Upwelling modulation of functional traits of a dominant planktonic grazer during “warm-acid” El Niño 2015 in a year-round upwelling area of Humboldt Current

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.

Biogeochemical Anomalies at Two Southern California Current System Moorings During the 2014–2016 Warm Anomaly‐El Niño Sequence

“…Bednaršek et al () showed negative pteropod responses in the northern CCS to the 2013–2016 combined marine heat wave, El Niño, and enhanced upwelling. Aguilera et al () similarly observed reductions in growth and egg production in the resident copepod Acartia tonsa in the Humboldt Current System under warm, acidic conditions in 2015. Our finding of increased pelagic molluscs in the southern CCS in springs 2014 and 2016 may be explained by several factors.…”

“…These feeding strategies may have allowed some molluscs to increase despite overall reduced primary production and smaller phytoplankton sizes during the Warm Anomaly and El Niño, perhaps also offsetting negative effects of thermal stress. Aguilera et al () observed that A. tonsa maintained normal body size despite unfavorable habitat conditions during both the 2015 warm period and general upwelling conditions. The authors attribute this to elevated phytoplankton prey, suggesting food availability may partially offset negative physical oceanographic conditions.…”

Intraseasonal Hydrographic Variations and Nearshore Carbonates System Off Northern Chile During the 2015 El Niño Event

“…Interestingly, both the El Niño 1997–1998 (Torres et al, 2003) and El Niño 2015 (current study) events largely differ in genesis and development (Dewitte & Takahashi, 2017; Mogollón, 2019), emphasizing the increasing complexity of El Niño manifestations observed in this productive EBUS (Aguilera et al, 2019; Chavez et al, 1999; Escribano et al, 2004; Silva et al, 2009; Torres et al, 2003). Strongly linked to wind forcing (Brady et al, 2019; Mogollón, 2019; Rödenbeck et al, 2014), the coastal response of the carbonates system to wind variations can be greatly modified due to reiterated upwelling events accumulating CO 2 in the upper layer (Torres et al, 2003), ocean acidification (Hauri et al, 2013), and El Niño conditions (Aguilera et al, 2019; Torres et al, 2003). Further assessments could be based on the analysis of the δ 13 C‐DIC since all DIC sources affecting carbonates system, such as those in the water masses fueling coastal upwelling (Goyet et al, 2009; Martínez‐Méndez et al, 2013) or the excess atmospheric CO 2 driving ocean acidification (Feely et al, 2008), differ in their δ 13 C‐DIC isotopic signal.…”

“…Also due to climate change, the amplitude and frequency of El Niño events is expected to increase (Cai et al, 2018), while its oceanographic manifestations can be strongly modulated by stochastic processes as they propagate poleward and impact EBUS across the northern and southern Pacific basins (Frischknecht et al, 2015; McPhaden, 2015). In fact, within the scope of equatorial oceanographic variability, drastic changes in aquatic conditions and biological productivity have been observed in the Humboldt EBUS under the influence of El Niño event (Aguilera et al, 2019; Escribano et al, 2004; Torres et al, 2002; Ulloa et al, 2001).…”

Antagonistic interplay between pH and food resources affects copepod traits and performance in a year-round upwelling system

“…Temperature-standardized pH ( @25 °C ) was measured in closed 25 mL cells thermostated using a Metrohm 827 pH meter (input resistance, >1 × 1012 Ohm, 0.1 mV sensitivity and nominal resolution at 0.01 pH units) and a glass combined double junction Ag/AgCl electrode (Aquatrode PT1000, N/P 6.0257.000) calibrated with 4 and 7 buffers within 1 h from time of collection. Samples for total alkalinity analysis were collected in borosilicate glass bottles with ground glass stoppers (250 mL) and poisoned with 10 μL HgCl 2 62 . Total alkalinity (A T ) was determined using the open-cell titration method 63 , using an automated Alkalinity Titrator AS-ALK2 Apollo SciTech.…”