Background ScenarioPeople in Peru have come to rely on cool, deep water to rise and bring nutrients to the surface. The small fish that are their livelihood eat the microscopic marine plants that use these nutrients. There are years where the cool, nutrient rich water comes to the surface, supporting enough fish for everyone. Some years there are hardly any fish. These years are tough on the coastal people who make their living from fishing. Can the fluctuations be predicted? Is Peru the only place where there are such a fluctuations?
Upwelling
Upwelling is a phenomenon that occurs in a number of places in the global ocean. The term refers to cold, nutrient rich water coming to the surface from depths of over 50 meters. It is created by wind blowing across the ocean surface and pulling the surface water with it. As the surface water leaves an area, the 'hole' left behind is filled in by water 'upwelling' from below.Mechanisms that Create Ocean Upwelling
In order to understand the different types of upwelling, the Coriolis Effect and Ekman Transport need to be introduced. The Coriolis Effect is the apparent curvature of the path of a moving object due to the fact that Earth rotates underneath it as it moves. For example, if a rocket is aimed directly from the North Pole to a point east of Brazil, the rocket will miss. While the rocket is en- route, Earth will rotate underneath it and the rocket will land somewhere west of its intended target, and the path would appear to someone on land to be a curved path between the North Pole and the landing point. The reason this is important is that water in the ocean moves in accordance with this Coriolis Effect as well. It is not 'attached' to the Earth and so moves in an apparently curved path. The curvature is to the right of movement in the Northern Hemisphere, and to the left of the movement in the Southern.QuickTime movie on the Coriolis Effect
Water that is near the surface of the ocean is affected by wind. As wind moves across the surface layer the water is dragged along due to frictional forces. The very top layer of water gets pulled along in the same direction as the wind, except that it is deflected due to the Coriolis Force. It has been determined that this layer moves approximately 45o to the wind (right or left determined by the hemisphere). This layer pulls frictionally on the layer below it, dragging it along but with this same 45o deflection. Some of the windÕs energy is lost to friction, so each layer feels less and less pull, but is similarly turned 45o from the layer above. Eventually, there is a depth where the effect of the wind is not felt, called the Ekman Depth. By averaging all the movement of the water from the surface to the Ekman Depth, Ekman Transport is derived. Ekman Transport is the net effect of the wind on all these frictionally bound surface layers and is 90o to the wind. For example, with a northward blowing wind in the northern hemisphere, Ekman Transport will be to the right of the path or eastward.
Types of Upwelling
There are three main types of upwelling; equatorial, coastal, and seasonal. Equatorial upwelling is caused by the winds known as the trade winds. The trade winds blow from east to west in the vicinity of the equator. On the northern side of the equator Ekman Transport is to the right (northward), and on the southern side it is to the left (southward). With water flowing directly away from the equator, both northward and southward, the equator itself has a deficit of water. Hence, water from below upwells to fill in the gap. Equatorial upwelling is most prominent in the Pacific Ocean.Coastal upwelling is caused by the same mechanism as above, however, rather than being in the open ocean it occurs along the border of a continent. For example, the main wind-driven current along the west coast of the United States is southward (the California Current). As the water moves southward, Ekman Transport is to the right of the path (westward), or away from the coast. As the coastal area has a deficit of water, upwelling will occur. Coastal upwelling is most prominent along the California and Oregon coasts, the Peru coast, the south-western tip of Africa and the north-western tip of Africa.
Seasonal upwelling occurs due to 180o shifts in the direction of wind due to land/water heating differences between summer and winter. In the summer, land heats a great deal compared to water and the heated air over land rises, thus inviting wind to blow toward land from water (onshore). In the winter, land cools more significantly than water so the relatively warm air over water rises, thus inviting wind to blow toward water from land (offshore). In the first case, there will be a pile up of water at the coastline. In the second case, there will be a removal of water from the coastline. This will be an invitation for upwelling along the coast. The coast of Oman is a good example of seasonal upwelling, although it is not quite correlated with the 'normal' seasonal shift described here because it is so close to the equator and is affected by winds/currents in the Indian Ocean.
Identifying Upwelling on Satellite-derived Maps
There are two main ways to identify areas of upwelling. One method is to look at Sea Surface Temperature (SST). Because the water that is upwelled is very cold, we can look at satellite-derived temperature maps of the water surface to find areas that show colder temperatures than surrounding water. The surrounding water will be warmer as the sun has warmed it for a longer period of time. Due to the way the earth is tilted and rotates around the sun, the sun warms the earth more in latitudes near the equator and less toward the poles. Therefore, areas of upwelling will stand out as 'too cold' for a given latitude.Another way to identify areas of upwelling is by using maps of Ocean Color. These are maps of chlorophyll on the surface of the ocean. Chlorophyll is a telling signal of upwelling due to the fact that upwelled water brings with it nutrients and dissolved gases that are important for plant growth. The list includes nitrogen, phosphorous, silica and carbon dioxide. With these being brought to the surface, where sunlight is also available, marine plants including microscopic phytoplankton (algae) grow very well. Hence, the chlorophyll that they contain can be used to identify areas where upwelling is occurring.
Ecological and Economic Effects of Upwelling
Due to the increase in phytoplankton growth, areas of upwelling make a large contribution to the fisheries of the world. Phytoplankton are food for many animals of the ocean, hence they are the base of oceanic food webs. Some fish species and the people who fish them are extremely dependent on upwelling to provide a good living. It has been shown that while upwelling areas only cover approximately 1% of the ocean surface, they are directly responsible for 50% of the world's fisheries.Upwelling areas can be highly variable. There are climatological events, such as El Nino, that reduce upwelling significantly. During El Nino events, the wind patterns change and thus the upwelling patterns change. The people whose economic well being depends on fishing would benefit from predictions of the stock. Also, if fishing effort is not reduced during low upwelling years, overfishing and/or extinctions of fish populations can occur.
Assessing Upwelling by Rate and Duration
Biological productivity of an area can be determined by two factors of upwelling. One factor is the rate of upwelling and the other is the duration of upwelling. The rate determines the size of phytoplankton cells that will grow. With too high or too low a rate of upwelling, phytoplankton are small. Small phytoplankton are grazed upon by zooplankton rather than small fish, therefore there is an additional trophic level between the algae and the fish. With additional trophic transfers, energy is lost. Therefore, small phytoplankton are not ideal for producing a lot of fish. With a moderate rate, the size of phytoplankton will be large, and fish will feed directly on the plants. This will lead to a more efficient energy transfer and more fish will be produced.Rate can be qualitatively assessed by looking at the temperature difference in an upwelling area versus a nearby warm area. Colder upwelling areas imply deeper water rising to the surface due to stronger wind. (Looking at the equatorial Pacific example in the power-point green is two color shifts away from the orange surrounding water, so let's call this 'Moderate'.)
The duration of upwelling determines the total amount of phytoplankton that will grow. Longer upwelling events will produce more phytoplankton while shorter events will produce less phytoplankton. (Here, long is defined as 8 months or more.) Combining rate and duration - the ideal situation for lots of phytoplankton growth is moderate rate for long periods.
By looking at satellite maps of the global ocean, areas of upwelling can be assessed in terms of their rate and duration of upwelling. By this assessment, phytoplankton production can be surmised. The amount of time that passes between an upwelling event and when fish will be harvestable is unknown and likely variable. However, it can be assumed that the trophic transfers occur within a season. Therefore, an upwelling event that lasts from January to April will probably yield fish by August or so.
