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Home > GCR > Vol. 4 > Iss. 3 (1974)

 

Alternate Title

A Review of Salinity Problems of Organisms in United States Coastal Areas Subject to the Effects of Engineering Works

Document Type

Article

Abstract

The nongaseous substances that normally move in and out of cells are metabolites, water and salts. The common salts in water determine its salinity, and the definition of sea water salinity and its composition are discussed. The relationships of salinity to all phyla of animals living in the coastal waters are reviewed, with emphasis on the estuaries of the Gulf and Atlantic coasts of the United States, which are particularly influenced by coastal engineering works and changes of salinity caused thereby.

The fauna of estuaries is made up of a few brackish water species which complete their life cycles there, marine species which spend only a part of their life cycles there and which have definite low-salinity limits, a few anadromous species passing through, and a few fresh water species with high salinity limits. Organisms of marine origin are dominant. Floods and drought periods, resulting in severe osmotic changes, may kill organisms or result in drastic changes of the biota, which may take extended time to reestablish itself, if the conditions return to normal.

Many invertebrate animals have few mechanisms for controlling the movement of waters and salts across their external membranes and thus are osmotic conformers. Species that cannot withstand wide salinity change are said to be stenohaline, but many species can tolerate rather wide changes; they have broad tolerance at a cellular level. Osmoregulators exhibit considerable control of their internal salinity by excretory mechanisms and permeability control of the body surface to water and salts. In general, the worms and molluscs are more often osmoconformers than the crustaceans, the latter having greater control over their internal osmotic environment than other invertebrate groups. Effects upon different stages of life history are different, but in general the limiting effects of salinity and other environmental factors bear upon the reproductive stages or the young. The limits of salinities are nearly always on the lower side with regard to estuarine organisms. Aside from osmotic adjustments, animals react to salinity changes by closing their shells, closing their burrows, burying in the bottom where interstitial water has higher salinity, or motile forms simply move out. In spite of these adjustments, none may be successful during large floods.

The salinity relationships of all phyla are reviewed, even though some of them are unimportant parts of the marine populations. Dinoflagellate Protozoa which cause red tides have precise salinity requirements and their outbreaks would be subject to control if reservoirs of fresh water were available. Sponges are generally high salinity organisms and not often damaged by low salinity. All the worms, including several phyla and the rather advanced Annelida, and the Mollusca are weakly motile at best and are thus subject to extermination by heavy floods in estuaries. However, most of these animals have short life cycles and their populations are quickly reestablished when the salinity regime returns to normal.

The dominant invertebrates in estuaries are crustaceans. The larger species are generally motile and not subject to catastrophic damage. Nevertheless, the lower limits of toleration of most species are quite limited and even a change of half part per thousand salinity will cause changes in the range in many species. If the salinity gradient falls, species drop out of the complex because their lower limits are reached, with the numbers of species becoming less in lower salinities.

There is an estuarine life history among the higher crustaceans such as shrimp and crabs which is also applicable to the fishes, and which is closely related to salinity. The adults spawn in high salinities of the open ocean and the young come back into the estuaries to raise. The young apparently prefer the lower salinities that the species will tolerate; and they move back out to sea as they grow, thus bringing about a correlation with risinpsalinity and increase in size, which may be quite precise as in the case of the commercial shrimp of the Gulf coast.

Although catastrophic changes in salinity are quite important to populations, the day-today generally normal salinity regimes also play a great part in determining the population picture of an estuary. Ninety-eight per cent of all the commercial fishery production in the Gulf of Mexico is from species connected with the estuary sometime in their life history.

The fishes of bays and sounds are made up of a relatively few species which spend their lives there, of semi-anadromous and anadromous fishes going from and coming to fresh water, of a few strays from fresh water, a few strays from the open ocean and a host of semicatadromous species that undergo the marine-estuarine type of life history. Largc specimens are found in higher salinity water for most species. Being strongly motile, few species are damaged by floods. The weakly motile amphioxus is the only chordate known to be destroyed in vast numbers by fresh water. The fishes are strong osmoregulators and in the changing salinity of the bays they regulate as fresh water fishes or marine fishes, depending upon whether or not the ambient salinity is higher or lower than that of the blood.

The salinity relations of salt water plants are less well known than those of the animals; however, a summary is given on what information we have concerning the flowering plants of the marshes and the totally submerged marine grasses along the Gulf coast.

Each separate bay, sound or estuary is an individual case with regard to salinity, but they can be appraised by biologists acquainted with the local fauna and flora so as to minimize and possibly even enhance their biotic potential in connection with salinity changes caused by engineering works.

First Page

380

Last Page

475

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