68
Chapter 4: Physical Control of Ecological Processes
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 processes per 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
Chapter 4: Physical Control of Ecological Processes
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 processes per 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