Ecosystems of the eastern Pacific and Atlantic boundary currents are regions of extraordinary biological productivity. The high levels of primary production among phytoplankton in these ecosystems are sustained by upwelling, a process by which wind drives away surface ocean water and nutrient-rich waters rise up from beneath the surface to replace it. These productive phytoplankton communities support zooplankton production, which in turn, nourishes massive populations of small pelagic fishes. Anchovy and sardine are the two dominant, commercially harvested species in these upwelling ecosystems, and they have contributed substantially to the fisheries landings around the world during the 20th and 21st centuries. Anchovy and sardine are particularly valuable to human communities of the California Current (eastern North Pacific), Humboldt Current (eastern South Pacific), Canary Current (eastern North Atlantic), and Benguela Current (eastern South Atlantic), where the species have supported immense commercial fisheries (both for reduction and human consumption) as well as predators such as large pelagic fish, seabirds, and marine mammals.
However, populations of anchovy and sardine exhibit dramatic fluctuations in biomass. These ‘boom-and-bust’ characteristics are a challenge to the industries dependent on the fisheries resources, and oceanographers, fisheries managers, and fishers have long sought a better understanding of the factors that influence the variability in the species’ productivity so that future changes in populations might be anticipated. These fisheries’ ecosystems have been the subject of intense oceanographic research over many decades, but only a few clues about how they function have been acquired.
In an attempt to better understand the factors driving the anchovy and sardine populations, oceanographers have gathered multiple clues. Although both species are small, pelagic, and planktivorous, they do exhibit certain differing characteristics that offer some insight into their sensitivities to climate change. Taken together, these clues indicate that anchovy and sardine may be adapted for planktonic prey fields associated with differing oceanic conditions. The cooler, nearshore waters associated with more rapid upwelling and larger phytoplankton and zooplankton communities may be most beneficial for anchovy, while the relatively warm and less nutrient-rich waters further offshore may benefit sardine populations. Based on this, future changes in the rates of inorganic nutrient supply might be a critical factor for resolving the impact of future climate change on populations of these species in upwelling ecosystems.
How can we expect the nutrient supply to upwelling systems to change in response to future climate? A number of hypotheses have been offered to describe the impacts of anthropogenic climate change in this regard. Three key things must be considered: changes in upwelling-favourable winds (including intensification, poleward shifts, and altered seasonality), changes in water column stratification, and modified characteristics of source waters supplied to the systems. Testing hypotheses through examination of observational records is hampered by science’s ability to distinguish the impacts of anthropogenic climate change from those of natural variability, but numerical model projections can offer some insight into changes in the upwelling process over the coming century.
In contrast to earlier hypotheses, models of future climate do not indicate that these favorable winds will intensify with continued global warming. Rather, the dominant process affecting them is associated with a poleward shift of atmospheric high-pressure systems over the subtropical ocean basins. These shifts stimulate a poleward migration of summertime upwelling in all four of the major upwelling ecosystems. That is, upwelling winds tend to increase in the higher-latitude portions of these ecosystems, but decrease in the lower-latitude portions.
Secondly, some models indicate that changes in the intensity of favourable winds may not be the most important factor to affect the nutrient supply rate. As the vertical density stratification of the oceans increases with future surface warming , much of the subtropical and subpolar ocean basins will be characterized by increased differences in density of nutrient concentrations between the ocean’s sunlit, surface layer (the euphotic zone) and its deeper ones. The euphotic zone will contain fewer nutrients, while the layers just below will contain increased concentrations. Overall, such a process will lead to a decrease in fisheries production in most regions. However, in upwelling zones, where subsurface layers are forced upwards, nutrient enriched deep source waters which feed the upwelling process may increase the nutrient supply. This is one trend that is unexpected and may have a positive impact on primary production in such ecosystems. The reliability of such projections continues to be explored, and there is hope that by combining these projections with small pelagic fish models, fisheries productivity forecasts will soon be available.