Coastal Upwelling

Coastal upwelling occurs when water along a coastline flows offshore and deeper water—usually relatively cool, rich in nutrients, and high in partial pressure of carbon dioxide—flows upward to fill its place. Upwelling areas are notable for their effect on carbon cycling, as upwelling not only brings dissolved inorganic carbon to the surface, where it is released into the atmosphere, but also stimulates phytoplankton blooms that further remove some of that carbon through photosynthesis; a small percentage of this bloom also sinks in the form of organic matter (organic carbon) to deep water and becomes buried in sediment, creating a long-term carbon sink. There is considerable interest among carbon-cycle scientists regarding the reciprocal interactions between upwelling systems and climate change. Although such upwelling can in principle occur along any coastline, marine or freshwater, some marine coastlines (e.g., Peru, the western United States, northwest Africa, and southwest Africa) are renowned for their annual upwelling events that are the source of major blooms of diatoms and dinoflagellates, which become the base for extensive marine food webs and coastal fishing industries.

Influence on Carbon Cycling

In the past several decades, major research programs have developed around the influence of coastal upwelling ecosystems on ocean carbon cycling and atmospheric carbon dioxide, how natural climate change (such as glacial–interglacial cycles) has affected coastal upwelling and associated biological productivity over a range of time scales, and how human-induced climate change is affecting coastal upwelling rates and timing and the associated fisheries.

Carbon dioxide exchange between coastal surface water and the atmosphere varies considerably in time and space. Because the pattern is complicated and dynamic relative to the number of direct measurements, considerable uncertainty lingers regarding the net carbon flux through the system over the course of a year. In general, outgassing occurs near the coastline, where upwelled water outcrops at the surface. This water is often rich in carbon dioxide arising from the respiration of organisms ingesting organic matter that sank from the surface to deeper water (which may be the sea bottom along the continental shelf). As upwelled water moves from shore, phytoplankton bloom in response to dissolved nitrogen, phosphorus, and other nutrients and begin to use up some of the dissolved inorganic carbon, reducing the partial pressure of carbon dioxide.

Because this process occurs over a period of several weeks, the rate of uptake of dissolved inorganic carbon also changes through time, so that net outgassing will occur early in an upwelling event, gradually changing to net ingassing. Much of the phytoplankton is recycled in the surface layer, prolonging the bloom, but some of the nutrients and carbon escape the system through the fecal material of heterotrophs feeding on the phytoplankton. The nutrients of remineralized organic matter that sink may come to the surface in future years through upwelling, or the organic matter may sink below the depth of upwelled water into the deep sea, or get buried in sediment.

The latter two processes can take carbon out of the atmosphere for thousands or millions of years, respectively. Although these processes occur in other aquatic areas, enough of the global ocean carbon flux in a given year occurs through coastal upwelling zones to affect atmospheric carbon dioxide. The strength and direction of surface winds that drive coastal upwelling vary over a broad

spectrum of time scales. Changes in global heat retention through time affect the potential for temperature gradients that influence wind speed, and the distribution of land masses and topographic

features such as mountains affect coastal shape, coastal currents, sea level and coastal profile, and atmospheric circulation patterns. 

Temperature and precipitation patterns and sea level, among other variables, affect nutrient distribution in the oceans. All of these affect upwelling strength, biological productivity, carbon burial, and net effect on the global carbon cycle. Much research has been dedicated to understanding upwelling changes during glacial–interglacial cycles, tracking responses to changed wind speeds and to lowered sea level, and therefore steeper coastal profiles. Other research has examined how to predict occurrences of upwelling in, for example, the Mesozoic, under the assumption that upwelling is responsible for the accumulation of some petroleum deposits.

Both models and empirical observations of several coastal upwelling areas, such as off the coast of California and northwest Africa, suggest that atmospheric warming is leading to greater rates of upwelling. This increase is driven by a greater land–ocean temperature gradient and therefore greater wind speeds. This can lead both to greater outgassing of carbon dioxide (if not balanced by increased productivity) and loss of certain fish that cannot maintain their population position because of higher offshore current velocities.