Upwelling Matters

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Weak upwelling offshore (yellow arrows) favors production of smaller plankton and spawning of the Pacific sardine, while stronger upwelling near the coast (red arrows) favors production of larger plankton and spawning of the northern anchovy. CCE-LTER region.
Ryan Rykaczewski, University Corporation for Atmospheric Research, Princeton, New Jersey

Years of previous research have focused on the process of coastal upwelling close to the continental boundary of the west coast of North America. A new study led by CCE-LTER (then) graduate student Ryan Rykaczewski and his advisor David Checkley identified, for the first time, important ecosystem contrasts between the strong nearshore upwelling in the classical coastal boundary region and the largely ignored, weaker offshore upwelling in the California Current. In the offshore region, a horizontal gradient in the intensity of the wind induces upwelling of nutrients through a different mechanism (wind-stress curl) than the process of Ekman transport that occurs close to the coast. The result is that lower upwelling velocities occur in the offshore region, leading to lower concentrations of nutrients that support phytoplankton growth. In these regions of lower nutrients, small phytoplankton cells are favored over larger ones because of their more efficient nutrient uptake. Small begets small: Rykaczewski and Checkley (2008) found that the body size of grazing zooplankton is also smaller in the offshore upwelling region than in the coastal boundary upwelling zone. Although the rate of upwelling per unit surface area of ocean is much lower in the offshore zone, the total ocean area affected by this offshore, curl-induced upwelling is much larger than in the coastal boundary region. Therefore the previously ignored, offshore upwelling zone may contribute a larger share of the total nutrient supply into surface waters of the California Current.

The significance of this finding extends to the level of two of the most abundant fishes in the California Current System, the Pacific sardine and northern anchovy. Both are plankton feeding fishes, but with a key difference: Pacific sardines typically eat smaller-bodied zooplankton prey, while northern anchovy eat larger-bodied zooplankton, because of differences in gill-raker spacing and prey retention capabilities. Thus, sardines preferentially feed and spawn in the offshore, where the smaller-bodied zooplankton prey are better matched to their feeding abilities, while the anchovies usually spawn inshore of this region, where they can capture larger-bodied zooplankton prey.

Rykaczewski and Checkley (2008) further discovered a major difference in long-term trends in winds between the offshore and the nearshore upwelling zones. While coastal upwelling-favorable winds have shown relatively little change over the past 55 years, the offshore, wind stress curl upwelling zone has shown a continuous increase in upwelling. These results suggest a progressive increase over time in the importance of nutrient supply in the offshore region relative to the nearshore. Putting the pieces together: the long-term increase in offshore, curl-driven upwelling seems to be enriching the offshore environment, thus favoring the Pacific sardine over the coastally-associated northern anchovy. This study provides a foundation for forecasting future changes in commercially important fishes.

Long-term variation in summertime upwelling, seawater density (red line), nutricline depth (blue line), and chlorophyll-a concentration (green line). These properties of the ocean water column are more highly correlated with curl-driven upwelling (gray line) than with coastal upwelling (black line) after 1970. CCE-LTER site.
Rykaczewski and Checkley (2008)
For further reading: 
Rykaczewski, R.R., Checkley, D.M., Jr., 2008. Influence of ocean winds on the pelagic ecosystem in upwelling regions. Proceedings of the National Academy of Sciences, U.S.A. 105, 1965-1970.
For further information: 
Ryan Rykaczewski, University Corporation for Atmospheric Research, Princeton, New Jersey
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