Upwelling is an oceanographic phenomenon that involves wind-driven motion of dense and nutrient-rich water towards the ocean surface, replacing the warmer nutrient-depleted surface water. The nutrient-rich upwelled water stimulates the growth and reproduction of primary producers such as phytoplankton. Due to the biomass of phytoplankton and presence of cool water in these regions, upwelling zones can be identified by cool sea surface temperatures and high concentrations of chlorophyll-a; the increased availability of nutrients in upwelling regions results in high levels of primary production and thus fishery production. 25% of the total global marine fish catches come from five upwellings that occupy only 5% of the total ocean area. Upwellings that are driven by coastal currents or diverging open ocean have the greatest impact on nutrient-enriched waters and global fishery yields; the three main drivers that work together to cause upwelling are wind, Coriolis effect, Ekman transport. They operate differently for different types of upwelling.
In the overall process of upwelling, winds blow across the sea surface at a particular direction, which causes a wind-water interaction. As a result of the wind, the water is transported a net of 90 degrees from the direction of the wind due to Coriolis forces and Ekman transport. Ekman transport causes the surface layer of water to move at about a 45 degree angle from the direction of the wind, the friction between that layer and the layer beneath it causes the successive layers to move in the same direction; this results in a spiral of water movement down the water column. It is the Coriolis forces that dictate which way the water will move. In the Southern Hemisphere, the water is transported to the left of the wind. If this net movement of water is divergent upwelling of deep water occurs to replace the water, lost; the major upwellings in the ocean are associated with the divergence of currents that bring deeper, nutrient rich waters to the surface. There are at least five types of upwelling: coastal upwelling, large-scale wind-driven upwelling in the ocean interior, upwelling associated with eddies, topographically-associated upwelling, broad-diffusive upwelling in the ocean interior.
Coastal upwelling is the best known type of upwelling, the most related to human activities as it supports some of the most productive fisheries in the world. Wind-driven currents are diverted to the right of the winds in the Northern Hemisphere and to the left in the Southern Hemisphere due to the Coriolis effect; the result is a net movement of surface water at right angles to the direction of the wind, known as the Ekman transport. When Ekman transport is occurring away from the coast, surface waters moving away are replaced by deeper and denser water; this upwelling process occurs at a rate of about 5–10 meters per day, but the rate and proximity of upwelling to the coast can be changed due to the strength and distance of the wind. Deep waters are rich in nutrients, including nitrate and silicic acid, themselves the result of decomposition of sinking organic matter from surface waters; when brought to the surface, these nutrients are utilized by phytoplankton, along with dissolved CO2 and light energy from the sun, to produce organic compounds, through the process of photosynthesis.
Upwelling regions therefore result in high levels of primary production in comparison to other areas of the ocean. They account for about 50% of global marine productivity. High primary production propagates up the food chain because phytoplankton are at the base of the oceanic food chain; the food chain follows the course of: Phytoplankton → Zooplankton → Predatory zooplankton → Filter feeders → Predatory fish → Marine birds, marine mammals Coastal upwelling exists year-round in some regions, known as major coastal upwelling systems, only in certain months of the year in other regions, known as seasonal coastal upwelling systems. Many of these upwelling systems are associated with a high carbon productivity and hence are classified as Large Marine Ecosystems. Worldwide, there are five major coastal currents associated with upwelling areas: the Canary Current, the Benguela Current, the California Current, the Humboldt Current, the Somali Current. All of these currents support major fisheries.
The four major eastern boundary currents in which coastal upwelling occurs are the Canary Current, Benguela Current, California Current, Humboldt Current. The Benguela Current is the eastern boundary of the South Atlantic subtropical gyre and can be divided into a northern and southern sub-system with upwelling occurring in both areas; the subsystems are divided by an area of permanent upwelling off of Luderitz, the strongest upwelling zone in the world. The California Current System is an eastern boundary current of the North Pacific, characterized by a north and south split; the split in this system occurs at Point Conception, California due to weak upwelling in the South and strong upwelling in the north. The Canary Current is an eastern boundary current of the North Atlantic Gyre and is separated due to the presence of the Canary Islands; the Humboldt Current or the Peru Current flows west along the coast of South America from Peru to Chile and extends up to 1,000 kilometers offshor
In economics, returns to scale and long run average total cost are related but different concepts that describe what happens as the scale of production increases in the long run, when all input levels including physical capital usage are variable. The concept of returns to scale arises in the context of a firm's production function, it explains behavior of the rate of increase in output relative to the associated increase in the inputs in the long run. In the long run all factors of production are variable and subject to change due to a given increase in size. While economies of scale show the effect of an increased output level on unit costs, returns to scale focus only on the relation between input and output quantities. There are three possible types of returns to scale: increasing returns to scale, constant returns to scale, diminishing returns to scale. If output increases by the same proportional change as all inputs change there are constant returns to scale. If output increases by less than that proportional change in all inputs, there are decreasing returns to scale.
If output increases by more than the proportional change in all inputs, there are increasing returns to scale. A firm's production function could exhibit different types of returns to scale in different ranges of output. There could be increasing returns at low output levels, decreasing returns at high output levels, constant returns at one output level between those ranges. In mainstream microeconomics, the returns to scale faced by a firm are purely technologically imposed and are not influenced by economic decisions or by market conditions; when the usages of all inputs increase by a factor of 2, new values for output will be: Twice the previous output if there are constant returns to scale Less than twice the previous output if there are decreasing returns to scale More than twice the previous output if there are increasing returns to scale Assuming that the factor costs are constant and the production function is homothetic, a firm experiencing constant returns will have constant long-run average costs, a firm experiencing decreasing returns will have increasing long-run average costs, a firm experiencing increasing returns will have decreasing long-run average costs.
However, this relationship breaks down if the firm does not face competitive factor markets. For example, if there are increasing returns to scale in some range of output levels, but the firm is so big in one or more input markets that increasing its purchases of an input drives up the input's per-unit cost the firm could have diseconomies of scale in that range of output levels. Conversely, if the firm is able to get bulk discounts of an input it could have economies of scale in some range of output levels if it has decreasing returns in production in that output range. Formally, a production function F is defined to have: Constant returns to scale if F = a F Increasing returns to scale if F > a F Decreasing returns to scale if F < a F where K and L are factors of production—capital and labor, respectively. In a more general set-up, for a multi-input-multi-output production processes, one may assume technology can be represented via some technology set, call it T, which must satisfy some regularity conditions of production theory.
In this case, the property of constant returns to scale is equivalent to saying that technology set T is a cone, i.e. satisfies the property a T = T, ∀ a > 0. In turn, if there is a production function that will describe the technology set T it will have to be homogeneous of degree 1; the Cobb–Douglas functional form has constant returns to scale when the sum of the exponents is 1. In that case the function is: F = A K b L 1 − b where A > 0 and 0 < b < 1. Thus F = A b 1 − b = A a b a 1 − b
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