Life in the Open Ocean. Joseph J. Torres
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of pelagic communities, with as many of the players being treated as possible and with an accent on the community as a whole. Oceanic communities are constrained to water masses, identifiable (sometimes very large!) parcels of water, because the species comprising those communities live out their life histories in a discrete region and those regions have predictable characteristics to which life has become adapted. Adjacent oceanic regions that harbor fundamentally different communities presumably must differ enough physically and be separated enough from a biological perspective, that selection can change species composition. Physical factors play a big part in that selection process.
The physical factors limiting the distributions of open‐ocean species are temperature, oxygen, light, and pressure. Salinity, an important variable in estuarine systems, is of far less importance in the open ocean. Salinities in the open sea vary from approximately 33 parts per thousand (ppt or ‰, a 3.3% salt solution) to 38 ppt (a 3.8% solution), which is not a sufficient fluctuation to act as an important selective pressure on pelagic fauna. However, salinity does act indirectly to influence oceanic communities as it is an important operator in ocean circulation and the formation of water masses. And it does vary enough to be useful in identifying water masses when plotted against temperature in a T‐S diagram, discussed later in this chapter.
An individual animal’s interactions with the open‐ocean environment are governed not only by temperature and pressure but also by the properties of water as a fluid. How fast a shark sinks relative to a jellyfish is within the province of basic fluid dynamics, as are the forces acting on the swimming individuals as they make their way quickly or slowly through the fluid medium.
Since the distribution and characteristics of oceanic life are dictated partly by the characteristics of the physical environment, it is imperative that one have a fundamental understanding of how that environment varies with location and of some of the principles that govern the variability. This chapter will cover enough of the basics to provide an understanding of the physical environment of the open sea and the biological interaction with it.
The Vastness of the Open Ocean
A few facts about the open ocean are important to help put the vastness of the oceans in perspective. To appreciate the total living space available to pelagic fauna, we need to consider both the ocean’s surface area and the volume beneath the surface: the ocean’s horizontal and vertical extent. The ocean basins cover 71% of the planet and their average depth is 3800 m (Sverdrup et al. 1942). Since the Earth is a sphere with a radius of 6371 km, its total surface area is calculated at 5.1 × 108 km2. The surface area of the world’s oceans at 3.6 × 108 km2 is ~71% of the total surface area of our planet. Consider volume. The average depth of the ocean basins is ~3800 m so the volume of water contained in the ocean is ~1.335 × 109 km3. The volume of the moon is 2.2 × 1010 km3; the volume of the world’s oceans could form a body over half (60% in fact) the size of our moon. Thus the oceans contain an immense volume of water that affords habitat to the creatures that live within it.
We can arrive at a similar volume measurement for terrestrial systems by assuming that biologically useful space on land is defined by the height of a tall tree (0.05 km). Using 29% of the total surface area of the Earth for the terrestrial ecosystems and multiplying that by the height of the tree, we arrive at one useful‐volume estimate of 7.4 × 106 km3, which is only 0.5% of the volume in the oceans (cf. Herring 2002).
It is interesting to look at extremes. The highest point on Earth above sea level is Mt Everest at 8.55 km. The lowest point on Earth is in the Pacific Ocean, the Challenger Deep in the Marianas Trench near the Philippines at 10.93 km below sea level.
The total living space in the pelagic realm can also be compared with that in the terrestrial biome by evaluating surface area. The Hypsographic Curve of the world in Figure 1.1 expresses graphically the amounts of the planet’s surface, by elevation, above and below sea level. From the Curve it is easy to see that over 60% of the Earth’s surface is covered by water of depths greater than 1000 m. Since 97% of all the water on Earth is contained in the oceans, the open sea at 1000 m deep and below is therefore the Earth’s “average environment”!
Freshwater in rivers and lakes accounts for about 1% of the Earth’s water; glaciers, polar ice, and groundwater contain about 2% more. The remainder of the Earth’s water is in the vastness of the oceans.
The Properties of Water
The physical properties of water do much to shape the Earth’s climate, while the density of water literally influences the shape of marine species. Water exhibits many unusual characteristics. It is the only compound that can be found in all three phases (solid, liquid, and gas) at the temperatures encountered at the Earth’s surface. Water has a high specific heat, which means that it must absorb a great deal of heat energy for its temperature to rise by 1 °C (1.0 cal g−1 at 10 °C). Because it can absorb a lot of heat, it can also deliver a lot of heat to regions far removed from where the heat was absorbed, and ocean currents such as the Gulf Stream do just that. The climate of Great Britain, for example, would be far colder than it is were it not for the warm maritime influence of the Gulf Stream.
Figure 1.1 The Earth's surface area. (a) Earth’s surface area as a function of elevation and depth. (b) The Hypsographic Curve; cumulative percentage of earth's surface area as a function of elevation and depth.
Source: Gage and Tyler (1991), figure 2.4 (p. 12). Reproduced with the permission of Cambridge University Press.
When water changes its physical state, a great deal of energy is gained or lost. When ice is formed, 80 cal g−1 is liberated as the latent heat of fusion. This property is particularly important at the poles during the freezing and melting of sea ice. Similarly, 540 cal g−1 must be applied to cause liquid water to evaporate (latent heat of evaporation) and an equal amount of energy is released upon condensation. Water’s freezing (0 °C) and boiling (100 °C) points are far higher than those of closely related compounds: H2S, e.g. boils at about −59 °C.
Water has a high thermal conductivity (0.587 W m−1°K−1 at 10 °C), so heat diffuses readily through it. When well mixed, like the wind‐mixed surface layer of the ocean, large volumes of water can be quite homogeneous in temperature. Water has a high surface tension, which makes it fairly “sticky.” Small volumes will form drops, capillary action will cause water to readily invade a small tube, and small water‐dwelling animals must contend with a fairly viscous environment.
Water’s most unusual characteristic by far is that it is less dense in its solid form than in its liquid phase. It is unique in the physical world in that regard, and it is because of that distinctive characteristic that ice floats. Pure water reaches its maximum density at 4 °C. Seawater does not reach maximum density until −3.5 °C (Vogel 1981), which is below its freezing point at −1.9 °C and ice crystals have already begun to form. When seawater freezes, the salt is excluded as a brine; the ice itself is fundamentally salt‐free.
Density
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