Abstract
Climate change is expected to exacerbate upwelling intensity and natural acidification in Eastern Boundaries Upwelling Systems (EBUS). Conducted between January-September 2015 in a nearshore site of the northern Humboldt Current System directly exposed to year-round upwelling episodes, this study was aimed at assessing the relationship between upwelling mediated pH-changes and functional traits of the numerically dominant planktonic copepod-grazer Acartia tonsa (Copepoda). Environmental temperature, salinity, oxygen, pH, alkalinity, chlorophyll-a (Chl), copepod adult size, egg production (EP), and egg size and growth were assessed through 28 random oceanographic surveys. Agglomerative clustering and multidimensional scaling identified three main di-similitude nodes within temporal variability of abiotic and biotic variables: A) “upwelling”, B) “non-upwelling”, and C) “warm-acid” conditions. Nodes A and B represented typical features within the upwelling phenology, characterized by the transition from low temperature, oxygen, pH and Chl during upwelling to higher levels during non-upwelling conditions. However, well-oxygenated, saline and “warm-acid” node C seemed to be atypical for local climatology, suggesting the occurrence of a low frequency oceanographic perturbation. Multivariate (LDA and ANCOVA) analyses revealed upwelling through temperature, oxygen and pH were the main factors affecting variations in adult size and EP, and highlighted growth rates were significantly lower under node C. Likely buffering upwelling pH-reductions, phytoplankton biomass maintained copepod reproduction despite prevailing low temperature, oxygen and pH levels in the upwelling setting. Helping to better explain why this species is among the most recurrent ones in these variable yet productive upwelling areas, current findings also provide opportune cues on plankton responses under warm-acid conditions, which are expected to occur in productive EBUS as a consequence of climate perturbations.
Highlights
A comprehensive understanding of oceanographic processes controlling spatial [1,2,3] and temporal [3,4,5] variations in pH and pCO2 conditions in coastal regions have improved our capacity to identify natural and anthropogenic signals and their future trends [6,7,8]
The uptake of anthropogenic CO2 has caused low pH, high pCO2 waters to shoal since preindustrial times, and in consequence are part of the waters currently being upwelled in some coastal regions of productive Eastern Boundary Upwelling Systems (EBUS) [2,8]
While there seems to be consensus regarding the trend of increased upwelling intensity in the future due to intensification of upwelling favorable winds [9] with consequences in ocean acidification (OA) [9,10], less attention has been devoted to understanding the impact of discrete low pH-events (“event effects”) [11]
Summary
A comprehensive understanding of oceanographic processes controlling spatial [1,2,3] and temporal [3,4,5] variations in pH and pCO2 conditions in coastal regions have improved our capacity to identify natural and anthropogenic signals and their future trends [6,7,8]. The uptake of anthropogenic CO2 has caused low pH, high pCO2 waters to shoal since preindustrial times, and in consequence are part of the waters currently being upwelled in some coastal regions of productive Eastern Boundary Upwelling Systems (EBUS) [2,8]. Upwelling areas are characterized by a high temporal variability, from hours to months, in consequence, upwelling driven pH-changes might constitute a significant environmental factor affecting short-term but ecologically functional plankton processes, such as growth and reproduction [13,14,15]. The omission of the environmental history can lead to uncertain interpretations of copepod physiology under pH-variations, which may not necessarily reflect current local environmental-biological coupling or future responses to global stressors [29]
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