ENSO Climate Forcing of the Marine Mercury Cycle in the Peruvian Upwelling Zone Does Not Affect Methylmercury Levels of Marine Avian Top Predators

Abstract

Climate change is expected to affect marine mercury (Hg) biogeochemistry and biomagnification. Recent modeling work suggested that ocean warming increases methylmercury (MeHg) levels in fish. Here, we studied the influence of El Ninõ Southern Oscillations (ENSO) on Hg concentrations and stable isotopes in time series of seabird blood from the Peruvian upwelling and oxygen minimum zone. Between 2009, La Ninã (2011 and El Ninõ conditions (2015−2016) were accompanied by sea surface temperature anomalies up to 3 °C, oxycline depth change (20−100 m), and strong primary production gradients. Seabird Hg levels were stable and did not co-vary significantly with oceanographic parameters, nor with anchovy biomass, the primary dietary source to seabirds (90%). In contrast, seabird Δ 199 Hg, proxy for marine photochemical MeHg breakdown, and δ 15 N showed strong interannual variability (up to 0.8 and 3‰, respectively) and sharply decreased during El Ninõ. We suggest that lower Δ 199 Hg during El Ninõ represents reduced MeHg photodegradation due to the deepening of the oxycline. This process was balanced by equally reduced Hg methylation due to reduced productivity, carbon export, and remineralization. The non-dependence of seabird MeHg levels on strong ENSO variability suggests that marine predator MeHg levels may not be as sensitive to climate change as is currently thought.

Elevated Mercury Concentrations and Isotope Signatures (N, C, Hg) in Yellowfin Tuna (Thunnus albacares) from the Galápagos Marine Reserve and Waters off Ecuador

“…During 2015 to 2016, a weak El Niño event was reported in the area, which caused shifts in the δ 15 N and δ 13 C baseline due to changes in nutrient regimes related to upwelling dynamics (Renedo et al, 2021; Zarn et al, 2020). When pooling the Δ 199 Hg and THg data for both the Galápagos Marine Reserve and Ecuadorian mainland coast, we infer the influence of foraging depth on THg exposure using Δ 199 Hg as a tracer (i.e., Δ 199 Hg values decrease from the surface to aphotic waters; Blum et al, 2013).…”