Biogeochemical Responses and Seasonal Dynamics of the Benthic Boundary Layer Microbial Communities during the El Niño 2015 in an Eastern Boundary Upwelling System

Verónica Molina,Guillermo Alarcón,Carla Acuña,Daniela Silva,Nelson Silva,Eulogio H Soto,Marcela Cornejo-D’Ottone,Eduardo Quiroga

doi:10.3390/w13020180

Abstract

The Eastern South Pacific coastal zone is characterized by seasonal and interannual variability, driven by upwelling and El Niño Southern Oscillation (ENSO), respectively. These oceanographical conditions influence microbial communities and their contribution to nutrient and greenhouse gases recycling, especially in bottom waters due to oxygenation. This article addresses the seasonal hydrographic and biogeochemical conditions in the water and sediments during El Niño 2015. Bottom water active microbial communities, including nitrifiers, were studied using amplicon sequencing of 16S rRNA (cDNA) and RT-qPCR, respectively. The results of the hydrographic analysis showed changes in the water column associated with the predominance of sub-Antarctic Waters characterized by warmed and low nutrients in the surface and more oxygenated conditions at the bottom in comparison with El Niño 2014. The organic matter quantity and quality decreased during fall and winter. The bottom water active microbial assemblages were dominated by archaea (Ca. Poseidoniales) and putative ammonia oxidizing archaea. Active bacteria affiliated to SAR11, Marinimicrobia and Nitrospina, and oxygen deficient realms (Desulfobacterales, SUP05 clade and anammox) suffered variations, possibly associated with oxygen and redox conditions in the benthic boundary layer. Nitrifying functional groups contributed significantly more during late fall and winter which was consistent with higher bottom water oxygenation. Relationships between apparent oxygen utilization nitrate and nitrous oxide in the water support the contribution of nitrification to this greenhouse gas distribution in the water. In general, our study suggests that seasonal oceanographic variability during an El Niño year influences the microbial community and thus remineralization potential, which supports the need to carry out longer time series to identify the relevance of seasonality under ENSO in Eastern Boundary Upwelling Systems (EBUS) areas.

Highlights

The coastal zone of central Chile is marked by high productivity, associated with upwelling events driven by the anticyclonic gyre seasonality, characteristic of EasternBoundary Upwelling Systems (EBUS)
The water column off Valparaíso was characterized by the presence of two water masses, the sub-Antarctic Water (SAAW) and the Equatorial Subsurface Water (ESSW) according to
Silva et al [24], the first usually located at the surface layer (

Summary

Introduction

The coastal zone of central Chile is marked by high productivity, associated with upwelling events driven by the anticyclonic gyre seasonality, characteristic of EasternBoundary Upwelling Systems (EBUS). During 2015–2016, strong temperature anomalies were observed in the equatorial region during the “Godzilla El Niño” event, in comparison with previous events since 1950 [8], impacting the biogeochemical conditions in the water, from moderate to strong, between early 2015 and mid-2015 at Callao coasts off Perú [9]. These authors reported higher temperatures, oxygen, pH and nitrate but lower silicate and phosphate concentrations at the surface layer during “El Niño”. The shifts in the nutrient reservoir in the surface water and oxygenation influences the phytoplankton community structure and primary productivity at different upwelling areas along the ESP coast [10,11,12]

Methods

Results

Conclusion