Ostreopsis cf. ovata abundances on different benthic substrata: how to compare them?

In the framework of benthic harmful algal bloom monitoring, the most common sampling strategy is based on the collection of macroalgae, and the abundance of epiphytic microalgae are mainly expressed as cells g −1 macroalgal fresh weight (fw). However, this methodology has some inherent problems, due to (i) the thallus-specific weights that markedly differ among algal species, (ii) the thallus architecture, and (iii) the production of allelopathic compounds that affects the epiphyte abundances among macroalgae, irrespective of the available colonizable surface. This study proposes a method to compare the abundances of Ostreopsis cf. ovata cells on different substrata, using a conversion factor that converts the abundances expressed as cells g −1 fw (or dry weight) to cells cm −2 . Expressing abundances in terms of cells cm −2 , the abundances can be compared (i) among different macroalgal species and (ii) between macroalgae and other substrata (such as rocks, pebbles, or shellfish shells). We also propose to normalize abundances when different macroalgae are sampled throughout the bloom period, considering the different epiphyte loads of different macroalgal species regardless of the available surface area.