Asymmetric, large - scale community responses to ENSOs in Galapagos subtidal ecosystems

Background/Question/Methods . Theory suggests that non-linear shifts in ecosystems occur as thresholds are exceeded, but most empirical studies of El Nino impacts in marine systems describe linear losses of biomass, abundance or diversity. To understand the influence of ENSO's, upwelling and local biotic processes shaping rocky subtidal communities, we've conducted experimental manipulations in a framework of long term monitoring at 12 sites spanning a 120 km region of the central Galapagos since 1999-2003. Here we document non-linear changes where corals were bleached and /or killed and barnacle populations dramatically increased during the 2006- 2007 ENSO.  

Results/Conclusions . . These changes were likely triggered by unusually cold water during the 2007 La Niña phase. Anomalously cold episodes (as low as 13.00 C; 6, 15 m depth) were common by May – June 2007 when average temperatures were 0.6 – 2.4 00 C lower than long term averages at some sites. Extensive coral bleaching occurred between June 2007 and January 2008 surveys. Coral bleaching and resilience was taxon specific, with the most damage incurred by Pocilloporid finger corals where all colonies encountered were at least partially bleached and over half were dead. Mounding corals (Porites, Pavona) sustained less bleaching and the majority of colonies had recovered by January 2009. La Niña impacts on the subtidal ecosystem were apparently asymmetric since significant increases in the percent cover of barnacles (Megabalanus) occurred via recruitment and growth at all 12 sites during the period when corals were stressed. Eastern sites showed the largest increases in barnacle abundance (up to 2 fold) during the La Nina, suggesting spatially coherent high barnacle recruitment over at least 100 km N-S distance. La Nina associated increases in barnacle abundance will likely have a bottom up effect on the rocky subtidal food web as such effects on predatory whelk populations were demonstrated over a 3 yr period leading up to the 2006-2007 ENSO. Effects on the broader food web will be evaluated. In contrast to corals and barnacles, densities of a key grazer and omnivore, the pencil urchin, Eucidaris galapagensis, remained stable during and after the 2006-2007 ENSO. Overall, the results suggest that regional climate oscillations may have simultaneously negative and positive effects on different components of the same large-scale ecosystem, underscoring the importance of long- term field observations to probe the impacts of climate change in the ocean.