Energy in the Oceans

Oceans are one of the earth’s most prominent features, covering seventy percent of the earth’s surface and containing approximately 97% of the earth’s water supply. Oceans provide us with food resources, means of transportation, recreation, minerals and energy.  All life originated in the oceans and today both the largest (whale) and smallest (bacteria and viruses) organisms on earth live in the oceans.

All of the energy in the oceans, which drives ocean circulation and productivity, is derived from sunlight. The sunlight heats the surface waters, causing a temperature gradient (known as a thermocline) with depth. Because of the tilt and curve of the earth, different regions receive more direct sunlight than others do. Thus, the water in the tropics is warmer, while the water at the poles is cooler. The temperature of the water also affects chemical characteristics of the water, such as salinity, density, and gas solubility, causing additional concentration gradients. These gradients drive ocean circulation because of the associated variations in potential energy.

In ocean circulation, warm surface waters travel pole-ward from the tropics. On the way, heat is lost to the atmosphere in the cooler climates. At the poles, the water is further cooled, which increases its density and causes it to sink to the deep ocean. Deep ocean water gradually returns to the surface and is carried back to the tropics, where the cycle begins again. The more efficient this cycle, the more heat is transferred to the atmosphere, and the warmer the climate.
 
 

Because it is the main global transport mechanism for heat, ocean circulation can have large effect on global weather patterns. Deep ocean currents (often referred to as the global conveyor belt) cause large scale water movement through the ocean basins, driven by wind and heat transfer. Ocean circulation also effects ocean currents. Changes to ocean currents can cause draughts, floods, or storms, and other extreme weather in various part of the world.  Variations in the ocean's circulation can lead to variations in heat transport and thus affect weather patterns. One important variation in circulation is the change to circulation around the equator, known as El Niño. El Niño occurs regularly only once every two to five years. However, there is concern among scientists that global warming effects on ocean circulation will cause El Niño- type events more regularly and in different parts of the world.
 

Ocean Life

There is a wide variety of life in the oceans, ranging from phosphorescent benthic organisms, to photosynthetic surface dwelling phytoplankton, and including creatures of all shapes, sizes and diets. However, sixty percent of life in the oceans occurs in only two percent of the oceans’ water, mainly near the surface. The amount of life in an area of the ocean is referred to as its productivity. Variations in the climate of an area can have a great impact on the amount of productivity in the region. The warmer tropical regions are more productive because they receive more sunlight, thus warming the waters and increasing the solubility of the nutrients essential to productivity. The primary producers of the ocean also derive all their energy from sunlight, and are thus more abundant in these regions.

The primary producers of the ocean ecosystem are phytoplankton. Phytoplankton are small, single celled ocean plants which derive their energy from sunlight though a process called photosynthesis. Trees and other plants also derive energy from the sun by photosynthesis. Through photosynthesis, these organisms use the sun’s energy to convert the carbon from CO2 into biomass, releasing oxygen as a by-product.

Phytoplankton, while relatively low in biomass, account for forty percent of the planets primary productivity. Larger organisms (like fish and zooplankton) feed on phytoplankton, thus deriving energy and nutrients from that stored in the phytoplankton’s cells. Larger organisms (including humans) then feed on these organisms and so on, each deriving energy from the organisms it consumes. This process is called a food chain.  Each “step” up the food chain is called a trophic level.

With each step up the food chain, some amount of the initial energy stored by the primary producer is lost. The amount of energy lost per trophic level can be as high as about 90 percent or as low as about 10 percent. This means that the energy a fish might use in finding, consuming and metabolizing food is equal to about 90 percent of the energy that is actually assimilated from it’s prey.

Phytoplankton are very sensitive to changes in their environment. Pollution and other disturbances to the climate or ecosystem can strongly inhibit phytoplankton growth, thus affecting the entire food chain. The amount of phytoplankton at the base of the food chain is directly proportional to the overall productivity of the ecosystem. Therefore, the more phytoplankton growth is inhibited, the lower the overall productivity of the ecosystem. It is therefore important that we protect this valuable resource (the oceans), as fish are the main food resource for many countries. Special precautions should be taken not only to protect the oceans from pollution, but also to prevent over-harvesting, whether intentional or as by-catch, of essential organisms.
 

The Carbon Cycle

The fundamental element of all living organisms is carbon. Therefore, the cycling of carbon through an ecosystem is essential to that ecosystem's survival. There are three principal processes involved in the carbon cycle. The first is a carbon exchange from the atmosphere to plants and other surface sinks, in the form of carbon dioxide. This occurs through photosynthesis in plants and through direct carbon dioxide exchange between the oceans’ surface water layer and the atmosphere. CO2 exchange between the atmosphere and oceans is limited by the solubility of CO2 in that region due to the water temperature.

Besides being the first link in the marine food chain, phytoplankton are also a critical part of the ocean carbon cycle. During photosynthesis, phytoplankton remove carbon dioxide that is dissolved in seawater. The oceans then absorb additional carbon dioxide from the atmosphere in order to maintain the equilibrium. Therefore, the less phytoplankton there are, less CO2 is taken up by them, and CO2 concentrations in the atmosphere increase.

The second process involved in the carbon cycle is the storage of carbon in one of two carbon sinks: the oceans or the soil. The soil is considered a sink because when organic matter dies and decomposes, the carbon becomes incorporated into the soil, where it is stored until it is used or converted into another form of carbon. In the oceans, carbon is transported between the oceans surface water and deeper layers through two mechanisms. The first is through the upwelling and mixing of the oceans due to ocean transport processes. The second mechanism for the transport of carbon into the deeper layers is by the death and decay of organic matter, including, plants, phytoplankton and fish.

When phytoplankton die, they sink to the ocean floor and are buried by settling sediment. Therefore, the oceans are a sink for carbon which otherwise would accumulate in the atmosphere as carbon dioxide. Terrestrial vegetation and soil are also carbon sinks, however, when these materials are burned as fuel, the carbon is released to the atmosphere as CO2. Deforestation therefore also contributes to the accumulation of carbon dioxide in the atmosphere by destroying this sink.

The third element of the carbon cycle is the release of stored carbon back to the atmosphere. This can occur through many different processes. Carbon that has been assimilated into an organism may be again released through respiration of CO2. Additionally, some carbon stored in the global sinks is eventually converted to a fossil fuel. Examples of fossil fuels are oil, coal, or natural gas. When fossil fuels are burned, the carbon is converted into CO2 and released to the atmosphere.

Click here to see a diagram of the carbon cycle.
 

Nutrients

Other nutrients besides carbon are essential to phytoplankton growth, including nitrogen, phosphorous and iron. Scientists use what is called the “Redfield ratio” to describe the elemental composition of phytoplankton. The Redfield ratio was discovered by ______ in ____. He found that the ideal nutrient ratio for phytoplankton growth was Carbon:Nitrogen:Phosphorous:Iron  = 106:16:1:0.005. Therefore, in theory one pound of available iron can lead to the production of 100,000 pounds of phytoplankton biomass when sufficient quantities of the other nutrients are available.

The inorganic nutrients contained in ocean surface waters are constantly being used up by growing phytoplankton. Most phytoplankton are eaten on the spot and the nutrients comprising their biomass are regenerated on the surface to be used for another round of phytoplankton production.  However, some of these nutrients are not regenerated on the surface, but sink to the ocean bottom in dead organisms and fecal matter. In the deep sea, these nutrients are assimilated and regenerated into inorganic forms by bacteria. This action creates a sharp concentration gradient of nutrients with depth. Only some of these nutrients are ever brought back to the surface through upwelling of the deeper waters in various regions of the world.
 
 

Another source of nutrients to the surface waters is from the settling of atmospheric dust (particularly iron) and gas exchange. The sources of atmospheric dust are mainly the desserts, and therefore the waters bordering the dessert generally have higher productivity than other areas. The one nutrient in lowest supply compared to the Redfield ratio is called the limiting nutrient. In most oceans, the limiting nutrient is nitrogen. However, in the Southern Ocean iron is the limiting nutrient because of the lack of iron containing dust deposition.