Review: Artificial Upwelling
In oceanography class one of the first lessons you learn is that the warm equatorial water’s of the world are often times essentially a desert, devoid of life, or any resources that life would need to survive. Nutrients such as nitrogen, phosphorus, potassium, and iron have all been exhausted by the few organisms that do live in the surface water. This is in contrast to coastal waters, especially the western coasts of continents where large amounts of natural upwelling occur. A combination of wind and the Coriolis effect of the earth spinning pulls the surface water away from the coast allowing cooler, nutrient rich water to upwell near the coast into the photic zone. This allows for primary producers, mostly phytoplankton, to flourish, giving them the two basic things they require, sunlight and nutrients. Since phytoplankton form the base of the aquatic food web their growth increases the growth of every trophic level above them. As a result, fish populations flourish and this protein can be caught and eaten by humans, as well as other marine life such as seals, whales, and bigger fish.
Unfortunately, all of this productivity is limited to just a tiny fraction of the surface area of the ocean, coastal Peru and California are two examples of where coastal upwelling is especially strong and subsequently are locations where fish populations can explode in size. The cruel reality is that in most of the ocean cooler, nutrient rich water is trapped about 200 meters below the water’s surface. At this depth almost no sunlight remains, having been filtered out and blocked by the water above it. As a result primary producers do not ever have the two things they require in one place, they have sunlight in the warm, surface water, and nutrients in the cool, deep water. These two bodies of water are separated by a difficult to penetrate barrier known as a thermocline. Just as there is a fluid boundary between air and water, there is also a boundary formed between two bodies of water with differing densities, the surface water has a lower density due to its warmer temperature and essentially floats on top of the cooler deep water which is often around 4 degrees celcius. As a result in most parts of the ocean these two bodies of water do not mix.
That is where artificial upwelling comes in. The goal of artificial upwelling is to get these two bodies of water to mix so that nutrients are not locked below the thermocline but instead are accessible to phyoplanton in the photic zone.
Artificial upwelling has been discussed since the 1970’s as a method to increase the primary productivity of the world’s oceans, and subsequently increase certain fish populations. A number of clever designs have been proposed, and we will discuss a few in the following paragraphs, but the basic principal is always the same. Find a way to move nutrient rich water that is trapped below the thermocline to above the thermocline and to the top of the water column where it can be utilized by primary producers in the photic zone.
There are five basic types of artificial upwellers, each has been tested to some degree and the actual efficacy of all five is still up for debate. The first type of upweller is a simple water pump, in this design a common pump is used to draw water up from the deep water and discharge it into the surface waters. The head pressure required for the pump is minimized since the water can be discharge below the water’s surface, the pump just needs to use energy to overcome the head loss in the suction end, pump body and discharge pipe, these head losses can all be minimized with large pipe diameters and slower pump speeds. Although the design is simple there are a number of disadvantages, first is the cost of electrical energy to drive the pump and the need for an electrical energy source. Upwelling often needs to occur in areas that are nowhere near a reliable electrical energy source, a solar or wind powered array would be too costly to justify. Finally a pump has a limited life expectancy especially in a submerged, high salinity environment. Because of these disadvantages pumps have not been explored to great deal and instead more clever and passive designs have been tried.
Similar to a traditional water pump is the use of an airlift pump. In an airlift, bubbles of air are discharged below the surface by a compressor into a large diameter pipe. The rising bubble cloud creates a current that also raises water with it. Airlifts are great for low head pumping applications like artificial upwelling and they are more efficient then traditional pumps for zero head applications. Despite this they still have many of the same drawbacks as traditional pumps. An energy source is needed to power a compressor that can pump air to a depth of 200 meters, and this compressor must be located on a raft above the water than can handle ocean waves. Airlifts have been used in near shore environments and they are often employed in lakes where thermal stratification is a problem during summer months. For large scale artificial upwelling the airlift pump is not scalable.
Wave powered artificial upwellers have been explored over the years in both field trials and numerical modeling. They have proven to work in a range of conditions and move water up to the surface without the use of an external power source. The basic design consists of a ring bouy on the surface below which extends a large flexible or ridged tube. The tube reaches down below the thermocline and as the bouy on the surface moves down after a wave passes underneath water moves upwards towards the surface. A one way valve prevents the water from going back down the tube and instead forces it overflow from the tube at the surface. In a regular ocean with large waves the water is pumped up the tube at a steady rate. The drawbacks of a wave powered upweller are it requires nice even waves, and it should be anchored for best performance. The advantage is no need for external power of any kind. An efficient wave powered upweller is maybe the best current design for artificial upwelling.
Another passive design is the use of a density changing upweller. In this design the cool, less salty bottom water moves up a long rigid or flexible tube due to density changes as the water warms. Since the warm surface water is saltier, it has a greater density then the cooler bottom water once it has warmed up. As the cool water rises it warms up and then floats towards the surface due to its lesser density. The drawback is the density differences are very small and the rate of water flow is extremely slow. It is so slow it is hard to measure when it has been tested in pilot studies. This design may prove viable where the surface water is particularly salty and long tubes can be easily anchored in water that does not have a strong current.
The final artificial upwelling design is the most complex, it is known as ocean thermal electric conversion (OTEC). In an OTEC system cold, deep water is pumped upwards to the surface, once there it goes through a heat engine. The heat engine generates electricity by utilizing the difference in temperature between the warm surface water and cooler, deep water. A byproduct of OTEC is lots of cold, nutrient rich water which can then be discharged back into the surface water or used for other cooling or aquaculture needs. OTEC is considered a viable alternative energy source. The drawback is it needs to be located on or near land and requires a complex system of equipment and skilled operators to run the system. Electricity is the main product though and the upwelling is just an added bonus. Research is ongoing in OTEC and there are demonstration plants in Hawaii and Japan.
All artificial upwelling techniques have to contend with some basic disadvantages and unknowns. Issues include the fact that cooler deep water is inherently denser than the warm surface water and it can easily sink back down to below the thermocline very quickly, negating the entire process. Similarly there is debate as t even if upwelling can be induced would be able to deliver enough nutrients in a given area to make a difference or would it still be too diffuse to allow for organisms to utilize it. All of the methods described above require some kind of gear, which costs money to make and deploy. Also the gear is being used in a harsh environment and it would take some time to know what the effective lifespan of various methods would be and thus what their lifetime costs would truly be. Despite these setbacks it is still possible that one day we will realize a time where we can induce upwelling in certain areas to increase ocean productivity.