Biodiversity of gelatinous macrozooplankton: Quantitative assessment of data and distribution patterns in the southern and central North Sea during August 2018.

This article describes the biodiversity of gelatinous macrozooplankton and presents quantitative field data on their community composition and distribution pattern in the North Sea during August 2018. The data set consists of jellyfish and comb jelly species abundance estimates which are based on sampling at 62 stations in the central and southern North Sea covering Danish waters, the German Bight, waters off the Dutch coast as well as the western North Sea off the UK coast and the central North Sea. The sampling gear was a 13 m long MIK-net (modified Methot Isaac Kidd net; Ø 2 m, mesh size 1 mm, mesh size cod end 500 μm) deployed in double oblique hauls from the surface to 5 m above the sea floor. Samples were visually analysed for gelatinous macrozooplankton (>2 mm) using a light table. Samples were processed within 1 hour after catch. In total, 6239 gelatinous macrozooplankton specimen were caught. Spatial distribution pattern described in this article include the jellyfish species sp, , sp., , , , , , , , sp as well as the comb jelly species and . Further, size frequency distributions of abundant taxa are provided together with a summary of abundances as well as average, maximum and minimum sizes of all species. This dataset has not previously been published and is of high value for comparison with other - and future - investigations of gelatinous macrozooplankton in the North Sea. The data were obtained during an internationally coordinated, standard fishery survey which is carried out annually (Quarter 3 - North Sea - International Bottom Trawl Survey - Q3 NS-IBTS). The gained information could be used as baseline for a monitoring of potential changes in gelatinous macrozooplankton abundances to address the long standing question if gelatinous zooplankton are on the rise due to climate change induced stressors.