De Troch, M.

Hostens, K.

Van Regenmortel, T.

Vansteenbrugge, L.

Vincx, M.

Open Marine Archive

ICES CM 2014/A:12 
 
 
Gelatinous zooplankton in the Belgian part of the North Sea and the adjacent Schelde 
estuary: a real problem?  
 
Lies Vansteenbrugge1,2, Tina Van Regenmortel2, Marleen De Troch2, Magda Vincx2 and Kris Hostens1 
 
1 Institute for Agricultural and Fisheries Research (ILVO), Animal Sciences Unit, Aquatic Environment and 
Quality, Bio-environmental Research Group, Ankerstraat 1, 8400 Oostende, Belgium 
2 Ghent University (UGent), Faculty of Sciences, Biology Department, Marine Biology Section, Sterre Campus, 
Krijgslaan 281 – S8, 9000 Gent, Belgium 
Corresponding author: Lies Vansteenbrugge, Institute for Agricultural and Fisheries Research (ILVO), 
lies.vansteenbrugge@ilvo.vlaanderen.be, Phone: +32 59 56 98 16  
 
Summary 
Gelatinous zooplankton has been largely understudied in ecosystems worldwide, and the absence of 
quantitative long-term abundance data impedes drawing conclusions on a potential rise of these 
organisms. This study presents results from monthly samples taken in 2011-2012 at six locations in the 
Belgian part of the North Sea and the adjacent Schelde estuary.  
Three Scyphozoa, three Ctenophora and 27 Hydrozoa taxa were identified. Densities of gelatinous 
zooplankton never outnumbered the non-gelatinous zooplankton. Average gelatinous zooplankton 
densities reached up to 18 ind.m-3 near the coast and gradually declined towards the open sea (60 km 
offshore). In the brackish Schelde estuary, average densities remained below 4 ind.m -3. Gelatinous 
zooplankton densities were highest in autumn throughout the study area.  
We conclude that the observed distribution of gelatinous zooplankton is driven by a limited number of 
environmental variables (temperature and salinity).  
One of the three non-indigenous gelatinous species found in the area is Mnemiopsis leidyi. The peak 
densities did not reach worrying densities but data from this and other studies indicate that Dutch 
estuaries and Belgian ports provide optimal conditions and potential for year round presence and thus 
reproduction. Further regular monitoring is needed to estimate the gelatinous future of the Southern 
North Sea.  
 
Introduction 
Global ocean ecosystems are thought to be heading towards a gelatinous future, a perception that is 
strengthened by an increased reporting of problems caused by jellyfish. However, gelatinous zooplankton 
has been largely understudied in most ecosystems worldwide, and the absence of quantitative long-term 
abundance data actually impedes drawing conclusions on the potential rise of these organisms. Jellyfish 
densities show large inter-annual fluctuations, which makes it difficult to distinguish natural fluctuations 
in jellyfish populations from changes caused by on-going anthropogenic perturbations. Global warming 
and overfishing, but also local phenomena such as eutrophication, habitat modification and transport of 
non-indigenous species might cumulatively affect the ecosystem and intensify the natural fluctuations in 
jellyfish abundance. The introduction of the ctenophore Mnemiopsis leidyi for example had a substantial 
effect on the ecosystem of the Black Sea when densities increased. Most jellyfish are carnivores feeding on 
almost anything, from unicellular organisms to much larger prey. Consequently, the predation and 
competition pressure on the co-occurring plankton community and on higher trophic levels (e.g. fish) can 
be considerable during jellyfish blooms. Considering the haziness regarding the inter-annual and seasonal 
fluctuations in jellyfish populations, and the potential impact of gelatinous zooplankton on the non-
gelatinous zooplankton community, this study contemplates the community structure and dynamics of 
ICES CM 2014/A:12 
 
gelatinous zooplankton in the Belgian Part of the North Sea (BPNS) and Schelde estuary in the light of a 
jellification risk as data from these two areas is scarce or absent. 
This study therefore aims to characterise the gelatinous zooplankton community in the BPNS and the 
adjacent Schelde estuary, to investigate the spatial and temporal distribution patterns of this community 
in relation to some structuring environmental variables, and to discuss the relation between gelatinous 
zooplankton and the rest of the zooplankton community. Ultimately, this study can form the baseline for 
the further evaluation of the jellification process in the BPNS. 
 
Materials and methods 
In 2011-2012, monthly samples were taken at six locations in the Belgian part of the North Sea and the 
adjacent Schelde estuary. Undulating WP3 net tows (Ø 1 m, mesh size 1000 µm) were executed in order to 
adequately sample the gelatinous zooplankton. Environmental variables were measured at each location. 
Spatial and temporal differences in the species composition of the samples were investigated using 
PERMANOVA (Permutational ANOVAs) to test for the factors season and location. Finally, the 
environmental variables were related to the patterns in gelatinous species composition and abundance via 
distance-based linear models (DistLM) using the Primer version 6, Permanova+ software. 
 
Results and Discussion 
Three Scyphozoa, three Ctenophora and 27 Hydrozoa taxa were identified. Densities of gelatinous 
zooplankton never outnumbered the non-gelatinous zooplankton. Average gelatinous zooplankton 
densities reached up to 18 ind.m-3 near the coast and gradually declined towards the open sea (60 km 
offshore). In the brackish Schelde estuary, average densities remained below 4 ind.m -3. Gelatinous 
zooplankton densities were highest in autumn throughout the study area. Spring had the lowest densities 
of gelatinous and non-gelatinous zooplankton. Overall, the same species assemblage was present across 
the study area and the seasons, however the little variation that was noted in terms of density was driven 
by a limited number of environmental variables: temperature, oxygen concentration and salinity. 
Temperature and oxygen concentration are both related to ‘season’. Temperature has a big influence on 
the budding or strobilation process of Hydrozoa and Scyphozoa with characteristic temperature ranges 
for each species. Salinity is related to sampling location. Most gelatinous zooplankton species prefer areas 
with high salinity in contrast to the brackish Schelde estuary. Other species, such as Rathkea octopunctata, 
have a very wide range of salinity and can be observed in areas with very low salinities. Pleurobrachia 
pileus is omnipresent in the coastal zone as they rely on tidal currents, moreover they do not depend on a 
sessile phase in their life cycle which means they can be observed year round. Three non-indigenous 
gelatinous species were found in the area: Nemopsis bachei, Lovenella assimilis and Mnemiopsis leidyi. The 
peak densities of the latter did not yet reach worrying densities: highest density at sea of 0.55 ind.m-3 
observed in autumn. However, in the ports, densities up to 18.37 ind.m-3 were found. Moreover, M. leidyi 
larvae were observed in the ports as well as in the Schelde estuary. In addition to other studies, this study 
shows that the Dutch estuaries and Belgian ports provide optimal conditions and potential for a year 
round presence and thus reproduction of M. leidyi. Further regular monitoring is needed to estimate the 
gelatinous future of the Southern North Sea.  
 


Gelatinous zooplankton in the Belgian part of the North Sea and the adjacent Schelde estuary: a real problem?

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