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The Oceans. Introduction. The increase in world population and the continued rise of industrialization have resulted in a need to further understand the world’s oceans. One of the most important factors that impact the biosphere is the condition of the world’s oceans.
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Introduction The increase in world population and the continued rise of industrialization have resulted in a need to further understand the world’s oceans. One of the most important factors that impact the biosphere is the condition of the world’s oceans. A basic understanding of the structure and composition of the ocean and knowledge of how life in the ocean affects life on land can increase the extent to which we are able to protect this important natural resource and the life that depends on it.
Misconceptions • The general understanding is that the ocean surface has no actual relief of its own and therefore is flat. • Another misunderstanding that people often have is that tides are caused by the action of the wind. Actually, tides are not caused by the wind, but by the gravitational pull of the moon and sun on the Earth.
Misconceptions • The ocean depths are devoid of life. • The seafloor is flat and the same age as the continents.
Regulating Mechanism • What is the biological pump? • Biologic activity, in particular primary productivity, draws in CO2 from the surrounding water column. • Dead organisms will sink in the water column. Some of it will remineralize and some will continue below the thermocline. That material that makes it below the thermocline is effectively segregated from the surface ocean, thereby completing the “pumping” of atmospheric CO2 into the deep ocean.
Regulating Mechanism What does the pump affect? • Global climate (perhaps) and carbon flow • Locally change CO2 levels • Also alterCH4, N2O, and DMS • 30 to 40% of fossil fuel CO2 goes into oceans • Small perturbations to the system can have large ramifications
Ocean Structure and Composition • Atmospheric pressure at sea level is 14.7 pounds per square inch (also referred to as "one atmosphere"), and pressure increases by an additional atmosphere for every 10 meters of descent under water.
Epipelagic Zone The surface layer of the ocean is known as the epipelagic zone and extends from the surface to 200 meters (656 feet). It is also known as the sunlight zone because this is where most of the visible light exists. With the light come heat. This heat is responsible for the wide range of temperatures that occur in this zone.
Mesopelagic Zone Below the epipelagic zone is the mesopelagic zone, extending from 200 meters (656 feet) to 1000 meters (3281 feet). The mesopelagic zone is sometimes referred to as the twilight zone or the midwater zone. The light that penetrates to this depth is extremely faint. It is in this zone that we begin to see the twinkling lights of bioluminescent creatures. A great diversity of strange and bizarre fishes can be found here
Bathypelagic Zone The next layer is called the bathypelagic zone. It is sometimes referred to as the midnight zone or the dark zone. This zone extends from 1000 meters (3281 feet) down to 4000 meters (13,124 feet). Here the only visible light is that produced by the creatures themselves. The water pressure at this depth is immense, reaching 5,850 pounds per square inch. In spite of the pressure, a surprisingly large number of creatures can be found here. Sperm whales can dive down to this level in search of food. Most of the animals that live at these depths are black or red in color due to the lack of light.
Abyssopelagic Zone The next layer is called the abyssopelagic zone, also known as the abyssal zone or simply as the abyss. It extends from 4000 meters (13,124 feet) to 6000 meters (19,686 feet). The name comes from a Greek word meaning "no bottom". The water temperature is near freezing, and there is no light at all. Very few creatures can be found at these crushing depths. Most of these are invertebrates such as basket stars and tiny squids. Three-quarters of the ocean floor lies within this zone. The deepest fish ever discovered was found in the Puerto Rico Trench at a depth of 27,460 feet (8,372 meters).
Hadalpelagic Zone Beyond the abyssopelagic zone lies the forbidding hadalpelagic zone. This layer extends from 6000 meters (19,686 feet) to the bottom of the deepest parts of the ocean. These areas are mostly found in deep water trenches and canyons. The deepest point in the ocean is located in the Mariana Trench off the coast of Japan at 35,797 feet (10,911 meters). The temperature of the water is just above freezing, and the pressure is an incredible eight tons per square inch. That is approximately the weight of 48 Boeing 747 jets. In spite of the pressure and temperature, life can still be found here. Invertebrates such as starfish and tube worms can thrive at these depths.
Ocean Currents • Ocean currents allow for mixing to occur. This in turn provides for redistribution of heat from low latitudes to high latitude, carry nutrients from deep waters to the surface, and shape the climates of coastal regions. • There are three primary ways for mixing to occur.
Ocean Currents • Waves and surface currents are caused mainly by winds. • The Ekman transport causes mixing by combining the effects of the wind and the Coriolis effect – deflecting the current approximately 45 to the direction of the wind – going to the right in the Northern hemisphere and left in the Southern hemisphere.
Ocean Currents • Thermohaline Circulation is responsible for mixing the ocean at deeper levels. • The density of water increases as it becomes colder and saltier so it sinks at high latitudes and is replaced by warm water flowing northward from the tropics.
Ocean Currents • When the Ekman Transport combines with the Thermohaline Circulation, gyres are formed. • Gyres rotate clockwise in the Northern Hemisphere and counter-clockwise in the Southern Hemisphere, driven by easterly winds at low latitudes and westerly winds at high latitudes.
The Ocean & the Earth’s Climate • The oceans redistribute heat from high to low latitudes by moving warm water from the equator toward the poles. • In areas where coastal upwelling brings cold water up from the depths, cold currents have the opposite effect. • Because water warms and cools more slowly than land, oceans tend to moderate climates in many coastal areas.
Thermohaline Circulation • Often referred to as the "global conveyor belt“, it moves large volumes of water along a course through the Atlantic, Pacific, and Indian oceans. • The thermohaline circulation is driven by buoyancy differences in the upper ocean that arise from temperature differences (thermal forcing) and salinity differences (haline forcing).
Thermohaline Circulation • Salinity differences are caused by evaporation, precipitation, freshwater runoff, and sea ice formation. • When sea water freezes into ice, it ejects its salt content into the surrounding water, so waters near the surface become saltier and dense enough to sink.
Driving Bodies of Water • North Atlantic Deep Water (NADW), the biggest water mass in the oceans, forms in the North Atlantic and runs down the coast of Canada, eastward into the Atlantic, and south past the tip of South America. • Antarctic Bottom Water (AABW), is the densest water mass in the oceans. • It forms when cold, salty water sinks in the seas surrounding Antarctica and flows northward along the sea floor underneath the North Atlantic Deep Water.
Ocean Circulation and Climate Cycles • Measuring the variables that signal switches in climate cycles, such as changes in ocean temperature and atmospheric pressure, is an important research focus for ocean and atmospheric scientists who are working to make better predictions of climate and weather cycles.
Ocean Circulation and Climate Cycles Monsoon rain clouds near Nagercoil, India, August 2006
Ocean Circulation and Climate Cycles • Monsoons are a well-known example of a seasonal climate cycle. • As land temperatures increase during summer months, hot air masses rise over the land and create low-pressure zones. • At the surface, ocean winds blow toward land carrying moist ocean air. • When these winds flow over land and are lifted up by mountains, their moisture condenses and produces torrential rainfalls.
Ocean Circulation and Climate Cycles • Hurricanes develop on an annual cycle generated by atmospheric and ocean conditions that occur from June through November in the Atlantic and from May through November in the eastern Pacific. • The main requirements for hurricanes to develop are warm ocean waters (at least 26.5°C/80°F), plenty of atmospheric moisture, and weak easterly trade winds.
Ocean Circulation and Climate Cycles • The best-known climate cycle is the El Niño Southern Oscillation (ENSO), which is caused by changes in atmospheric and ocean conditions over the Pacific Ocean. • Atmospheric pressure rises over Asia and falls over South America, equatorial trade winds weaken, and warm water moves eastward toward South and Central America and California. • Coastal upwelling in the eastern Pacific dwindles or stops. • Warm, moist air rises over the west coasts of North and South America, causing heavy rains and landslides as droughts occur in Indonesia and other Asian countries.
Ocean Circulation and Climate Cycles • The Pacific Decadal Oscillation (PDO) is a 20- to 30-year cycle in the North Pacific Ocean. • Positive PDO indices (warm phases) are characterized by warm Sea Surface Temperature (SST) anomalies along the Pacific coast and cool SST anomalies in the central North Pacific. • Negative PDO indices (cold phases) correspond to the opposite anomalies along the coast and offshore. • Cool PDO phases are well correlated with cooler and wetter than average weather in the western United States. • During the warm phase of the PDO, the western Pacific cools and the eastern Pacific warms, producing weather that is slightly warmer and drier than normal in the western states.
Ocean Circulation and Climate Cycles • The North Atlantic Oscillation (NAO), another multi-decadal cycle, refers to a low-pressure region south of Iceland and a high-pressure region near the Azores. • Positive NAO periods occur when the differences in Sea Level Pressures (SLP) are greatest between these two regions. Under these conditions, the westerly winds that pass from North America between the high and low pressure regions and on to Europe are unusually strong, and the strength of Northeast Trade Winds is also strengthened. • This strong pressure differential produces warm, mild winters in the eastern United States and warm, wet winters in Europe as storms crossing the Atlantic are steered on a northerly path. • In the negative phase, pressure weakens in the subtropics, so winter storms cross the Atlantic on a more direct route from west to east. • Both the eastern United States and Europe experience colder winters, but temperatures are milder in Greenland because less cold air reaches its latitude.
Biological Activity in the Upper Ocean • Most life in the ocean does not have fins or flippers. • Single celled organisms called phytoplankton far outnumber the sum of all the marine organisms most of us think of first. • They convert huge quantities of carbon dioxide (CO2) into living matter. • In that process they release a major percentage of the world's oxygen into the atmosphere.
Biological Activity in the Upper Ocean • Derived from the Greek words phyto (plant) and plankton (made to wander or drift), phytoplankton are microscopic organisms that live in watery environments, both salty and fresh. • Some phytoplankton are bacteria, some are protists, and most are single-celled plants. Among the common kinds are cyanobacteria, silica-encased diatoms, dinoflagellates,green algae, and chalk-coated coccolithophores.
Biological Activity in the Upper Ocean • Like land plants, phytoplankton have chlorophyll to capture sunlight, and they use photosynthesis to turn it into chemical energy. They consume carbon dioxide, and release oxygen. • Phytoplankton, like land plants, require nutrients such as nitrate, phosphate, silicate, and calcium at various levels depending on the species.
Biological Activity in the Upper Ocean • Other factors influence phytoplankton growth rates, including water temperature and salinity, water depth, wind, and what kinds of predators are grazing on them. • When conditions are right, phytoplankton populations can grow explosively, a phenomenon known as a bloom.
Biological Activity in the Upper Ocean Phytoplankton can grow explosively over a few days or weeks. This pair of satellite images shows a bloom that formed east of New Zealand between October 11 and October 25, 2009. (NASA images by Robert Simmon and Jesse Allen, based on MODIS data.)
Biological Activity in the Upper Ocean • Many of these events are not harmful in themselves, but they deplete oxygen in the water when the organisms die and decompose. • Sometypes of phytoplankton algae produce neurotoxins, so blooms of these varieties are dangerous to swimmers and consumers of fish or shellfish from the affected area. • Most plankton blooms are beneficial to ocean life because they increase the availability of organic material.
Biological Activity in the Upper Ocean • Climate cycles can have major impacts on biological productivity in the oceans. • An El Niño event – reduction of phytoplankton results in no sardines and anchovies reduces food for large predators like tuna, sea lions, and seabirds • A PDO event – results in reduction of salmon, several ground fish, albacore, seabirds, and marine mammals in the North Pacific.
The "Biological Pump" • Deep waters provide nutrients that plankton need for primary production in the upper ocean, but how do these nutrients get to the ocean depths? • They are carried down from the surface in a rain of particles often referred to as marine snow, which includes fecal pellets from zooplankton, shells from dead plankton, and other bits of organic material from dead or dying microorganisms.
The "Biological Pump" • When marine snow reaches deep waters, some is consumed by bottom-dwellers and microbes who depend on it as a food source. • Some is oxidized, releasing CO2, nitrate, and phosphate and recycling nutrients into deep waters. • The remainder is buried in sediments and is the source of today's offshore oil and gas deposits.
The "Biological Pump" Three sediment trap designs. The original funnel design uses a large collection area to sample marine snow that falls to great depths. Surface waters contain enough sediment that traps there don't require funnels. Neutrally buoyant, drifting sediment traps catch falling material instead of letting it sweep past in the current.
The "Biological Pump" • This flow of particles to ocean depths is a critical link in the global carbon cycle. • Plankton take up carbon from the atmosphere in two ways: they fix CO2 as organic carbon during photosynthesis and form shells from calcium carbonate (CaCO3). • Marine snow carries both of these forms of carbon away from the atmosphere and surface waters to reservoirs in the deep oceans and ocean sediments, where it remains stored for centuries.
The "Biological Pump" • Without this mechanism, concentrations of CO2in the atmosphere would be substantially higher. • The overall efficiency of the biological pump depends on a combination of physical and biogeochemical factors. • Both light and nutrients must be available in sufficient quantities for plankton to package more energy than they consume.
Resources • http://people.duke.edu/~ts24/ENSO/ - El Nino video • http://www.nasa.gov/vision/earth/lookingatearth/plankton.html - chlorophyll productivity map • http://earthobservatory.nasa.gov/Features/Phytoplankton/ - phytoplankton information
