Semi-quantitative macroalgae survey on abundance and lower depth distribution of dominant brown algal species collected at Hansneset, Kongsfjorden, Spitsbergen in 2021

Surveys were performed of abundance and lower depth distribution of biomass dominant brown algae species at Hansneset, Blomstrand in Kongsfjorden, Svalbard, between 2 m and 20 m depth in June - August 2021. This dataset is part of a time series currently spanning over 25 years and complements the studies conducted in 1996/98 (Hop et al., 2012, doi:10.1515/bot-2012-0097) and 2012-14 (Bartsch et al., 2016, doi:10.1007/s00300-015-1870-1). The aim was to document changes in Arctic kelp forests dynamics in an Arctic fjord system influenced by glacial melt by repeatedly sampling the same site in a standardized manner. As ocean temperatures in the Arctic have risen substantially and underwater light climate continuously deteriorated over this time period alterations were observed in the seaweed community, especially kelps as major coastal foundation species. The depth distribution of biomass dominant brown algae was semi-quantitatively (visually) investigated by scientific divers in five parallel transects off the coastline covering the vertical gradient between 2 m and 20 m depth. For each replicate a 1x1 m quadrat divided into four 50 x 50 cm subquadrats was placed on the ground or above the kelps at every depth meter along the transect and species occurrence was documented as attached frequency within each subquadrat. This resulted in a relative frequency of 0 - 4 per replicate (Frequency count). Additionally, the visual presence of the species in the close surroundings of each replicate was documented 0 or 1 (Additional Presence). In summary this generates in a maximum presence score of 5 for each species per replicate. The target replication was five per depth but due to depth corrections to chart datum this resulted in n = 3–8 for each depth and species. The presence (1) or absence (0) of rocks, pebbles, and/or sand was all documented.