Host performance
To obtain measures of host performance, we recorded the final wet weight
at the end of the experiment (t12), summed this with the
weight of the fragments harvested for the sampling of epibiota, divided
this by the initial wet weight, and multiplied it by 100 to express
relative growth rate (RGR) in average percentual growth per day. In
addition, we measured the maximum quantum yield of photosystem II using
a Diving-PAM fluorometer (Heinz Walz GmbH, Effeltrich/Germany). For this
purpose, algae were dark-adapted for 15 min after the final sampling.
During the experiment, we also observed the development of two types of
disease or stress symptoms. The most prevalent of those symptoms was an
increased brittleness of the thallus. We also observed thallus bleaching
and decay (similar to (Saha & Weinberger, 2019)) which was often
combined with fragmentation. Both thallus brittleness and decay were
binary scored (0 or 1 for absence or presence of symptoms; see also
Krueger‐Hadfield & Ryan, 2020; Sotka et al., 2018). To investigate
correlations, we calculated Spearman rank coefficients among all four
performance measures.