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Time-series are crucial to understand the status of zooplankton communities and to anticipate changes that might affect the entire food web. Long-term time series allow us to understand impacts of multiple environmental and anthropogenic stressors, such as chemical pollution and ocean warming, on the marine ecosystems. Here, a recent time series (2018-2022) of abundance data of four dominant calanoid and one harpacticoid copepod species from the Belgian Part of the North Sea was combined with previously collected (2009-2010, 2015-2016) datasets for the same study area. The time series reveals a significant decrease (up to two orders of magnitude) in calanoid copepod abundance (Temora longicornis, Acartia clausi, Centropages spp., Calanus helgolandicus), while this was not the case for the harpacticoid Euterpina acutifrons. We applied generalized additive models to quantify the relative contribution of temperature, nutrients, salinity, primary production, turbidity and pollution (anthropogenic chemicals, i.e., polychlorinated biphenyls and polycyclic aromatic hydrocarbons) to the population dynamics of these species. Temperature, turbidity and chlorophyll a concentrations were the only variables consistently showing a relative high contribution in all models predicting the abundances of the selected species. The observed heat waves which occurred during the summer periods of the investigated years coincided with population collapses (versus population densities in non-heatwave years) and are considered the most likely cause for the observed copepod abundance decreases. Moreover, the recorded water temperatures during these heatwaves correspond to the physiological thermal limit of some of the studied species. As far as we know, this is the first study to observe ocean warming and marine heat waves having such a dramatic impact (population collapse) on the dominant zooplankton species in shallow coastal areas., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 Elsevier Ltd. All rights reserved.)

Marine zooplankton are increasingly being affected by recent environmental changes, such as climate change, and respond with profound spatial relocations and shifts in phenology and physiology. In order to predict whether populations are able to persist or adapt to such new conditions, it is essential to understand the molecular basis of such adaptations, which ultimately get translated into these physiological responses. To explore variation in population gene expression across time and space, we investigated transcriptome-level profiles of the calanoid copepod Temora longicornis, that were collected at four different locations in the Belgian Part of the North Sea (BPNS) on three different time points (April, June, October) in 2018. RNA-seq analysis of field collected adults identified large seasonal differences in gene expression, mainly between spring-summer and autumn samples. The largest log-fold changes occurred in a set of genes encoding for ribosomal and myosin (heavy chain) transcripts. Enrichment analysis revealed a strong seasonal pattern in vitellogenin, cuticle and glycolytic gene expression as well. We also found a positive correlation between vitellogenin expression and densities of T. longicornis. No clear spatial variation in expression patterns was found in the BPNS. This study underlines the potential of field gene expression studies for biomonitoring purposes and the significance of considering seasonal variation in future studies., (Copyright © 2020 Elsevier Ltd. All rights reserved.)

Understanding the impacts of global change in zooplankton communities is crucial, as alterations in the zooplankton communities can affect entire marine ecosystems. Despite the economic and ecological importance of the calanoid copepod Temora longicornis in the Belgian part of the North Sea, molecular data is still very limited for this species. Using HiSeq Illumina sequencing, we sequenced the whole transcriptome of T. longicornis, after being exposed to realistic temperatures of 14 and 17 °C. After both an acute (1 day) and a more sustained (5 days) thermal exposure to 17 °C, we investigated gene expression differences with animals exposed to 14 °C, which may be critical for the thermal acclimation and resilience of this copepod species. We also studied the possibility of a short term stress recovery of a heat shock. A total of 179,569 transcripts were yielded, of which 44,985 putative ORF transcripts were identified. These transcripts were subsequently annotated into roughly 22,000 genes based on known sequences using Gene Ontology (GO) and KEGG databases. Temora only showed a mild response to both the temperature and the duration of the exposure. We found that the expression of 27 transcripts varied significantly with an increase in temperature of 3 °C, of which eight transcripts were differentially expressed after acute exposure only. Gene set enrichment analysis revealed that, overall, T. longicornis was more impacted by a sustained thermal exposure, rather than an immediate (acute) exposure, with two times as many enriched GO terms in the sustained treatment. We also identified several general stress responses independent of exposure time, such as modified protein synthesis, energy mobilisation, cuticle and chaperone proteins. Finally, we highlighted candidate genes of a possible recovery from heat exposure, identifying similar terms as those enriched in the heat treatments, i.e. related to for example energy metabolism, cuticle genes and extracellular matrix. The data presented in this study provides the first transcriptome available for T. longicornis which can be used for future genomic studies., (Copyright © 2018 Elsevier Ltd. All rights reserved.)

Perfluorinated compounds (PFCs), which are extensively used in a wide variety of applications because of their specific surfactant properties, have recently appeared as an important new class of global environmental pollutants. Quantitative analysis of PFCs in aqueous matrices remains, however, a challenging task. During this study, a new analytical method for the determination of 14 PFCs in surface-, sewage- and seawater was developed and validated. The target analytes were extracted using solid-phase extraction followed by liquid chromatography coupled to a time-of-flight mass spectrometer (LC-ToF-MS). The use of very narrow mass tolerance windows (< 10 ppm) resulted in a highly selective MS-technique for the detection of PFCs in complex aqueous matrices. Validation of this analytical method in surface-, sewage- and seawater resulted in limits of quantification (LOQs) varying from 2 to 200 ng L⁻¹, satisfying recoveries (92-134%), and good linearity (R²=0.99 for most analytes). Analysis of samples of the North Sea, the Scheldt estuary, and three harbours of the Belgian coastal region led to the detection of four different PFCs. Perfluorooctane sulfonate (PFOS) was found to be the most abundant PFC in levels up to 38.9 ng L⁻¹., (Copyright © 2010 Elsevier B.V. All rights reserved.)