Gelatinous macrozooplankton in the North Sea: Biodiversity and distribution pattern during winter 2023-Q1 dataset

The biodiversity and distribution of gelatinous macrozooplankton in the North Sea and adjacent waters during winter (January/February) 2023 is presented both quantitatively and qualitatively. The data include species-specific jellyfish and comb jelly community data, encountered during the North Sea - Midwater Ring Net (MIK) survey [1]. The MIK survey targets ichthyoplankton and is conducted at night during the quarter 1 (Q1) International Bottom Trawl Surveys (IBTS). Presented data about the gelatinous macrozooplankton community stems from Danish (DTU Aqua), the Swedish (SLU), and German (TI) partners. A total of 158 stations were investigated using a MIK net (2 m diameter, 13 m long, 1.6 mm mesh size with 0.5 mm for net end and cod end) [2]. Samples were collected by double oblique hauls from the surface to 5 m above the seafloor with a maximum depth of ∼100 m [2]. Eighteen gelatinous macrozooplankton species were encountered during the Q1 2023 survey. Species encountered are hydrozoans (i) Aequorea vitrina, (ii) Agalma elegans (siphonophore), (iii) Aglantha digitale, (iv) Apolemia uvaria (siphonophore), (v) Clytia spp., (vi) Eutima spp., (vii) Leuckartiara octona, (viii) Melicertum octocostatum, (ix) Muggiaea atlantica (siphonophore), (x) Nanomia cara (siphonophore), (xi) Tima bairdii; scyphozoans (i) Cyanea capillata and (ii) Cyanea lamarckii as well as the ctenophores (i) Beroe spp., (ii) Bolinopsis infundibulum, (iii) Pleurobrachia bachei, (iv) Pleurobrachia pileus, and (v) the non-indigenous Mnemiopsis leidyi. In total 12,093 individual specimens from samples and sub-samples were analyzed and extrapolated to generate a database with 77,099 records of gelatinous macrozooplankton caught in the investigation area during Q1 2023. For rare species, the entire sample was processed, while abundances were estimated from sub-samples for abundant taxa. Flow-meter recordings and maximum net depths during each haul were used to convert raw counts to volume-specific densities (individuals m^-3) and area-specific abundances (individuals m^-2). Further, sizes for the different species were obtained from a total of 5,566 individual gelatinous macrozooplankton organisms. Sizes are presented in the accompanying database and were used to calculate species-specific wet weights, using published size-weight regressions [3] and regressions outlined in Table 1. In addition, we present spatial wet weight distribution patterns for (i) the total gelatinous macrozooplankton community, (ii) hydrozoans only, (iii) scyphozoans only and (iv) ctenophora only. The presented data contribute to a time series describing the gelatinous macrozooplankton diversity and distribution in the extended North Sea area during winter [3,4] and summer [5] and are an important baseline to understand response of jellyfish to climate change.