The seas and oceans are rich in animal life, from whales and fish to starfish and sponges. The nature of marine communities depends upon depth, with the surface and the ocean floor being particularly rich in biodiversity. Similar communities are often found at similar depth, even though they may be widely separated geographically. There are also complex marine habitats, such as mangrove swamps, estuaries, and coral reefs, occurring near the shore.
Marine ecosystems depend largely upon phytoplankton, which are photosynthetic algae living near the surface of the water where the sun penetrates. Tiny herbivores feed on the phytoplankton and these, in turn, are eaten by increasingly larger animals, ending with larger fish and sharks at the top of the marine food web. The seas and oceans are important for humans as a resource for fish and other marine products, but its ecosystems are threatened by exploitation, such as overfishing and pollution.
Historical Background and Scientific Foundations
Until the middle of the nineteenth century, it was assumed that few, if any, animals and plants lived in the seas and oceans because the waters were dark and cold. The first clues to the existence of rich and complex marine ecosystems came from broken underwater telegraph wires. When these were retrieved, various unusual and previously unknown creatures were found clinging onto them. The HMS Challenger, a British naval vessel, carried out the first-ever oceanographic survey between 1872 and 1876, exploring as deep as 18,700 ft (5,700 m) in the Pacific Ocean. The expedition returned with thousands of specimens, many of which were previously unknown to science.
The marine environment is where life evolved in the first place. Seawater contains sodium chloride (NaCl) and other mineral salts, and has remained at roughly the same salt concentration for millions of years. Salinity, as the salt concentration is called, is around one ounce per liter, of which 90% is composed of sodium chloride. This happens to be the same sodium chloride concentration as living cells, making it a natural environment for organisms to deal with.
Water covers around 70% of Earth’s surface. Its average depth is just over 2 mi (3.2 km)—ranging from a few inches close to the shore to around 7 mi (11.2 km) at its greatest depth. The oceans alone provide more than 170 times more living space than land, air, and freshwater put together. The weight of water does exert pressure upon those organisms living there. This pressure will increase by one atmosphere for every 33 ft (10 m) of depth. But, like organisms living with atmospheric pressure on land, deep-sea animals such as fish and snails have the same pressure inside and outside their bodies. Most sea creatures are composed mainly of water and, since liquids are incompressible, they do not experience adverse effects on moving from one depth to another.
The seas are constantly in motion due to surface currents and deeper circulation currents, which means that cold salty water, which is heavier, sinks and is replaced by warmer, less salty water. This mixes the water, making its chemistry uniform as well as carrying oxygen (O) from the surface to deeper layers, making life down there possible. The temperature of surface water varies from 104°F (40°C) in tropical waters to about 35°F (1.9°C) for seawater in the Arctic and Antarctic. The depths of the oceans are always very cold, even in tropical regions, at 32 to 37°F (0 to 3°C).
Compared to the land, the marine environment is not so rich in different species. Only one tenth of the
WORDS TO KNOW
BENTHIC: Living on, or associated with, the ocean floor.
MARINE: Living in, or associated with, the sea.
PELAGIC: Living in, or associated with, open areas of ocean away from the bottom.
PHYLUM (PLURAL, PHYLA): A biological classification group lying between kingdom and class.
nearly 2 million animal species known are found in the sea, and only around 4,000 plants, compared to a quarter of a million on land. But marine ecosystems tend to be more diverse, with 28 different phyla existing in the oceans, compared to only 11 on land.
Temperature, salinity, and the availability of oxygen, light, and nutrients shape the marine ecosystems. Thousands of different invertebrates make their home in the ocean, often providing food for larger species. For instance, there are about 5,000 species of sponges, of a wide variety of colors and shapes. These animals have the least complex body structure of all multi-celled creatures, consisting of an outer layer of tissue and an innerlayer of either silica (SiO2) or calcium carbonate (CaCO3). The echinoderms, which include starfish, sea urchins, and sea cucumbers, are a group of exclusively marine bottom-dwelling invertebrates, characterized by hard, spiny skin. Around 6,000 species dwell in the world’s salt waters, often in rocky pools around beaches.
The cnidarians are another important marine invertebrate group and include the jellyfish, corals, and sea anemones. These organisms are characterized by their soft, watery bodies. The molluscs, which are found on land and sea, include the gastropods (limpets, slugs, and snails), cephalopods (octopus, squid, and cuttlefish), and bivalves (clams, mussels, cockles, oysters, and scallops). Most crustaceans, of which there are about 39,000 species, are marine and include crabs, lobsters, shrimps, barnacles, and woodlice. The copepods, which are tiny crustaceans ranging from 0.02 to 0.7 in (0.05 to 1.8 cm) in length are particularly abundant. There could be hundreds of thousands of copepods in a cubic meter of surface water and they are an important source of food for predators like marine worms and the smaller jellyfish.
Marine worms are another diverse group, many of which are completely different from the well-known earthworm. For instance, the arrow worm is a major predator of the copepods. Only 1-in (2.5-cm) long, it is also known as the chaetognath worm, which means “bristle jaw.” The worm darts and grabs its prey in its jaws. The nematodes, or thread worms, are among the tiniest of creatures in the marine environment, being just a fraction of an inch long. They live in sediments and feed off bacteria. As a group, the nematode worms have not been studied much, and researchers believe there may be thousands of species remaining to be discovered. At the other extreme are the tube worms found on the ocean floor, which may be up to 6 ft (1.8 m) in length. These creatures are so unusual that they have been placed in a phylum of their own. They have no mouths, but their bodies are filled with chemosynthetic bacteria, which extract energy from minerals rather than sunlight, and these provide much of their food supply.
Birds, mammals, and fish all live in or around the marine environment, but fish are the only vertebrates that are purely aquatic and are found in both freshwater and saltwater around the world. Around half of the 25,000 known species of fish live in the marine environment, mainly in shallower, warmer waters. Around 1,000 fish species occupy the open ocean. Fish that live in the deep ocean are generally black, brown, or gray, without the silvery camouflage that characterizes those living nearer the surface. Some are buoyant and swim up and down the depths of the water, searching for food, while others, including sharks and rays, are heavier and sink if they cease swimming. They tend to stay in place, catching food as it passes, or making just short hunting excursions.
Whales are marine mammals, breathing with lungs, and the largest animals in the oceans. There are two suborders: the baleen whales, which do not have teeth and filter feed on massive amounts of plankton; and the toothed whales, which feed on fish and squid. Toothed whales have the remarkable ability to navigate through the ocean by echolocation. The whales, sharks, and giant squid represent the top end of the marine ecosystem. Their sheer size has made them famous in marine culture and folklore.
In the open ocean, the ecosystems are vertically stratified. That is, different plants and animals are found at different depths. Sunlight penetrates to a depth of only 3 ft (0.9 m) or so in the cloudy waters of an estuary, compared to up to 300 ft (90 m) in the clearer waters of the open ocean. In this relatively light region, known as the euphotic zone, there is a net primary production of food by photosynthesis carried out by 4,000 or so species of phytoplankton.
The column of water extending down from the surface to a depth of about 2.5 mi (4 km) is called the pelagic zone and is composed of the epipelagic (top), mesopelagic (middle or twilight), and bathypelagic (bottom) zones. Below this, extending to about 4 mi (6.4 km), are the abyssal and hadal zones. Hadal comes from the Greek word for unseen, abyssal from the Greek word for bottomless. There is no light in these zones close to the ocean floor other than what is emitted from the organisms themselves. Thousands of species, including bacteria, squid, and fish, emit flashes of light by a process
known as bioluminesence to distract predators and locate prey.
The oceans and seas contain some specific ecological niches. For instance parts of the ocean floor, or benthos, contain hydrothermal vents, where jets of water containing sulfur (S) compounds gush up, heated by the magma beneath the ocean floor. The ecological communities of microbes, worms, and mussels living around hydrothermal vents were known only since 1977. They can withstand temperatures of up to 660°F (350°C), and the microbes are capable of extracting biochemical energy from the sulfur compounds.
Shallow areas of the seas and oceans have rich ecosystems too. The sea floor slopes gradually from the shore out to the deeper ocean. The area near the shore is called the littoral zone, and many fish and shellfish are found here. Filter feeders such as mussels and barnacles live in rocky-bottomed shores, while soft-bottomed beaches are home to scavengers like shrimps and polychaete worms. The number of species increases nearer to water. On sand
IN CONTEXT: THE GULF OF MEXICO
The Gulf of Mexico is a unique, semi-enclosed sea located between the Yucatan and Florida peninsulas, at the southeast shores of the United States. The Gulf of Mexico borders 5 of the 50 United States (Alabama, Florida, Louisiana, Mississippi, and Texas), and also Cuba and the eastern part of Mexico. Sometimes it is also called America’s Sea. The Straits of Florida divides the Gulf from the Atlantic Ocean, while the Yucatan Channel separates it from the Caribbean Sea. The Gulf of Mexico covers more than 600,000 square mi (almost 1.5 million square km), and in some areas its depth reaches 12,000 ft (3,658 m; called Sigsbee Deep, or the “Grand Canyon under the sea”). About two thirds of the contiguous United States (31 states between the Rocky Mountains and the Appalachian Mountains) belongs to the watershed area of the Gulf of Mexico, while it receives freshwater from 33 major river systems, and many small rivers, creeks, and streams. This watershed area covers a little less than two million square mi (almost 5 million square km).
The Gulf of Mexico has several environmental quality problems originating either from natural processes, or from anthropogenic (human-caused) pollution, or their combination. The problems range from erosion and topsoil washing from the land into the Gulf, to oil spills and hazardous material spills, or trash washing ashore. These problems not only affect the estuaries, wetlands, and water quality in the Gulf, but have led to problems such as hypoxia (a zone of oxygen-depleted water), declining fish catch, contaminated fish, fish kills, endangered species, and air and water quality problems.
that is never completely submerged, it is necessary to dig in to find worms and sand crabs, while sea anemones and sea cucumbers are found near rocky shores. Estuaries, where river meets the sea, mixes freshwater and mud sediments with seawater and are often rich in fish because the nutrient levels in the water are high.
Mangrove swamps or forests are another important area near the tropical shores. Mangroves are salt-resistant trees that support fish or shrimp. Coral reefs such as the Great Barrier Reef, which stretches for around 1,200 mi (1,930 km) along the eastern coast of Australia, are probably one of the best-known marine ecosystems. They consist of animals called stony corals, each of which is a polyp with tentacles that can trap organisms. The polyps live in symbiosis with photosynthetic algae. The algae provide them with food, and the polyps provide the algae with protection. Each polyp lies in a shell of calcium carbonate and they are joined together by a sheet of tissue to form a giant organism and ecosystem in its own right. Coral reefs tend to occur in warm, shallow, clear waters where they provide a home for a diverse community of fish, worms, and crustaceans, protecting the small fish from larger predatory fish.
The marine food web is based upon the photosynthetic activity of phytoplankton, which is often greatest near the shore, where the water tends to be richer in nutrients like nitrogen (N) and phosphorus (P) because of runoff from land. Many tiny herbivores feed directly on phytoplankton, as well as the juvenile stages of certain squid and fish. These miniature predators will, in turn, be meals for slightly bigger creatures. The pattern goes on, up to the larger fish and sharks that are the equivalent of wolves and lions on the land. Generally, animals do not feed on those that are more than a tenth their size. However, there are exceptions to this. Some sharks will feed on animals as big or bigger than themselves, taking bite-sized lumps out of them. Fish with wide mouths and big teeth in the deeper layers can swallow bigger animals. The blue whale, the largest animal in the ocean, dines solely on tiny shrimp called krill. Meanwhile, dead phytoplankton and other organisms sink to the bottom of the ocean, forming a “marine snow” that benthic organisms, such as crabs and fish, depend upon as a food source.
Impacts and Issues
Human activity impacts upon marine ecosystems just as it does on land, with the areas nearest the coast being most affected. For instance, the coral reefs are among the world’s most threatened biomes. They are affected by a range of factors, including destructive fishing practices, pollution, and sewage. Meanwhile, global warming is already causing bleaching of coral reefs. Increased temperatures destroy the symbiotic relationship between the coral and the algae that live there. According to research from the United Nations, one third of coral reefs around the world are destroyed already, 60% are damaged and will likely be dead by 2030. Mangrove swamps are similarly at risk.
Estuaries and shore areas are also at risk from pollution, which can cause eutrophication by raising nutrient levels in the water. Eutrophication encourages the growth of decomposers that consume available oxygen in the water. As oxygen levels fall, fish and other organisms begin to die off. Eutrophication also causes an overgrowth of algae in the water, often visible as a red, yellow, or green scum on the surface and a visible sign of a threatened ecosystem. These dead zones are found in many areas around the world, such as the Mediterranean and the East Coast of the United States.
The deep ocean has also been used as a dump for low-level radioactive waste, although this was banned in 1993. There have also been discussions on burying medium and high-level radioactive waste from nuclear power stations. If land alternatives prove too risky, these options may be put into practice, but no one knows what the long-term impact might be upon marine ecosystems, however secure the waste was made.
Humans do not just put things into the seas and oceans. They take things out as well. Fishing is a traditional activity, with fish being an important protein source in the human diet. However, global fish harvest-
ing has increased 5 times during the last 50 years or so, partly because fishing technology has become more efficient and partly because of the increase in human population, which has increased demand for food. The oceans can probably support a fish harvest of about 100 million tons (90 million metric tons) of fish caught per year. As the fishing industry has expanded, these limits are being reached. Fishing vessels now have to travel farther and farther to get catches. Not only does this hurt the economy, there could also be as-yet-unknown disturbances to the marine ecosystem by driving fish stocks down in this way.
People are also seeking to exploit the ocean for oil and gas. Drilling offshore began in 1947 in the Gulf of Mexico, and now there are thousands of such developments. It may be that efforts to extract oil and gas will go deeper still, despite the difficulties of the technology, with unknown effects on the marine environment. There has also been discussion about whether it might be possible to exploit the sea bed as a source of minerals. As the world’s population grows and industrial development spreads, the pressure to use the ocean as a resource can only increase, with unknown impacts on marine ecosystems.
See Also Algal Blooms; Aquatic Ecosystems; Bays and Estuaries; Benthic Ecosystems; Coastal Ecosystems; Coastal Zones; Commercial Fisheries; Estuaries; Marine Fisheries; Ocean Salinity; Ocean Tides; Oceans and Coastlines; Overfishing; Reef Ecosystems
Cunningham, W.P., and A. Cunningham.Environmental Science: A Global Concern. New York: McGraw-Hill International Edition, 2008.
Kaufmann, R., and C. Cleveland. Environmental Science. New York: McGraw-Hill International Edition, 2008.
Rice, Tony. Deep Ocean. London: Natural History Museum, 2000.