Depression and recovery of pelagic biomass in response to El Niño events and a warming trend

Climatic variability exerts enormous pressures on the structure and function of open ocean ecosystems. Although the responses of primary producers and top predators to these pressures are being increasingly well-documented, little is known about how organisms at the center of oceanic food webs respond to environmental variability. We address this knowledge gap through a study of the effects of El Niño Southern Oscillation (ENSO), the strongest climatic anomaly on the planet, on a midtrophic community in the Gulf of California (GOC), Mexico. Using hydroacoustic data collected during 2007 – 2017, we define acoustic proxies for biomass and density of the midtrophic community in the upper 200 m of the water column and describe its response to environmental variability using generalized additive mixed models. Acoustic biomass in the central GOC decreased significantly during warm conditions and in the positive phase of the El Niño Southern Oscillation. Following strong El Niño events in 2010 and 2015, acoustic biomass was depressed by a factor of two and took more than three years to recover. Acoustic density increased linearly with acoustic biomass, suggesting that prey availability decreases exponentially with acoustic biomass, likely an important factor determining the response of predators to environmental variability. This study provides unique insight into the effects of El Niño on midtrophic communities and highlights the need to quantify variability at all levels of oceanic food webs to improve our understanding of how ecosystem function and services will change under future ocean warming and the intensification of climatic anomalies.