So far, we have been concerned only with the consequences of ocean physics for the development of pelagic ecosystems characteristic of the open ocean. We should also note, very briefly, the fact that in the shallow seas, whether enclosed or over continental shelves fronting the open ocean, ideal physical processes are dominated by the consequences of land form. This brief excursion into the asymmetries of coastal oceans, and their
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consequences for the forcing of ecological processes, will touch only on two aspects—
tides and tidal streams, and the nature and distribution of terrigenous sedimentary material. You have to know something about the special regional characteristics of each of these if you want to understand the ecological processes occurring in any region of the coastal seas.
This is not the place for a treatise on tides and tidal streams, nor am I equipped to write one, but it will be useful to bear in mind the extraordinary complexity of the processes that determine regional tidal characteristics when considering the ecology of coastal seas. Complexity derives from the fact that tides are driven by an equally complex suite of gravitational forces, each of which produces an individual effect on sea level so that the observed tides represent the interaction between several tidal elements. Consider then the fact that the ideal oceanic tides are modified by the location and the shape of the continents and are further molded by the shape of the open coastline, islands, and adjacent enclosed seas as the tides run up over the continental shelf into shallow water.
No wonder that the understanding of tidal phenomena dominated research on ocean physics from classical times until the mid-19th century.
It is no more than common knowledge that tides are raised in the ocean by the gravitational effect of moon and sun. Because the gravitational effect of the moon is rather more than twice that of the sun, we are accustomed to thinking of tides as related exclusively to pull of the moon, but that is far from the case. Sun, moon, and Earth dance a complex ballet for which you will find a description in any good text; here, we shall need only to recall that the consequence of this ballet, in which the relative positions of the three spheres, and the relative distances between them, are changing constantly, but in a repetitive manner. These changing orbital relationships produce 4 semidiurnal tides, of which 2—the lunar and solar M2 and S2 tides—command most attention, together with 3 diurnal and 3 long-period tides. The observed tides are raised, then, by a series of harmonic oscillations (or “partial tides”) having the periods of the changing orbital relationships.
The interactions between these forces are expressed differently in each region. In the Atlantic, with the minor exception of the Gulf of Mexico, semidiurnal tides prevail: that is, there are two high and two low waters each day. In the NW and W Pacific, tides are predominantly diurnal, with only one high and one low water daily. Some enclosed seas, such as the Baltic and most of the Mediterranean, have extremely small tidal ranges and hence weak tidal streams. Further regional complications arise because the ideal tidal sequence and the ideal relative heights of tides of various periods is under topographic control and differs strongly from place to place on each coast. The orientation of the coastline, and the arrangement of its headlands and bays, will tend to favor one or another of the various tidal components. A single explanation cannot account for the great tides of places like the Bristol Channel and the Bay of Fundy: in the Bay of St. Malo, for example, the tidal range is greater than can be accounted for simply by the narrowing and shoaling of the bay. This is due to the fact that the tide, advancing up the English Channel, takes the form of a Kelvin wave with small magnitudes on its left side, on the English coast, but great magnitude on its right, French side.
But we are not concerned with coastal processesper se, and it will be the consequences of coastal morphology for the velocity of tidal streams and the consequential overturning of stratification that will be of greater significance to us in thinking about the pelagic ecosystem over continental shelves. Tidal velocities are, of course, modified by coastal form and the placement of headlands, and the most direct effect will be the acceleration of flow in shoal water or through straits, or wherever else flow is constricted. On great continental shelves that are relatively flat, such as parts of the coasts of the Arctic Ocean, friction between the rough sea bed and the superjacent tidal stream may almost dissipate
Coastal Asymmetry, Geomorphology, and Tidal Forcing 69
the flow. The consequences of all this for the location of shallow-water fronts between tidally mixed and stratified water was discussed, as you will recall, in Chapter 3.
The form of continental terraces reflects the movement of the continents themselves:
trailing-edge terraces along the eastern coast of the Americas differ strongly in profile from the “collision” or leading-edge terraces along the western coastline (see, for example, Eisma, 1988). Because mountain building occurs preferentially along leading-edge coasts, rivers opening there are short and carry little organic material; in contrast, great rivers tend to flow to trailing-edge coasts and carry larger sediment loads, richer in humus.
Note, however, that the present-day sediment loads carried to the sea may be very different from the pristine state: the Amazon carries only about half as much sediment as the smaller Ganges/Brahmaputra system, because of the relative intensity of human development in the two drainage basins.
We shall expect these facts to have major consequences for the ecology of each type of continental terrace. Thus, “upwelling” coasts associated with eastern boundary currents have narrow, steep-to continental terraces, whereas coasts associated with western boundary currents tend to have broader terraces with a less steep slope. On geologically active, collision coasts the continental crust is thinly covered with sediments, whereas on trailing-edge shelves the sediment cover may become very deep. In extreme cases, on prograding shelves along trailing-edge coasts, the crustal topography becomes covered with a thick blanket of sediments within which bottom currents, themselves heavily loaded with sediments, may cut deep canyons. Slumping and folding of sediments may also occur by local tectonic activity. It is also helpful to consider the relative ages, or maturity, of continental shelves, depending on the era in which the large continental blocks separated. Young shelves are characteristic of the eastern Asian region, from New Zealand to the Sea of Okhotsk, with the exception of those around the South China Sea.
More mature shelves are characteristic of much of the Atlantic coastline.
The relative strength of tidal streams on the shelf has, of course, a second significance for ecological geography. This has to do with the transport, sorting, and redistribution of terrigenous sediments, which largely determine the location of characteristic faunas of benthic invertebrates and demersal fish. It may therefore be useful to use a little space in a discussion of how shelf deposits were constituted and came to be distributed as we observe them to be.
Recall that the continental shelves were largely dry land during the recent glaciations and that their superficial geology is no more than an extension of that of the continents, partially overlain by terrigenous and pelagic sediments, and calcareous biogenic material of benthic origin. Therefore, present-day bottom types range from rocky ridges, cobbles, and water-worn gravel, through sand and shell sand, to organic muds, in addition to biogenic reefs of calcareous skeletal material. During glacial periods, coarse sediments were preferentially deposited near the actual shelf edge, and this pattern still dominates the pattern of sedimentation on many shelves. Along eastern coasts of both the North American and Asian continents, relic sediments from glacial epochs still dominate much of the shelf, and modern sand and silt lies preferentially shallower, closer to the coast, dominating in bights and gulfs where sedimentation from suspension is induced.
It was originally supposed that there would be a simple particle-size gradient toward the shelf edge and that a “mud line” would be found everywhere at about 50 m, shoaler than which wave action would prevent the deposition of the smallest organic particles. This approximately describes the situation off Western Europe, where the earliest investigations were made, but it is not a useful generalization.
Sedimentation is not, as it might appear to be from the resultant arrangement of sediments, a continuous process, but is rather episodic and the arrangement of sediments is evolutionary. Episodic settlement of fine-grained material occurs when tidal streams (or other sources of turbulence) weaken. The deposit at first tends to form patches,
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reflecting the topography of the bottom terrain, filling hollows and bowls, until a plane surface is formed. Within a few hours, the new sediments become dewatered and their surface becomes rippled by subsequent water movement, the dimension of the ripples depending on current speed and sediment particle size. In mature sediments, under rapid water movement, ripples may become parallel sand waves or ridges several meters high, arranged at right angles to mean flow. Resuspension occurs continuously, and sorting of particles by their density follows, sand being deposited as ripples, and mud mainly in the troughs between. Bedding of sediments of different density occurs also at tidal and event scales. Bioturbation by burrowing benthic organisms powerfully modifies the ideal, physically driven arrangement.
Terrigenous material delivered by rivers to the shelf comprises mineral sand particles and organic material derived from the decay of terrestrial and estuarine vegetation. The latter tends to remain incompletely oxidized in estuarine regions and close inshore, whence the black, organic-rich, reducing and acidic muds that may dominate here where tidal streams weaken. Pelagic organic material sinking to the shelf deposits comprising fecal pellets, phytoplankton cells, and zooplankton carcasses is more completely oxidized and forms blue or gray muds, often with high calcium content from the skeletons of foraminifera. Sedimentary material of benthic origin comprises fragments of the shells of mollusks, and of the exoskeletons of echinoderms and crustacea. These various material are sorted and transported by coastal currents and tidal streams according to their relative density and particle size to form the mosaic of bottom types we observe on continental shelves. Where, as off California, the topography of the shelf is characterized by the existence of deep basins and offshore islands and the input of terrigenous material is negligible, the grain size and organic content of sediments follows the general pattern of depth. Close inshore and in the shoal water around islands, grain size is large and organic content low. Progressively into the deep basins and troughs, grain size decreases and organic content increases.
Where, as in the many tropical regions, wave action at the coast is relatively slight and input from rivers is significant, terrigenous mud deposits may occur close inshore, while offshore muds are principally of pelagic origin. The distribution of sediments and water types on the shelf determines, quite precisely, the composition of the benthic invertebrate and demersal fish faunas on the continental shelves.
Thus, ecological processes on continental shelves are fragmented spatially, and pre- dictable only from a specific knowledge of the forces that drive this fragmentation. It is this problem that makes it so much more difficult to generalize satisfactorily about the geography of ecological processes in coastal seas than in the open ocean. Research into benthic ecology has, in recent decades, turned very largely toward an understand- ing of fluxes of energy within the invertebrate and microbial community, and between this and the pelagic ecosystem. Consequently, the exploratory surveys characteristic of benthic ecology of the first half of the 20th century are no longer done, and we are left with adequate descriptions of the distribution of benthic species associations covering only a very small fraction of the continental shelves of the world. From these, we must extrapolate to the unknown regions.