cc-by-4.0Soininen, Janne2025-04-292024-01-092024-01-09https://datakatalogi.helsinki.fi/handle/123456789/4461We sampled nine brackish-watered, isolated rock pools 12 times at roughly ten-day intervals in May-September 2019 on a granitic outcrop in Pihlajasaari island (66°68‵N, 38°40‵E), ca. 2 km south of Helsinki on the coast of the northern Baltic Sea. We examined patterns and drivers of temporal beta diversity, and whether the temporal variation in community composition is related to temporal environmental variation among the diatom communities. We measured water pH, conductivity, and temperature in situ with YSI field meter. We measured pool morphometrics (i.e., max depth, length, and width) and calculated pool area by multiplying pool length by pool width. We collected a 0.5L water sample from each pool preserved at 4°C for the determination of total P (SFS-EN ISO 2004) and total N (SFS-EN ISO 1998) concentration, and a nutrient supply (i.e., N:P) ratio. We determined pool location by latitude and longitude with a GPS and measured pool distance from the sea and pool mean isolation as a mean Euclidean distance (i.e., the mean distance to five closest pools; Vanschoenwinkel et al. 2007) from an aerial photograph. We sampled benthic diatoms following standard methodology (EN 13946 2003) by collecting ten epilithic subsamples (ca. 25 cm2) from each pool bottom with a toothbrush, combined as a single composite sample in the field. A minimum of 500 valves per slide were counted and identified to the lowest taxonomic level possible (mostly species level) with a light microscope. We created a binomial site-species matrix based on species presence-absence after the count abundances. We classified the diatom species into 21 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 their morphological adaptations (i.e., cell motility, posture, and type of adhesion) to physical and chemical disturbance following Rimet and Bouchez (2012). Any taxon with various successive life-forms was classified into multiple life-form categories (Berthon et al. 2011). We further divided the species after their preferences for nutrient concentration and physical disturbance into four ecological guilds after Passy (2007) and Rimet and Bouchez (2012): high profile, low profile, motile, and planktic guild. Finally, we separated between acid-tolerant (acidobiontic or acidophilous species with pH optimum < 7 in Van Dam et al. 1994) and nitrogen-fixing species capable of fixing atmospheric nitrogen by cyanobacterial endosymbionts) (Soininen et al. 2016). We created a binomial site-trait matrix, in which each species belonging to a given guild was given a value of 1; otherwise, the value was set to 0. A species characterized by multiple traits could belong to more than one guild.dataset