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 Aequorea sp., Aurelia aurita, Beroe sp., Chrysaora hysoscella, Clytia hemisphaerica, Cyanea capillata, Cyanea lamarckii, Eirene viridula, Leuckartiara octona, Melicertum octocostatum, Obelia sp. as well as the comb jelly species Mnemiopsis leidyi and Pleurobrachia pileus. 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.

Data

This data article presents a description of the biodiversity of the gelatinous macrozooplankton community sampled at 62 stations across the southern and central North Sea (Fig. 1) during August 2018. The data consist of spatial distribution patterns and abundance data (Fig. 2, Fig. 3, Fig. 4) of 13 major gelatinous macrozooplankton species along with size frequency distributions of the five most important species (Fig. 5). Further, a table presenting total numbers, average and maximum abundance data across stations as well as average, maximum and minimum sizes is provided (Table 1). A total of 6239 jellyfish and comb jelly specimen were caught belonging to the following species: Aequorea sp., Aurelia aurita, Beroe sp., Chrysaora hysoscella, Clytia hemisphaerica, Cyanea capillata, Cyanea lamarckii, Eirene viridula, Leukartiara octona, Melicertum octocostatum, Mnemiopsis leidyi, Obelia sp. and Pleurobrachia pileus (see Appendix 1 for detail). All raw data are included in Appendix 2.

Fig. 1

Investigation area of the North Sea where gelatinous macrozooplakton has been sampled at 62 stations (indicated by red number) during August 2018. Basin names are provide (in blue) along with borders of exclusive economic zones - modified from Copejans & Smiths 2011.

Fig. 2

Spatial distribution of the comb jelly Pleurobrachia pileus in the North Sea in August 2018. Black dots indicate sampling stations where no animals have been caught.

Fig. 3

Spatial distribution of the scyphozoan jellyfish species Cyanea capillata (A) and C. lamarckii (B) in the North Sea in August 2018. Black dots indicate sampling stations where no animals have been caught.

Fig. 4

Spatial distribution of gelatinous macrozooplankton species in the North Sea in August 2018 with A) Aurelia aurita (light blue) and Aequorea sp. (dark blue); B) Beroe sp. (yellow) and Chrysaora hysoscella (pink); C) Obelia sp. (green), Clytia hemisphaerica (red) and Melicertum octocostatum (lightblue); and D) Eirene viridula (blue), Mnemiopsis leidyi (red) and Leuckartiara octona (yellow). Note: A, B, C use the same scale for depicting abundance data, while D uses a different scale accounting for 1 order of magnitude higher abundance data. Black dots indicate sampling stations where no animals have been caught.

Fig. 5

Relative size frequency distribution of selected gelatinous macrozooplankton species in the North Sea during August 2018. Size bins for 1 or 2 cm size classes depicted for the species Cyanea lamarckii (A), Eirene viridula (B), Aequorea sp. (C), Mnemiopsis leidyi (D) and Cyanea capillata (E).

Table 1

Total counts (N), average (av.) and maximum (max.) abundance as well as average, minimum (min.) and maximum size of gelatinous zooplankton caught in the North Sea during August 2018.

Species	N	Av. abund. 1000m−³ ± SD	Max. abund. 1000m−³ ± SD	Av. size (cm) ± SD	Min. size (cm)	Max. size (cm)
Aequorea spp.	48	1.0 ± 1.1	3.7	7.4 ± 1.7	2	12
Aurelia aurita	20	0.8 ± 0.8	2.2	10.7 ± 5.9	4	26
Beroe spp.	78	2.3 ± 2.8	8.9	2.1 ± 1.1	1	5
Chrysaora hysoscella	22	0.4 ± 0.3	1	5.5 ± 2.2	2	11
Clytia hemisphaerica	186	10.9 ± 12.2	40.5	0.3 ± 0.1	0.3	1
Cyanea capillata	159	0.7 ± 0.9	5	8.2 ± 6.7	1	32
Cyanea lamarckii	225	1.3 ± 1.4	5.7	5.7 ± 2.2	1	12
Eirene viridula	759	54.7 ± 107.2	341	1.8 ± 1.1	0.5	5
Leuckartiara octona	442	26.9 ± 70.4	362.7	0.4 ± 0.2	0.3	1
Melicertum octocostatum	15	2.5 ± 1.5	4.6	0.3 ± 0.1	0.3	0.5
Mnemiopsis leidyi	424	23.8 ± 40.9	109.7	2.6 ± 1.1	1	6
Obelia spp.	85	4.4 ± 7.6	26.8	1.8 ± 1.1	0.5	5
Pleurobrachia pileus	3776	70.7 ± 128.7	598.7	0.4 ± 0.3	0.3	2
total	6239

Experimental design, materials, and methods

Samples were collected in the central and southern North Sea (Fig. 1) at night time (19:30–5:30 GMT) in August 2018 (30.7.2018–16.8.2018) during the Danish contribution to the International Bottom Trawl Survey (IBTS) on board the Danish R/V DANA (DTU Aqua, Denmark). The IBTS is a long-term fishery monitoring program [2] which is conducted twice a year, both in the 1st and 3rd quarter. The standard procedure during the survey is bottom trawling during daytime to provide abundance indices for a range of commercially important fish species, as well as standard CTD casts to describe the physical parameters. During the Danish 3rd quarter IBTS in 2018 these standard procedures were supplemented by plankton sampling during nighttime.

During this additional plankton sampling, gelatinous macrozooplankton was sampled on a total of 62 stations across the southern and central North Sea (Fig. 1) by use of a MIK-net (modified Methot Isaac Kidd) net. The MIK-net is a large ring net with a 2 m diameter mouth opening and a 13-m-long net with a mesh size of 1 mm. The last metre of the net as well as the cod end bucket have a finer mesh size of 500 μm. The net was hauled at a speed of 3 knots in a double oblique tow from the surface to 5 m above the sea floor. A calibrated flow meter in the center of the gear opening was used to assess the water volume filtered during the tow. A total of 69 hauls were analysed for gelatinous macrozooplankton, with duplicated hauls at stations 9, 12, 38, 42 and 55 and triplicated hauls at station 59. After the net had been retrieved and carefully washed, the un-preserved cod end contents were sorted for gelatinous macrozooplankton (>2 mm) and fish larvae on a light table in R/V DANA's laboratory. All jellyfish and comb jellyfish were identified to genus or species level [1]. Jellyfish and comb jellyfish were rinsed and individually removed from the sample, whereafter they were counted and sized to the nearest mm using a caliper. The remaining zooplankton sample was concentrated on a sieve (mesh size 150 μm) and preserved in 96% ethanol within 1 h after catch.

The amount of filtered water in m³ per station was calculated using the following formula:where flowmeter count = the difference of the flowmeter values before and after the haul, 34.96 = a flowmeter calibration coefficient determined during calibration hauls and radius = 1 m. Total counts of organisms per station were divided by the amount of filtered water per station to calculate individuals per m³. In order to avoid very small values, these abundance data were further standardized to 1000 m3. Average abundance data along with size information are provided in Table 1.

The spatial distribution of gelatinous macrozooplankton were visualized using the Software Surfer® (Golden Software LLC). Bathymetry data for surfer maps were obtained as ESRI ASCII files from the EMODnet Data Portal (http://portal.emodnet-bathymetry.eu/) for tiles D3, D4, E3 and E4, using the DTM version 2018. Sampling stations were plotted according to their coordinates and associated gelatinous macrozooplankton abundance data were depicted as circles with the size of the circle proportional to the abundances (Fig. 2, Fig. 3, Fig. 4). Size frequency distributions were calculated for the most important five species only (Fig. 5).

Hydrography: Physical parameters from the North Sea during the investigation period can be downloaded from the ICES hydrographic database.

Specifications table

Subject area	Biological oceanography, zoology, biodiversity
More specific subject area	Plankton, gelatinous zooplankton, jellyfish, comb jelly
Type of data	5 Figures, 1 Table, 2 Appendices (Appendix 1: specific data description; Appendix 2: link to raw data table).
How data was acquired	Research vessel based plankton survey using a 13 m long MIK-net (modified Methot Isaac Kidd; Ø 2 m, mesh size 1 mm, mesh size cod end 500 μm). Unpreserved samples were analysed on a light table within 1 hour after catch.
Data format	Raw and analysed - including species specific abundances per area (m−2) and volume (m−3).
Experimental factors	Samples have been analyzed right after catch without preservatives. Data have been standardized to volume specific abundance data by using calibrated flow meter values.
Experimental features	Species identification based on Russell 1953 [1], along with size acquisition of unpreserved material using a caliper on a light table.
Data source location	National Institute of Aquatic Resources, Technical University of Denmark, DTU Aqua, 2800 Kgs. Lyngby, Denmark.
Data accessibility	Data are provided within this article.

Value of the data• This dataset is important for assessing the biodiversity of gelatinous macrozooplankton in the North Sea (>2 mm) and for addressing the long standing scientific questions i) if gelatinous zooplankton are on a rise due to climate change induced stressors (e.g. warming, acidification) and ii) if we observe an increase in non-indigenous macrozooplankton species in the North Sea due to global change (e.g. increased shipping activity, over-fishing). • The applied methodology to obtain this dataset could be used as model for including gelatinous macrozooplankton analyses into existing, standard fishery surveys. • The here presented data could be used as a baseline for future monitoring initiatives of gelatinous macrozooplankton in the North Sea. • Considering increased pressures marine ecosystems are facing due to global change, the data are of high relevance in relation to the European Commission's Marine Strategy Framework Directive (MSFD) and could foster collaboration with European partners which are obliged to assess and achieve good environmental status of marine habitats in their national waters.