Although many features of the marine environment are virtually uniform over wide areas, different parts of the sea are populated by different communities of organisms. The aim of marine ecological studies is to discover what these differences are and why they exist and to evaluate the factors responsible for them. These investigations encounter many difficulties.

There are the obvious problems of working in an environment to which we have no easy access. Observations and measurements have mostly to be made 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 and remote videos and submersibles in deep water. Organisms can be brought into the laboratory and kept alive for a time, but here their behaviour may not be the same as in natural surroundings because it is obviously impossible to simulate closely in a tank all the conditions of the open sea.

Because several properties of the marine environment usually vary together, the effects of variation in single factors are seldom evident in natural conditions. There are two major zonations of distribution in the sea - between the tropics and the poles, and between the surface and the depths. Both are associated with differences of penetration and absorption of solar radiation, and therefore with gradients of temperature, illumination, and to a lesser extent salinity. Vertical distribution is also influenced by pressure. The distribution of a species is consequently associated with a complex of variables and it is not easy to assess the role of each factor independently.

The effects of variation in single factors can be studied to some extent in controlled conditions in the laboratory but in this unnatural environment the responses may be abnormal. There is also the complication that several factors often interact in their effects; for example, in some species the tolerance to salinity change is modified by temperature, and temperature tolerance may itself vary with salinity. Furthermore, observations on specimens from one locality may not hold for an entire population of wide distribution because each species exhibits a range of variation for each character, and these may be related to the geographical situation due to selection or acclimatization.

Apart from the effects of the inorganic environment, there are also many ways in which organisms influence each other. Even where physical and chemical conditions seem suitable, a species may not flourish if the presence or absence of other species has an unfavourable effect. Predation may be too severe. Other competing forms may be more successful in the particular circumstances. The environment may be lacking in some essential resources contributed by other species, such as food, protection, an attachment surface or some other requirement. These biological factors are obviously of great importance, but their evaluation is extremely difficult.

Generally, the distribution of a species is an equilibrium involving many complex interactions between population and environment which are at present very incompletely understood. Nevertheless, a start can be made in tracing the complicated web of influences which control the lives of marine organisms by first studying the individual environmental variables, noting the extent to which each can be correlated with the distribution and activity of different species, and observing the effects of change both in natural conditions and in the laboratory. The variable conditions of obvious biological importance which we shall refer to in this chapter are temperature, the composition of the water, specific gravity and hydrostatic pressure, viscosity, illumination and water movements.

For detailed descriptions of the composition, properties and conditions for life in seawater, the student should refer to one of the many general texts available on oceanography. A selection of these is listed in the references at the end of this chapter and Chapter 1.

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