Climate driven decadal variations of biological production and plankton biomass in the equatorial Pacific Ocean: is this a regime shift?

Abstract

Recent studies indicate strengthened trade winds and intensified upwelling in the tropical Pacific since the late 1990s, suggesting implications for the biogeochemical processes. We employed a fully coupled physical-biogeochemical model to test the hypothesis that there were climate driven decadal variations in biogeochemical fields of the equatorial Pacific. We quantified changes in nitrate and iron concentrations, primary and secondary productions, and phytoplankton and zooplankton biomass between 1988–1996 and 1999–2007. Our modeling simulation showed that the intensified upwelling during 1999–2007 resulted in significant increases of nitrate and iron concentrations in the mixed layer of the central equatorial Pacific. In addition, the upwelling front moved westward, causing shifts of oligotrophic conditions to mesotrophic conditions in some parts of the western equatorial Pacific. As a result, there was an overall enhancement of biological activity in the western and central equatorial Pacific, leading to an increase in primary production and secondary production by 10–15% and 15–50%, respectively. Our study also indicated that there were changes in ecosystem states in the equatorial Pacific Ocean, suggesting alternative new states with more zooplankton biomass during 1999–2007. Additionally, our study showed significant changes in seasonal variations of biogeochemical fields. Particularly, there was a much stronger seasonality in biological production and plankton biomass near the dateline during 1999–2007 relative to 1988–1996

Implications of changing El Niño patterns for biological dynamics in the equatorial Pacific Ocean

“…The resulting anomalies in equatorial surface chlorophyll over the past century are highly coherent with El Niño oscillation indices [ Boyce et al , 2010]. Recent studies also suggest a decadal change and possible regime shift in ecosystem structure [ Wang et al , 2010; Masotti et al , 2011].…”

Effects of fluctuations in sea-surface temperature on the occurrence of small cetaceans off Southern California

“…In contrast, during the cool, negative PDO phase, the California Current is stronger, bringing cool water farther south and east into the region (Mantua and Hare, 2002). A PDO regime shift from cool to warm occurred around 1977, before our study, and a shift back to a cool PDO may have occurred during the last decade starting in 1998-99 (Peterson andSchwing, 2003;Zhang and McPhaden, 2006;Wang et al, 2010).…”

“…In contrast, during the cool, negative PDO phase, the California Current is stronger, bringing cool water farther south and east into the region (Mantua and Hare, 2002). A PDO regime shift from cool to warm occurred around 1977, before our study, and a shift back to a cool PDO may have occurred during the last decade starting in 1998-99 (Peterson andSchwing, 2003;Zhang and McPhaden, 2006;Wang et al, 2010).Two long-term sets of ship-based surveys have been conducted in Southern California waters, making it an ideal region for this investigation. The California Cooperative Oceanic Fisheries Investigations (CalCOFI) has been conducting quarterly cruises that have sampled a breadth of oceanographic and lower-trophic-level biological data since 1949.…”