Reconstructing Primary to Secondary Paleo-productivity Using Nitrogen Isotopes: a Case Study from Bainbridge Lake, Galápagos

Abstract:

El Niño Southern Oscillation (ENSO), a pattern of increased sea surface temperatures in the eastern tropical Pacific, is responsible for disturbances in atmospheric circulation and precipitation, as well as lowered primary productivity. Because seabird populations are so often affected by variations in climate and productivity, δ¹⁵N in bird guano deposited in sediments can be used as an indicator of climate patterns. Previous studies have applied this approach to systems dominated by piscivores, but this is the first time this approach has been applied to secondary (rather than tertiary) consumers— American flamingos feeding on krill in Bainbridge Crater Lake, Galápagos. As observed at other sites, we demonstrate that flamingo guano δ¹⁵N was enriched relative to both algae and sediment trap samples. Applying this approach to replicate sediment cores, we find that sediment δ¹⁵N decreased over time. This suggests that from the late 1960s to the present day, productivity at Bainbridge Lake has decreased, as local sea surface temperatures increased. Finally, applying a preliminary (Pb-210 based) age-depth model to the sediment core reveals short-term δ¹⁵N variations associated with regional climate patterns, such as ENSO. More negative δ¹⁵N during strong and very strong El Niño events suggest that the warm phase of ENSO had a negative effect on local primary productivity and bird populations. As our samples produced similar δ¹⁵N paleo-productivity records to those recorded at nearby sites using tertiary consumers, our findings suggest that secondary consumers’ guano can be used as indicators of climate shifts. Ongoing work will extend these records to reconstruct the impact of climate variability on productivity at the base of the critical eastern equatorial Pacific food web.