The seawater habitat—physical and chemical conditions

Many features of the marine environment are virtually uniform over wide areas; different parts of the sea are populated by different communities of organisms. Observations and measurements are made mostly with remotely controlled instruments. Some of the physical and chemical conditions can be measured with precision, but biological measurements involve many uncertainties because sampling apparatus such as nets, dredges, and grabs are not instruments of high accuracy. Measurements of the activities of marine organisms in their natural surroundings are limited to diver observations in shallow water, remote videos, and submersibles in deep water. There are two major zonations of distribution in the sea between the tropics and the poles, and between the surface and the depths are discussed in this chapter. The continual circulation of the oceans and their enormous heat capacity ensure that the extent of temperature variation in the sea is small despite great geographical and seasonal differences in absorption, and radiation of heat. The specific gravity of seawater varies with temperature and salinity, and very slightly with pressure. Illumination of the surface layers varies with place, time and conditions depending upon the intensity of light penetrating the surface, and upon the transparency of the water.