Major declines in NE Atlantic plankton contrast with more stable populations in the rapidly warming North Sea.

Plankton form the base of marine food webs, making them important indicators of ecosystem status. Changes in the abundance of plankton functional groups, or lifeforms, can affect higher trophic levels and can indicate important shifts in ecosystem functioning. Here, we extend this knowledge by combining data from Continuous Plankton Recorder and fixed-point stations to provide the most comprehensive analysis of plankton time-series for the North-East Atlantic and North-West European shelf to date. We analysed 24 phytoplankton and zooplankton datasets from 15 research institutions to map 60-year abundance trends for 8 planktonic lifeforms. Most lifeforms decreased in abundance (e.g. dinoflagellates: -5 %, holoplankton: -7 % decade-1), except for meroplankton, which increased 12 % decade-1, reflecting widespread changes in large-scale and localised processes. K-means clustering of assessment units according to abundance trends revealed largely opposing trend direction between shelf and oceanic regions for most lifeforms, with North Sea areas characterised by increasing coastal abundance, while abundance decreased in North-East Atlantic areas. Individual taxa comprising each phytoplankton lifeform exhibited similar abundance trends, whereas taxa grouped within zooplankton lifeforms were more variable. These regional contrasts are counterintuitive, since the North Sea which has undergone major warming, changes in nutrients, and past fisheries perturbation has changed far less, from phytoplankton to fish larvae, as compared to the more slowly warming North-East Atlantic with lower nutrient supply and fishing pressure. This more remote oceanic region has shown a major and worrying decline in the traditional food web. Although the causal mechanisms remain unclear, declining abundance of key planktonic lifeforms in the North-East Atlantic, including diatoms and copepods, are a cause of major concern for the future of food webs and should provide a red flag to politicians and policymakers about the prioritisation of future management and adaptation measures required to ensure future sustainable use of the marine ecosystem.

Methodology for defining homogeneous water bodies for management purposes.

European legislation requires monitoring of toxic algae in marine areas where shellfish are harvested for consumption. Monitoring assumes the existence of homogeneous water bodies, the definition of which have important implications for stakeholders and consumers. Yet, the definition of homogeneous water bodies remains unclear. Here we present a methodology to divide coastal and estuarine waters into homogeneous water bodies to monitor toxic algae. The proposed method is mainly based on water transport, and secondarily on oceanographic characteristics; salinity and sea surface height. We apply the methodology to the Limfjord in Denmark and demonstrate its usefulness in areas with a complicated coastal morphology. The oceanographic descriptors applied in the method are standard outputs from coastal hydrodynamical models. Provided that validated and high resolution model output is available for a given area, the technique is thus adaptable to other morphologically and oceanographically complicated estuarine and coastal areas where toxic algae monitoring is necessary.

Metabolomics-derived marker metabolites to characterize Phaeocystis pouchetii physiology in natural plankton communities.

Phaeocystis pouchetii (Hariot) Lagerheim, 1893 regularly dominates phytoplankton blooms in higher latitudes spanning from the English Channel to the Arctic. Through zooplankton grazing and microbial activity, it is considered to be a key resource for the entire marine food web, but the actual relevance of biomass transfer to higher trophic levels is still under discussion. Cell physiology and algal nutritional state are suggested to be major factors controlling the observed variability in zooplankton grazing. However, no data have so far yielded insights into the metabolic state of Phaeocystis populations that would allow testing this hypothesis. Therefore, endometabolic markers of different growth phases were determined in laboratory batch cultures using comparative metabolomics and quantified in different phytoplankton blooms in the field. Metabolites, produced during exponential, early and late stationary growth of P. pouchetii, were profiled using gas chromatography-mass spectrometry. Then, metabolites were characterized that correlate with the growth phases using multivariate statistical analysis. Free amino acids characterized the exponential growth, whereas the early stationary phase was correlated with sugar alcohols, mono- and disaccharides. In the late stationary phase, free fatty acids, sterols and terpenes increased. These marker metabolites were then traced in Phaeocystis blooms during a cruise in the Barents Sea and North Norwegian fjords. About 50 endometabolites of P. pouchetii were detected in natural phytoplankton communities. Mannitol, scyllo-inositol, 24-methylcholesta-5,22-dien-3ß-ol, and several free fatty acids were characteristic for Phaeocystis-dominated blooms but showed variability between them. Distinct metabolic profiles were detected in the nutrient-depleted community in the inner Porsangerfjord (< 0.5 µM NO3-, < 0.1 µM PO 4 3- ), with high relative amounts of free mono- and disaccharides indicative for a limited culture. This study thereby shows how the variable physiology of phytoplankton can alter the metabolic landscape of entire plankton communities.

Non-indigenous species refined national baseline inventories: A synthesis in the context of the European Union's Marine Strategy Framework Directive.

Refined baseline inventories of non-indigenous species (NIS) are set per European Union Member State (MS), in the context of the Marine Strategy Framework Directive (MSFD). The inventories are based on the initial assessment of the MSFD (2012) and the updated data of the European Alien Species Information Network, in collaboration with NIS experts appointed by the MSs. The analysis revealed that a large number of NIS was not reported from the initial assessments. Moreover, several NIS initially listed are currently considered as native in Europe or were proven to be historical misreportings. The refined baseline inventories constitute a milestone for the MSFD Descriptor 2 implementation, providing an improved basis for reporting new NIS introductions, facilitating the MSFD D2 assessment. In addition, the inventories can help MSs in the establishment of monitoring systems of targeted NIS, and foster cooperation on monitoring of NIS across or within shared marine subregions.

Application of Cytosense flow cytometer for the analysis of airborne bacteria collected by a high volume impingement sampler.

Characterization of airborne bacterial cells requires efficient collection, concentration, and analysis techniques, particularly to overcome the challenge of their dilute nature in outdoor environments. This study aims to establish a rapid and reliable approach for quantification of bacteria in air samples. To do this, a high volume impingement sampler was applied to collect airborne bacteria from a wastewater treatment plant (WWTP). The bacterial cell density was estimated by a Cytosense flow cytometer (Cytobouy) and compared to quantitative PCR (qPCR) data based on 16S rRNA genes. The average bacterial cell density measured by Cytosense ranged from 1.1 to 2.5 × 104 cells m-3 of air and that estimated by qPCR ranged from 0.08 to 3.8 × 104 cells m-3 of air. Regression analysis showed no systematic difference in bacterial cell densities between two methods applied when the cells were analyzed in vivo, and statistical tests confirmed that Cytosense counts of unfixed samples provided realistic values. Bacterial cell densities and the amount of DNA extracted from the sample were significantly correlated with relative humidity on a sampling day. The results showed that the present method was reliable to estimate bacteria densities from the outdoor environment, and the analysis given by Cytosense was faster and more sensitive than qPCR method. In addition, the Cytosense gave valuable information about cell characteristics at different sampling conditions.

Future climate scenarios for a coastal productive planktonic food web resulting in microplankton phenology changes and decreased trophic transfer efficiency.

We studied the effects of future climate change scenarios on plankton communities of a Norwegian fjord using a mesocosm approach. After the spring bloom, natural plankton were enclosed and treated in duplicates with inorganic nutrients elevated to pre-bloom conditions (N, P, Si; eutrophication), lowering of 0.4 pH units (acidification), and rising 3°C temperature (warming). All nutrient-amended treatments resulted in phytoplankton blooms dominated by chain-forming diatoms, and reached 13-16 µg chlorophyll (chl) a l-1. In the control mesocosms, chl a remained below 1 µg l-1. Acidification and warming had contrasting effects on the phenology and bloom-dynamics of autotrophic and heterotrophic microplankton. Bacillariophyceae, prymnesiophyceae, cryptophyta, and Protoperidinium spp. peaked earlier at higher temperature and lower pH. Chlorophyta showed lower peak abundances with acidification, but higher peak abundances with increased temperature. The peak magnitude of autotrophic dinophyceae and ciliates was, on the other hand, lowered with combined warming and acidification. Over time, the plankton communities shifted from autotrophic phytoplankton blooms to a more heterotrophic system in all mesocosms, especially in the control unaltered mesocosms. The development of mass balance and proportion of heterotrophic/autotrophic biomass predict a shift towards a more autotrophic community and less-efficient food web transfer when temperature, nutrients and acidification are combined in a future climate-change scenario. We suggest that this result may be related to a lower food quality for microzooplankton under acidification and warming scenarios and to an increase of catabolic processes compared to anabolic ones at higher temperatures.

Co-transport of polycyclic aromatic hydrocarbons by motile microorganisms leads to enhanced mass transfer under diffusive conditions.

The environmental chemodynamics of hydrophobic organic chemicals (HOCs) are often rate-limited by diffusion in stagnant boundary layers. This study investigated whether motile microorganisms can act as microbial carriers that enhance mass transfer of HOCs through diffusive boundary layers. A new experimental system was developed that allows (1) generation of concentration gradients of HOCs under the microscope, (2) exposure and direct observation of microorganisms in such gradients, and (3) quantification of HOC mass transfer. Silicone O-rings were integrated into a Dunn chemotaxis chamber to serve as sink and source for polycyclic aromatic hydrocarbons (PAHs). This resulted in stable concentration gradients in water (>24 h). Adding the model organism Tetrahymena pyriformis to the experimental system enhanced PAH mass transfer up to hundred-fold (benzo[a]pyrene). Increasing mass transfer enhancement with hydrophobicity indicated PAH co-transport with the motile organisms. Fluorescence microscopy confirmed such transport. The effective diffusivity of T. pyriformis, determined by video imaging microscopy, was found to exceed molecular diffusivities of the PAHs up to four-fold. Cell-bound PAH fractions were determined to range from 28% (naphthalene) to 92% (pyrene). Motile microorganisms can therefore function as effective carriers for HOCs under diffusive conditions and might significantly enhance mobility and availability of HOCs.

Grazer cues induce stealth behavior in marine dinoflagellates.

Chain formation is common among phytoplankton organisms but the underlying reasons and consequences are poorly understood. Here we show that chain formation is strongly impaired by waterborne cues from copepod grazers in the dinoflagellate Alexandrium tamarense. Chains of Alexandrium cells exposed to copepod cues responded by splitting into single cells or shorter chains. Motion analysis revealed significantly lower swimming velocities for single cells compared with chains, with two- to fivefold higher simulated predator encounter rates for two- and four-cell chains, respectively. In addition, the few remaining two-cell chains in grazed treatments were swimming at approximately half the speed of two-cell chains in treatments without grazers, which reduced encounter rates with grazers to values similar to that of single cells. Chain length plasticity and swimming behavior constitute unique mechanisms to reduce encounters with grazers. We argue that dinoflagellates can regulate the balance between motility and predator avoidance by adjusting chain length. The high predator encounter rate for motile chains may have contributed to the low prevalence of chain formation in motile phytoplankton compared with in nonmotile phytoplankton where chain formation is more common.

Unsteady motion: escape jumps in planktonic copepods, their kinematics and energetics.

We describe the kinematics of escape jumps in three species of 0.3-3.0 mm-sized planktonic copepods. We find similar kinematics between species with periodically alternating power strokes and passive coasting and a resulting highly fluctuating escape velocity. By direct numerical simulations, we estimate the force and power output needed to accelerate and overcome drag. Both are very high compared with those of other organisms, as are the escape velocities in comparison to startle velocities of other aquatic animals. Thus, the maximum weight-specific force, which for muscle motors of other animals has been found to be near constant at 57 N (kg muscle)(-1), is more than an order of magnitude higher for the escaping copepods. We argue that this is feasible because most copepods have different systems for steady propulsion (feeding appendages) and intensive escapes (swimming legs), with the muscular arrangement of the latter probably adapted for high force production during short-lasting bursts. The resulting escape velocities scale with body length to power 0.65, different from the size-scaling of both similar sized and larger animals moving at constant velocity, but similar to that found for startle velocities in other aquatic organisms. The relative duration of the pauses between power strokes was observed to increase with organism size. We demonstrate that this is an inherent property of swimming by alternating power strokes and pauses. We finally show that the Strouhal number is in the range of peak propulsion efficiency, again suggesting that copepods are optimally designed for rapid escape jumps.