CR Earth Science 1/2

Marine biologists have identified more than

250,000 marine species. This number is constantly increasing as new organisms are discovered.

Desalination

A process of removing salt and other chemicals from ocean water to make drinkable water. Large-scale desalination plants pump seawater through a series of superfine membranes. Water molecules are small enough to pass through the membranes, but salt and other dissolved chemicals are not. The freshwater is treated, stored, and distributed to homes, schools, and business as needed. The wastewater, which is much saltier than seawater, is diluted and returned to the sea. Hydrologists, scientists who study how water moves through the Earth's crust, and a variety of engineers, including environmental and structural engineers, design and oversee the construction of desalination plants. Systems engineers and operators keep the plants running.

What are the primary sources of dissolved substances in the ocean?

Chemical weathering of rocks on the continents is one source of elements found in seawater. These dissolved materials reach oceans through runoff from rivers and streams at an estimated rate of more than 2.3 billion metric tons per year. The second major source of elements found in seawater is from Earth's interior. Through volcanic eruptions, large quantities of water vapor and other gases have been emitted into the atmosphere during much of geologic time. Scientists believe that this is the principal source of water in the oceans.

Gas Hydrates

Gas hydrates are compact chemical structures made of water and natural gas. The most common type of natural gas is methane, which produces methane hydrate. Gas hydrates occur beneath permafrost areas on land and under the ocean floor at depths below 525 meters. Most oceanic gas hydrates are created when bacteria break down organic matter trapped in ocean-floor sediments. The bacteria produce methane gas along with small amounts of ethane and propane. These gases combine with water in deep-ocean sediments in such a way that the gas is trapped inside a lattice-like cage of water molecules.

Where are ocean surface temperatures the greatest?

High surface water temperatures occur in low latitude regions around the equator.

The ocean, like Earth's interior, is layered according to density

Low density water exists near the surface, and higher-density water occurs below. Except for some shallow inland seas with a high rate of evaporation, the highest-density water is found at the greatest ocean depths. Oceanographers generally recognize a three-layered structure in most parts of the open ocean: a shallow surface mixed zone, a transition zone, and a deep zone. These zones are shown in the figure.

Nekton

Nekton (nektos = swimming) include all animals capable of moving independently of the ocean currents, by swimming or other means of propulsion. Nekton are able to determine their position within the ocean and in many cases complete long migrations. Nekton include most adult fish and squid, marine mammals, and marine reptiles. The figure shows an example of nekton.

What is the difference between pure water and seawater?

One of the most obvious differences is that seawater contains dissolved substances that give it a salty taste. These dissolved substances include sodium chloride, other salts, metals, and even dissolved gases.

Plankton

Plankton (planktos = wandering) include all organisms—algae, animals, and bacteria—that drift with ocean currents. Just because plankton drift does not mean they are unable to swim. Many plankton can swim but either move very weakly or move only vertically. Among plankton, the algae that undergo photosynthesis are called phytoplankton. Most phytoplankton, such as diatoms, are microscopic. Animal plankton, are called zooplankton. Zooplankton include the larval stages of many marine organisms such as fish, sea stars, lobsters, and crabs.

Which zone experiences the greatest temperature variation?

The mixed zone at the surface of the ocean experiences the greatest diversity of temperature changes as it is the location where solar radiation has the strongest effect on surface temperatures.

Benthos

The term benthos (benthos = bottom) describes organisms living on or in the ocean bottom. The shallow coastal ocean floor contains a wide variety of physical conditions and nutrient levels. Most benthos organisms can be found living in this area. Shallow coastal areas are the only locations where large marine algae, often called seaweeds, are found attached to the bottom. These are the only areas of the seafloor that receive enough sunlight for the algae to survive.

What factors influence a region's photosynthetic productivity?

The two factors that influence a region's photosynthetic productivity are the availability of nutrients like nitrogen, phosphorus, and iron for primary producers to thrive and the amount of solar radiation, or sunlight.

The thermocline is a barrier that cuts off the supply of nutrients from deeper waters below. Productivity in tropical regions is limited by the lack of nutrients. These areas have so few organisms that they are considered biological deserts.

Water layers in the tropics (1) - Depth (m) 0-150 - Warm, nutrient-depleted surface waters (2) 150-250 m Thermocline (3) 250-500- m Cold, nutrient-rich deep water

The sun rises higher in the sky during spring, creating

a greater depth at which photosynthesis can occur. A spring bloom of phytoplankton occurs because solar energy and nutrients are available, and a seasonal thermocline develops. The thermocline traps algae in the euphotic zone. This creates a tremendous demand for nutrients in the euphotic zone, so the supply is quickly depleted, causing productivity to decrease sharply. Even though the days are lengthening and sunlight is increasing, productivity during the spring bloom is limited by the lack of nutrients.

You may be surprised to learn that productivity is low in tropical regions of the open ocean. Because the sun is more directly overhead, light penetrates much deeper into tropical oceans than in temperate and polar waters. Solar energy also is available year-round. However, productivity is low because

a permanent thermocline prevents mixing between surface waters and nutrient-rich deeper waters.

The abyssal zone is

a subdivision of the benthic zone. The abyssal zone includes the deep-ocean floor, such as abyssal plains. This zone is characterized by extremely high water pressure, consistently low temperature, no sunlight, and sparse life. Food sources at abyssal depths typically come from the surface. Some food is in the form of tiny decaying particles that steadily "rain" down from the surface. These particles provide food for filter-feeders, brittle stars, and burrowing worms. Other food arrives as large fragments or entire carcasses of organisms that sink from the surface. These pieces supply meals for actively searching fish, such as the grenadier, tripodfish, and hagfish.

Benthos organisms such as giant kelp, sponges, crabs, sea anemones, sea stars, and marine worms that attach to, crawl upon, or burrow into the seafloor occupy parts of the benthic zone. The benthic zone includes

any sea-bottom surface regardless of its distance from shore and is mostly inhabited by benthos organisms.

Two factors influence the photosynthetic productivity of a region of ocean water. The two factors are solar radiation and _____

availability of nutrients

Three factors are used to divide the ocean into distinct marine life zones:

availability of sunlight, the distance from shore, and the water depth.

Productivity is limited by

available sunlight in polar regions and by nutrient supply in the tropics. In temperate regions, which are found at mid-latitudes, a combination of these two limiting factors, sunlight and nutrient supply, controls productivity. Nutrient levels are highest in the winter. These levels drive increases in phytoplankton levels early in the spring and again in the fall. Zooplankton peak after phytoplankton. Sunshine is greatest in the summer, but the increased warmth in the water prevents mixing of water layers.

Gas hydrates resemble

chunks of ice but ignite when lit by a flame. The hydrates burn because methane and other flammable gases are released as gas hydrates evaporate. An estimated 20 quadrillion cubic meters of methane are locked up in sediments containing gas hydrates. This amount is double the amount of Earth's known coal, oil, and natural gas reserves combined.

By sampling ocean waters, oceanographers have learned that temperature and salinity—and the water's resulting density—vary with

depth. The figure shows two graphs of density versus depth. One graph shows the density for low-latitude regions and the other for high-latitude regions. Temperature is the most important factor affecting seawater density. It also shows that temperature is inversely proportional to density.

Marine organisms can be classified according to where they live and how they move. They can be classified as

either plankton (floaters) or nekton (swimmers). All other organisms are benthos, or bottom dwellers.

Other processes remove large amounts of fresh water from seawater, increasing salinity. These processes include

evaporation and the formation of sea ice. High salinities, for example, are found where evaporation rates are high, as is the case in the dry subtropical regions. This trend is illustrated in the graph on the peaks between 20° and 40° north and south latitude. In areas where large amounts of precipitation dilute ocean waters, as in the mid-latitudes and near the equator, salinity is lower. This trend is illustrated in the graph at the low point of 0° latitude. Surface salinity in polar regions varies seasonally due to the formation and melting of sea ice. When seawater freezes in winter, salts do not become part of the ice. Therefore, the salinity of the remaining seawater increases. In summer when sea ice melts, the addition of relatively fresh water dilutes the solution and salinity decreases.

Chemical energy stored in the mass of the ocean's algae is transferred to the animal community mostly through

feeding. Zooplankton are herbivores (herba = grass, vora = eat), so they eat algae. Larger herbivores feed on the larger algae and marine plants that grow attached to the ocean bottom near shore.

Which resource of the ocean floor is not currently being used, but in the future could become an important energy source?

gas hydrates

In high latitudes, this three-layered structure of the open ocean does not exist as seen in the figure. The three layers do not exist because there is no rapid change in temperature or density with depth. Therefore,

good vertical mixing between surface and deep waters can occur in high-latitude regions. Here, cold high-density water forms at the surface, sinks, and initiates deep-ocean currents.

Higher sea surface temperatures are found in

low-latitude regions

Most marine organisms live within the sunlit surface waters. Strong sunlight supports photosynthesis by

marine algae. Algae either directly or indirectly provides food for the majority of organisms. All marine algae live near the surface because they need sunlight to survive. Most marine animals also live near the surface because this is where they can find food.

Density is defined as

mass per unit volume. It can be thought of as a measure of how heavy something is for its size. For example, an object that has low density is lightweight for its size, such as a dry sponge, foam packing, or a surfboard. An object that has high density, such as cement or most metals, is heavy for its size. Density is an important property of ocean water because it determines the water's vertical position in the ocean. Density differences cause large areas of ocean water to sink or float. When high-density seawater is added to low-density fresh water, the denser seawater sinks below the fresh water.

The sun rises even higher in the summer, so surface waters in temperate parts of the ocean continue to warm. A strong seasonal thermocline is created that prevents the

mixing of surface and deeper waters. So, nutrients depleted from surface waters cannot be replaced by those from deeper waters. Throughout summer, the phytoplankton population remains relatively low. Global temperatures have increased rapidly over the last few centuries. Scientists believe phytoplankton in the ocean could help cool off Earth.

In which region of the ocean is water the densest?

polar regions and the deep ocean floor

Primary productivity is

production of organic compounds from inorganic substances through photosynthesis or chemosynthesis. Photosynthesis is the use of light energy to convert water and carbon dioxide into energy-rich glucose molecules.

Seawater density is influenced by two main factors:

salinity and temperature. An increase in salinity adds dissolved substances and results in an increase in seawater density. An increase in temperature results in a decrease in seawater density. Temperature has the greatest influence on surface seawater density because variations in surface seawater temperature are greater than salinity variations. However, in the very cold, polar areas of the ocean, salinity affects density significantly. Cold water that also has high salinity is some of the highest-density water in the world.

One drawback to using gas hydrates as an energy source is

that they rapidly break down at surface temperatures and pressures. In the future, however, these ocean-floor reserves of energy may help provide our energy needs.

The ocean's surface water temperature varies with

the amount of solar radiation received, which is primarily a function of latitude. The intensity of solar radiation in high latitudes is much less than the intensity of solar radiation received in tropical latitudes. Therefore, lower sea surface temperatures are found in high-latitude regions.

Although photosynthesis cannot occur much below 100 meters, there is enough light in the lower photic zone for marine animals to avoid predators, find food, recognize their species, and locate mates. Below this zone is

the aphotic zone, where there is no sunlight

Two factors influence a region's photosynthetic productivity:

the availability of nutrients and the amount of solar radiation, or sunlight. Primary producers need nutrients such as nitrogen, phosphorus, and iron. Lack of nutrients can be a limiting factor in productivity. Thus, the most abundant marine life exists where there are ample nutrients and good sunlight.

Surface water temperatures in high latitudes are much cooler than in low latitudes, so

the curve begins at the surface with a low temperature. Deeper in the ocean, the temperature of the water is similar to that at the surface, so the curve remains vertical. There is no rapid change of temperature with depth. A thermocline is not present in high latitudes. Instead, the water column is isothermal (iso = same, thermo = heat).

Below the transition zone is

the deep zone. Sunlight never reaches this zone, and water temperatures are just a few degrees above freezing. As a result, water density remains constant and high. The deep zone includes about 80 percent of ocean water.

Solar radiation decreases in

the fall as the sun moves lower in the sky. Surface temperatures drop and the summer thermocline breaks down. Nutrients return to the surface layer as increased wind strength mixes surface waters with deeper waters. These conditions create a fall bloom of phytoplankton, which is much less dramatic than the spring bloom. The fall bloom is very short-lived because sunlight becomes the limiting factor as winter approaches to repeat the seasonal cycle.

Marine life zones can also be subdivided based on distance from shore. The area where the land and ocean meet and overlap is

the intertidal zone. This narrow strip of land between high and low tides is alternately covered and uncovered by seawater with each tidal change. It appears to be a harsh place to live with crashing waves, periodic drying out, and rapid changes in temperature, salinity, and oxygen concentrations. However, the species that live here are well adapted to the constant environmental changes.

When water temperature increases, its density decreases. The pycnocline (pycno = density, cline = slope) is

the layer of ocean water between about 300 meters and 1,000 meters where there is a rapid change of density with depth. A pycnocline presents a significant barrier to mixing between low-density water above and high-density water below. A pycnocline is not present in high latitudes; instead, the water column is about the same density throughout.

The thermocline (thermo = heat, cline = slope) is

the layer of ocean water between about 300 meters and 1,000 meters, where there is a rapid change of temperature with depth. The thermocline is a very important structure in the ocean because it creates a vertical barrier to many types of marine life.

Marine algae, plants, bacteria, and bacteria-like organisms are

the main oceanic producers. As producers make food available to the consuming animals of the ocean, energy passes from one feeding population to the next.

Seaward from the low-tide line is

the neritic zone. This zone covers the gently sloping continental shelf. The neritic zone can be very narrow or may extend hundreds of kilometers from shore. It is often shallow enough for sunlight to reach all the way to the ocean floor, putting it entirely within the photic zone. Although the neritic zone covers only about 5 percent of the world ocean, it is rich in both biomass and number of species. Many organisms find the conditions here ideal because photosynthesis occurs readily, nutrients wash in from the land, and the bottom provides shelter and habitat. This zone is so rich that it supports 90 percent of the world's commercial fisheries.

Beyond the continental shelf is

the oceanic zone. The open ocean reaches great depths. As a result, surface waters typically have lower nutrient concentrations because nutrients tend to sink out of the photic zone to the deep-ocean floor. This low nutrient concentration usually results in smaller populations than the more productive neritic zone.

A third method of classifying marine habitats is based on water depth. Open ocean of any depth is called

the pelagic zone. Animals in this zone swim or float freely. The photic part of the pelagic zone is home to phytoplankton, zooplankton, and nekton, such as tuna, sea turtles, and dolphins. The aphotic part of this zone has giant squid and other species that are adapted to life in deep water.

The offshore sand-and-gravel industry is second in economic value only to

the petroleum industry. Sand and gravel, which include rock fragments that are washed out to sea and shells of marine organisms, are mined by offshore barges using suction devices. Sand and gravel are used for landfill, to fill in recreational beaches, and to make concrete.

The upper part of the ocean into which sunlight penetrates is called

the photic zone (photos = light). The clarity of seawater is affected by many factors, such as the amount of plankton, suspended sediment, and decaying organic particles in the water. In addition, the amount of sunlight varies with atmospheric conditions, time of day, season of the year, and latitude.

The euphotic zone is

the portion of the photic zone near the surface where light is strong enough for photosynthesis to occur. In the open ocean, this zone can reach a depth of 100 meters, but the zone will be much shallower close to shore where water clarity is typically reduced. In the euphotic zone, phytoplankton use sunlight to produce food and become the basis of most oceanic food webs.

Chemosynthesis is

the process by which certain microorganisms create organic molecules from inorganic nutrients using chemical energy. Bacteria in hydrothermal vents use hydrogen sulfide as an energy source. Acting as producers, these bacteria support the hydrothermal vent communities

Salinity (salinus = salt) is

the total amount of solid material dissolved in water. It is the ratio of the mass of dissolved substances to the mass of the water sample. Many common quantities are expressed in percent (%), which is parts per hundred. Because the proportion of dissolved substances in seawater is such a small number, oceanographers typically express salinity in parts per thousand. The average salinity of seawater is 3.5 percent or 35 parts per thousand. Most of the salt in seawater is sodium chloride, common table salt.

Below the sun-warmed zone of mixing, the temperature falls abruptly with depth. Here, a distinct layer called

the transition zone exists between the warm surface layer above and the deep zone of cold water below. The transition zone includes a thermocline and associated pycnocline. This zone accounts for about 18 percent of ocean water.

Oceanic productivity, however, varies dramatically because of

the uneven distribution of nutrients throughout the photosynthetic zone and the availability of solar energy due to seasonal changes.

When seawater evaporates, the salts increase in concentration until

they can no longer remain dissolved. When the concentration becomes high enough, the salts precipitate out of solution and form salt deposits. The most economically important salt is halite—common table salt. Halite is widely used for seasoning, curing, and preserving foods. It is also used in agriculture, in the clothing industry for dying fabric, and to de-ice roads.

Only a small percentage of the energy taken in at any level is passed on to the next because energy is consumed and lost at each level. As a result, the producers' biomass in the ocean is many times greater than the mass of

top consumers, such as sharks or whales.

The herbivores are then eaten by carnivores (carni = meat, vora = eat). Smaller carnivores are eaten by another population of larger carnivores, and so on. Each of these feeding stages is called a

trophic level.

The transfer of energy between trophic levels is

very inefficient. The efficiencies of different algal species vary, but the average is only about 2 percent. This means that 2 percent of the light energy absorbed by algae is ultimately changed into food and made available to herbivores.

Productivity in temperate oceans is very low during

winter, even though nutrient concentration is highest at this time. The reason is that solar energy is limited because days are short, and the sun angle is low. As a result, the depth at which photosynthesis can occur is so shallow that phytoplankton do not grow much.

Hydrothermal Vents

Among the most unusual seafloor discoveries of the past 30 years have been the hydrothermal vents along the oceanic ridge. Here, seawater seeps into the ocean floor through cracks in the crust. The water becomes super-heated and saturated with minerals. Eventually the heated water escapes back into the ocean. When the hot water comes in contact with the surrounding cold water, the minerals precipitate out, giving the water the appearance of black smoke. These geysers of hot water are referred to as black smokers. At some vents, water temperatures of 100°C or higher support communities of organisms found nowhere else in the world. In fact, hundreds of new species have been discovered surrounding these deep-sea habitats since scientists found some vents along the Galápagos Rift in 1977. Chemicals from the vents become food for bacteria. The bacteria produce sugars and other foods that enable them and many other organisms to live in this very unusual and extreme environment.

Density and temperature change very little with depth in polar regions and mixing occurs between surface waters and deeper, nutrient-rich waters. In the summer, however, melting ice creates

a thin, low-salinity layer that does not readily mix with the deeper waters. This lack of mixing between water masses is crucial to summer production, because it helps prevent phytoplankton from being carried into deeper, darker waters. Instead, they are concentrated in the sunlit surface waters where they reproduce continuously. Because of the constant supply of nutrients rising from deeper waters below, high-latitude surface waters typically have high nutrient concentrations. The availability of solar energy, however, is what limits photosynthetic productivity in polar areas.

The percentage of the world's oil extracted from offshore regions has increased from trace amounts in the 1930s to more than 30 percent today. Part of this increase is due to

advances in technology. Geologists and geophysicists use satellite imaging and powerful computers to pinpoint geological structures that indicate the presence of an oil or natural gas reserve. Engineers then use computer-generated maps to plan the best way to drill into the reservoir and extract the oil or gas. Accumulated oil and gas located thousands of meters underwater may be extracted by deep-water drill ships. These ships use the global positioning system (GPS) and satellite data to stay stationary above an exploration target.

Manganese nodules are

hard lumps of manganese and other metals that precipitate around a smaller object. Manganese nodules contain high concentrations of manganese, iron, and smaller concentrations of copper, nickel, and cobalt, all of which have a variety of economic uses. Cobalt, for example, is important because it is required to produce strong alloys with other metals. These alloys are used in high-speed cutting tools, powerful permanent magnets, and jet engine parts. With current technology, mining the deep-ocean floor for manganese nodules is possible but not economically profitable. Manganese nodules are widely distributed along the ocean floor, but not all regions have the same potential for mining. Good locations for mining must have a large amount of nodules that contain an optimal mix of copper, nickel, and cobalt. Sites like this are limited. In addition, it is difficult to establish mining rights far from land. Also, there are environmental concerns about disturbing large portions of the deep-ocean floor.