The Global Aquatic Ecosystem

Even if you've never seen the ocean, you are connected to it. That's because water is always making its way to the ocean through a network of streams, ponds, marshes, lakes, and rivers. All these water bodies collect moisture that drains from nearby land. All land is divided into watersheds — that is, areas of land that drain into particular bodies of water.

A drainage basin includes all the watersheds that flow into a salty water body such as an ocean. Canada's six drainage basins are named after their final destinations: the Pacific, Atlantic (St. Lawrence River), and Arctic Oceans, plus Hudson Bay, the Gulf of Mexico, and the Internal drainage basin. In which drainage basin do you live? Now can you see how you're connected to the ocean through the network of flowing water in which you live?

An ecosystem is a community of biotic (living) creatures that interact with each other and abiotic (non-living) elements. An ecosystem can be as big as a planet or as small as a puddle. But whatever its size, an ecosystem consists of biotic elements (all living things) along with abiotic elements (soil, rocks, air, and water). Different ecosystems are linked by common elements, such as water, air, or even wildlife species that travel from one ecological area to another.

No living thing can survive without moisture, so all ecosystems need water. Aquatic ecosystems contain either fresh or salt water. Freshwater ecosystems include both standing and flowing waters. Ponds, lakes, marshes, swamps, sloughs, potholes, and puddles are examples of standing water ecosystems. Streams and rivers are examples of flowing water ecosystems. Marine ecosystems include both shores and oceans. Shore ecosystems range from rocky and sandy beaches to tide pools, estuaries (tidal mouths of rivers where salt and freshwater mix), salt marshes, and mud-flats. Ocean ecosystems include pelagic (moving water) and benthic (ocean floor) communities.

Waters of the continental shelf (sometimes called neritic waters) and those of the open ocean are examples of pelagic environments. The continental shelf is that area near the coast where the ocean floor lies less than 200 to 500 metres beneath the surface. These waters represent less than 10 percent of the ocean, yet contain 90 percent of its life forms.

No one knows how many different species live on the planet. About 1.4 million have been identified so far, but scientists think there could be anywhere from 10 to 80 million.

This amazing conglomeration of living things is known as biodiversity, which simply means a "variety of life."

There are three important types of biodiversity — genetic, species, and ecosystem diversity:

• Genetic diversity. Each species has unique genes that are found in no other life form. These genes provide the "blueprints", or operating instructions, that adapt a species to its particular living conditions. They tell a creature how to grow and how to survive. Genes tell whales how and where to migrate, or fish the best spots to spawn. Such blueprints are found in humans, too, and have evolved over millions of years.
• Species diversity. Every ecosystem contains a variety of species that depend on each other for survival in many ways. Sometimes the connection is obvious. A fish may be eaten by a bird or seal, but may itself eat even smaller fish, insects, and plants. But many connections between species are not all that clear to us, and some are very complex. For instance, the seeds of the Galapagos tomato must be eaten and digested by the giant Galapagos tortoise before they can sprout.

Ecosystems with a wide variety of species are more capable of adjusting to disruptions, such as Hudson fluctuations in food supplies or wildlife populations. On the other hand, if even a single species in a less diverse ecosystem disappears or is threatened, the whole system could collapse. That's because all living things are linked together in one way or another. If we take away one species, all the rest will feel the impact — including humans.

• Ecosystem diversity. There are many kinds of ecosystems. Each has a special assortment of species, some of which are found nowhere else on Earth. A few examples of ecosystems are mountains, prairies, oceans, rocky coasts, and swamps — but there are many others. What's more, similar ecosystems in distant parts of the world can support quite different wildlife species. For instance, a kelp forest off the coast of Nova Scotia is not the same as one off New Zealand's shores.

Scientists define oceans as the bodies of salt water that cover about three-quarters of the Earth's surface. Imagine — an ocean can be 10 kilometres deep! Sunlight only reaches 30 to 120 metres under the waves. That's why sun-loving ocean plants flourish near the surface. These plants — especially tiny floating algae called phytoplankton — produce enough food and oxygen to support an incredible array of marine creatures.

The complex interactions that occur within ecosystems are all-important — often in ways we haven't begun to understand. For instance, populations of American oysters in Chesapeake Bay fell 99 percent between 1870 and 1992. Once they were so plentiful that they filtered the waters of the bay every three days. Now the same process takes a full year, which is one reason why the water is so muddy and lacking in oxygen.

Oceans provide most of the water we need to survive. Since time began, the same amount of water has splashed around in an everlasting circle called the hydrologic cycle. This process begins when the sun's heat changes water into vapour (evaporation). Plants also give off vapour through their leaves. When the vapour cools, it falls back to earth as rain, snow, or sleet. Oceans play a huge part in this cycle: each year about 330,000 cubic kilometres of water evaporate from oceans! In all, oceans provide 97.5 percent of the Earth's water and cover about 75 percent of its surface.

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