The paralic domain supplies fossil energy resources, coal and oil and the fossil or present-day evaporitic areas abundantly supply minerals and elements indispensable for human activities:
- halite (NaCl) comes almost exclusively from the paralic domain: world production was 143 million t in 1971
- gypsum (CaSo4, 2H2O) is mainly used for making plaster but is also a constituent of cements
- potassium salts, sylvite (KCl) and carnallite (KMgCl3, 6H2O), essential constituents of fertilizers (world production in 1971: 17 million t
- elements extracted from connate brines or from the mother-waters of salt marshes (Mg, Br, K, etc.)
There seem to be genetic connections (which are incidentally variable) between the confined milieux and certain metal-bearing deposits, particularly of lead, zinc, copper, magnesium (Routhier, 1980: Lagny, 1980: Quemeneur, 1974). As seen before, the processes of biological concentration of some of these elements are probably closely related to the notion of confinement.
The rich biological resources and the high productivity of paralic milieux explain their having been exploited since antiquity and probably even since long-distance prehistoric times.
This is, of course the first form of exploitation, and can be intensive, for - unlike the very fragile marine ecosystems which can be seriously perturbed by over-harvesting (even to the extent of a possible disappearance of certain species) and furthermore highly sensitive to “pollution” - the paralic domain, because of its biological stability and high productivity, appears to be practically inexhaustible; for example, the conchylicultural yield of the lagoons of Diana, Thau and Urbino is around 15 t/ha/year of marketable products.
Amanieu and Lasserre (1981) demonstrated the production levels of the Mediterranean coastal lagoons and their contribution to the enrichment of demersal fishing. The lagoon fishing-grounds represent an 8–10% contribution to the entire Mediterranean catch (the Black Sea excluded) 1; but it must also be emphasized that these lagoons represent feeding nurseries for young fish during their first years of life; later these same fish will be found in the marine fishing-grounds.
(a) Levels of halieutic production in the Mediterranean lagunar sites
After analysis of data collected from 46 peri-Mediterranean lagunar sites, Amanieu and Lasserre (1981) demonstrate "the extreme disparity of the outputs which vary from 6 kg/ha/year (Nador lagoon, Morocco), to 149 kg/ha/year (Venice lagoon, Italy) or even 172 kg/ha/year (the Ebro estuary, Spain): therefore, an average figure would be meaningless. On the contrary, it would be useful to have a better knowledge of the Mediterranean lagoons' production potentialities. A preliminary estimate would be different from the above, the variations noted at present being largely due to the conditions of exploitation. If the true potentiality is equal to the highest yield of the Spanish or Italian lagoons, i.e., around 150 kg/ha/year and by extrapolating to the 883 720 ha of available lagoons around the Mediterranean, a potential annual production is obtained of 125 000 t, more than twice the present yield. However, such an estimate remains speculative and artificial.
(b) Contribution of the lagunar sites for the enrichment of inshore fishing
According to Amanieu and Lasserre (1981) : “The lagunar sites are very irregularly distributed along the coast of the Mediterranean. The level of inshore captures also varies considerably and, so it seems, in relation to the proximity of lagoons likely to contribute to trophic supplies, or indeed recruitment. Thus, the lagunar milieux of the Adriatic and the Gulf of Lions, which cover around 280 000 ha, i.e., nearly 34% of the totality of the Mediterranean lagoons, are themselves among the most prolific sites (between 80 and 150 kg/ha/year). They furthermore run parallel with the coastline where the production of demersal fishing grounds is particularly high, about 35–50 t/km of coast. On the contrary, such regions as the French Riviera, the south of the Peloponnese in Greece, or the Libyan coast, are characterized both by a low rate of productivity (between 0 and 5 t/km of coast) and by the absence of coastal lagoons. Such an estimate can only remain rough insofar as it does not take sufficiently into account the surface areas of the fishing grounds, which depend on the local width of the continental shelf”.
Thus, compared with the sea coast, the lagoons contribute largely to the increase of the halieutic demersal potential : the zones of high demersal production correspond to the zones where the lagoons are large and productive. The paralic domain appears here again as a highly productive ecosystem which exports a good deal of its excess energy towards the sea (Fig. 36).
As confinement contributes to the reduction of species and simultaneously the increase in their density, it is clear that the paralic milieux are propitious for mono-or pauci-specific culture, provided that the species chosen are adequate: it is from this point of view that aquaculture has developed, first empirically, and now more rationally. Today, most aquaculture takes place in paralic milieux.
The considerable productivity of the paralic domain can no doubt be used for the creation of energy if, for instance, the production of organic matter by phytoplankton or by the algal fields of Cyanophyceae, or again the amazing rate of growth of certain algae such as the Ulva and the Enteromorpha (Appendix 10) are considered.
A reasonably long-term exploitation of such potential resources can therefore be envisaged.
Given that the hypotheses concerning the paralic domain and its dynamics continue to be proved correct through observed facts, an overall strategy for the development of these milieux is proposed.
First, the place of each basin on the confinement scale and the shape of the various confinement zones it includes determine its biological potentialities (species likely to develop, productivity rate, location of the beds, etc.). The ecological characterization in terms of confinement of these stretches of water (Guelorget et al., 1983) is then an essential phase of their development, without modifying the initial natural environment (Appendix 6).
However, if, in the marine or continental domain, man has only limited means of action over the fundamental parameters of the milieu, the situation is different in the paralic domain, since it is possible, often quite cheaply, to influence the confinement. This is exactly what salt-producers do in the salt marshes, when they adjust the inflow of seawater and the displacement speed of the brines, according to meteorological variations and, with a specific aim, the production of salt.
It is therefore possible to envisage, for each paralic basin, the regulation of the confinement, and possibly its gradient values, to the quantitative and qualitative requirements of a coherent economic policy. Thus, the present effort to develop the coast must necessarily be accompanied by the study of the paralic milieux.
