A budget of carbon cycling in the Belgian coastal zone: relative roles of zooplankton, bacterioplankton and benthos in the utilization of primary production

Anthropogenic activities influence the mobilization of trace metals and oxyanions in coastal sediment porewaters

The biogeochemical cycle of trace metals and oxyanions in marine sediments is mainly controlled by early diagenesis of organic material, but anthropogenic activities such as dumping of dredged sludge or shipyard activities can strongly perturb these natural processes. Therefore, the impact of dumping of dredged sludge, shipyard activities such as blasting of boat hulls and the mixing of polluted Scheldt estuary water with less polluted coastal water on the mobilization of trace metals and oxyanions in the sediments was studied in the pre- and post-bloom period (March and October) in Belgian Coastal Zone (BCZ). Vertical profiles of total and labile dissolved trace element concentrations, dissolved sulfide, pH and redox potential were assessed in sediment porewaters. Generally, total and labile dissolved element profiles in the porewaters are well correlated, showing higher concentrations in March when the redox potential is lower and sulfide concentrations higher. Low pH levels were documented together with high amounts of Fe and associated elements in March at the dumping station. At the coastal station in the vicinity of the shipyards, sediment porewaters are enriched in dissolved Zn while at the mouth of the Scheldt estuary the highest dissolved Cu levels were found. Theoretical calculations with Visual MINTEQ confirm the increased Fe and associated element concentrations in March (lower pH) and the lower metal concentrations in October (lower Eh). R values, ratio of labile dissolved to total dissolved concentrations, reflect the potential of the solid sediment phase to supply labile elements to the porewater. They were lowest for Cu and highest for V. This study showed that anthropogenic activities in the BCZ such as dumping of dredged sludge and blasting of boat hulls have a serious impact on the levels, distribution and bioavailability of contaminants in the sediments, which may form a real threat to the benthic ecosystem.

Bacterial communities in temperate and polar coastal sands are seasonally stable

Coastal sands are biocatalytic filters for dissolved and particulate organic matter of marine and terrestrial origin, thus, acting as centers of organic matter transformation. At high temporal resolution, we accessed the variability of benthic bacterial communities over two annual cycles at Helgoland (North Sea), and compared it with seasonality of communities in Isfjorden (Svalbard, 78°N) sediments, where primary production does not occur during winter. Benthic community structure remained stable in both, temperate and polar sediments on the level of cell counts and 16S rRNA-based taxonomy. Actinobacteriota of uncultured Actinomarinales and Microtrichales were a major group, with 8 ± 1% of total reads (Helgoland) and 31 ± 6% (Svalbard). Their high activity (frequency of dividing cells 28%) and in situ cell numbers of >10% of total microbes in Svalbard sediments, suggest Actinomarinales and Microtrichales as key heterotrophs for carbon mineralization. Even though Helgoland and Svalbard sampling sites showed no phytodetritus-driven changes of the benthic bacterial community structure, they harbored significantly different communities (p < 0.0001, r = 0.963). The temporal stability of benthic bacterial communities is in stark contrast to the dynamic succession typical of coastal waters, suggesting that pelagic and benthic bacterial communities respond to phytoplankton productivity very differently.

Warming the phycosphere: Differential effect of temperature on the use of diatom-derived carbon by two copiotrophic bacterial taxa

Heterotrophic bacteria associated with microphytoplankton, particularly those colonizing the phycosphere, are major players in the remineralization of algal-derived carbon. Ocean warming might impact dissolved organic carbon (DOC) uptake by microphytoplankton-associated bacteria with unknown biogeochemical implications. Here, by incubating natural seawater samples at three different temperatures, we analysed the effect of experimental warming on the abundance and C and N uptake activity of Rhodobacteraceae and Flavobacteria, two bacterial groups typically associated with microphytoplankton. Using a nano-scale secondary ion mass spectrometry (nanoSIMS) single-cell analysis, we quantified the temperature sensitivity of these two taxonomic groups to the uptake of algal-derived DOC in the microphytoplankton associated fraction with ¹³ C-bicarbonate and ¹⁵ N-leucine as tracers. We found that cell-specific ¹³ C uptake was similar for both groups (~0.42 fg C h^-1 μm^-3 ), but Rhodobacteraceae were more active in ¹⁵ N-leucine uptake. Due to the higher abundance of Flavobacteria associated with microphytoplankton, this group incorporated fourfold more carbon than Rhodobacteraceae. Cell-specific ¹³ C uptake was influenced by temperature, but no significant differences were found for ¹⁵ N-leucine uptake. Our results show that the contribution of Flavobacteria and Rhodobacteraceae to C assimilation increased up to sixfold and twofold, respectively, with an increase of 3°C above ambient temperature, suggesting that warming may differently affect the contribution of distinct copiotrophic bacterial taxa to carbon cycling.

Interactions between polystyrene microplastics and marine phytoplankton lead to species-specific hetero-aggregation

To understand the fate and impacts of microplastics (MP) in the marine ecosystems, it is essential to investigate their interactions with phytoplankton as these may affect MP bioavailability to marine organisms as well as their fate in the water column. However, the behaviour of MP with marine phytoplanktonic cells remains little studied and thus unpredictable. The present study assessed the potential for phytoplankton cells to form hetero-aggregates with small micro-polystyrene (micro-PS) particles depending on microalgal species and physiological status. A prymnesiophycea, Tisochrysis lutea, a dinoflagellate, Heterocapsa triquetra, and a diatom, Chaetoceros neogracile, were exposed to micro-PS (2 μm diameter; 3.96 μg L^-1) during their growth culture cycles. Micro-PS were quantified using an innovative flow-cytometry approach, which allowed the monitoring of the micro-PS repartition in microalgal cultures and the distinction between free suspended micro-PS and hetero-aggregates of micro-PS and microalgae. Hetero-aggregation was observed for C. neogracile during the stationary growth phase. The highest levels of micro-PS were "lost" from solution, sticking to flasks, with T. lutea and H. triquetra cultures. This loss of micro-PS sticking to the flask walls increased with the age of the culture for both species. No effects of micro-PS were observed on microalgal physiology in terms of growth and chlorophyll fluorescence. Overall, these results highlight the potential for single phytoplankton cells and residual organic matter to interact with microplastics, and thus potentially influence their distribution and bioavailability in experimental systems and the water column.

Permeability shapes bacterial communities in sublittoral surface sediments

The first interaction of water column-derived organic matter with benthic microbial communities takes place in surface sediments which are acting as biological filters catalyzing central steps of elemental cycling. Here we analyzed the bacterial diversity and community structure of sediment top layers at seven sites in the North Sea where sediment properties ranged from coarse-grained and highly permeable to fine-grained and impermeable. Bacterial communities in surface sediments were richer, more even and significantly different from communities in bottom waters as revealed by Illumina tag sequencing of 16S rRNA genes. Sediment permeability had a clear influence on community composition which was confirmed by CARD-FISH. Sulfate-reducing Desulfobacteraceae (2-5% of total cells), Flavobacteriaceae (3-5%) were more abundant in impermeable than in highly permeable sediments where acidobacterial Sva0725 dominated (11-15%). Myxobacterial Sandaracinaceae were most abundant in medium permeable sediments (3-7%). Woeseiaceae/JTB255 and Planctomycetes were major groups in all sediments (4-6%, 8-22%). Planctomycetes were highly diverse and branched throughout the phylum. We propose Planctomycetes as key bacteria for degradation of high molecular weight compounds and recalcitrant material entering surface sediments from the water column. Benthic Flavobacteriaceae likely have restricted capabilities for macromolecule degradation and might profit with Sandaracinaceae and Acidobacteria from low molecular weight compounds.

Respiration in the open ocean

A key question when trying to understand the global carbon cycle is whether the oceans are net sources or sinks of carbon. This will depend on the production of organic matter relative to the decomposition due to biological respiration. Estimates of respiration are available for the top layers, the mesopelagic layer, and the abyssal waters and sediments of various ocean regions. Although the total open ocean respiration is uncertain, it is probably substantially greater than most current estimates of particulate organic matter production. Nevertheless, whether the biota act as a net source or sink of carbon remains an open question.

The importance of the planktonic ecosystem of the North Sea in the context of oil and gas development

Planktonic organisms are the primary source of food for the top level of the marine food chain, the fish. Yet only part of the plankton is ingested by fish, the remainder sediments to the bottom to provide food for benthic organisms (which may in turn be grazed by demersal fish) and to contribute to a detrital sink. Although the relative proportions of the plankton entering each of these compartments is still a matter of debate, some indication of its importance as a resource can be gauged from the North Sea fishery that has yielded 2-3 Mt per year since the mid-1960s. Calculations for the North Sea of the annual production of phytoplankton, zooplankton, fish and benthos as energy equivalents are contrasted with the annual energy yields of oil and gas. Since 1948 the plankton of the North Sea has been monitored at a depth of 10 m by the Continuous Plankton Recorder (CPR) survey. Results for two large areas which encompass the Brent, Beryl and Forties oilfields are presented. Between 1948 and 1982 the plankton of these areas showed similar large changes in population structure in both phytoplankton and zooplankton with almost tenfold changes in levels of biomass during this 35 year period. The development of oil-related activities is discussed in relation to the plankton time-series with comment on possible causes of the changes which are believed to be the result of natural variability.

Organochlorines in different fractions of sediments and in different planktonic compartments of the Belgian continental shelf and the Scheldt estuary

PCB levels in sediments (bulk and fraction <63 microm), suspended matter and zooplankton from the Belgian continental shelf of the North Sea and the Scheldt estuary were evaluated in relation to their organic carbon content, their lipid content and, for sediments, their particle size distribution. PCB accumulation mechanisms are discussed, considering the importance of direct contamination (adsorption onto the cell surfaces, absorption through the cell walls and partitioning into the cell lipids) for suspended matter and sediments, and of indirect contamination through the food for zooplankton. Geographical and seasonal variations are described.