In this chapter we shall introduce the reader to the shelf seas, their extent and position in the global ocean and the motivation, both fundamental and applied, behind our efforts to understand and model the complex processes which control the shelf sea environment and ecosystem. We shall then briefly explain the historical development of shelf sea science and describe the technical tools which are now available and which have facilitated the relatively rapid advances of recent years. As well as discussing the principal observational techniques, in a final section we shall consider the role of numerical modelling and its potential contribution to developing understanding.

Definition and relation to the global ocean

Between the deep oceans and the continents lie the seas of the continental shelf. These shallow areas usually have rather flat seafloors and extend out to the shelf break, where the seabed inclination generally increases rapidly at the top of the continental slope leading down to the abyssal ocean. This abrupt change of slope is clear in the map of global bathymetry shown in Fig. 1.1a. It typically occurs at a depth of ~200 metres and a contour, or isobath, at this depth is often taken as defining the outer limit of the shelf seas. This choice is not critical, however, since the continental slope is so steep (~1:10); moving from the 200- to the 500-metre isobath involves little horizontal movement. Using the basis of a 500-metre definition, Fig. 1.2a shows that the shelf seas account for ~9% of the total area of the global ocean and less than 0.5% of the volume. The shelf seas have an influence and importance quite out of proportion to these numbers.

Preface

The seas of the continental shelf where the depth is less than a few hundred metres experience a physical regime which is distinct from that of the abyssal ocean where depths are measured in kilometres. While the shelf seas make up only about 7% by area of the world ocean, they have a disproportionate importance, both for the functioning of the global ocean system and for the social and economic value which we derive from them. Approximately 40% of the human population lives within 100 km of the sea, and the coastal zones of the continents are host to much of our industrial activity. Biologically, the shelf seas are much more productive than the deep ocean; phytoplankton production is typically 3–5 times that of the open ocean, and globally, shelf seas provide more than 90% of the fish we eat. They also supply us with many other benefits ranging from aggregates for building to energy sources in the form of hydrocarbons and we use our coastal seas extensively for recreation and transport. The high biological production of the shelf seas also means that these areas are important sources of fixed carbon which may be carried to the shelf edge and form a significant component of the drawdown of atmospheric CO2 into the deep ocean.

Understanding of the processes operating in shelf seas and their role in the global ocean has advanced rapidly in the last few decades. In particular, the principal processes involved in the workings of the physical system have been elucidated, and this new knowledge has been used to show how many features of shelf sea biological systems are underpinned and even controlled by physical processes. It is the aim of this book to present the essentials of current understanding in this interdisciplinary area and to explain to students from a variety of scientific backgrounds the ways in which the physics and biology relate in the shelf seas. Our motivation to write such a book came from our extensive experience of teaching undergraduate and post-graduate courses in physical oceanography and biological oceanography to students from diverse disciplinary backgrounds and the realisation that there was an unfulfilled need for a textbook to present the maturing subject of shelf sea oceanography combining the physical and biological aspects.

The tidal mixing fronts identified in Chapter 6 are the transition zones between areas which are vertically well mixed and those where weaker tidal stirring allows stratification to develop. In this chapter we will focus on these transition zones. Tidal mixing fronts have special properties which distinguish them from the mixed and stratified regimes that lie on either side of them. The large temperature gradients exhibited by the fronts are clearly apparent in satellite infra-red (I-R) imagery of the sea surface which provides a useful way of keeping track of the position of fronts and following their evolution. The large horizontal temperature gradients in the fronts also involve corresponding changes in density, and hence pressure gradients, which can result in jet-like flows along the fronts and a degree of cross-frontal flow. These frontal currents together with the rapid changes in water column stability which can occur in fronts and the consequent modification of light and nutrient availability in the frontal zone have important implications for primary production and higher levels in the food chain. In the final sections of the chapter we will consider these implications and examine the hypothesis that fronts are zones of significantly enhanced primary production, and assess the reasons for corresponding increases in activity at higher levels in the food chain.

Frontal positions from satellite I-R imagery

As we noted in Section 2.2.2, long wave energy radiated by the sea surface has a spectral peak at a wavelength λ ~ 10 μm. This maximum in emission coincides with a minimum in atmospheric absorption by gases. Under cloud-free conditions, satellite I-R sensors can see though this ‘window’ and map the sea surface temperature (SST), by inversion of the Planck radiation law, with a resolution of a few tenths of a degree Centigrade.

In this chapter we consider the powerful forces that drive the shelf seas, and supply the large amounts of energy which are dissipated within them. We shall see that these forces act mainly through the transfer of properties (momentum, heat, freshwater, etc.) at the sea surface and through the lateral boundary where the shelf seas meet the deep ocean. Together the various forcing mechanisms produce an energetic regime which, in most shelf seas, maintains a high level of energy dissipation far greater than that of the deep ocean. We begin by identifying the principal energy and momentum sources and then consider, in turn, the forcing mechanisms involved and the extent to which the resultant inputs are known and can be related to measurable parameters.

Energy sources

Perhaps the most obvious and striking form of mechanical energy input to the sea arises from surface wind stresses and pressure gradients imposed by the atmosphere. These forces drive ocean currents and generate surface waves whose impact at the coast can be dramatic and is often seen as symbolic of the ocean's power. In many shelf seas, however, energy input through tidal forcing is a more consistent and more powerful source of mechanical energy. Most tidal energy is delivered to the shelf in the form of energy fluxes in tidal waves which originate in the deep ocean, although there is also a (usually small) contribution arising from tidal body forces acting directly on the waters of the shelf seas. Both winds and tides inject very large amounts of kinetic energy to the ocean as a whole; total inputs have been estimated recently as ~1 and ~3.5 TW for wind and tidal inputs respectively (Munk and Wunsch, 1998).

In this chapter we shall look at waves and turbulence, two forms of motion which are of particular importance in the shelf seas because of their roles in bringing about the mixing which re-distributes properties such as heat, salt, momentum and substances dissolved or suspended in the water. There is a marked contrast in character between the two: waves are generally highly ordered motions which are amenable to precise mathematical description, while turbulence is chaotic in nature and it can usually only be represented in terms of its statistical properties. Both waves and turbulence are large scientific topics in their own right and are the subject of more than a few specialised textbooks. Here, we shall focus on those aspects of surface waves, internal waves and turbulence theory which are necessary to the understanding of processes in shelf seas, and we shall leave the more specialised aspects for the interested student to pursue from the further reading list.

Surface waves

We have already developed the theory of long waves in Chapter 3 from the basic equations and shown how such waves can help us to understand tidal motions in shelf seas. The more general theory of surface wave motions, in which there is no restriction on wavelength, is more involved and a full treatment is beyond the scope of this book. Here we shall simply present the assumptions and the principal results of the theory of infinitesimal waves and give a physical description of the motion involved. As well as being useful in themselves, the results of surface wave theory introduce us to many of the concepts relevant to the understanding of the more complicated motions involved in internal waves.

Much of the physics in the preceding chapters can be traced back to the fundamentals of fluid flow encapsulated in the equations of motion and continuity, along with the eddy description of turbulence. By contrast, describing the basics of life in the sea presents us with the difficulty of trying to distil a broad set of concepts from a system which is inherently very complex. Our experience of working at sea alongside biologists has been stimulating and fruitful, but there is always a tension: physicists can get exasperated at the complexity of the systems that biologists like to describe, while the biologists roll their eyes at the physicists’ insistence on boiling problems down to as simple a level as possible. In this chapter we will take more of a physicist's view of biology in the ocean, focusing mainly on those aspects of the biology that are relevant to understanding how organisms’ access to resources and growth are controlled by the structure and motion of the fluid environment.

Broadly, we are aiming to understand how organic compounds are produced in the ocean, and their fate. The schematic illustration of Fig. 5.1 provides us with a framework for the chapter; you could also have a look at the final schematic in Fig. 5.19 if you would like some idea of the details that we will be adding to this framework. We will begin by describing the fundamental biochemistry that lies at the heart of the growth of the autotrophs, the single-celled, photosynthesising phytoplankton which produce the organic matter and so power both the rest of life in the ocean and the cycling of carbon. In contrast, the heterotrophs consume organic material, either recycling it back to inorganic matter or passing it further up the food chain by being food for larger heterotrophs. Heterotrophs are much more varied in form and in their methods used for finding and consuming their prey, so instead of trying to detail this variety we will identify their broad roles in the ecosystem and some of the constraints that life in a turbulent fluid imposes on them. The biological processes that we will describe are in general common to the open ocean and to the shelf seas. We will use shelf sea examples to illustrate the processes, and identify where the important contrasts are between shelf and open ocean biology.

At the outer edge of the shelf is a region where the gentle slopes of the shelf give way to the much steeper topography of the continental slope, and bottom depths rapidly increase down to the abyssal plains of the deep ocean. On average, the depth at which this slope transition occurs is about 130 metres, but this varies through the world's oceans. Off NW Europe the shelf edge is at a depth of 200 metres, while at high latitudes the shelf edge is deeper, typically 400–500 metres around Antarctica and off Greenland. Because the topography is steep, with slopes as large as 1:10, the transition between shelf and the deep ocean is usually limited in extent (~50 km). It is in this rather narrow region that the very different regimes of the shelf and the deep ocean adjust to each other. In this chapter, we shall consider how this adjustment occurs and how it controls the important exchanges between shelf and deep-ocean. We will look at wind-driven upwelling, the most studied process linking the physics and the ecology of the shelf edge which, in many parts of the world, supports important stocks of plantivorous fish. We shall also consider the upwelling (and downwelling) driven by the bottom Ekman layer of along-slope flows, and the consequences for nutrient supply to, and organic material export from, shelf seas. We will describe the density contrasts that develop in winter between temperate shelf seas and the adjacent ocean that can lead to downslope cascades of shelf seawater and its constituents. Finally, we consider the role of the internal tide, a prominent shelf edge process which strongly influences the biochemistry and is important in relation to commercial fisheries.

As we saw in Chapter 2, the input of buoyancy over much of the shelf seas is dominated by heating and cooling through the sea surface. There are additional exchanges of buoyancy at the sea surface through the processes of evaporation and precipitation, both of which modify the salinity of surface layers, but in temperate latitudes their contribution is generally small in comparison with heat exchange. In areas of the shelf adjacent to estuaries, however, freshwater discharge from rivers can make a dominant contribution to buoyancy input (Section 2.3) and maintain strong horizontal gradients of salinity.

In this chapter, we shall consider these Regions Of Freshwater Influence (ROFIs) (Simpson, 1997), the suite of processes which operate within them and the distinctive environment that results. In terms of physical processes, ROFIs have much in common with estuaries, but they differ insofar as estuaries are confined by land barriers and are generally of a smaller horizontal scale than that which is characteristic of ROFIs. In many cases, the circulation in an estuary is predominantly an along-channel flow while transverse motions are relatively small and the effects of the Earth's rotation can be neglected. We shall initially develop the theory of density-driven circulation for such a simplified, non-rotating system before proceeding to consider how rotation modifies the density-driven circulation in ROFIs. We shall next examine the way in which the density-driven circulation is involved in determining the water column structure of ROFIs in competition with stirring and then explore the implications for the biology and environmental health of ROFIs.

In the last chapter we developed an understanding of the basic seasonal competition between heating and stirring, in which the mixing was driven by frictional stresses at the water column boundaries. In this chapter we shall describe the generally far weaker mixing which occurs across density interfaces within the interior of the water column. We will illustrate the physics involved using more detailed models of the interaction between buoyancy input and vertical mixing processes in the seasonally stratified regime. We will show where the models fail in their descriptions of mixing and how correcting these failings is vital if we are to understand and model the survival and growth of phytoplankton in stratified waters.

Pycnoclines often separate biochemically distinct regimes in the water column: high light and low nutrients near the sea surface, low light and high nutrients near the seabed. The inherent stability of a pycnocline can provide a niche for phytoplankton that contains both sufficient light and nutrients for survival. We will describe the links between physical and biological processes that lead to the survival of phytoplankton; you will see that understanding the processes that drive turbulence within and across pycnoclines lies at the heart of the growth and distribution of the primary producers.

We have now provided the foundations in physics and biology that can be used to understand how shelf sea ecosystems are driven by the underlying physics. In this chapter we will begin our journey through the links between physics and biology in shelf seas by identifying the role of physics in partitioning the shelf seas into biogeochemically contrasting regimes. Recall from Chapter 5 that the vertical structure of the water column is critical in determining the availability of both light and nutrients to phytoplankton. In this chapter we first consider the fundamental physical controls that determine whether or not a region will thermally stratify in spring. We derive a condition for seasonal stratification to occur which indicates that, during the summer season, the shelf seas will be divided into stratified and mixed regimes. We then consider the spring stratification as the physical trigger for the spring bloom of phytoplankton. This highlights the spatial partitioning into mixed and stratifying regions as controlling whether or not regions experience a spring bloom.

Buoyancy inputs versus vertical mixing: the heating-stirring competition

The vertical structure of the water column is the result of an ongoing competition between the buoyancy inputs, due to surface heating and freshwater input on the one hand and stirring by the tides and wind stress on the other. Our focus here is on temperate shelf sea regions where the dominant buoyancy input is a seasonally varying surface heat flux. We will deal with freshwater as a source of buoyancy in Chapter 9. During the winter months, when heat is lost from the surface, the buoyancy term contributes to stirring by increasing surface density and making all or part of the water column convectively unstable. As a result, apart from limited areas influenced by large positive buoyancy inputs from estuaries, the shelf seas are vertically well mixed during the winter months. This vertically mixed regime continues until the onset of positive heating at, or close to, the vernal equinox (21 March/September in the northern/southern hemisphere) after which the increasing input of positive buoyancy tends to stabilize the water column. Whether or not the water column stratifies is then dependent on the relative strengths of the surface heating and the stirring due to frictional stresses imposed at the bottom boundary by the tidal flow and at the surface by wind stress.

In this final chapter we shall try to provide a perspective of the science of shelf seas and an indication of some of the important challenges which remain. In looking to the future of the subject, we shall highlight the need to move from temperate latitude shelf seas, which have been the focus of most research to date, into the shelf seas of the Arctic and the tropics. In both these areas, the suite of physical processes controlling the shelf sea environment and its biogeochemistry is substantially different from that operating in temperate latitudes. As well as looking at the prospects for future research in these areas, we shall also consider new ideas on the role of the shelf seas in the global ocean system and their putative influence on climate change since the last ice age. But first, we shall try to identify the big questions which remain in relation to the scientific understanding of the mid latitude shelf seas.

Remaining puzzles in the temperate shelf seas

As we have seen in previous chapters, the dominant physical processes controlling the environment of the shelf seas in temperate regions have been identified. The different shelf sea regimes have been defined in terms of the particular processes which dominate them, and the interaction of these processes has, in several cases, been simulated at least to first order in numerical models.