UC San Diego

Physical processes in surface ocean circulation are critical in shaping pelagic communities. On spatial scales that include entire ocean basins, climate oscillations drive changes in ocean physics that in turn shape biological production. The El Nin̋o Southern Oscillation (ENSO), for example, persists for 6 to 18 months, spatially enveloping physical changes in the tropical and eastern Pacific. On the other hand, the Pacific Decadal Oscillation (PDO), while having a similar spatial fingerprint, has "regimes" lasting 20 to 30 years. Changes in these regimes, or regime shifts, can happen abruptly (within a year), affecting pelagic ecosystems by altering processes regulating nutrient supply that in turn drive biological production (bottom-up forcing). With the benefit of hindsight, we now recognize that regime shifts have impacted ecosystems in the eastern Pacific, particularly in extra-tropical regions. Despite covering the largest portion of the worlds oceans, few long-term ecological data sets exists for tropical oceanic ecosystems. Thus, there is a lack knowledge as to how tropical open ocean systems react to regime shifts. In this dissertation, I retrospectively built physical and biological data sets to test hypotheses linking the 1976/ 77 Pacific Ocean regime shift to bottom-up effects of ecosystem change in a tropical and oceanic system, the eastern Pacific warm pool. I approached my research goals by analyzing three components of the eastern Pacific warm pool ecosystem during the 1960-2006 time period. First, I used historical hydrographic data from the World Ocean Database 2009 to characterize trends in thermocline depth and water column stratification in the upper 200 meters. Second, I tested hypotheses linking the 1976/77 Pacific Ocean regime shift to bottom-up control of ecosystem change in the eastern Pacific warm pool for mid-trophic- level organisms and apex predators. For the mid-trophic- level organisms I used ichthyoplankton samples collected during historical and contemporary expeditions to the eastern tropical Pacific. For the apex predators I used carbon and nitrogen stable isotope ratios measured from seabird feathers of a suite of ecologically and phylogenetically diverse seabird species collected in the eastern Pacific warm pool in 1960-2006 to gauge diet variability during this time period. I found evidence suggesting that multidecadal changes occurred in the thermal structure of the upper 200 meters in the eastern Pacific warm pool. Furthermore, I found evidence suggesting that organisms from two trophic levels responded differently to these environmental changes. Temoral variability in species assemblages of mid-trophic organisms, ichthyoplankton, appeared to be higher in regions of the study area where upwelling is prevalent, while assemblages from oceanic regions with less or no upwelling were stable. In contrast to variability of mid trophic-level organisms, the carbon and nitrogen stable isotope proxy for diet of apex predators, seabirds, showed little variation over time. These results are in agreement with the notion that physical forcing shapes nutrient fluctuations driving biological production and that lower trophic levels are more likely to respond to these fluctuations than long-lived apex predators. However, stable isotope proxy data and biological survey of apex predators in the northeastern Pacific have shown fluctuations coherent with ocean warming and the 1976/77 regime shift. The eastern tropical Pacific supports a unique multispecies community of apex predators comprised of around 50 resident seabird species and 30 cetacean species including several endemics and the world's largest yellowfin tuna fishery. The most recent regime shift is thought to have occurred in 1998/99. Future research should focus on more robust data sets that can further improve our knowledge of ecosystem effects of regime shifts in tropical systems