SOME RELATIONS OF ESTUARINE ORGANISMS TO SALINITY

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Abstract
Collectors often ignore salinity while carefully gathering other data with locality records of estuarine organisms, although there has been a great deal of work showing that salinity is a limiting factor to the distribution of many marine organisms, especially as it varies downward, and these limits are often quite sharp. Some recent evidence is reviewed. Hypersalinities are not common in the sea, but where such areas are found on certain coasts, high salinities also limit biotic distributions. The endemic fauna of estuaries is largely sessile. There are a few endemic motile species, chiefly fishes and crustaceans, but the motile fauna is made up mostly of the young of species which spawn offshore in high salinity water and these are sometimes present in vast hordes. Examples are menhaden, mullet, croakers, blue crabs and penaeid shrimp, which all support major fisheries in the United States. Estuaries may be characterized as nursery grounds. They are not refuges for spent races. There are three known types of biological gradients that correspond with salinity gradients. Sessile or only slightly motile marine organisms have optimal salinity ranges for best growth and when the salinities vary away from the optima, either upward or downward, the populations become stunted. A second correlation is between the size of motile organisms and the salinity. In general the smaller and younger ones initially distribute themselves in lower salinity water and migrate towards the sea as they grow larger; thus, the correlation between salinity and size is direct. This correlation is probably dependent upon intrinsic physiological conditions, but more information is needed. The greatest numbers of species of marine plants and animals are found in the photic zone of the shallow sea in full sea water and not in estuaries, as has sometimes been stated. All marine organisms and most estuarine organisms can withstand full sea water, but some of them cannot withstand lowered salinities and thus the species numbers decline with the salinity gradient decline in estuaries. The same thing is noted with the salinity gradient rise in hypersaline areas. The reasons for this situation are not clear, but it shows similarities to the warm to cold decline in numbers of species, away from the tropics, which holds both on land and in the sea. This temperature‐species gradient has been explained on the basis that the colder regions of the earth are relatively new and organisms have not yet had time to populate them fully. That explanation is probably not valid, for a parallel situation holds over shorter distances in estuaries, which are not new in terms of the Earth’s history.