Dynamics of Marine Ecosystems

Biological-Physical Interactions in the Oceans

K. H. Mann, John R. N. Lazier

• Publisher: Wiley-Blackwell
• Year: 2013
• ISBN:
  9781118687918
• Edition: 3
• Topic: Physical Sciences
• Subtopic: Oceanography

The new edition of this widely respected text provides comprehensive and up-to-date coverage of the effects of biological–physical interactions in the oceans from the microscopic to the global scale.

• considers the influence of physical forcing on biological processes in a wide range of marine habitats including coastal estuaries, shelf-break fronts, major ocean gyres, coral reefs, coastal upwelling areas, and the equatorial upwelling system
  
• investigates recent significant developments in this rapidly advancing field
  
• includes new research suggesting that long-term variability in the global atmospheric circulation affects the circulation of ocean basins, which in turn brings about major changes in fish stocks. This discovery opens up the exciting possibility of being able to predict major changes in global fish stocks
  
• written in an accessible, lucid style, this textbook is essential reading for upper-level undergraduates and graduate students studying marine ecology and biological oceanography

1

Marine ecology comes of age

Marine ecology of the open ocean, as traditionally understood, is the study of marine organisms and their relationships with other organisms and with the surrounding environment. The subject parallels similar studies of organisms on land but, while terrestrial organisms are relatively easy to observe and manipulate, marine organisms are much more inaccessible. This inaccessibility has led to a slower growth of knowledge. The physical factors leading to fertile and infertile areas are very different on land than in the ocean. The nutrients required by land plants are generated nearby from the decaying remains of previous generations, but decaying matter in the ocean tends to sink and leave the sunlit euphotic layer where phytoplankton grow. The nutrients supplied by the decay are thus unavailable for phytoplankton growth unless some physical mechanisms bring the nutrients back up to the surface. This book is largely concerned with those mechanisms and the resulting biological phenomena. Compared with the extensive body of knowledge about physical–biological interactions in open water, much less is known about physical–biological interactions in intertidal communities. Hence the greater part of this book is about the ecology of open-ocean communities.

It is now possible to add an extra dimension to marine ecology. Instead of putting the organisms at the center of the picture and considering them in relationship to other organisms and the environment, it is possible to work with marine ecosystems in which physical, chemical, and biological components are equally important in defining total system properties. Those properties include production of living organisms such as fish, but flux of carbon dioxide as determined by both physical and biological processes may be more important in the context of climate change.

Interest in and research activity in marine ecology are intensifying. There are many reasons for this trend, of which four may be mentioned:

1 The physical processes underlying some of the large-scale biological phenomena are now better understood. For example, the North Pacific Ocean and the North Atlantic Ocean are seen to undergo oscillations in their near-surface physical properties on a time scale of about five decades, and these oscillations have a profound effect on biological processes, including the production of fish. The changes in physical oceanography appear to be driven by changes in the atmospheric circulation. In tropical regions, an atmospheric cycle known as the Southern Oscillation is seen to drive major changes in the coastal upwelling system in the Humboldt Current, and to have links to changes in climate and biological production in many parts of the world.

2 There have been important advances in our ability to make continuous, fine-scale biological measurements by means of automated sensors feeding into computers. It is now possible to collect biological data with a coverage and resolution comparable with the best physical data and to make integrated biological–physical studies at a wide range of time scales. For example, on a global scale the satellite image in Plate 1 shows the distribution of chlorophyll in surface waters, and reveals a great deal about the incidence of upwelling and the exchange of gases with the atmosphere. On a scale of tens of meters, Plate 5 shows how timely deployment of an intensive array of instruments made it possible to investigate the functioning of a breaking internal wave and its relationship to plankton.

3 The need to understand marine ecological processes influencing the greenhouse effect and other aspects of world climate is becoming more urgent. The flux of carbon dioxide from the atmosphere into surface waters and on down into the deep ocean, as a result of biological processes, is believed to be an important part of the mechanism of climate change. In this connection, there is important new information on the limitation of phytoplankton production in some areas by low concentrations of iron in the water, and on the stimulation of primary production in otherwise unproductive areas by a variety of intermittent mechanisms.

4 Our enormous increase in understanding fundamental processes over second to decadal time scales and centimeter to megameter space scales is beginning to influence the management of the ocean’s living resources. We are seeing that year-to-year and decade-to-decade changes in the atmosphere are reflected in property changes in the near-surface ocean. The way in which these changes affect the growth and survival of fish larvae and the distribution of fish are two topics that will receive a great deal of attention in the coming decades.

For all of these reasons, marine ecology has changed rapidly and may be said to have come of age. The dominant theme of this book is that physical processes create the conditions for many important biological processes; the biology cannot be understood in isolation. One good example is the jump in understanding why shelf-break fronts are so productive. This came about through a combination of a high-resolution numerical model and some clever field experiments. The model revealed details of the physical processes that would be impossible to observe with fixed instruments such as moored current meters. The field experiment tracked dye to reveal flow details that bring nutrient-rich water from deeper to shallower water within the front. In this volume, the connections between the physical and biological processes are emphasized and brought into focus more sharply than before.

The nature of the relationships between physical and biological processes is subtle and complex. Not only do the physical processes create a structure, such as a shallow mixed layer, or a front, within which biological processes may proceed, but they also influence the rates of biological processes in many indirect ways. Discussion of this relationship has most often been in terms of energy flow. Biologists often model food-web relationships in terms of the flow of solar energy, captured in photosynthesis by the phytoplankton and passed from organism to organism by means of feeding transfers. The physical phenomena such as currents, turbulence, and stratification also rely on solar energy, transmitted to the water directly as heat or indirectly as momentum from the wind. These two fluxes of solar energy are in one sense quite distinct: organisms do not use the energy of water motion for their metabolic needs. In another sense, they are interrelated. Water movement alters the boundary layers around organisms, transports nutrients and waste products, assists migrations, and influences the rate of encounter between planktonic predators and their prey. Stratification causes the retention of planktonic organisms in the upper layer of the ocean, making light more available but limiting access to inorganic nutrients. Water temperature has a profound influence on the rates at which biological processes proceed, and differences in water motion, from place to place, largely determine the kinds of organisms colonizing those places. From a biological point of view, the physical energy is termed auxiliary energy, which literally means “helping energy.”

However, it is important not to fall into the trap of assuming that there are strict and unvarying relationships between physical oceanography and the dynamics of biological communities. It is now becoming clear that interactions between organisms modify the responses of communities to physical conditions. For example, the size composition of a phytoplankton community may be determined by the types of zooplankton feeding upon it, and when nitrate-rich water is brought up into the mixed layer the response of a community of large phytoplankton cells will be very different from the response of a community of very small cells.

LENGTH SCALES

In approaching the subject it is useful to have a feeling for the dimensions of the organisms and phenomena to be discussed (Fig. 1.01). Ocean basins are typically 10,000 km wide and confine the largest biological communities. The average depth of the ocean is 3800 m but the depths of the euphotic layer (~100 m) and the mixed layer (~100 m) are more often critical to open-ocean biological processes.

The Coriolis and gravitational forces give rise to the Rossby internal deformation scale or radius, a frequently encountered length scale in physical/biological oceanography (see Section 5.2.3). It arises in flows of stratified water when a balance between the two forces is established. This scale, which varies strongly with latitude, is the typical width of ocean currents such as the Gulf Stream, the width of the coastal upwelling regions, or the radius of the eddies in the ocean.

Fig. 1.01 The size scale from 1 μm to 100,000 km, showing some characteristic size ranges of organisms and physical length scales.

The viscous or Kolmogoroff length is the scale where viscous drag begins to become important, that is, where viscosity starts to smooth out turbulent fluctuations in the water (see Section 2.2.6). The scale represents the size of the turbulent eddies where the viscous forces are roughly equal to the inertial forces of the turbulent eddies. The scale also indicates an important change in the methods of locomotion and feeding. Organisms larger than ~10 mm are not seriously affected by viscous drag, while for the smallest organisms swimming is akin to a human swimming in honey. Because of the change in the turbulent motions the smallest organisms must depend on molecular diffusion for the transfer of nutrients and waste products. For the larger animals nutrients and wastes are moved rapidly by turbulent diffusion, which is not affected by viscosity. These topics are developed in Chapter 2.

TIME SCALES

As a first approximation, time scales change in direct proportion to length scales. On the global scale, the thermohaline circulation may take 1000 years to complete a circuit. On the ocean-basin scale, the major gyres may require several years to complete a circuit. Eddies and gyres spun off from the major currents have lifetimes of weeks to months, and as energy cascades through smaller and smaller scales of turbulence, the characteristic time for rotation decreases to seconds at the smallest scale.

While physical features determine the spatial scales of ecological processes, the organisms determine the time scales. While the life span of a large marine mammal may be close to 100 years, those of fish are more like 1–10 years, and zooplankton may complete a generation in a few days or weeks. Phytoplankton have doubling times on the order of days, and bacteria of hours. It follows that small organisms are likely to undergo more rapid fluctuations in numbers than large ones. Since, in general, each type of organism tends to feed on organisms smaller than itself, the process of trophic transfer has the effect of smoothing out the rapid fluctuations. Conversely, predators may impose on their prey longer-term fluctuations that correspond with fluctuations in predator numbers.

PLAN OF THE BOOK

Part A begins by introducing turbulent motion and viscous boundary layers, which determine the unusual feeding and locomotion techniques of the very small organisms. These phytoplankton and zooplankton are the base of the food chain and account for about half of the total biomass of the ocean. Their survival depends on a variety of physical processes outlined in Chapters 3 and 4. In the open ocean survival depends on the annual creation and destruction of the seasonal pycnocline. In shallow coastal waters the effects of freshwater run-off and tidal mixing can be the dominant processes.

In Part B, Chapter 5 describes the consequences of winds near coasts and of the Coriolis force that lead to the Ekman drift in the surface layers and coastal up-welling. This process is responsible for some of the most productive regions in the ocean. The enhanced biological activity near various types of fronts is covered in Chapter 6 and is followed by a discussion of tides including explanations of tidally generated internal waves that transport nutrients onto the continental shelves.

Large-scale phenomena are treated in Part C, beginning with an explanation of the wind-driven circulation, the intense western boundary currents such as the Gulf Stream (Plate 2), and the warm- and cold-core rings that are generated by instabilities in the boundary currents. The unique biological properties of the rings and other circular circulation patterns such as gyres are then reviewed. The El Niño – Southern Oscillation story in Chapter 9 introduces the effect on biological productivity of changing circulation in the ocean. It is now clear that regular multi-decadal cycles in the atmosphere–ocean interactions of the major ocean basins cause predictable large-scale cycles in the abundance of fish. Chapter 10 reviews the greenhouse effect and the role of the oceans in this cycle, emphasizing the biological pump that is an important mechanism transferring carbon dioxide from the upper layers to the bottom of the ocean.

In the final chapter we discuss questions for the future. There is a sense in which the whole book is an exploration of these questions, so we give them here:

1 Is there a common mechanism to account for the occurrence of high biological productivity in a variety of physical environments?

2 To what extent are events in marine ecosystems determined by physical processes, and to what extent are the outcomes modified by interactions within the biological community?

3 How can we develop concepts and models that span the enormous range of scales in marine ecology, from the microscopic to the global and from seconds to geological ages?

4 How do we explain an apparent synchrony in the variations in the biomasses of fish stocks worldwide?

We shall see that a tentative answer to the first question was provided by Legendre (1981). He said, in effect, that vertical mixing followed by stratification of the water column leads to a phytoplankton bloom, and that this effect can be seen to happen in a variety of habitats and at a range of temporal and spatial scales. Our review supports this answer, but are there other mechanisms?

One is tempted to respond to the second question by saying that physical factors obviously determine the course of biological events, and the converse rarely happens. In fact, if we take a long-term view, we see that the greater part of the carbon dioxide released into the atmosphere during the life of the earth has been fixed by phytoplankton and deposited in marine sediments as carbonates or organic matter. Without these processes the carbon dioxide content of the atmosphere would be much higher, the earth would be much hotter, and the circulation of the oceans would be totally different. Even on the short time scale there are examples of phytoplankton altering the penetration of light and heat into the water column and hence the functioning of the ecosystem. Interactions between physics and biology are not entirely, or even mainly, in one direction. Moreover, while physical processes have predictable effects on individual organisms, their effects on whole biological communities are much less predictable. Community responses may be modified by the substitution of one species by another, or by predator–prey interactions.

The third question has been much discussed without any real resolution. It is a problem for ecologists generally, for we do not understand how to include bacterial processes on scales of millimeters and seconds in the same models that deal with animals that live for decades and may range over thousands of kilometers. Marine ecologists have the added difficulty that the biological events take place in a medium that exhibits physical processes on the same range of scales, thus compounding the difficulties.

The fourth question came into sharp focus at the end of the twentieth century. Multi-decadal changes in global patterns of atmospheric circulation correlate well with biomass changes in many of the major fish stocks. An enormous amount of work will be required to investigate, at a range of scales, the mechanisms responsible for the links between atmospheric changes and changes in marine ecosystems.

We have found it useful to keep these questions in mind as we review the developments of marine ecology as an integrated physical, chemical, and biological discipline.

Part A

Processes on a scale of less than 1 kilometer

2

Biology and boundary layers

2.1 Introduction

2.2 Phytoplankton and boundary layers

2.2.1 Turbulent motion

2.2.2 Sources of turbulent energy

2.2.3 Viscosity

2.2.4 Comparing forces: the Reynolds number

2.2.5 Molecular diffusion

2.2.6 Scales ...