PhD Student (Adv. Thomas Schwaha)
Unit for Integrative Zoology, Department of Evolutionary Biology
University of Vienna

Abstract

Physical and biological forces in the intertidal zone can cause dynamic pH fluctuations from micro-scale (diffusive boundary layer, DBL) up to ecosystem-scale (benthic boundary layer, BBL). These natural pH variations may alter an organism’s response to ocean acidification (OA) by providing temporal refugia.

We used a mesocosm system to investigate how pH fluctuations, generated by the biological activity of Fucus serratus under OA, modulates the responses of two calcifying epibionts in the Western Baltic Sea, namely Balanus improvisus and Electra pilosa. For this, both epibionts were grown on inactive and biologically active substrates and exposed to (i) constant pH scenarios under ambient (pH 8.1) or OA conditions (pH 7.7), or (ii) oscillating pH scenarios mimicking BBL conditions at ambient (pH 7.7-8.6) or OA scenarios (pH 7.4-8.2).  All treatment combinations were tested at 10 and 15°C, simulating mild and warm conditions of the calcifiers’ main growth period.

Microscale measurements showed that F. serratus activity adds additional variability, exacerbating diurnal pH fluctuation experienced by their epibionts. However, fluctuation conditions of the DBL did not necessarily provide temporal refugia from OA. We propose that factors other than pH (e.g., surface characteristics, macrophyte antifouling defense or dynamics of oxygen and nutrient concentrations) may play larger roles for epibiont growth on macrophytes. Furthermore, OA treatments did not affect epibiont growth under constant or fluctuating (BBL) pH conditions, indicating rather high tolerance to predicted OA scenarios.