Living Shores: Interacting with southern Africa's marine ecosystems, by George Branch and Margo Branch

Living Shores: Interacting with southern Africa's marine ecosystems, by George Branch and Margo Branch. Penguin Random House South Africa. Imprint: Struik Nature. 2nd revised edition. Cape Town, South Africa 2018. ISBN 9781431700813 / ISBN 978-1-43-170081-3

Living Shores: Interacting with southern Africa's marine ecosystems, by George Branch and Margo Branch should be the recommended textbook for every undergraduate studying marine biology, every marine communicator, and anyone passionate about the ocean.

George Branch  Margo Branch  

The properties of seawater

The nature of seawater strongly influences marine life. Water buoys the body, so submerged animals and plants use less energy than landlubbers and need less supportive tissue to hold up their bodies against gravity. As a result, seaweeds can increase their chlorophyll-bearing tissues - they are soft and floppy compared to woody land plants, but grow faster because they photosynthesise more efficiently (Fig. 1.4). Supported by seawater, whales can grow much larger than land animals (Fig. 1.5). Temperature fluctuations are much smaller in the sea than on land. Land warms rapidly by day and cools quickly at night. For example, desert temperatures swing from below zero to 40°C in the course of a day; but in the sea, daily fluctuations of more than a few degrees are rare. Water stores about 1,000 times more heat than land or air. The 'skin' of the ocean - the top 3.2m -holds as much heat as all of the world's air, and releases and gains it slowly, so air temperatures are buffered. Temperature declines with depth. At about 200m, it usually decreases abruptly at the 'thermocline' (Fig. 1.6). This establishes a barrier between the upper and lower layers, with little mixing between them. Terrestrial organisms lose water to the air, while freshwater life has to avoid gaining too much water from its dilute medium; but most marine animals have body fluids similar to seawater, and consequently have few problems of water loss or gain. Thus, in many ways, seawater is a comfortable and benign medium for life. However, oxygen is less readily available in seawater than on land. It diffuses slowly through water and thus is not rapidly replaced when it used up by respiration or bacterial decomposition. As a result, most marine animals have large external gills and need to circulate water over them to obtain sufficient oxygen and eliminate carbon dioxide. Species that lack these adaptations are often sluggish and slow moving. Below the thermocline, oxygen declines rapidly and an 'oxygen minimum' tends to develop there (Fig. 1.6); but oxygen increases again in very deep waters because they are supplied with cold, oxygen-rich waters from polar regions and there is little life at these depths that consumes oxygen. An important limitation is that water filters light, so enough light for plants to photosynthesise is present only in the upper (euphotic) layers of the sea, in the top 50-100m. A chlorophyll maximum zone is often present just above the thermocline (Fig. 1.6), where there is still sufficient light, and nutrients are readily available from the deeper, nutrient-rich waters. As most animal life depends on plants for food, limitations on where plant life can grow are important. In the euphotic zone, animals can feed directly on phytoplankton or seaweeds, but in deeper waters animals depend on dead organic matter sinking from the upper layers. In nearly all cases, life ultimately depends on energy derived from sunlight. The only exception occurs around deep-sea vents at depths of up to 6,500m, where volcanic eruptions release gases and minerals. These 'hot smokers' produce copious hydrogen sulphide (H2S) and methane (CH4), which bacteria can use to produce carbohydrates by chemosynthesis entirely without solar energy. Even more remarkably, Jillian Peterson and colleagues showed in 2011 that bacteria associated with vents employ hydrogen (H2) as an energy source - perhaps this will inspire human efforts to obtain energy from hydrogen! [...]

This is an excerpt from Living Shores: Interacting with southern Africa's marine ecosystems, by George Branch and Margo Branch.

Title: Living Shores
Subtitle: Interacting with southern Africa's marine ecosystems
Authors: George Branch Margo Branch
Publisher: Penguin Random House South Africa
Imprint: Struik Nature
2nd revised edition. Cape Town, South Africa 2018
ISBN 9781431700813 / ISBN 978-1-43-170081-3
Hardcover, 22 x 29 cm, 336 pages, throughout colour photographs and illustrations