritivorous species at the expense of suspension feeders. The interest of measuring this parameter is thus undeniable.
(c) The mineralogical composition (qualitative and quantitative) of the carbonated matrix depends on various factors among which rank temperature, salinity, the photosynthetic activity of the milieu, the activity of the bacterian flora, the mineralogical composition of the tests on species living in the milieu, etc. Moreover, and this complicates the situation, certain types of minerals, such as aragonite and higher magnesian calcite, are metastable under natural conditions and are transformed at speeds varying with the chemical characteristics of the environment: carbonates are extremely elusive.
In the present-day, Near paralic milieux, associations with calcite, highly magnesian calcite and aragonite are very generally found. The variations of relative proportions of these minerals, along with the Mg content in the calcites are likely to provide indications as to the milieu. Thus, for example, high aragonite content can indicate zones of contact between bodies of water of different confinement levels (Perthuisot et al., 1983; Frisoni et al., 1983) but the spatial variations of the relative proportions of the different types of minerals in these milieux are difficult to interpret in detail (Fig. 37) given the present level of knowledge.
In the Far paralic (beyond zone VI) in the evaporitic or pre-evaporitic milieux, highly magnesian carbonates, dolomite, huntite, magnesite appear (Perthuisot, 1971, 1975, 1980) while in freshwater milieux, the carbonated phase is generally calcic (calcite, aragonite).
6. THE ROLE AND EXPRESSION OF CONFINEMENT IN THE MANGROVE LAGOONS: BELLE PLAINE AND MANCHE A EAU (GUADELOUPE 1 From Gaujous (1981) and Gaujous and Guelorget (1983)
1 Section 1 of the main text discusses these two paralic basins
The sedimentological and hydrological parameters measured at the different stations (temperature, salinity, dissolved oxygen, pH, bathymetry and vegetable debris content of sediments) were analysed together into principal components. From this, it is clear that the various stations fall into two distinct groups corresponding to each of the lagoons (Figs. 38, 39 and 40). The lagoon of Belle Plaine has on the whole warmer and more saline waters and higher pH, than those of Manche à Eau: the increase in the value of these parameters appears on the chart (Figs. 39 and 40) as a shift to the northeast (towards the right and upwards). This can be explained by the greater influence of marine waters in the lagoon at Belle Plaine, while Manche à Eau communicates with the sea only by a complex system of canals. However, within each group a southeast/northwest gradient can be observed going from the stations close to the sea (2, 9, 10), which are low in oxygen, shallow, and rich in vegetable dibris: this hydrological and sedimentological gradient is one of the expressions of the confinement gradient.
A similarity study (Legendre and Legendre, 1979) carried out at the same stations comparing the densities of each zoological group (echinoderms, crustaceans, gastropods, pelecypods, annelids) gives a totally different picture from the preceding one (Fig. 41). The stations form three groups subdividing the previous ones:
- A first group includes the stations (1, 6, 8, 7, 11) where all the zoological groups are represented. In particular, this is the only group where echinoderms are present.
- A second group stations (2, 4, 9, 10) where only annelids and pelecypods are present.
- Lastly, stations 3 and 5 where very few thalassoid species and individuals are to be found.
The first group corresponds to stations under pronounced marine influence (zone II), the second group to typical paralic stations (zones III and IV), and stations 3 and 5 marginal backwaters (zone V) where the fauna is considerably impoverished.
Thus, even the mathematical treatment of the abiotic data on the one hand and the biological data on the other, shows that the biological organization of paralic milieux is dependent of the physiochemical characteristics of the milieu, and follows exactly the confinement gradient. This is expressed, in each of the mangrove basins studied, by apparently concording salinity gradients, levels of dissolved oxygen, and vegetable debris contents, but the comparison of the two basins shows that there is not an unequivocal relation between the value of one or other of these parameters and the confinement level.
If, from among these parameters, the best indicators of confinement has to be singled out, the vegetable debris content in the sediment would emerge: this extrinsic factor measures the continental influence at a given point, owing to the omnipresence of an abundant continental biomass. Nevertheless, it is emphasized that if the quantity of vegetable debris gives a clear picture of the confinement in this type if milieu, the other abiotic parameters (dissolved oxygen, pH) are closely subordinated to it.
7. THE CASPIAN SEA
The Caspian Sea is a unique example of a basin of marine origin separated from the sea a long time ago geologically, and which still shows all the qualitative and quantitative abiotic and biological characteristics of the paralic domain. Zenkevitch and other Soviet authors (Zenkevitch, 1957) have made interesting studies.
The Caspian Sea is 1 200 km long, between 200 an d500 km wide, with a surface of 436 000 km2 and a volume of 77 000 km3. It comprises three regions (Fig. 42): the northern basin, which is very shallow and receives the majority of the continental water inflow notably from the Volga and the Ural rivers; the central basin reaches over 700 m in depth and flows particularly into the Kara Bogaz Gol Bay, where evaporates are deposited (Dzens-Litovskij, 1956, 1962); the southern basin which also belongs partly ton Iran, reaches over 900 m in depth.
Form the hydrological point of view, the same divisions as above are found and each basin has its own hydrodynamic system, as each large mass of water rotates upon itself (Fig. 43). Hydrous exchanges between the different basins seem to be above all due to movements of perilittoral drift.
The content of salts dissolved in the surface waters follows a north-south gradient with freshwaters at the mouths of the large rivers coming from the north, and obviously in the peripheral evaporitic annexes (Fig. 44). The two deep basins have very homogeneous waters from the surface down to the bottom, with concentrations of between 12 and 13 g/l. The chemical composition of the waters of the Caspian Sea does not (or no longer correspond (s) to a dilute seawater, notably by its richness in sulphate ion.
The flora and funa comprise a small number (around 400) of species of macrofauna 1, among which three groups can be distinguished.
1 The determinations of these species are for the most part outdated, and systematic classification has greatly evolved in the last few decades. Because of this, the specific denominations used may, in certain cases, be unreliable. In the interpretation of Zenkevitch's data, the generic classification has been maintained.
- Common freshwater species are principally localized in the extreme north of the basin (Abramis brama, Salmo trutta, Cyprinidae, Lucioperca lucioperca, Eriocheir sinensis, Astacus);
- Common paralic species:
|Phytoplankton:||Exuviella, Rhizosolenia, Skeletonema|
|Phytobenthos:||Potamogeton pectinatus, Ruppis spiralis, Zostera noltii|
|Zoobenthos:||Cerastoderma glaucum, Corophium, Nereis succinea, Pontogammarus, Mytilaster|
|Fish:||Atherina, Syngnathus, Pomatoschistus, Gobius|
- Endemic species - 60% of the fauna is endemic including for instance certain Cardiidae (Adacna, Mondacna, Didacna) the Micromelaniidae, a great number of crustaceans, and among the fish, Clupeidae (Caspialosa, Clupeonella and Acipenser stellatus. These endemic species appear to derive from common paralic species, such as the Micromelaniidae which some consider as Hydrobiidae, or mixed (migrant) species, such as Leander, the Acipenseridae, Clupeidae, Pleuronectidae, Petronyzonidae, Mugilidae.
From these data, so general hat they tend to conceal the real diversity of the Basin, it is reasonable to try to establish coherent zoning in terms of confinement. The absence of echinoderms (Table 7) and a great number of other thalassic groups (Radiolaria, corneous and siliceous sponges, Siphonophora, Anthozoa, Ctenophora, Nemertea, cephalopods, Tunicata) shows that the basin as a whole is situated beyond zone II on the scale of confinement of the Mediterranean paralic domain.
The hydrochemical gradients, the presence of freshwater populations in the northenmost part of the basin and the presence of strictly paralic species (Cerastoderma glaucum, Nereis succinea lead to the conclusion that the Caspian Sea occupies confinement zones III, IV and V and VI, with a fringe situated in the Far paralic, the Volga Delta and similar regions (freshwater pole) Kara Bogaz Gol (evaporitic pole). Furthermore, given the elongated shape of the basin, the disposition of the continental inflows and the bathymetric profile, the possibility of a confinement gradient increasing from the waters of the central and southern basins towards the northern limits can fairly safely be assumed.
The quantitative biological data given in Zenkevitch's publication permit this zoning to be specified:
The map of benthic biomass distribution (Fig. 45) shows a maximum of biomass in a curved zone between the northern and central basins, as well as along the northwest and northeast shores of the latter. This region corresponds clearly to zone III, a zone where, in the paralic domain, the benthic biomass generally - if not always - reaches its maximum.
The map of the distribution of Nereis succinea biomass (Fig. 46) traces the outlines of zones IV and V very exactly: this species, which is strictly paralic, characterizes these two zones. It is probably in this region that Zenkevitch notes the phytoplanktonic blooms which are practically exclusive to Exuviella (a strictly paralic species, zone V) sometimes reaching 4.5 to 6.5 g/m3: these blooms clearly indicate the region of maximal phytoplanktonic biomass. Here, in a basin separated from the marine domain, is evident the organizational system of the paralic domain, and, in particular, the remarkable feature of the crossing of the benthic biomass curve and that of the phytoplanktonic biomass in Zone IV (Fig. 47).
Thus, in spite of the many centuries since it was in contact with the marine domain, the Caspian Sea is far from being totally “continetalized”: the qualitative and quantitative organization of the paralic domain is found there. It seems that this phenomenon is linked to the greater depth of the basin which has still not had time to become highly confined since its separation from the marine domain, except in the deepest parts of the basin which are affected by bathymetric confinement and anoxia (Fig. 48) as shown by the very low levels of biomass encountered.
Thus, similarly to normal paralic basins where the sea represents a vitalizing source, the upper water level in the two deep basins of the Caspian sea acts as a vitalizing reserve for the more confined zones, and the organization of the confinement fields depends on it.
8. LIVING FOSSILS AND PANCHRONIC FORMS
The stability of paralic settlements and species have been mentioned. It seems that a certain number of coincidences in present-day nature tend to indicate that the paralic domain could represent a “sanctuary” or a “depository” of panchronic forms and living fossils. This idea is based on several considerations:
- the similarity and morphological invariability of paralic molluscs and crustaceans since the beginning of the Tertiary era and for certain among them, since the beginning of the Secondary era;
- the presence, particularly in the Far paralic, of archetypal forms such as the phyllopod Artemia salina, the mystococarid Derocheilocaris, the archaic Chlorophyceae Dunaliella salina the agnathe Petromyzon, the white whales in Saint Lawrence Bay;
- the obligatory biological relationship, for tropical or genetic reasons, between the paralic domain and species descending from very ancient and not highly evolutive origins. Examples are “green” turtles, the Galapagos iguana, sturgeon, eels, sea lice (Polyphemus whose larval form is said to be trilobitic);
- the obligatory palebiological relationship, in what was the newly-born paralic domain, of species nowadays separated by the effects of global tectonics.
Obviously there is the problem of the “panchronic forms” and the “living fossils” which in present-day nature are not clearly geographically linked to the paralic domain. The coelacanth (Crossopterygian, can be mentioned, certain species which inhanbit medio-oceanic rifts (Pognophora, the Monoplacophore Neopilina galathea, the Nautilus, etc.
It is noticeable that in most cases, in contrast with species developed for deep-sea living (bathyal and abyssal forms), these species do not present morphological pr anatomical characteristics corresponding to their natural environment (Latimeria's large eyes and “legs” which seem incongruous in its habitat and which would tend to indicate a coastal ethology). Moreover, these species are found in limited, often highly restricted geographical areas, and what is more, present anomalous hydrological characteristics. There are paralic basins isolated in the interior of continents (the Caspian Sea, Fayoum). Could they not have counterparts here and there in the midst of the marine domain, relict paralic bubbles, a heritage from the geological past of the Earth?
Vecelet (1983) proposes that submarine caverns conserve living fossils.
9. PALEONTOLOGY, CONFINEMENT, AND EVAPORITIC DEPOSITS
The fact that there is no unequivocal relationship between salinity and confinement leads to doubt about the relationship between biological activity and evaporitic sedimentation, that is, concerning the chronological relationship between saline deposits and biogenic facies.
First, and examination of the paleontological content of paralic formation (associated or otherwise with evaporites) does not permit the reconstitution of the salinity of their place of deposit. Thus the classification of a large number of paralic formations from the Fresh Paleogene as having a brackish water facies, i.e., hypohaline, needs to be totally re-examined, particularly in cases which are associated with evaporites. Other arguments, notably geochemical and isotopic, might no doubt be better used, although the latter should be subject to great caution (Jauzein, 1982).
Even the attempt at reconstitution of paleosalinities from fossil ichthyofauna, which are often remarkably conserved in paralic formations (Gaudant, 1982) runs into two problems:
- incomplete knowledge of present-day natural phenomena where much still remains to be discovered, and where many surprises are possible, in particular concerning “salinity tolerances” of a given species;
- the geographical and/or ethological drift, always possible, in any case always conceivable, of descendence in the course of geological time.
Even obviously thalassic fossil settlements are not indicators of a “normal” salinity of the environment, since in present-day natural conditional, typically thalassic flora and fauna are to be found in milieux deviating appreciably from the marine “norm”. Moreover, in the case of large deep basing where communication with the sea is becoming limited, climatic and hydrographical conditions may be such that the concentration of the waters by evaporation takes place more rapidly than the progression of the confinement of the basin; depth is an important factor here, since in zones of great depth, biological activity impoverishes the milieu at only a very slow rate. It becomes possible to imagine highly hyperhaline basins where communities characteristic of low confinement still thrive, even thalassic communities. These considerations perhaps explain the presence of Coccolithophorida planktonic nanoflora in the Mediterranean Messinian (Rouchy, 1981). In fact, the possibility of a growth of reef-type structures in basins depositing evaporites cannot be completely excluded: if this is hardly conceivable for coral reefs, the majority of Coelenterata being highly sensitive to confinement, it is however more so for Stromatoporidae or Cyanobacteria. The latter, for example, form reef-type structures contained within the Miocene gypseous series of the Gulf of Suez.
10. ALGAL PRODUCTION IN THE ETANG DU PREVOST
Riouall (1976) establishes monthly maps of the macrophytic biomass distribution in the Etang du Prévost (Fig. 49) and thus calculates the total lagunar algal biomass (Table 8) and the specific lagunar biomass for the four principal types (Ulva, Enteromorpha, Stictyosiphon, and Gracilaria) (Table 9). He estimates lagunar productivity at around 6 900 t (drained wet weight) during October (Fig, 50) giving a yield of 25 t/ha. This yield may reach 50 t/ha in certain zones. He calculates the lagunar production during the rest of the year (20 t/ha) at 5 400 t, giving a total for the whole year of 12 300 t (47 t/ha). Converted into weight of dry matter, the total lagunar plant productivity reaches 2 000 t, representing a yield of 7 t/ha.
For comparison, Mercier (1973) evaluates the productivity of the entire lagunar complex of Bages-Sigean at 1 t of dry matter per hectare (1.4 to 2 t/h for the surfaces alone covered by vegetation, 4 t/h of dry matter for the richest vegetations). There too, the dominant types of algae are Ulva and Enteromorpha.
11. THE INTERTIDAL ZONE OF ROCKY COASTLINES
Many studies have been made of the biological zoning of rocky coasts; to give an example, a description follows of he eastern coasts of the USA (Doty, 1957):
The so-called sublittoral fringe, situated in the vicinity of the lowest tideline, i characterized by a flora of Laminaria associated with Zostera and Phillopadix. The highly diversified fauns includes clearly thalassic groups, Nudibranchia, Tectibranchia, echinoderms, etc.
The coastal zone possesses a different flora and fauna, with fewer species represented (Anthopleura elegantissima, Pahygrapsus crassipes, Pagurus granosimanus, Thais emerginata, etc.). The lower part of this zone is characterized by Mytilus californianus, Iridophycus coriaceum, Mitella polymerus, Tegula brunnea. The upper part still maintains a flora (Fucus and Pelvetia) and a fauna which although not highly diversified is rich in individual specimens (Balanus glandula, Littorina scutulata, Patella).
The supra-littoral fringe, situated in the vicinity of the highest tideline, is often referred to as the “black zone” because of its colour, due to the presence of Cyanohyceae such as Calothrix. The fauna consists only of Littorina, a grazing gastropod, and a very small number of detritivorous species such as Lygia.
This zoning (Fig. 51), which proceeds from a typically thalassic settlement to an association comprising only Cyanophyceae, a gastropod, and a small number of isopods, is to be found naturally with certain specific variations, on all rocky coasts, even those with the lowest tides (Pérès and Picard, 1964). The analogy between this coastal zoning and that in the Mediterranean lagunar milieux is striking, in spite of notable differences in the specific composition of the settlements, in the one case adapted to a soft bottom and a permanent depth of water, and in the other to a hard bottom and intermittent changes in water levels.
It may be presumed that the essential factor influencing the zoning of intertidal species is undoubtedly resistance to emergence.
Depending on current meteorological conditions, as far as the film of water covering the rocks is concerned, emersion will mean either an increase in salinity and possibly total dessication, or else dilution and transition to freshwater; here are found the two hydrochemical extremes of the paralic domain.
A similar zoning may be observed in the intertidal zones on a soft bottom (beaches).
Thus it would seem that the “pattern” of the intertidal zone is a typically paralic pattern, in which confinement may be measured by the mean length of time of emergence.
Here, an attempt is made to apply the biological zoning of the paralic domain to a fossil basin using a method which is particularly efficient for present-day conditions, to see how far it is possible to retrace the paleozonings - or the paleobiogeography - of ancient paralic basins, including their quantitative aspect.
The transposition of an outline concerning present-day natural conditions to a sedimentary basin of the geological past encounters certain difficulties:
- First, the fossil fauna are different from present-day fauna, and this difference increases with the age of the basin studied. This explains the choice of the Paleogene in which the species differ only moderately from present-day forms, particularly concerning the malacofauna.
- In practice, it is very difficult, even impossible, to define with precision a time-surface that can be located on a sufficient number of section or logs, especially in the neritic or paralic basins where changes in facies prevail. Even in the Paris Basin, which has been subjected to innumerable geological studies, it is rash to descend below a time-interval corresponding to the layer stage. One is thus led to integrate the greater part of each stratigraphic cycle and the outline becomes much less precise.
First, an attempt was made to reconstitute the evolution of the Paris Basin, in the Paleogene within the paralic domain, working from global lists of paleofauna; then, a more precise reconstitution of one layer only, the Stampian.
1. EVOLUTION OF THE PARIS BASIN IN THE ZONING OF THE PARALIC DOMAIN DURING THE PALEOGENE
The main reference document consulted was the “Catalogue des fossiles tertiaires du Bassin de Paris” by Furon and Soyer (1947), without which much bibliographic research would have been necessary and totally out of proportion with the subject. It is possible that this list may be incomplete for a number of recently discovered species. Moreover, the stratigraphic divisions adopted by the author (Table 10) have been considerably modified over the last few decades, but this is of secondary importance to the discussion here.
It is very clear that this universal and synthetic list can only offer a very general view of the picture. Nevertheless, given the large numbers of thalassoid fauna species recovered over the entire basin for each whole layer, the presence or absence of certain zoological groups, particularly echinoderms and cephalopods, and the relative proportions of the different groups (Table 11), the dominant zone of confinement and the position of the basin in the diagram of the paralic domain, can be defined for each period (Figs. 52, 53).
The Montian comprises an essentially thalassic fauna, with very few paralic or continental species. It is therefore situated entirely in the marine domain (zones O and possibly I).
The Thanetian contains few zoological groups. The fauna is almost exclusively composed of molluscs including numerous examples of pelecypods and gastropods. The majority of species listed are paralic and with freshwater tendencies. The major part of the Paris Basin in the Thanetian is therefore situated in the zones of high confinement (zone IV and V dominant).
In the Sparnacian, the fauna is also almost exclusively limited to molluscs, but the number of species present is considerably reduced; there is also a very high proportion of gastropods. Further, paralic species (Hydrobiidae, for example) are well represented, and even freshwater pelecypods are to be found. This situation is characteristic of a very highly confined basin, where zones V and VI dominate, in a position which approaches nearer to the freshwater pole. This point of view is confirmed by the existence of lignitic layers in the Sparnacian formations of the Paris Basin.
By comparison with the preceding layers, the Cuisian includes a great number of different species and numerous zoological groups are represented. However, the absence of echinoderms is a notable fact to be emphasized. Here too, the molluscs largely dominate the fossil settlements. The basin is in closer communication with the sea although without going beyond zone III. Zone III dominates largely, and the importance of biological production during this period can be affirmed.
In the Lutetian and Bartonian the maximum representation of the various zoological groups is reached with a very high total number of different species. The species listed are practically all obviously thalassic: these two layers clearly represent the maximum of marine influence on the Paris Basin in the Paleogene. Nevertheless, the Bartonian shows a slight tendency towards confinement, notably by the reduction in echinoderms. Obviously, the edges of the basin cover more highly confined zones, particularly at the end of the Lutetian in the direction of the evaporitic pole (Toulemont, 1980).
In the Ludian a considerable reduction in the number of zoological groups can observed and of the number of species within each group. The basin becomes disinctly more confined and zones IV and V are the most widely represented. However, the contact with the sea remains evident, at least temporarily, during this period, for echinderms are still present. This high level of confinement on several occasions draws the basin towards the evaporitic pole.
The Sannoisian fauna is almost exclusively represented by a small number of typically paralic gastropods (9 Hydrobiidae out of 18 gastropods). The other members of the fauna indicate the importance of continental inflows. This layer is dominated by zones V and VI.
The Stampian is characterized by a fresh increase in numbers of species with the reappearance of thalassic species (echinoderms). Moreover, the abundance of molluscs, and notably of pelecypods, indicates the dominance of the highly productive zone III.
In the Chattian, the settlement is limited to a small number of paralic gastropods. Freshwater and terrestrial forms are very abundant: the basin is situated partly in the extreme paralic (zone VI and Far paralic) and partly in the continental lacustrian domain. Communication with the sea has no doubt disappeared at this time.
The outline proposed (Fig. 53) comes to no original conclusions as to the paleogeographical reconstitution nowadays accepted for the Paleogene in the Paris Basin. It may however be that there is too great a tendency to consider thalassoid fauna as being marine: thus only the Lutetian, Bartonian, and, to a lesser extent, the Stampian layers - except for the highly localized Montian - correspond to typical marine transgressions. The other periods correspond to greater or lesser extensions of the paralic domain over the Paris Basin, which must have remained in connection with a relatively far-off marine domain.
For greater precision it is necessary to establish a zoning pattern from fauna collected in different quarries and within as short a period of time as possible.
2. TEST-ZONING OF THE STAMPIAN IN THE PARIS BASIN
An attempt to transpose the zoning pattern revealed by the present-day paralic domain onto a bygone period required access to a study dealing with a particular layer and to the most complete description possible of the paleofauna collected in numerous different deposits. The Stampian was chosen because the thesis by Alimen (1936) corresponds to all these requirements (Fig. 54). In an attempt to be even more precise, two periods were distinguished: the lower Stampian, excluding the Sannoisian, which at that time was considered to be independent of the Stampian (Table 10); it includes the Etrechy limestone site deposits, the faluns at Jeurs and Morigny, the giant oyster marls, the clays at Corbules de Frépillon, the sands and sandstones at Cormeilles, etc. Second, the upper Stampian (Shelly deposits at Pierrefitte and Ormoy and lateral equivalents). Finally, account was taken of the fact that certain deposits present mixed or indeterminate fauna (the Shelly deposit at Vauroux, for example). A total of 38 fossil deposits was counted.
The parameters taken into consideration in this study were the number of different species of benthic fauna present, the number of different zollogical groups, and their relative proportions. It is however possible to pursue the matter further, in view of the large numbers of species present on the one hand and the density, when it is possible to have an idea of this, on the other, and to try to define which species are strictly paralic and characteristic of a high degree of confinement. In this category are species such as Cyrena convexa, Melania decussata, M. semidecussata, the Hydrobiidae, and a certain number of cerithiids, Cerithium conjunctum, C. plicatum, C. trochleare, and of course Cerithium (Potamides) lamarcki. There is some doubt regarding the oysters, Ostrea longitostris and O. cyathula which by comparison with present-day conditions, should rather be considered as mixed species. However, there is at Evreux an Ostrea cyathula var. minor which could very well be paralic or indicate a highly confined milieu (dwarfism). Similarly, Natica carasstina seems to characterize the most confined zones and could well be a paralic species in spite of its large size.
Considered as a whole, the biological characteristics of the different fauna of the Stampian deposits permit an outline of the biological zoning of the two levels in this layer:
- Lower Stampian (Fig. 55)
Echinoderms and nummulites are present only in the southernmost part of the basin, where the paleofaunistic associations also show the maximum numbers of different species: only this southern zone is under direct marine influence (zone II in present-day paralic terms).
On either side of the corridor formed by the zone and towards the north of the basin, a significant decrease is found in diversity of species which is accompanied locally by a proliferation of certain species. The fauna here consists only of molluscs, with. gastropods becoming largely dominant. This denotes a much higher degree of confinement (zones III and IV).
- Upper Stampian (Fig. 56)
The outline is schematically the same, but, on the whole, the confinement is more pronounced: except in the region of Pierrefitte and Ormoy, the diversity of species is considerably reduced, in particular in the entire northwest of the basin where the fauna is reduced to a few pelecypods (Ostrea cyathyla, Cyrena convexa) together with Potamides lamarcki and Hydrobiidae. Moreover, the local presence (Evreux) of layers rich in organic matter indicates the high degree of confinement of this whole region (zones IV and V) and its typically paralic character.
The pattern of the biological organization of the Paris Basin in the Stampian and the confinement gradients which can be inferred from it show clearly that the influence of the sea during this period was limited to the south of the basin: it is therefore in this direction that communication with the marine domain must be sought. This conclusion, in contradiction with those of Alimen, seems to be corroborated by recent discoveries (Pomerol, 1973) in the south of the Paris Basin. Communication towards the Atlantic approximately following the Seine Valley, suggested by Alimen, should be discounted, particularly in the Upper Stampian (Figs. 57, 58).
The application of the biological organization pattern of the present-day paralic domain to fossil paralic basins shows the usefulness of the classification categories which defined the confinement scale, in particular the benthic organisms. It is among these zoological groups that the most fossilizable species are to be found, leaving exploitable traces in sedimentary rocks. What is more, a study of the structure and organization of benthic communities often gives with a certain precision a good picture of their natural environment, assimilating as they do the fluctuations in the conditions of the milieu, in the more or less long term. Further, present-day species of benthic macrofauna, especially the paralic forms, in general have their counterparts in geological formations, particularly these which are not too remote in time. This permits a relatively viable reconstitution, without too much systematic distortion (specific or generic) of the structure, organization, and even, to a certain extent, the production level of fossil paralic basins: the organizational pattern of the present-day domain appears to be transposable in its broad outline on the Paleogene of the Paris Basin.
Without minimizing the difficulties linked with the evolution of thalassoid stock of the possibility of movements and modifications of fossil fauna, the study of paralic paleofauna allows the ecological dynamics of the formations under consideration to be retraced and may be, in the light of these propositions, an efficient tool for paleogeographical reconstitution.
The Baltic Sea has been the subject of substantial biological studies (segerstråle, 1957), but the recent works of Andersin et al.(1976) on the community structure of soft-bottom benthic macrofauna in different parts of the Baltic illustrate well the biological organization pattern of the paralic domain which proposed the present work. The results obtained by these authors show that the pattern is applicable to a biogeographical region very different from that of the Mediterranean. The large number of sampling stations, together with the existence of a pronounced hydrochemical gradient and a diversified bathymetry, reveals the majority of the aspects of confinement and their consequences which are shown clearly but gradually.
However, the studies of Andersin et al. cover neither the Kattegat nor the Øresund, the strait separating the towns of Helsingør in Denmark and Halsingborg in Sweden. This region is of great interest, as it corresponds to the transit zone where waters of marine origin enter the Baltic; various other works have been consulted to complement these of Andersin et al., including these of Bråttstrom (in Sergerstråle, 1957).
The Baltic Sea (420 000 km2, 1 500 km long from Copenhagen to the Bay of Bothnia, maximum depth 495 m) separates continental Scandinavia from the rest of Europe. It forms a Y shape whose vertical line is the main part of the Baltic with Denmark as its base. the two oblique arms represent respectively the Gulf of Bothnia and the Gulf of Finland (Fig. 59). This “sea” is the remnant of the basin which constituted the bed of the North European ice Scandinavian shield and flow into the Baltic: this, along with the low rate of evaporation, explains the generalized low salinity level of the basin, with a very pronounced negative gradient from the Kattegat towards the Bay of Bothnia where salinity is lower than 2 ‰.
Andersin et al. calculated for each station the density (in numbers of individual samples per m2) the biomass (in g of dry weight per m2) and the diversity of species.
The principal conclusions which can be drawn from the analysis of these three parameters are as follows:
- The density increases notably (Fig. 60) from the Arkona Basin in the south, where it is around 2 000 individuals/m2, to the Gulf of Bothnia in the north, where it exceeds 3 000 individuals/m2 and may reach total of over 5 000 individuals/m2. The density decreases slightly in the Bay of Bothnia. It is noted immediately that the deep zones of the Central Basin are devoid of benthic macrofauna resulting from the lack of oxygen and the presence of H S. The significance and origin of this phenomenon are examined2 later.
- The biomass decreases significantly from the Arkona Basin which contains the highest levels of biomass, in general more than 200 g/m2, towards the Gulf of Bothnia where the biomass totals vary between 10 and 20 g/m2 (Fig. 61). Here also, the deep zones appear to be anomalous; when the level of dissolved oxygen falls below 2 ppm, the biomasses fall below 2.5 g/m2 and in places decrease to 1 g/m2. Andersin et al. emphasize (and this is the case for other basins) that the decrease in biomass accompanying the increase in density towards the more remote zones is due to the dwarfism which affects paralic species: they notice particularly that the size and weight of the crustacean Pontoporeia affinis which dominates the communities of the northern regions of the Baltic are larger in the Gulf of Bothnia than in the Bay of Bothnia further north. They ascribe this reduction in size and weight to the salinity, the temperature, the trophic conditions and the combined effect of other factors. (The authors of this document ascribe it to the confinement.)
- The diversity of species decreases drastically from the Arkona Basin towards the outer limits of the Baltic. In the Arkona Basin, Andersin et al., record 41 species, 20 of which are found only in this area. The Central Basin contains only 20 species, the gulf of Bothnia no more than 8. A marked diversity of species gradient is demonstrated very clearly in Figure 62, which shows also that the majority of stations situated in the northern part of the Gulf of Bothnia are colonized by only one species, the detritivorous crustacean, Pontoporeia affinis. This very pronounced diversity of species gradient may also be observed in Figure 63 which shows the decrease in the number of taxa constituting the total community in the different basins from Denmark towards the outer limits of the Baltic.
The study by Andersin et al. of the faunal composition of the benthic communities allows the biological organization pattern of the paralic domain to the Baltic Sea to be applied.
In the southern part of the basin, molluscs largely dominate since they represent between 70 and 90% of the total biomass; nearly all these molluscs are pelecypods including Macoma baltica. The high biomass totals recorded in this region and the absence of echinoderms - confined to the Kattegat and the straits and disappearing immediately to the east of the Danish coast (Fig. 64) - situate it in zone III (Fig. 65).
The communities of the central basin (particularly in its northern half) and the Gulf of Finland are no longer dominated by pelecypods but by crustaceans, particularly Pontoporeia affinis, which may alone represent 90% of the community. These two regions can thus be placed in zone IV.
In the Gulf of Bothnia, the communities are always dominated by crustaceans and Pontoporeia affinis alone represents nearly the whole of the total benthic macrofauna. Macoma baltica is still occasionally found. The greater part of the Gulf of Bothnia (southern and central parts) can again be situated in zone IV.
In the extreme northern part of this Gulf (Bay of Bothnia) nothing but Pontoporeia affinis is found at the great majority of the stations studied: this region is situated in zone V.
Andersin et al. emphasize the anomaly represented by the zones where the oxygen level is low. These zones are sites particularly subject to the accumulation of organic matter which, as demonstrated, brings about a transformation of the faunal composition of the communities. This is shown, in these anomalous zones of the Baltic, by7 the clear predominance of the polychaete Capitella capitata, a species which indicates “organic pollution”.
Finally, in the deepest zones where water movement is nil (bathymetric confinement) large quantities of organic matter accumulate, originating not only from the biological activity of the superficial water layers, but also from the considerable inflow from the tributaries draining regions of high production and good conservation of vegetable debris. These deep areas are obviously situated in zone VI.
Thus, the Baltic presents an interesting confinement gradient but it also demonstrates various aspects of the conditions of this confinement, notably in relation to bathymetry: here, as seen in other examples, it is characterized by an increase in organic matter and a tendency to anoxia.
Andersin et al. attribute the variations in the composition of soft-bottom benthic communities to the parameters of salinity and dissolved oxygen alone. Neither of these parameters is a determining ecological factor, but both are tracers expressing confinement: one longitudinally, salinity; the other, vertically, the level of dissolved oxygen. These authors also emphasize in their conclusion that the “stability” of the communities in the poorly oxygenated deep zones is greater in the Gulf of Bothnia than in the more southerly zones: this confirms the stability of paralic communities.
Taking into account the qualitative and quantitative biological organization of paralic ecosystems, it can first be described in terms of confinement which permits a rapid description - simply and precisely - of the natural resources of each basin and of their distribution.
The second step could be the optimalization of the potential resources through the implantation in an adequate confinement zone of an intensive conchyliculture with species that live naturally fixed onto a hard substratum (oysters and mussels) in soft bottom only biotopes. In that case, zone III should be chosen for it corresponds to the maximum productivity of the malacofauna. This optimalization, which of course is at least partially dependent on local socio-economic conditions, entails no artificial development liable to modify the natural site, i.e., the various abiotic and biological gradients of the ecosystem.
A later step would be to envisage, by means of installations - mainly hydraulic - the adaptation of the ecosystem, through an adequate translation within the confinement scale, to an aquacultural activity defined by socio-economic imperatives.
This gradual approach is akin to what is known as “ecological planning” as defined by Falque et al. (1975), and employed again by Jouveniaux and Palanchon (1978). This approach has been applied to the aquacultural development possibilities of the Nador lagoon (Frisoni et al., 1982).
The definition of the natural resources of this lagoon can be based on the study of currents and hydrology (Fig. 66), bathymetry and granulometry (Fig. 67), the hydrochemical gradients and the phytoplanktonic biomass (Fig. 68) and above all the biological zoning calculated according to the benthic fauna, which makes possible a mapping of the basin in terms of confinement (Fig. 69).
The Nador lagoon is located mostly in zone II; and this situation in the lower part of the confinement scale is rather a disadvantage, for it means that most of the water is in a zone of average productivity, with a diversified fauna little adapted to an intensively aquacultural orientation.
Taking into consideration the “positive” and “negative” aspects (Guelorget et al., 1983) resulting from the human environment of this lagoon (Figs. 70, 71 and 72), it is possible to envisage without too “massive” a development, two types of aquacultural activity:
- an extensive tapiliculture (breeding of Veneridae), in the zones close to the sand bar where the granulometry of the sediments and the bathymetry are propitious;
- intensive suspended mussel and oyster breeding in the more central zones which are deeper and slightly more confined, near to the zone of maximum phytoplanktonic production (Fig. 73).
It is clear that an adequate arrangement reducing the communication between the lagoon and the sea, would in time increase the average confinement of the basin and would shift it towards zone III, optimal for malacological production. This displacement would also extend zones IV and V, principal phytoplankton producers, which would result in an increase of the basin's richness with regard to the suspension-feeding species cultivated there. This being so, it is possible to envisage the further improvement of aquacultural activities by the establishment of hatcheries, the transformation of the outlying salt marshes into breeding basins, the setting-up of fish cages, etc. (Fig. 74).
This brief explanation shows all the interest of the zoning of the paralic domain which can be perceived on the spot quickly, reliably and at little expense applied to the development of the coastal regions.
Perhaps this technique could be applied to the oceans?
One of the most remarkable features of confinement is that it can be applied to all the spatial scales. Compare, for example, the tiny Bermuda Triangle passing from zone III into zone IV in the space of a few hundred metres; with the Great Baltic Sea, where zones III to V stretch over more than 1 500 km. This is because, in every paralic basin, confinement depends on the effectiveness of the communication which the sea and of the currents within the basin with regard to its size and volume. Thus, the large paralic basins are similar - in the geometrical sense of the word - to the smaller ones, as far as their biological zoning is concerned. In this respect, it should be remembered that the intertidal zoning on rocky surfaces is simply the transposition over several metres - or indeed centimetres -of subvertical wall, of the horizontal zoning of the lagunar milieux which can stretch over several kilometres.
Apart from the biological zoning, confinement controls the hydrochemical zoning (for a given freshwater balance), and one notes the similarity of the large paralic basins of the past and the smaller basins of today: for example, the Vosgian sandstones and the Rhone Delta, the Silurian formation of Salina, Michigan and the Sebkha el Melah, the evaporitic Trias of the Saharan shelf and the salt marsh of Salin-de-Giraud.
All this, in spite of the space/time scale differences separating the knowledge of the Earth's past which man is trying to acquire from the present-day knowledge of nature, justifies the study of the present to better understand the past.