Читать книгу Life in the Open Ocean - Joseph J. Torres - Страница 12

For many beginning the study of oceanic fauna, the ocean itself is a fairly mysterious place. We know that it is vast, deeper in some places than others, and that the deep sea is cold and dark. What is less clear is how physical factors vary over the global ocean and why they are the way they are. The purpose of this first chapter is to briefly describe the physical factors impacting pelagic (open ocean) animal life and how those factors are distributed in the world ocean in the horizontal and vertical planes. Physical factors play an important role in shaping the adapted characteristics of animal life, particularly physiological characteristics, and by virtue of being physical factors they vary predictably in space and time.

One of the main purposes of this book is to give the reader an appreciation 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.