Читать книгу Life in the Open Ocean - Joseph J. Torres - Страница 15

As Vogel (1981) observed, the concept of mass when applied to the inherent shapelessness of fluids is a bit awkward and for practical purposes is replaced by density. The density of pure water is about 830 times that of air, or about 1000 kg m⁻³ at 0 °C and atmospheric pressure. It only varies by about 0.8% in density over the biological range of temperatures (0–40 °C) despite our attention to the fact that its maximum density is above its freezing point. Density of water is even less sensitive to pressure; it increases by only 0.5% with each kilometer of depth (Denny 1993).

Salinity alters the density of water considerably more than does temperature or pressure. The difference in density between freshwater and seawater is substantial. Salinity itself is the amount of dissolved material expressed in g per kg of seawater. The material consists mainly of salts; the principal salt is sodium chloride. The nicely intuitive definition of salinity as g per kg, or parts per thousand, has been replaced in some circles by the introduction of practical salinity. Practical salinity (S_p; UNESCO 1983) is the ratio of the electrical conductivity of a seawater sample and to a standard solution of potassium chloride. Since it is a ratio, practical salinity has no units. It is very close to actual salinity, though. To get back to actual salinity in g per kg from practical salinity, you use the following equation: S = 1.00510 S_p (Bearman 1989).

The fact that water is far denser than air has its good points and bad points from the perspective of a swimming animal. On the plus side, it means that much less structural investment is required to support the weight of an organism in water than on land. A popular analogy compares a tree and a kelp of equal height above the substrate. Clearly, the kelp has far less energy invested in its 0.05 m diameter stipe than the tree has in its 0.5 m trunk. The principle works equally well for a jellyfish elegantly trailing its tentacles in the ocean or piled up in a soggy mass on the beach.

The aquatic medium provides buoyant support according to the difference in density between the body and the medium in which it is immersed. The weight of an object in water is described by the equation

where ρ (rho) is the density of the object, ρ _w is the density of water, g is the acceleration of gravity (9.8 m s⁻²), and V is the volume of the object in question. The expression ( ρ − ρ _w) is the effective density or ρ _e of our submerged body and determines whether it will float, remain suspended, or sink. Its effective weight in water is thus g· ρ _e V, and it follows logically that a body will weigh more in air than in water, usually by 5‐ to 50‐fold (Denny 1993). Do not be guilty of synonymizing weight and mass. Mass is a scalar quantity measured in kilograms; weight is a force that is measured in newtons. You will notice that the mass of an object in water does not change, but its weight does.

Seawater at a salinity of 35‰ has a density of 1024 kg m⁻³, meaning that marine species enjoy more buoyant support from their medium than do their freshwater counterparts. Knowing what we know about relative weights in air and water, neutral buoyancy for marine species will be achieved with a density equal to that of seawater. Let us compare the density of some common biological materials. Mollusk shells at 2700 kg m⁻³ are quite dense, providing protection and support for the soft tissues beneath but also assuring that they are most useful in bottom‐dwelling species. Cow bones are also quite dense, 2060 kg m⁻³, providing the skeletal support needed by a heavy animal in air. Neither structure is appropriate for a species concerned with remaining suspended in mid‐water, so the likelihood of cows invading the marine environment remains low. In contrast, muscle is 1050–1080 kg m⁻³, only about 5% higher than the density of seawater. Fats are slightly less dense than seawater, 915–945 kg m⁻³ so they provide a source of static lift for marine species. It is instructive to note that small changes in an animal’s density can confer big advantages to its weight in water but would do little to affect its weight in air. The energetic advantages of neutral buoyancy have done much to influence how pelagic species are put together. In succeeding chapters, we shall explore buoyancies and mechanisms for achieving neutral buoyancy in open‐ocean taxa.