The rise of deep, cold water towards the surface to replace water is called an upwelling (National Oceanic and Atmospheric Administration, 2013). Successful fisheries all over the globe are situated in areas with strong coastal upwelling due to the presence of highly productive marine systems. Oxygen-depleted conditions and available diverse electron donors, such as hydrogen sulfide and organic matter, favor processes involved in nitrogen loss resulting in high productivity of the ecosystem (Galán, Faúndez, Thamdrup, Santibañez, & Farías, 2014).
Upwelling is caused by a variety of conditions. One is the replacement of warm surface water with cold water from beneath produced by upward deflection of tidal currents by underwater ridges, shoals, and other bottom bumps of channels. A less dynamic upwelling process is also observed in mid-oceans where slow currents are deflected upward by submarine mountain ridges or peaks.
Coriolis effect associated with the planet’s rotation also plays an important role in the induction of upwelling areas. Winds push surface water away prompting the unforced rise of deeper water to replace it. Earth’s rotation causes drift currents to deflect to the right of the wind. This process generates very cold summer water along the west coast of the United States of America although a change in the coastal circulation through other factors may also trigger the same effect.
Lastly, winds that push water away from lee shores also produces upwelling and on the down-current side of prominences that project into a current. As current flows beyond an obstruction, such as land, some surface water will be drawn behind the barrier which in turn causes upwelling. The limited area of water affected in this situation renders the Coriolis effect negligible.
All causes of upwelling bring deep, nutrient-rich water into surface which supports hordes of fish species. On the other hand, the first cause of upwelling mentioned above is surface water responding to what takes place below while the second cause is a response to what takes place near the surface in which cold water from depths of 100 to 300 meters rises. This is very slow with an upward speed of 1 to 10 meters per day compared to forced upwelling observed in the first case. Nevertheless, thousands of square kilometers are affected at the same time, thus the total effect is still greatly significant (Thomson, 1981).
References
Galán, A., Faúndez, J., Thamdrup, B., Santibañez, J.F., & Farías, L. (2014). Temporal dynamics
of nitrogen loss in the coastal upwelling ecosystem off central Chile: Evidence of
autotrophic denitrification through sulfide oxidation. Limnology and Oceanography,
59(6), 1865-1878. doi: 10.4319/lo.2014.59.6.1865
National Oceanic and Atmospheric Administration. (2013). Upwelling is a process in which
deep, cold water rises toward the surface. Retrieved from
http://oceanservice.noaa.gov/facts/upwelling.html
Thomson, R.E. (1981). Oceanography of the British Columbia Coast. Ottawa, Canada: Minister
of Supply and Services Canada.
