Abstract
Linking pH/pCO2 natural variation to phenotypic traits and performance of foundational species provides essential information for assessing and predicting the impact of ocean acidification (OA) on marine ecosystems. Yet, evidence of such linkage for copepods, the most abundant metazoans in the oceans, remains scarce, particularly for naturally corrosive Eastern Boundary Upwelling systems (EBUs). This study assessed the relationship between pH levels and traits (body and egg size) and performance (ingestion rate (IR) and egg reproduction rate (EPR)) of the numerically dominant neritic copepod Acartia tonsa, in a year-round upwelling system of the northern (23° S) Humboldt EBUs. The study revealed decreases in chlorophyll (Chl) ingestion rate, egg production rate and egg size with decreasing pH as well as egg production efficiency, but the opposite for copepod body size. Further, ingestion rate increased hyperbolically with Chl, and saturated at ~1 µg Chl. L−1. Food resources categorized as high (H, >1 µg L−1) and low (L, <1 µg L−1) levels, and pH-values categorized as equivalent to present day (≤400 µatm pCO2, pH > 7.89) and future (>400 µatm pCO2, pH < 7.89) were used to compare our observations to values globally employed to experimentally test copepod sensitivity to OA. A comparison (PERMANOVA) test with Chl/pH (2*2) design showed that partially overlapping OA levels expected for the year 2100 in other ocean regions, low-pH conditions in this system negatively impacted traits and performance associated with copepod fitness. However, interacting antagonistically with pH, food resource (Chl) maintained copepod production in spite of low pH levels. Thus, the deleterious effects of ocean acidification are modulated by resource availability in this system.
Highlights
Anthropogenic CO2 emissions to the atmosphere since the industrial revolution have reduced the pH of the surface open ocean at a steady rate of 0.02 pH units per decade[1], giving rise to an unprecedented ocean acidification (OA) process in millions of years[2]
Biological-environmental relations were independently evaluated through a Distance based Linear Model (DistLM) and a Principal Coordinate (PCO) analysis, which scores
This study showed a link between variations in pH and copepod traits and performance in the upwelling system of the Humboldt Eastern Boundary Upwelling system (EBUs)
Summary
Anthropogenic CO2 emissions to the atmosphere since the industrial revolution have reduced the pH of the surface open ocean at a steady rate of 0.02 pH units per decade[1], giving rise to an unprecedented ocean acidification (OA) process in millions of years[2]. Biologically productive Eastern Boundary Upwelling Systems (EBUs) are naturally low in pH5, but the synergy with OA has decreased pH levels below thresholds[5,6] that impact the tolerance of the biota and threaten the social livelihood these globally relevant marine areas provide[7]. Natural pH variation regulates phenotypic plasticity and adaptive potential of local populations[15], and its omission in the design of OA experiments can lead to results which may not necessarily reflect future responses to global stressors[21]. The neritic copepod Acartia tonsa (Copepoda, Calanoida) is among the most abundant and temporarily prevalent species in upwelling areas of the productive Humboldt EBUs, where it inhabits near surface waters[33,34] and recruits continuously[33,35] in neutral to acidic (i.e., low pH values)[36], yet productive conditions
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