Tides
Introduction
The great English poet, Geoffrey Chaucer, wrote “Time and tide wait for no man.” Like time, the flow of the tides is steady, predictable, and unstoppable. Tides are the regular, alternating rise and fall of sea level caused by the gravitational pull of the moon and sun. The changing of the tide is often rapid and dramatic. On a smaller scale, similar motions occur on large lakes, in the atmosphere, and even within the solid earth.
During a tidal cycle in the oceans, sea level rises, or floods, until it reaches its highest normal point, called high tide. Sea level drops, or ebbs, to its lowest point at low tide. The difference between high and low tide is the tidal range. These tides occur because forces generated by the gravitational pull of the sun and the moon, and by the rotation of the earth, tug seawater into two enormous bulges. One bulge is located on the edge of the planet closest to the moon. The other bulge forms on the opposite side of the planet.
The sun and the moon both contribute to the formation of these tidal bulges. The tidal force generated by each body is determined by its mass and its distance from Earth. The sun is 27 million times more massive than the moon, but it is also almost 400 times farther away. As a result, the moon exerts more than twice as much tidal force on the oceans, so that a tidal bulge forms beneath and follows the moon. Solar gravity acts only to reinforce or diminish the moon’s pull.
The other tidal bulge forms on the far side of the planet. As the earth rotates through space, the water on it tends to keep moving in a straight line. This tendency is able to tug the sea up into a bulge where the moon’s gravitational force is weakest. These bulges are the basis of Earth’s tides, but many other factors also influence the behavior of tides.
On a global scale, the dynamic interaction between the pull of the sun and moon determines the range and magnitude of the tides. When the sun and moon align, their gravitational forces combine to produce the very highest and lowest tides, called spring tides. When the sun and moon are at right angles, they pull at the sea from different directions and moderate tides, with a smaller range, form. These are called neap tides.
On a more local scale, the timing, size, and speed of tides varies with the shape of the coastline, seafloor topography, river discharge, and even wind and weather conditions. For example, many areas have two, equal high tide – low tide cycles each day. In some regions though, the two cycles have different heights, and in still others, there is only one high and low tide each day.
In all of these circumstances, the once or twice daily rise and fall of sea level poses enormous challenges and opportunities for the plants and animals that live in the dynamic space between high and low tides. This challenging habitat is known as the intertidal zone. It has four subdivisions—the spray zone, and the upper, middle, and lower intertidal zones.
The spray zone is wetted by the splash of breaking waves, but rarely submerged. This zone supports very limited life. The high intertidal zone is underwater only during high tide. The middle intertidal zone is mostly submerged except for brief periods once or twice a day during low tide. The lower intertidal zone is only exposed to air during the lowest spring tides.
The organisms in the two highest zones are out of the water for long periods of time – up to several weeks at a stretch in the spray zone, and several hours once or twice a day in the upper intertidal. In order to survive here, they must be able to avoid drying out, endure temperature extremes, withstand intense sunlight, and survive a wide range of salinities. They also must escape predation by land animals during their long exposure.
Intertidal organisms cope with these harsh conditions in several ways. Seaweeds and other plants have tough leathery leaves that resist water loss. They grow in clumps that collapse upon one another when stranded so that only the top layer is exposed to sunlight. Soft bodied animals, like anemones, crowd together to reduce their surface area. Others scrunch up their bodies and secrete a protective coating of mucous when out of water. Shelled animals, like snails and clams, close up tightly to conserve moisture and repel predators. Mobile animals retreat with the falling tide, burrow into sandy or rocky shelters, or hide beneath overhanging rocks and amid plants.
In contrast, the plants and animals living in the deeper intertidal zones are less affected by the challenges posed by exposure, because they are usually protected by the cover of water. However, the lower, middle, and even upper intertidal zones are relentlessly pounded by every breaking wave. They must be able to bear up under these repeated blows, as well as maintain their position despite the powerful surge of tides, waves, and currents.
Organisms have developed a variety of adaptations to survive wave and tidal action. Sea stars have a thick skin and strong skeleton that can handle wave energy. They also have powerful arms lined with suction like tube feet to hold on tight in moving water. Sea slugs and other snails have a large foot that grips tightly to surfaces as they move or rest. Many other invertebrates, such as clams, and mussels have very thick shells, and cement themselves to rocks and one another for strength against the waves. Lower intertidal plants sway easily with the waves and currents because of firmly anchored roots and flexible stems. Fish that frequent the intertidal zone tend to have sturdy bodies and strong muscles.
Although the challenges of the intertidal zone are extreme, the marine organisms that survive there are well rewarded. Food is plentiful, washed in both from the land and the deeper sea. And the difficult conditions exclude many competing species.
Global Impact
The rise and fall of the tides has powerful effects on both deep ocean and coastal environments. Twice a day, tides roll through the open sea and tidal currents bounce off of sea floor ridges and shelves. The resulting turbulence stirs and mixes ocean waters, redistributing heat and nutrients and helping to regulate climate. Some recent studies suggest that approximately half of the energy needed to power the ocean’s global circulation pattern comes from the tides.
However important the role of tides in the deep sea, their impact on human activity is most apparent along the shore. Many of these effects are expensive, and some are dangerous. The constant shifting of local sea level poses a number of problems for coastal residents, and the designers of facilities such as marinas, ports, bridges, and offshore oil platforms. In areas where the tidal range is large, tides can worsen the damage caused by storms and tsunamis. If these destructive waves come onshore at high tide, flooding and erosion can occur much further inland than if they arrive at low tide.
Although tides can be destructive, they also have the potential to become an important source of reliable, renewable energy. Electricity can be generated by both the up and down motion of sea level, and by the sideways flow of tidal currents. There are technological barriers to harnessing tidal power, as well as environmental concerns. One of the most promising designs involves the use of offshore turbines, which function much like underwater windmills.
The economic impact of tides on coastal inhabitants is significant. But tides also play a powerful role in the emotional relationship between people and the sea. Tidepools, small pockets of water and life that appear at low tide, provide a uniquely accessible, intimate means of experiencing the mystery and wonder of the ocean realm. These transient windows into the sea have made many of those who explore them more aware, more appreciative, and more protective of oceanic ecosystems.