Upwelling ecosystems

Upwelling ecosystems occur where cold, often nutrient-rich waters from the ocean depths rise to the surface. They can also occur in freshwater bodies such as lakes. Upwelling ecosystems are considered some of the most productive ecosystems in the world. Upwelling is the process in which surface currents move away from one another, or when winds push surface waters away from the shoreline. This draws deeper water upward to replace the surface water. This process of the rising of cold nutrient-rich waters is essential for productivity in marine and freshwater ecosystems. The importance of the nutrient-rich waters has a direct tie to the food web. This is because most primary productivity in the oceans occurs in surface waters, but most of the organic matter can be found at the bottom of the sea. The reason for this is because almost all food chains begin with photosynthesis, and photosynthesis occurs in sunlight. Marine plants, therefore, will only live near the surface where light can penetrate, keeping herbivores and their predators up near the surface. But once these organisms die or shed scales, eggs, leaves, shells, or feces, they sink down toward the seafloor and decompose.

Upwelling thus brings all these organic nutrients back up to the surface water. With these nutrients, the phytoplankton and seaweed population grow, forming the basis of many food webs in marine and freshwater ecosystems. These blooms form the ultimate energy base for large animal populations higher in the food chain, including marine and freshwater fish, mammals, and seabirds. The raising of benthic nutrients to the surface waters occurs in regions where the flow of water brings currents of differing temperatures together. There are at least five types of upwellings: coastal upwelling, large-scale wind-driven upwelling in the ocean interior, upwelling associated with eddies, topographically associated upwelling, and broad-diffusive upwelling in the ocean interior.

Coastal Upwelling

Winds affected by the rotation of the Earth create a phenomenon known as the Coriolis effect. Along a coastline oriented north–south, like much of the west coast of the United States, winds that blow from the north tend to drive ocean surface currents to the right of the wind direction, thus pushing surface waters offshore. As surface waters are pushed offshore, water is drawn from below to replace them; these are coastal upwellings. Coastal upwelling usually occurs in the subtropics along the continental coasts, where prevailing trade winds drive the surface water away from shore, drawing deeper water upward to take its place. Because of the abundance of krill and other nutrients in the colder waters, these regions are rich feeding grounds for a variety of marine species, including whales, sharks, and many other fish species, as well as avian species (coastal birds). Some of the most well-known examples of this type of upwelling include: the Canary Current in Northern Africa, the Benguela Current in Southern Africa, the California Current off of the west coast of the United States, the Humboldt Current in South America, and the Somali Current off the western side of India. All of these currents support major fisheries. Along the west coast of southern Africa, the Benguela upwelling distinguishes itself from other upwelling systems by the intrusion of warm waters at its two boundaries: to the north, via the Angola Benguela front, and to the south, via the Agulhas Current that terminates the western boundary current of the Indian Ocean.

To spawn, sardines (Sardinops sagax) and anchovies (Engraulis encrasicolus) migrate toward the Agulhas Bank, where the warm waters transported by the Agulhas Current create a highly stratified environment. Eggs and larvae are then rapidly transported northward by a coastal current. After several days, they reach the upwelling region of the east coast of Africa, where certain numbers are transported out to the open sea by wind-driven currents, a source of mortality. Other sources of early life cycle mortality must not be overlooked, in particular, those linked to starving larvae and predation of eggs and larvae. A week after hatching, larvae must feed. As their swimming abilities are limited, they need not only a high density of suitably sized prey (mainly microzooplankton), but also optimal turbulence conditions. Excessive turbulence disperses plankton aggregations and reduces capture. Using more precise modeling of currents and production should shed new light on these issues. Although primary productivity of the Benguela system seems globally in excess, deficits may occur in certain spatiotemporal strata, such as that of the Agulhas Bank or the offshore feeding area during the reproductive season. In these conditions, feeding oases may be found within the large offshore eddies caused by the meeting of different currents and water masses. The low productivity of the Agulhas Bank creates intense food competition among the millions of tons of spawners that congregate there during the spawning season. This encourages parental cannibalism and the predation of eggs and larvae by other species. Because of the absence of monthly spatialized data on fish distribution, modeling of this phenomenon remains difficult but efforts at quantification are possible.

The Benguela ecosystem can be divided into two subsystems, the northern Benguela (southern Angola and Namibia) and the southern Benguela (South Africa and Namibia), separated by the permanent upwelling cell of Luderitz, the strongest in the world. In South Africa, historical fisheries data suggest a stock collapse of sardines in the late 1960s, followed by slow recovery, whereas the first level of collapse was noted in Namibia during the same period, but worsened in the late 1970s. The biomass of the Namibian stock has remained at a very low level. Scientists are not sure if these collapses are because of an environmental process (upwelling) or overfishing.

The structure and ecosystem dynamics of the southern Benguela have been undergoing progressive changes for over 30 years, whereas the northern Benguela ecosystem has been completely “reorganized” following a “regime shift.” Today, the northern Benguela is dominated by jellyfish and fish feeding on detritus (gobies). Pelagic fishes have become low in abundance, and fisheries for most commercial species (hake and horse mackerel) are threatened.

Upwelling in the Ocean Interior

Upwelling can also occur in the middle of the oceans where cyclonic circulation is relatively permanent or where southern trade winds cross the equator. The churning of a cyclone eventually draws up cooler water from lower layers of the ocean, which, in turn, causes the cyclone to weaken. Another very important example of upwellings that occur in very special habitats are upwellings that occur off the shore of islands, ridges, or seamounts because of a deflection of deeper currents. Just like other upwelling habitats, this provides a nutrient-rich area in otherwise low productivity ocean areas. Upwelling in seamounts has made them a very unique habitat with much endemism, and can be considered a biodiversity hot spot. A specific example includes upwellings around the Galapagos Islands with the Cromwell Current, and the Seychelles Islands, both which have major pelagic fisheries. In the Galapagos, the Crowmwell Current is a very important feeding ground for the endemic Galapagos penguin, which heavily depends on the smaller fish that feed on the plankton caused by the upwelling. This upwelling is also very important for the marine iguana, also endemic to the Galapagos Islands. When this upwelling fails to occur during an El Niño year, there is great devastation to the populations of these endemic animals that depend so heavily on it.

Another type of powerful upwelling occurs around Antarctica. The deepwater in the global conveyer belt flows across the seafloor all the way from the North Atlantic before it reaches Antarctica, so it has plenty of time to fill up with nutrients. Antarctica has many surface currents flowing around it, which stir up the waters, including the southern parts of the Pacific, Atlantic, and Indian Ocean gyres and the Circumpolar and Subpolar Currents. Also causing upwelling in this region are the strong polar winds that usually blow offshore.

Freshwater Upwelling

Upwelling in freshwater ecosystems tends to be seasonal. It is most likely to occur in lakes in temperate regions. During the summer, the lake surface is heated with infrared radiation from the sun, causing a thermal stratification (also known as thermocline) in the lake. It is because of this stratification that upwelling does not occur. By the time fall comes around, the stratification breaks down, and the temperature of the epilimnion (surface layers) becomes the same as the temperature at the hypolimnion (deeper layers). This means that the thermocline shifts and allows upwelling to occur; also, stronger seasonal winds aid the process of upwelling. When winter comes, the surface water of the lake freezes and upwelling or mixing does not occur. As the ice begins to melt during the spring, lakes become isothermal, which means that the epilimnion and the hypolimnion have the same temperature because of the wind action that brings deeper waters to the surface as a result of mixing.

Canaries Current

In the Canaries Current, there are three major ecosystems: the northern Moroccan coast with seasonal upwellings in the summer; the south Moroccan and north Mauritanian coast with permanent upwellings (Sahara Desert); and the south Mauritanian and Senegalese coast, with boreal upwellings. The southern part of this ecosystem is characterized by high seasonal variability, alternating between an ecosystem under tropical influence in the summer and a coastal upwelling ecosystem in the winter. This alternation is accompanied by a migration of certain tropical species (tuna and tuna-like fishes) up to 20 degrees north during summer, and by a southward extension of the habitat of temperate species such as the sardine S. pilchardus during the winter. The Canaries Current system has wide continental shelves in the south, whereas the eastern regions are generally characterized by narrow continental shelves because of their young geological age. Principal spawning areas have been linked to regions with wide shelves. This association would be the result of physical processes developing over wide and shallow continental shelves, which would limit exchanges between the coast and pelagic areas.

The Canaries Current ecosystems were dominated by large demersal fish, which were rapidly overexploited. In the southern part of the area, the adaptability of artisanal fisheries took advantage of the seasonal migration of numerous species (a balance between seasonal upwelling and the warm season dominated by tropical influences). Over a longer period of time, West African fisheries have been spared the sudden collapses observed in other systems. A lower fishing exploitation rate until the 1980s and high seasonal fluctuations could have contributed to this relative resilience. Three countries experienced rapid and unexpected growth of octopus stocks in the 1970s.

Because of its commercial value, octopus fishing has become one of the essential components of West African fisheries. This shift in species was interpreted as the result of the absence of top-down control, following the overexploitation of demersal species that favored the development of short-lived prey species, such as the octopus (as well as shrimp and pelagic fishes). Further to the north, the reasons that have lead to the southern displacement of seiner fleets exploiting sardines remain largely unknown.

Humboldt Current

The Humboldt Current system, with its permanent upwellings off the coast of Peru and its seasonal upwelling along the coast of Chile, is by far the most productive in fish landings. With less than 1 percent of the world's ocean surface, it provides 15 to 20 percent of world marine catches (up to nearly 20 million tons per year for Peru and Chile combined). A second particularity lies in the presence of a very intense, extensive shallow zone that is very low in oxygen. A final particularity is its position under the direct influence of the El Niño Southern Oscillation (ENSO) mechanism. Alternate upwellings of nutrient-poor and nutrient-rich waters off the coast of Ecuador and Peru are associated with El Niño and La Niña episodes in the tropical Pacific. During El Niño, the pycnocline is so deep that the upwelled waters come from the nutrient-poor waters above the pycnocline. In extreme cases, nutrient-deficient waters coupled with overfishing cause fisheries to collapse, bringing about severe, extended economic impacts.

Coastal upwelling ecosystems, like the west coast of the United States, are some of the most productive ecosystems in the world and support many of the world's most important fisheries. Although coastal upwelling regions account for only 1 percent of the ocean surface, they contribute roughly 40 to 50 percent of the world's fisheries landings (this refers to the part of the fish catch that is put ashore. Frequently, landings provide the only record of total catch).

In upwelling systems, large variations in fish recruitment in fisheries appear to be from fluctuations in mortality during the early life-history stages of the fish, and thereby must be essentially associated to climatic variability. Survival through these stages is essentially linked to hydrodynamic structures which, in certain spatiotemporal strata, favor the retention, enriching, and concentration of plankton and ichthyoplankton. Fluctuations in abundance of pelagic fish stocks reflect significant changes in the structure and functioning of upwelling ecosystems.

High mortality rates have been observed at upper trophic levels (birds, marine mammals, and large predatory fish) in response to the diminished abundance of prey. Effects at lower trophic levels may also bring to light evidence of reduced predation by pelagic fishes on planktonic species, bringing about in turn changes throughout the entire trophic web. An example is the change in relative abundance of two species of anchovy associated with the El Niño phenomenon. Long-term fluctuations in dominant species have been observed in most upwelling ecosystems, such as the alternating dominance of sardines and anchovies in the Humbolt, Benguela, and Kuroshio (offshore Japan) Currents.

Climate and Climate Change

Coastal upwellings also influence weather and climate. Along the northern and central California coast, upwellings lower sea-surface temperatures and increase the frequency of summer fogs. Relatively cold surface waters chill the overlying humid marine air to saturation, so that thick fog develops. Upwelling cold water inhibits formation of tropical cyclones (e.g., hurricanes) because tropical cyclones derive their energy from warm surface waters. During El Niño and La Niña, changes in sea-surface temperature patterns associated with warm- and cold-water upwelling off the northwest coast of South America and along the equator in the tropical Pacific affect the interannual distribution of precipitation around the globe. A great impact on upwelling is posed by the new installation of large wind farms.

Some upwellings are wind-driven; these new large wind farms exert a significant disturbance on the wind speed in the vicinity of the installations. A recent study shows that the size of the wind wake is an important factor for the oceanic response to the wind farm. At certain wind speeds, sufficient upwelling may be generated that the local ecosystem will most likely be strongly influenced by the presence of a wind farm.

Another factor that is currently affecting upwelling is climate change. In any one region, upwelling is intermittent; it can be strong in some years, and weak in others. The success of fishermen is greatly affected by this, since a weakening of an upwelling system can bring economic disaster. Upwelling brings nutrient-rich deepwaters to the surface, where algae can thrive in the sunlight, feeding the fish. Without nutrients, there will be no fish (as happens during El Niño conditions).

Scientists are trying to determine how climate change will affect upwelling and coastal productivity. The strong dependency of upwelling processes on the strength of trade winds contains one hint. Trade winds are zonal winds, which feed off the latitudinal temperature gradient; as this gradient weakens (computer simulations show that high latitudes become warmer than low ones), the zonal flow will weaken. Thus, the upwelling that derives its energy from trade winds will weaken. Off California, upwelling of cold water has become less common since 1975, and the productivity of the California Current has diminished accordingly. Experts contend that coastal warming caused by climate change increases water stratification, which might limit the effectiveness of bringing nutrient-rich deep water to the surface. Upwelling that depends on monsoon activity (as in the Arabian Sea) should be much less affected, or may even benefit from the change.

According to some scientists, wind-driven upwellings along the California coast have increased over the past 30 years, and some scientists postulate that the increase in wind-driven upwellings is largely because of increased greenhouse gas, but such an association has been speculative. A study on the effects of future climate change and upwellings in California showed negative results from the intensification of wind-driven upwelling events in that specific area.

According to a model scientists have made, the intensification of upwelling will likely have impacts on terrestrial and marine ecosystems. With intensified upwelling, enrichment can be increased that would be beneficial to organisms; however, concentration may decrease because of increased mixing, and retention may also be decreased by increased seaward transport of surface water. Overall, this could have a negative effect on marine ecosystems, as the current balance of these three factors will change with changes in upwelling.