Decadal decline of dominant copepod species in the North Sea is associated with ocean warming: Importance of marine heatwaves.

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.

Origin and partitioning of mercury in the polluted Scheldt Estuary and adjacent coastal zone.

Estuaries and coastal zones are areas with complex biogeochemical and hydrological cycles and are generally facing intense pollution due to anthropogenic activities. An emblematic example is the Scheldt Estuary which ends up in the North Sea and has been historically heavily contaminated by multiple pollutants, including mercury (Hg). We report here Hg species and their levels in surface waters of the Scheldt Estuary and the Belgian Part of North Sea (BPNS) from different sampling campaigns in February-April 2020 and 2021. Along the estuary, Hg concentration on suspended particles ([HgSPM]) progressively decreased with increasing salinity and was strongly correlated with organic matter content (%Corg) and origin (identified with δ13Corg). While [HgSPM] drives total Hg concentration in the estuary (total dissolved Hg, HgTD is only 7 ± 6 %), annual and daily variations of total Hg levels were mostly attributed to changes in SPM loads depending on river discharge and tidal regime. In the BPNS, a significant fraction of total Hg occurs as HgTD (40 ± 21 %) and the majority of this HgTD was reducible (i.e. labile Hg), meaning potentially available for microorganisms. Compared to the '90s, a significant decrease of [HgSPM] was observed in the estuary, but this was not the case for [HgTD], which can be due to (1) still significant discrete discharges from Antwerp industrial area, and (2) higher Hg partitioning towards the dissolved phase in the water column relative to the '90s. Our results highlight the important contribution of the Scheldt estuary for the Hg budget in North Sea coastal waters, as well as the need for seasonal monitoring of all Hg species.

Marked changes in diatom and dinoflagellate biomass, composition and seasonality in the Belgian Part of the North Sea between the 1970s and 2000s.

In the last decades, the North Sea has undergone intense environmental changes which have led to regime shifts that affected all trophic levels. Since the 1970s, both increases and decreases in phytoplankton biomass and production have been reported from different parts of the North Sea. Such conflicting observations may be partly caused by methodological differences, but also reflect regional differences related to bathymetry, hydrodynamics, climate, riverine and Atlantic influence. The Belgian part of the North Sea (BPNS) is a hydrodynamically and bathymetrically complex area under strong human influence, which has been characterized by eutrophication (up to the 1980s) and de-eutrophication (1990s onwards), and pronounced long-term changes in turbidity and water temperature. We used a newly recovered and standardized historic dataset, the Belgian Phytoplankton Database (Nohe et al., 2018), to compare the biomass, composition and seasonality of diatom and dinoflagellate assemblages, two key components of the plankton in the BPNS, between the 1970s and 2000s. Diatoms, especially large-sized taxa, showed an increase from late winter to summer, resulting in a more intense and extended growing season in the 2000s. Dinoflagellates increased year-round but especially in summer. Both diatom and dinoflagellate blooms showed a clear shift towards an earlier bloom start. In addition, while in the 1970s distinct seasonal community types were present, a striking seasonal homogenization in community structure had occurred by the 2000s. Finally, we observed a pronounced increase in the abundance of harmful diatom and dinoflagellate genera. The observed changes are most likely due to an increase in sea surface temperature and water transparency, and changes in nutrient loads and ratios. Our study underscores the importance of recovering previously inaccessible historic data as they can offer unprecedented insights into long-term change in marine ecosystems, which are essential for properly evaluating the impact of human activities on these ecosystems.

Multi-residue quantification and screening of emerging organic micropollutants in the Belgian Part of the North Sea by use of Speedisk extraction and Q-Orbitrap HRMS.

Knowledge about the occurrence of emerging organic micropollutants in the marine environment is still very limited, especially when focusing on the Belgian Part of the North Sea (BPNS). This study therefore optimized and validated a Speedisk® based SPE and LC-Q-Orbitrap HRMS method to tackle the challenge of measuring the expected ultra-trace concentrations in seawater. This method was applied to 18 samples collected at different locations in the open sea and harbor of the BPNS. Forty-eight compounds, among which several pharmaceuticals, personal care products or pesticides described in the EU Watchlist, were detected - some for the first time in seawater - at concentrations ranging up to 156 ng L-1. Moreover, the untargeted screening potential of the newly developed HRMS method was highlighted by revealing the presence of up to 1300 unknown components in a single sample and by assigning molecular formulae to those components demonstrating high discriminative potential between samples.