Growth response of the barnacle Balanus improvisus and the bryozoan Electra pilosa from Kiel Fjord to biogenic pH fluctuations and global ocean acidification scenarios

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 in two temperatures (10 °C and 15 °C) and under ocean acidification (OA). For this, epibionts were grown on inactive (Plexiglas) and biologically active (F. serratus) surfaces and exposed to (i) constant pH scenarios under ambient (pH 8.1) or OA conditions (pH 7.7), or (ii) oscillating pH scenarios mimicking benthic boundary layer conditions at ambient (pH 7.7-8.6) or OA conditions (pH 7.4-8.2). The initial size of barnacles (diameter of basal plate) was determined for ~50 randomly chosen individuals on Plexiglas and algal surfaces by analysing pictures taken under the microscope one day prior to the start of the experiment, using ImageJ. The resulting start sizes were 0.72 ± 0.09 mm on Plexiglass and 0.70 ± 0.09 mm on algal surfaces, respectively. Final sizes of B. improvisus were determined from pictures taken at the end of the experiment following the same procedure as for initial size measurements. Note that data of single individuals within one cylinder (independent replicate) has to be combined to a mean before analyses. For the bryozoan E. pilosa, colony growth determinations differed slightly between the two surface types. For colonies on inactive surfaces, growth (addition of new zooids) was determined by comparing the number of zooids per colony in the beginning and at the end of the experiment using photographs. For colonies on active surfaces, the number of new zooids were counted directly by counting the zooids established after calcein staining on pictures taken at the end of the experiment, under a fluorescence microscope. The different procedures were employed to minimize handling of the active surface samples. The growth efficiency (GE) of colonies on both surfaces were calculated based on the number of zooids before the experiment (Ns) and the number of new zooids (Nn) following the equation GE = log10 (Nn / Ns).