We sampled 30 brackish‐watered, isolated rock pools once a month (17 May, 22 June, and 22 July) in 2016 on a granitic outcrop in the western island of Pihlajasaari (66°68′449″N, 38°40′48″E), ca. 2 km south of Helsinki, Finland on the coast of the northern Baltic Sea. We examined the drivers and covariance of taxonomic and functional diversity among the rock pool communities. We measured water pH, conductivity, and temperature in the field, and pool morphometrics (i.e., max depth, length, and width) to the nearest centimeter, and calculated pool area (length * width). We collected a 0.5 L water sample from each pool for the determination of total P (SFS‐EN ISO 2004). We estimated pool X and Y coordinates (based on the perpendicular pool distance from the shore and the horizontal pool distance from the map origin in the southern end parallel to the shoreline, respectively) and mean isolation as a mean Euclidean distance (i.e., the sum of distances to five closest pools divided by five; Vanschoenwinkel et al. 2007) for each pool from a drawn grid map of the study area showing the relative location of the sampled pools to each other and to the seashore.

We sampled benthic diatoms by collecting epilithic samples (ca. 25 cm²) from each pool bottom with a toothbrush, following EN 13946 standard (2003). A total of 500 valves per sample were counted and identified to the lowest taxonomic level possible (mostly species level) with a light microscope. We created a taxonomic site‐species matrix based on species relative abundances.

The identified diatom species were classified into 21 partly overlapping functional groups. We first divided the species into five size classes after their biovolume (determined by cell length, width, thickness, and shape) and 14 life‐form categories after interspecific morphological adaptations to physical and chemical disturbance (i.e., cell motility, posture, and type of adhesion) following Rimet & Bouchez (2012). A single taxon may have various successive life forms and may thus be classified into multiple life‐form categories. We further classified the species after their preferences for nutrient concentration and physical disturbance into four ecological guilds (high profile, low profile, motile and planktonic) after Passy (2007) and Rimet & Bouchez (2012). Finally, we separated between acid‐tolerant (acidobiontic or acidophilus species with pH optimum <7 in Van Dam et al. (1994), and nitrogen‐fixing species (with cyanobacterial endosymbionts capable of fixing atmospheric nitrogen). In the species‐trait matrix, each species belonging to a given guild (other than continuous biovolume guild on a scale 1–5) was given a value of 1; otherwise, the value was set to 0. Each species could be characterized by multiple traits and could thus belong to more than only one guild.