Water returning to the oceans from the land carries huge amounts of sediment. The buildup of these sediments, together with erosion shores by pounding waves, has created the continental shelves that border the coasts.
In general, a sharp drop, the continental slope, marks the edge of the continental shelf and the boundary of the deep oceanic basins. The shelf, slope, and in many places the ocean bottom are covered with a fine mud or ooze consisting of slit, minerals precipitated from sea water, and the microscopic shells of dead marine animals.
Much of the ocean floor is a broad abyssal plain. Here and there the floor is studded with underwater mountains, often of volcanic origin. Where these seamounts just above the water they form small, isolated islands, such as those of Hawaii.
Also interrupting the flatness of the abyssal plain are deep trenches and mountainous submarine ridges. The Japanese islands are the exposed tops of one such ridge.
Drifting Continents and Changing Oceans
Recently it has been widely agreed that the continents were once jointed together, and that the present oceans were formed after the continents drifted apart.
This is the theory of plate tectonics, or as it is more popularly known, continental drift. There is a significant body of evidence in support of this theory. Equally important, it explains many previously puzzling features of the earth's surface.
Close inspection of a world map reveals that some continental coastlines have complementary shapes. Like pieces of a jigsaw puzzle, they seem to fit together.
Furthermore, the present ocean floors appear to be relatively young. Examination of cores drilled from bottom sediments suggests that the Atlantic Ocean formed only about 200 million years ago. That was probably when the Americas began to separate from Africa and Europe. The separation was a slow process which still continues.
Drifting of continents from one climatic zone to another may be part of the explanation for why dinosaurs and lush tropical vegetation once existed in what are now temperate areas of the United States and Europe. It certainly explains distribution of closely related species on distant continents-for example, the southern beeches found only in Chile, New Zealand, Tasmania, and Australia.
Are you asking yourself how continents move?
Crustal plates
It is clear that the earth has an unstable crust. Catastrophic earthquakes and volcanic eruptions all too often reveal the tremendous forces at work inside the earth. Luckily for us, most internal stresses, such as those that fold the earth's crust into mountain ranges, act so slowly that they do not affect us during our lifetime. Yet these forces move continents.
Present information indicates that the earth's crust is not an unbroken skin. Instead it consists of at least six-and probably more- fairly rigid blocks or plates.
These plates do not correspond exactly to any of our customary geographic boundaries but often include both continental masses and ocean bottom. Being composed of rather light rock, these crustal plates float on top of the heavier, more plastic material beneath. And like ice cubes floating in water, adjacent plates rub together and slide post or over each other as they are pushed apart by upheavals of molten rock from below.
Tectonics means building
The Atlantic Ocean floor is spreading as the result of activity at mid-oceanic ridges. Here new crustal material seems to be seeping out from fissures and forming additional sea floor. This, in turn, pushes the continents apart. In other oceans similar crusted plates are also growing.
But since the earth is not expanding in circumference, old parts of the crust must be undergoing destruction. This seems to be happening at Pacific Ocean trenches, among other places, such oceanic tranches; some more than six miles deep, develop when one plate is focused under another.
In places, molten crust comes to the surface, forming volcanoes. Volcanoes are particularly common along ocean ridges at places where the plates are growing. Volcanoes also appear near trenches where one crustal plate is slipping over another. In such locations, strings of volcanoes create island arcs. Not surprisingly, earthquake zones are associated with these island arcs as well as being common along other plate margins. Collisions between plates are also responsible for folded mountains.
The union of India (which once was an island) with Asia is believed responsible for the crumpled crust we know as the Himalayas.
Crustal movements are an ongoing characteristic of the earth, extending for back in time. Before the Old and New Worlds were united in one supercontinent, there was a time when they were separated by a body of water called the Protoatlantic Ocean.
Considering the behaviour of the crustal plates, no marine environment is changeless, although some are exceptionally stable compared with most habitats.
Ocean Habitats
Salts are constantly added to the ocean in river water and rainwater runoff. Depending on drainage, currents, and temperature, the saltiness of the oceans varies at different locations. The average salinity is about 3.5 percent, or about 35 parts by weight of salt to each 1000 parts of water.
Most of the salt in seawater is ordinary table salt (sodium chloride), but sulphate, magnesium, calcium, and potassium salts are also abundant. In fact, seawater contains traces of just about every element.
These accumulated salts make seawater much denser than fresh water. This is the reason it is easier to swim or float in the ocean than in a river or a lake. It also means that salt water has a much lower freezing point than fresh water. In addition to salt, seawater also contains dissolved atmospheric gases. But seawater has less oxygen than fresh water, since salt decreases the solubility of oxygen in water.
Ocean layers and life
The sun heats and illuminates the ocean, although few of the sun's rays penetrate very deep into the water. As a result, water temperature, illumination, and salinity (which is temperature dependent) are related to depth. Of course, all these characteristics influence the kind of organisms to be found at a given location, but light is of special importance.
Because photosynthetic plants need light in order to live, they are found only in shallow water and in the surface layer of the open ocean. Almost all photosynthesis occurs in the upper 80 meters (260 feet) designated as the euphotic (good light) zone.
Underneath the euphoric zone is a region of ever-deepening twilight. No sunlight penetates below 600 metres (1967 feet). As a result, most of the ocean is in perpetual darkness and can be inhabited only by animals and decomposer bacteria and fungi.
Life at the top
Microscopic floating algae, known collectively as phytoplankton, are the main producers in the oceans. Diatoms and din flagellates are the most prominent phytoplankton. Their collective biomass far out weights that of the more obvious algae known as "seaweeds." Most seaweeds are attached to rocks near the coast.
Phytoplankton is never evenly distributed. Not only are they limited to the top layers where there is light, but within that layer their density follows distribution of minerals.
Organisms that die in deep water usually drift to the bottom. Similarly, faces settle out, and the valuable nutrients they contain are lost from surface waters. Currents and diffusion only slowly return precious nitrogen and phosphorus from the bottom to the euphotic zone.
Shallow water over the continental shglves is usually fairly well mixed and contains more nutrients than surface water over the open ocean. Here the deep water is quite undisturbed, and minerals are concentrated far below the euphotic zone.
During the winter, because sunlight striking temperate waters is less intense than at other seasons, photosynthesis is limited to the top few metres of water. With the coming of spring the phytoplankton multiply rapidly in response to increasing illumination.
Exploding plankton populations are termed plankton blooms, because the masses, of algae often colour the water bright red, brown, or green. Although clearly visible, marine plankton blooms are never as dramatic as those of freshwater lakes. This difference in productivity may result from mineral deficiencies in illuminated marine waters.
Microscopic animals, often referred to as zooplankton, feed on phytoplankton. Copepods and krill, tiny relatives of crabs and shrimp, are among the most abundant zooplankton. Others are numbered among the unicellular protozoa and include foraminifera and radiolarians.
Typically, zooplanktons bear numerous projections that increase their surface area and help suspend them in the water. Most zooplankton both swims and floats.
For the most part, relatively small fish, such as anchovies, herrings, and sardines, prey on zooplankton. There are exceptions, however. In the cold but fertile water around Antarctica the penguins, fish, squid, and even baleen whales feed directly on krill. The amount of zooplankton consumed by a large animal can be astounding. A single blue whale can eat three tons of krill a day.
The briny deep
Beneath the productive upper layer of the ocean lies as much as six miles of water. Throughout most of this space, life is sparse. Food from the top decomposes quite slowly.
Fish in deep, dark waters are mostly mouth Good meals are rare here and few items are rejected as being too big. This is the province of the famous angler fish, which is equipped with luminescent lures that attract prey into its gaping mouth.
The low population level in deep water makes finding a mate at the right time a chancy proposition. In some species the males attach to the females during adolescence and maintain this parasitic relationship throughout life. As you might expect, the males of such species are tiny compared with their mates.
Rocks and Sand: Marsh and Muck
In contract to the relatively uniform conditions of the open sea, a variety of habitats occur where water meets land. Each of these environments is unique; each is intriguing in its own way. Perhaps rocky shores are the most interesting. Here we find a diverse community attuned to the rhythmically changing environment.
Living between the tides
In the band that spans high and low tide, the first priorities are to stay put, to keep wet, and to avoid being crushed. Waves pound the shores so vigorously that delicate organisms can be destroyed or carried away. Alternately submerged and exposed, intertidal species (those that live between lowest low tide and highest high tide) must also withstand cold, heat, and a wide range of salinity.
Summer sun can cook tissues, and evaporation of seawater deposits a crust of salt on every surface. Only a few months later, winter tides pull the protective layer of water away and leave organisms exposed to freezing cold or to torrents of fresh water. As adaptations to this extreme environment, intertidal organisms either cling tenaciously or dart about to find shelter. Their bodies are protected by shells, tough body walls, leathery surfaces; or muscus.
Since tidal exposure varies every day, there are distinct life zones along the shore. The following applies to the central California coast, but similar areas occur on rocky shores from Mexico to Alaska and along the coast of Maine and the Maritime Provinces, as well as on other continents.
A dark band of cyanobacteria (blue-green algae) marks the highest zone where there is much marine life. Here numerous periwinkles scavenge. A distinct band of white barnacles grows below the black zone. Barnacles lie on their backs, stuck to the rocks. When the tide goes out, barnacles close their limy shells to conserve water.
When they are submerged, they open their shell plates and feed, using delicate, jointed appendages to kick food into their mouths.
Limpets are abundantly distributed among the barnacles. These marine snails use a tremendous suction foot to pull their shells tightly against the rock. This traps water under the shells and prevents dehydration during low tide.
A crowded strip of mussels stands just below the mean-sea-level mark that coincides with the bottom of the barnacle region. The clams like mussels spin proteinaeous fibers that attach their blue-black shells to the rocks. Tidal pools harbour rock crabs, hermit crabs, and large green sea anemones.
Farther out, on the underside of large rocks and steep ledges hide sponges, sea cucumbers, sea squirts, chitons and abalone, Shrimp and spiny lobsters lurk in the lowest tide pools. No one has made a complete census of animals in the intertidal zone, but biologists familiar with the California coast estimate that there may be 300 species there.
The showy life of rocky shore appears missing from sandy beaches, but even though beaches appear barren, animals are present. It's just that most of them are hidden from view. Burrowing, digging, and tunnelling creatures, among them clams, mole crabs, tube worms, and olive shells, bury themselves in the sand.
Low tides expose some species periodically; others are permanently protected within their burrows. Typical residents below the low-tide line include whelks, swimming crabs, sand dollars, and hermit crabs.
Wetlands and estuaries
Bays and river mouths, where fresh and salt water meet, are unusually fertile environments. The most valuable of such estuaries are broad, shallow basins created by silt deposits.
Here mud flats and tidal marshes line channels of open water. Most of the silt and organic matter dumped from the river becomes trapped in its estuary. Slow currents, a large surface for evaporation, and presence of rooted vegetation all helps make estuaries places where nutrients tarry.
Salt marsh grasses, eel grasses, and other rooted plants take advantage of rich water and soft bottoms. Algal scums cover mud flats, larger plants, and every solid surface, and dense phytoplankton blooms colour the water. There is so much food that only a tiny part of the photosynthetic product finds its way directly into the mouth of herbivores. Instead, most plants die and, in the absence of swift currents, simply sink to the bottom and rot.
The abundant organic matter supports a teaming broth of bacteria and fungi that use all available oxygen from the muddy bottom. As a result, other anaerobic microorganisms thrive there and release hydrogen sulphide. This noxious gas blackens marshland mucks and imparts a characteristic stench.
Their tremendous photosynthetic productivity and extensive decomposer food chain permit estuaries to literally teem with life. Microscopic decomposers are eaten by filter-feeding zooplankton, as well as by larger filter feeders, such as worms, clams, and oysters. Estuaries serve as nurseries for numerous coastal fish.
Menhaden, striped mullet, summer flounder, king whiting, croakers, striped, bass, smelt, and strugeon are all commercially important species that rely on estuaries sometime during their lives. The shrimp fisheries also depend on this resource, because young shrimp require the estuary habitat. Oysters, blue crabs, and several commerically valuable elam species are permanent estuarian residents. Furthermore, salt water marshes are the natural home of most waterfowl.
As valuable as they are in their native state, estuaries are often prized more for their land. To take advantage of a protected harbor, most seaports are built on estuaries.
As early ports grew, the surrounding marshes and mud flats were usually drained, and regions of shallow water were dredged or filled to provide areas for docks, industries, airports, and even residences. In Washington, D.C., only the Tidal Basin remains to remind us that the Jefferson Memorial, Lincoln Memorial, and Washington Monument were built on land "reclaimed" from a marsh.
Once leached free of their load of salt, drained tidal marshes become fertile farmland. Over the centuries, carefully engineered diking and pumping schemes created much of the Netherlands.
But present generations seem little aware of how much humans are responsible for familiar shorelines and of how important estuaries are in the schemes of aquatic life. Even now, many of our least-modified estuaries are threatened with "development" into resorts.
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