Methods

A. Taxonomic screening The targets of this data set are habitat-forming seaweed species in rocky coastal communities, spreading from the warm to the cold temperate zones (Spalding et al. 2007) in Japan. Many of these species belong to the class Phaeophyceae, which includes Fucales (fucoids, mainly Sargassum spp.) and Laminariales (kelps) found in the temperate zone (Undaria spp., Ecklonia spp., Saccharina japonica, and S. japonica var. religiosa). We updated scientific names based on the nomenclature that is currently accepted in Algaebase (Guiry & Guiry 2019). Overall, we registered 58 species. Refer Kumagai et al. (2016) for detailed information on taxonomic screening. B. Literature collection First, we searched for literature that included seaweed-quantity records among the 355 literature sources listed in Kumagai et al. (2016). We compiled not only those quantity records but also seasonal and elevational (depth) changes, which are hardly described in Kumagai et al. (2016) because of the difference in the aim of their study. Second, we obtained additional quantitative records of seaweed by asking professional researchers and via internet searches of literature using Google Scholar (http://scholar.google.com/) and CiNii (http://ci.nii.ac.jp/en) with the key words “seaweeds, biomass, cover, or survey(s)” as well as the scientific and Japanese name(s) of each species in Japan. We started this search in April 2017 and finally updated the search at the end of Jul 2018. The descriptions obtained from the compiled literature are listed in LiteratureDB. The descriptions of seaweed quantity included percent cover, dry weight, wet weight, shoot density, and shoot length. Although most of them are described at the species level, several descriptions of wet weight pertain to total seaweed. Therefore, we also allowed the inclusion of wet weight for total seaweeds. Furthermore, several reports only include the range of seaweed quantity, particularly in percent cover (e.g., “50%–75%,” “<5%”). In these cases, we quantified them using the average value and the accuracy. Moreover, two types of “percent cover” were found: the total cover of all species was 100% or >100%. The location (latitude and longitude) and the elevation (i.e., water depth) of each quadrate were obtained from the description provided in each reference. We also found other minor types of quantity records (e.g., dry weight of total seaweeds); however, we did not include them in our data set because the number of such records was too small (<50). Most of the literature only provides a map or name of the study locations from which we obtained their approximate coordinates using Google Maps (https://maps.google.com/, Google Inc., Mountain View, CA, USA). Most of the sites were identified because Japanese lands along the coastal line are well discriminated by local names. The geographical position of 88.5% of the locations was identified with an accuracy of 1 km. There were two peaks in the number of records: 1970s because of the comprehensive surveys of the Fishery Agency and 2000s because of the increase in the number of various local surveys (Fig. 1) in response to the degradation of the Japanese seaweed community (Kumagai et al. 2018). The number of records was largest for the cover (6,462), and the second largest for wet weight (1,848) (Table 1). Each of the parameters widely distributed along Japanese coats (Fig. 2), although the geographic range of the cover was biased to the southern half of Japan (Fig. 2D). In terms of the number of records and the geographic range, the wet weight was the most representative variable in this data set (Fig. 2B). Moreover, most frequently, the wet weight was recorded with the other type of quantitative parameter (Table 1).