Abstract
AbstractIn coastal marine environments, physical and biological forces can cause dynamic pH fluctuations from microscale (diffusive boundary layer [DBL]) up to ecosystem‐scale (benthic boundary layer [BBL]). In the face of ocean acidification (OA), such natural pH variations may modulate an organism's response to OA by providing temporal refugia. We investigated the effect of pH fluctuations, generated by the brown alga Fucus serratus' biological activity, on the calcifying epibionts Balanus improvisus and Electra pilosa under OA. For this, both epibionts were grown on inactive and biologically active surfaces and exposed to (1) constant pH scenarios under ambient (pH 8.1) or OA conditions (pH 7.7), or (2) oscillating pH scenarios mimicking BBL conditions at ambient (pH 7.7–8.6) or OA scenarios (pH 7.4–8.2). Furthermore, all treatment combinations were tested at 10°C and 15°C. Against our expectations, OA treatments did not affect epibiont growth under constant or fluctuating (BBL) pH conditions, indicating rather high robustness against predicted OA scenarios. Furthermore, epibiont growth was hampered and not fostered on active surfaces (fluctuating DBL conditions), indicating that fluctuating pH conditions of the DBL with elevated daytime pH do not necessarily provide temporal refugia from OA. In contrast, results indicate that factors other than pH may play larger roles for epibiont growth on macrophytes (e.g., surface characteristics, macrophyte antifouling defense, or dynamics of oxygen and nutrient concentrations). Warming enhanced epibiont growth rates significantly, independently of OA, indicating no synergistic effects of pH treatments and temperature within their natural temperature range.
Highlights
IntroductionHydrodynamic conditions increase or dampen the effect of metabolic processes (e.g., photosynthesis and Epibiont responses to pH variability respiration) and lead to the enrichment or depletion of metabolic compounds (e.g., O2, CO2, CO23− , H+) in the surrounding seawater (Waldbusser and Salisbury 2014; Kapsenberg and Cyronak 2019)
Strongest diel pH fluctuations occurred in an active diffusive boundary layer (DBL) under fluctuating BBL conditions
Epibiont growth was higher on inactive surfaces compared to active surfaces, for E. pilosa, which suggests that factors other than the carbonate chemistry drive epibiont growth on F. serratus
Summary
Hydrodynamic conditions increase or dampen the effect of metabolic processes (e.g., photosynthesis and Epibiont responses to pH variability respiration) and lead to the enrichment or depletion of metabolic compounds (e.g., O2, CO2, CO23− , H+) in the surrounding seawater (Waldbusser and Salisbury 2014; Kapsenberg and Cyronak 2019) This can result in strong diurnal variations of, for example, pH in macrophyte beds, especially when water flow is low (Cornwall et al 2013a; Wahl et al 2018). They can alter the physical state of the surrounding seawater and play an important role in the resource availability to organisms within proximity (Hurd 2015; Teagle et al 2017) Their metabolic activity coupled with seawater flow intensity can create several boundary layers of varying thickness and chemical concentration gradients (Hermansen et al 2001; Noisette and Hurd 2018). The most drastic changes are found in DBLs where diurnal changes in hydrogen ion concentration of up to 30-fold occur, resulting in pH variations of 0.3–1.2 pH units between day and night (Wahl et al 2018)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
