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Water stable isotopes (δ18O and δ2H) were analyzed in samples collected in lakes, associated with riverine systems in northeastern Germany, throughout 2020. The dataset (Aichner et al., 2021; https://doi.org/10.1594/PANGAEA.935633) is derived from water samples collected at (a) lake shores (sampled in March and July 2020), (b) buoys which were temporarily installed in deep parts of the lake (sampled monthly from March to October 2020), (c) multiple spatially distributed spots in four selected lakes (in September 2020), and (d) the outflow of Müggelsee (sampled biweekly from March 2020 to January 2021). At shores, water was sampled with a pipette from 40–60 cm below the water surface and directly transferred into a measurement vial, while at buoys a Limnos water sampler was used to obtain samples from 1 m below the surface. Isotope analysis was conducted at IGB Berlin, using a Picarro L2130-i cavity ring-down spectrometer, with a measurement uncertainty of (δ18O) and (δ2H). The data give information about the vegetation period and the full seasonal isotope amplitude in the sampled lakes and about spatial isotope variability in different branches of the associated riverine systems.

WOS:000753551000001; International audience; In recent decades, the increase in terrestrial inputs to freshwater and coastal ecosystems, especially occurring at northern latitudes, has led to a process of water color darkening known as "brownification." To assess how brownification affects plankton community composition and functioning in northern coastal areas, an in situ mesocosm experiment using a highly colored humic substance to simulate a brownification event was performed in a North Atlantic bay (Hopavagen, Norway) in August 2019. Manual sampling for analyses of nutrient concentrations, phytoplankton pigments and zooplankton abundances was combined with high-frequency (every 15 min) monitoring of key environmental variables to investigate the response of the plankton community in terms of oxygen metabolism and community composition. In response to brownification, the oxygen gross primary production (GPP) and community respiration (R) slowed down significantly, by almost one-third. However, GPP and R both decreased to the same extent; thus, the oxygen metabolic balance was not affected. Moreover, the chlorophyll-a concentration significantly decreased under brownification, by 9% on average, and the chemotaxonomic pigment composition of the phytoplankton changed, indicating their acclimation to the reduced light availability. In addition, brownification seemed to favor appendicularians, the dominant mesozooplankton group in the mesocosms, which potentially contributed to lowering the phytoplankton biomass. In conclusion, the results of this in situ mesocosm experiment suggest that brownification could induce significant changes in phytoplankton and zooplankton community composition and significantly alter the overall oxygen metabolism of plankton communities in a northern Atlantic bay.

Winter conditions, such as ice cover and snow accumulation, are changing rapidly at northern latitudes and can have important implications for lake processes. For example, snowmelt in the watershed-a defining feature of lake hydrology because it delivers a large portion of annual nutrient inputs-is becoming earlier. Consequently, earlier and a shorter duration of snowmelt are expected to affect annual phytoplankton biomass. To test this hypothesis, we developed an index of runoff timing based on the date when 50% of cumulative runoff between January 1 and May 31 had occurred. The runoff index was computed using stream discharge for inflows, outflows, or for flows from nearby streams for 41 lakes in Europe and North America. The runoff index was then compared with summer chlorophyll-a (Chl-a) concentration (a proxy for phytoplankton biomass) across 5-53 years for each lake. Earlier runoff generally corresponded to lower summer Chl-a. Furthermore, years with earlier runoff also had lower winter/spring runoff magnitude, more protracted runoff, and earlier ice-out. We examined several lake characteristics that may regulate the strength of the relationship between runoff timing and summer Chl-a concentrations; however, our tested covariates had little effect on the relationship. Date of ice-out was not clearly related to summer Chl-a concentrations. Our results indicate that ongoing changes in winter conditions may have important consequences for summer phytoplankton biomass and production. Vermont Space Grant Consortium under NASA Cooperative Agreement [NNX15AP86H]; NASA Earth and Space Science Fellowship under NASA Cooperative Agreement [80NSSC18K1394P0000]; Swedish Infrastructure for Ecosystem Science (SITES); MANTEL [H2020-MSCA-ITN-2061]; Limnoscenes (Biodiversa); Limnoscenes (Belmond Forum); Limnoscenes (German Science Foundation); RSCF project, of Ministry of Higher Education and Research [18-44-06201, FZZE-2020-0026, FZZE-2020-0023]; Foundation for support of applied ecological studies of Lake Baikal; ERDF/ESF project Biomanipulation [CZ.02.1.01/0.0/0.0/16_025/0007417]; Estonian Research Council grant [PUT PSG32]; MANTEL ITN [H2020-MSCA-ITN-2061]; Estonian Research Council grants [PRG705] Published version Vermont Space Grant Consortium under NASA Cooperative Agreement; NASA Earth and Space Science Fellowship under NASA Cooperative Agreement; Swedish Infrastructure for Ecosystem Science (SITES); MANTEL; Limnoscenes (Biodiversa); Limnoscenes (Belmond Forum); Limnoscenes (German Science Foundation); RSCF project, of Ministry of Higher Education and Research; Foundation for support of applied ecological studies of Lake Baikal; ERDF/ESF project Biomanipulation; Estonian Research Council grant(Estonian Research Council); MANTEL ITN; Estonian Research Council grants Public domain – authored by a U.S. government employee

Global freshwater biodiversity is declining dramatically, and meeting the challenges of this crisis requires bold goals and the mobilisation of substantial resources. While the reasons are varied, investments in both research and conservation of freshwater biodiversity lag far behind those in the terrestrial and marine realms. Inspired by a global consultation, we identify 15 pressing priority needs, grouped into five research areas, in an effort to support informed stewardship of freshwater biodiversity. The proposed agenda aims to advance freshwater biodiversity research globally as a critical step in improving coordinated actions towards its sustainable management and conservation. ispartof: ECOLOGY LETTERS vol:25 issue:2 pages:255-263 ispartof: location:England status: published

Changes in land use and agricultural intensification threaten biodiversity and ecosystem functioning of small water bodies. We studied 67 kettle holes (KH) in an agricultural landscape in northeastern Germany using landscape-scale metatranscriptomics to understand the responses of active bacterial, archaeal and eukaryotic communities to land-use type. These KH are proxies of the millions of small standing water bodies of glacial origin spread across the northern hemisphere. Like other landscapes in Europe, the study area has been used for intensive agriculture since the 1950s. In contrast to a parallel environmental DNA study that suggests the homogenization of biodiversity across KH, conceivably resulting from long-lasting intensive agriculture, land-use type affected the structure of the active KH communities during spring crop fertilization, but not a month later. This effect was more pronounced for eukaryotes than for bacteria. In contrast, gene expression patterns did not differ between months or across land-use types, suggesting a high degree of functional redundancy across the KH communities. Variability in gene expression was best explained by active bacterial and eukaryotic community structures, suggesting that these changes in functioning are primarily driven by interactions between organisms. Our results indicate that influences of the surrounding landscape result in temporary changes in the activity of different community members. Thus, even in KH where biodiversity has been homogenized, communities continue to respond to land management. This potential needs to be considered when developing sustainable management options for restoration purposes and for successful mitigation of further biodiversity loss in agricultural landscapes.

Light pollution is an environmental stressor of global extent that is growing exponentially in area and intensity. Artificial skyglow, a form of light pollution with large range, is hypothesized to have environmental impact at ecosystem level. However, testing the impact of skyglow at large scales and in a controlled fashion under in situ conditions has remained elusive so far. Here we present the first experimental setup to mimic skyglow at ecosystem level outdoors in an aquatic environment. Spatially diffuse and homogeneous surface illumination that is adjustable between 10 mlx and 10 lx, resembling rural to urban skyglow levels, was achieved with white light-emitting diodes at a large-scale lake enclosure facility. The illumination system was enabled by optical modeling with Monte-Carlo raytracing and validated by measurements. Our method can be adapted to other outdoor and indoor skyglow experiments, urgently needed to understand the impact of skyglow on ecosystems.

Eutrophication of inland waters is an environmental issue that is becoming more common with climatic variability. Monitoring of this aquatic problem is commonly based on the chlorophyll-a concentration monitored by routine sampling with limited temporal and spatial coverage. Remote sensing data can be used to improve monitoring, especially after the launch of the MultiSpectral Instrument (MSI) on Sentinel-2. In this study, we compared the estimation of chlorophyll-a (chl-a) from different bio-optical algorithms using hyperspectral proximal remote sensing measurements, from simulated MSI responses and from an MSI image. For the satellite image, we also compare different atmospheric corrections routines before the comparison of different bio-optical algorithms. We used in situ data collected in 2019 from 97 sampling points across 19 different lakes. The atmospheric correction assessment showed that the performances of the routines varied for each spectral band. Therefore, we selected C2X, which performed best for bands 4 (root mean square error—RMSE = 0.003), 5 (RMSE = 0.004) and 6 (RMSE = 0.002), which are usually used for the estimation of chl-a. Considering all samples from the 19 lakes, the best performing chl-a algorithm and calibration achieved a RMSE of 16.97 mg/m3. When we consider only one lake chain composed of meso-to-eutrophic lakes, the performance improved (RMSE: 10.97 mg/m3). This shows that for the studied meso-to-eutrophic waters, we can reliably estimate chl-a concentration, whereas for oligotrophic waters, further research is needed. The assessment of chl-a from space allows us to assess spatial dynamics of the environment, which can be important for the management of water resources. However, to have an accurate product, similar optical water types are important for the overall performance of the bio-optical algorithm.

To understand underlying mechanisms of aquatic ecosystem functioning in relating to the global Grand Challenges (climate change, biodiversity loss, eutrophication, emerging pollutants, etc.), it is necessary to consider processes in adjacent systems, such as atmosphere and adjacent aquatic and terrestrial systems. For freshwater and coastal systems, the aquatic-terrestrial coupling on the watershed level is specifically important. We argue that for a better understanding of both aquatic and terrestrial ecosystems a combination of long-term data from connected environments, coupled with experimental ecosystem-scale experiments, have a greater potential for successful model testing and development of predictive concepts, than using only long-term data (without experiments) from separate systems. This talk will present the EU-funded RI-project AQUACOSM-plus (www.aquacosm.eu, 2020-2024) that offers access to >60 research facilities across the EU and is linked to world-wide cooperation through the MESOCOSM.EU portal, a virtual network of >100 research facilities. These networks comprise mesocosm facilities in all aquatic systems, including rivers, ponds, lakes, estuaries and marine systems – offering unique opportunities to conduct ecosystem-scale experimental studies of relevance to aquatic-terrestrial coupling. These facilities allow for process studies to test models based on trend or response observations from long-term-data, in order to better understand underlying mechanisms of ecosystem responses to the present global Grand Challenges. The AQUACOSM-plus mesocosm facilities, are also open for conducting ecosystem solution-based experiments to enable effective management in aquatic ecosystems. The AQUACOSM network will open calls to fund access to >13.000 days for a wide range of external users. We will also present examples of developing RI-RI collaborations and development of technological solutions and instrumentation to enhance the mobility of mesocosms and increase opportunities for relevant scenario-testing by the scientific community at large.

The diversity and composition of biological communities in ecosystems is typically linked to land use. Consequently, intensive agriculture has a strong influence on these patterns, including a reduction and homogenization of species diversity. Kettle holes (KH; also known as potholes) are small water bodies (2. They often function as drainage centers, linking various landscape elements in their surroundings and are hotspots of aquatic biodiversity in terrestrial landscapes. In the 1950s, the land use surrounding KH in Europe (e.g., Germany, Poland, The Netherlands) and North America (USA and Canada) shifted towards intensive agriculture, likely affecting the local and regional biodiversity of the KH. Reports on the biodiversity of ponds in agricultural landscapes are scarce, and mostly focus on single taxa. We used deep eDNA amplicon sequencing of eukaryotic, bacterial, and archaeal SSU rRNA genes to assess effects of land use on the overall species biodiversity of 67 KH in either arable fields, grasslands, or forests, all embedded in a landscape subjected to intensive agriculture for decades. These were later contrasted with Berlin city ponds. Metatranscriptomic analyses were conducted in parallel to investigate the taxonomic diversity and functionality of the active communities. The amplicon data, >300 million reads, were analyzed with phyloFlash and Kraken2 using the SILVA_SSU_Ref database. Subsequently, the sequences were grouped based on taxonomy. The use of different annotation pipelines resulted in similar patterns. A subset of the data was analyzed using DADA2, generating amplicon sequence variants. Clustering the sequences as variants or according to taxonomy resulted in identical patterns, suggesting that broad taxonomical groupings provide sufficient resolution for general overviews. Annotation of the eukaryotic community was challenging, as none of the currently available databases contains sufficient high-quality sequences to cover the entire phylogenetic breadth. To overcome this limitation, we used the SILVA_SSU_Parc database, which also contains short sequences excluded from SILVA_SSU_Ref, thus extending the taxonomic base of the analysis. The curated, high-quality PR2 eukaryotic database did not provide better results than the SILVA_SSU_Ref database. An incompatibility between the SILVA and classical taxonomic nomenclature resulted in a ‘language barrier,’ which could be partly resolved by using the parallel EMBL taxonomy provided in the SILVA databases. Nevertheless, we propose that short-read SSU-based eukaryotic taxonomic annotations should be validated by other means. Despite such methodological limitations, our study demonstrates that deep amplicon sequencing of eDNA return a reliable picture of the biodiversity in complex ecosystems. Such information is sufficient to identify biodiversity patterns across the three domains of life and can serve to pinpoint taxonomic groups which should be investigated by a more detailed approach. Our study concluded that long-term exposure to intensive agriculture results in biodiversity homogenization across diverse taxonomic groups, removing most differences in biodiversity patterns among land-use types. This outcome contrasts with biodiversity patterns associated with sediments of the KH, where temporal coverage by the eDNA analyses extends back to at least the onset of intensive agriculture. However, metatranscriptomic analyses, reflecting the distribution of activities rather than relative abundance, revealed temporal differences in the structure of the active community in KH of the investigated land-use types, matching times of field fertilization. Thus, even in a landscape where biodiversity has been homogenized, inputs from the surroundings result in short-term activity changes of different organisms. This needs to be considered when developing new management schemes needed to counteract the current biodiversity loss.

By shunting material out of the predatory pathway toward detritus and dissolved material, viruses are believed to have an important impact on biogeochemical functions of the pelagic microbial food web. To include viruses as a single plankton functional type (PFT) in dynamic food web models is, however, not trivial since they will then compete with predators for the same host/prey community as a shared limiting resource. As recently shown, one can solve this problem by introducing adaptation in the defensive and competitive traits of the host (prey) community. We here show how this can reproduce central aspects of viral dynamics as observed in a set of Arctic mesocosm experiments. In these experiments, contrasting microbial trophodynamics have previously been linked to the trophic cascades generated by seasonal vertical migration of large Arctic copepods. This approach thus produces a quantitative theory for the mechanisms regulating virus‐to‐prokaryote and lysis‐to‐predation ratios, and integrates this with a central role of predator top‐down control in pelagic microbial food webs. publishedVersion

Although processes in aquatic systems are closely connected to the terrestrial environment, these environments are often studied separately. We argue that for a better understanding of both aquatic and terrestrial ecosystems a combination of long-term data from connected environments, coupled with experimental ecosystem-scale experiments, have a greater potential for successful model testing and development of predictive concepts, than using only long-term data (without experiments) from separate systems. This talk will present the new EU-funded RI-project AQUACOSM-plus (www.aquacosm.eu, 2020-2024) that offers access to >50 research facilities across the EU and is linked to world-wide cooperation through the MESOCOSM.EU portal, a virtual network of >100 research facilities. Both networks include mesocosm facilities in all aquatic systems, including rivers, ponds, lakes, estuaries and marine systems – offering unique opportunities to conduct ecosystem-scale experimental studies of relevance to aquatic-terrestrial coupling. This network of research facilities can be used for large-scale process-based studies to test models based on trend or response observations from long-term-data, in order to understand underlying mechanisms of ecosystem functioning relating to the present global Grand Challenges (climate change, biodiversity loss, eutrophication, emerging pollutants, etc.). Interested parties are also welcome to suggest other uses of these research facilities, such as conducting ecosystem solution-based experiments to enable effective management in aquatic ecosystems. The network will fund access to >10.000 days for a wide range of external users, including scientists, students, industry and developers, from the whole world.

1. Global pressures on freshwater ecosystems are high and rising. Viewed primarily as a resource for humans, current practices of water use have led to catastrophic declines in freshwater species and the degradation of freshwater ecosystems, including their genetic and functional diversity. Approximately three-quarters of the world's inland wetlands have been lost, one-third of the 28 000 freshwater species assessed for the International Union for Conservation of Nature (IUCN) Red List are threatened with extinction, and freshwater vertebrate populations are undergoing declines that are more rapid than those of terrestrial and marine species. This global loss continues unchecked, despite the importance of freshwater ecosystems as a source of clean water, food, livelihoods, recreation, and inspiration. 2. The causes of these declines include hydrological alterations, habitat degradation and loss, overexploitation, invasive species, pollution, and the multiple impacts of climate change. Although there are policy initiatives that aim to protect freshwater life, these are rarely implemented with sufficient conviction and enforcement. Policies that focus on the development and management of fresh waters as a resource for people almost universally neglect the biodiversity that they contain. 3. Here we introduce the Alliance for Freshwater Life, a global initiative, uniting specialists in research, data synthesis, conservation, education and outreach, and policymaking. This expert network aims to provide the critical mass required for the effective representation of freshwater biodiversity at policy meetings, to develop solutions balancing the needs of development and conservation, and to better convey the important role freshwater ecosystems play in human well-being. Through this united effort we hope to reverse this tide of loss and decline in freshwater biodiversity. We introduce several short- and medium-term actions as examples for making positive change, and invite individuals, organizations, authorities, and governments to join the Alliance for Freshwater Life.

Browning caused by colored dissolved organic matter is predicted to have large effects on aquatic ecosystems. However, there is limited experimental evidence about direct and indirect effects of browning on zooplankton in complex field settings. We used a combination of an ecosystem‐scale enclosure experiment and laboratory incubations to test how prolonged browning affects physiological and life‐history traits of the water flea Daphnia longispina, a key species in lake food webs, and whether any such effects are reversible. Daphnids and water were collected from enclosures in a deep clear‐water lake, where the natural plankton community had been exposed for 10 weeks to browning or to control conditions in clear water. Daphnid abundance was much lower in the brown than in the clear enclosure. Surprisingly, however, daphnids continuously kept in brown enclosure water in the laboratory showed increased metabolic performance and survival, and also produced more offspring than daphnids kept in clear enclosure water. This outcome was related to more and higher‐quality seston in brown compared to clear water. Moreover, daphnids transferred from clear to brown water or vice versa adjusted their nucleic acid and protein contents, as indicators of physiological state, to similar levels as individuals previously exposed to the respective recipient environment, indicating immediate and reversible browning effects on metabolic performance. These results demonstrate the importance of conducting experiments in settings that capture both indirect effects (i.e., emerging from species interactions in communities) and direct effects on individuals for assessing impacts of browning and other environmental changes on lakes., German Federal Ministry of Education and Research, German Research Foundation http://dx.doi.org/10.13039/501100001659, IGB's Frontiers in Freshwater Science program

Ecological studies of some of the most numerous organisms on the planet, zooplankton, have been limited by manual analysis for more than 100 years. With the development of high-throughput video systems, we argue that this critical bottle-neck can now be solved if paired with deep neural networks (DNN). To leverage their performance, large amounts of training samples are required that until now have been dependent on manually created labels. To minimize the effort of expensive human experts, we employ recent active learning approaches to select only the most informative samples for labelling. Thus training a CNN using a nearly unlimited amount of images while limiting the human labelling effort becomes possible by means of active learning. We show in several experiments that in practice, only a few thousand labels are required to train a CNN and achieve an accuracy-level comparable to manual routine analysis of zooplankton samples. Once trained, this CNN can be used to analyse any amount of image data, presenting the zooplankton community the opportunity to address key research questions on transformative scales, many orders of magnitude, in both time and space, basically only limited by video through-put and compute capacity.

The seawater samples were stained with primuline (Direct Yellow 59, Sigma-Aldrich Co), fixed with a solution containing 3.6% (v/v) glutaraldehyde and 10% (v/v) glycerol (final concentrations), and gently filtered onto black-stained 25 mm diameter 0.4-µm pore-size Nucleopore filters. Filters were mounted on slides and analyzed by epifluorescence microscopy. The method is a modification from Grebecki (1962), Hobbie et al. (1977) and Caron (1983) with the glycerol added to reduce the damage of particularly small delicate protists during filtration as described in Sazhin et al. (2007). FlowCAM analysis of mesocosm seawater The FlowCAM was run in autoimage-mode, using 4x magnification to analyze particles ranging between 15 and 1000 µm. The water samples were kept in dim light at 4°C until analyzed, within 4 h after sampling. Each sample was run for ca. 30 min, corresponding to 5.7 ml of analyzed volume. The relevant context capture property chosen to do the analysis was distance to nearest neighbor = 50 µm. All the image collages were manually post-analyzed in order to identify the particles in question. Particle sizes were estimated from area-based diameter determined by the FlowCAM VISP 2.2 software following Jakobsen & Carstensen (2011). Colonies of P. pouchetii are composite clusters of cells spaced in colonies of various sizes. When the colonies are forming dense blooms, the FlowCAM cannot distinguish whether a cluster is a part of colony or if a cluster is a part of a neighbouring colony. The FlowCAM therefore captures clusters of colonies. S. marinoi and P. pouchetii are colonial structures of multiple cells, and we developed scaling relationships between colony area and cell number by manually counting the number of cells in either a diatom chain or a P. pouchetii colony (n > 100) and related the cell number to particle area. We further developed scaling relationships between cluster areas by manually counting the number of cells in P. pouchetii clusters (n > 100). The same procedure was used for estimating S. marinoi cells, where we manually counted the number of cells per chain. The cell numbers were related to either cluster area (P. pouchetii clusters) or chain length (S. marinoi), and the relationships were then used to estimate the total number of P. pouchetii or S. marinoi cells in colonies or chains, respectively, by sample. Cell concentrations were estimated by correcting for the analyzed volume. The resulting regression equation used for estimation the number of P. pouchetii cells in colonies and colony fragments was Cells = ABD 1.834 * 0.028 (R 2 = 0.83), where ABD (area base diameter) is an output parameter from

Microzooplankton dilution grazing experiments conducted in phytoplankton rich waters, particularly in polar and subpolar seas, often result in calculation of nonsignificant or negative grazing coefficients. We hypothesized that preparation of filtered seawater (FSW) from water containing high biomass of phytoplankton results in release of allelochemicals that inhibit phytoplankton growth, lowering the net growth of phytoplankton in the more diluted treatments. We tested this hypothesis during blooms of Skeletonema marinoi and Phaeocystis pouchetii in a nutrient-enriched mesocosm in the Raunefjord, Norway. During the S. marinoi bloom, inhibition of phytoplankton growth occurred in the diluted treatments. Simultaneously the concentration of total as well as dissolved polyunsaturated aldehydes (PUAs) was elevated. Passage of the FSW through a carbon cellulose cartridge to remove dissolved organic material reduced, or eliminated, the inhibition. In the early phase of the P. pouchetii bloom that followed the diatom bloom in the mesocosm, PUA concentration was relatively low and the untreated FSW had a less drastic, but often significant, inhibitory effect on phytoplankton growth. Laboratory experiments with cultures of S. marinoi and P. pouchetii confirmed that material present in filtrate prepared from diluted cultures was self-inhibitory. Many phytoplankters, particularly during late stages of a bloom, produce inhibitory metabolites that may be released during filtration of the relatively large volumes of seawater needed for dilution experiments. Under some conditions, dilution grazing experiments may underestimate phytoplankton growth coefficients and microzooplankton grazing coefficients.

Anthropogenic atmospheric loading of CO2 raises concerns about combined effects of increasing ocean temperature and acidification, on biological processes. In particular, the response of appendicularian zooplankton to climate change may have significant ecosystem implications as they can alter biogeochemical cycling compared to classical copepod dominated food webs. However, the response of appendicularians to multiple climate drivers and effect on carbon cycling are still not well understood. Here, we investigated how gelatinous zooplankton (appendicularians) affect carbon cycling of marine food webs under conditions predicted by future climate scenarios. Appendicularians performed well in warmer conditions and benefited from low pH levels, which in turn altered the direction of carbon flow. Increased appendicularians removed particles from the water column that might otherwise nourish copepods by increasing carbon transport to depth from continuous discarding of filtration houses and fecal pellets. This helps to remove CO2 from the atmosphere, and may also have fisheries implications.

In order to characterize copepod feeding in relation to microbial plankton community dynamics, we combined metabarcoding and metabolome analyses during a 22-day seawater mesocosm experiment. Nutrient amendment of mesocosms promoted the development of haptophyte (Phaeocystis pouchetii)- and diatom (Skeletonema marinoi)-dominated plankton communities in mesocosms, in which Calanus sp. copepods were incubated for 24 h in flow-through chambers to allow access to prey particles (

Numerous biological activities such as grazer defense and intraspecific signaling have been described for diatom oxylipins, fatty acid derived secondary metabolites produced by some diatom species. As the function and importance of these compounds are still controversial, the production of a subclass of these molecules, nonvolatile oxylipins, was studied during an induced bloom of Skeletonema marinoi (Samo et Zingone) in a mesocosm setup. Reproductive parameters of one of the main grazers, Calanus finmarchicus, were also examined during the bloom. Oxylipins detected during the bloom were the same as those previously described for S. marinoi and were detected predominantly in the mesocosm inoculated with this diatom. Reproductive success of C. finmarchicus remained unaffected during the course of the bloom. This may have been due to a dilution effect by the availability of alternative suitable prey or to the limited exposure of the copepods to the oxylipins generated during the short bloom. Follow up laboratory studies showed that oxylipin composition changed both when the S. marinoi clone used for inoculation was grown in the laboratory and in comparison to the well-studied Adriatic clone of S. marina These results highlight the necessity of quantitatively measuring oxylipin concentrations during diatom blooms at sea to be able to correctly evaluate their ecological significance. (C) 2013 Elsevier B.V. All rights reserved.

Increasing pCO2 is hypothesized to induce shifts in plankton communities towards smaller cells, reduced carbon export rates and increased roles of gelatinous zooplankton. Appendicularians, among the most numerous pan-global ‘gelatinous’ zooplankton, continuously produce filter-feeding houses, shortcutting marine food webs by ingesting submicron particles, and their discarded houses contribute significantly to carbon fluxes. We present a first mesocosm-scale study on the effects of temperature, pCO2 and bloom structures on the appendicularian, Oikopleura dioica. There were effects of temperature and nutrients on phytoplankton communities. No shifts in functional phytoplankton groups, nor changes in particle sizes/morphotypes, known to impact appendicularian feeding, were observed under manipulated pCO2 conditions. However, appendicularian abundance was positively correlated with increased pCO2, temperature and nutrient levels, consistent with hypotheses concerning gelatinous zooplankton in future oceans. This suggests appendicularians will play more important roles in marine pelagic communities and vertical carbon transport under projected ocean acidification and elevated temperature scenarios.

Ptacnik, Robert ... et al.-- 10 pages, 4 figures, 1 table, supplementary information https://dx.doi.org/10.1038/srep29286, Mixotrophs combine photosynthesis with phagotrophy to cover their demands in energy and essential nutrients. This gives them a competitive advantage under oligotropihc conditions, where nutrients and bacteria concentrations are low. As the advantage for the mixotroph depends on light, the competition between mixo- and heterotrophic bacterivores should be regulated by light. To test this hypothesis, we incubated natural plankton from the ultra-oligotrophic Eastern Mediterranean in a set of mesocosms maintained at 4 light levels spanning a 10-fold light gradient. Picoplankton (heterotrophic bacteria (HB), pico-sized cyanobacteria, and small-sized flagellates) showed the fastest and most marked response to light, with pronounced predator-prey cycles, in the high-light treatments. Albeit cell specific activity of heterotrophic bacteria was constant across the light gradient, bacterial abundances exhibited an inverse relationship with light. This pattern was explained by light-induced top-down control of HB by bacterivorous phototrophic eukaryotes (PE), which was evidenced by a significant inverse relationship between HB net growth rate and PE abundances. Our results show that light mediates the impact of mixotrophic bacterivores. As mixo- and heterotrophs differ in the way they remineralize nutrients, these results have far-reaching implications for how nutrient cycling is affected by light, The experiment was financed by the European Union Seventh Framework Program MESOAQUA (FP7/2009-2013) coordinated by JCN under grant agreement no. 228224. R.P. further acknowledges financial support by the German Research Council (DFG Pt 5/3-1), JMG by Spanish project REMEI (CTM2015-70340) and AC by project PROTOS (CTM2009-08783) from the Spanish Ministry of Economy and Competitiveness

Extreme weather events can pervasively influence ecosystems. Observations in lakes indicate that severe storms in particular can have pronounced ecosystem-scale consequences, but the underlying mechanisms have not been rigorously assessed in experiments. One major effect of storms on lakes is the redistribution of mineral resources and plankton communities as a result of abrupt thermocline deepening. We aimed at elucidating the importance of this effect by mimicking in replicated large enclosures (each 9 m in diameter, ca. 20 m deep, ca. 1300 m 3 in volume) a mixing event caused by a severe natural storm that was previously observed in a deep clear-water lake. Metabolic rates were derived from diel changes in vertical profiles of dissolved oxygen concentrations using a Bayesian modelling approach, based on high-frequency measurements. Experimental thermocline deepening stimulated daily gross primary production (GPP) in surface waters by an average of 63% for > 4 weeks even though thermal stratification re-established within 5 days. Ecosystem respiration (ER) was tightly coupled to GPP, exceeding that in control enclosures by 53% over the same period. As GPP responded more strongly than ER, net ecosystem productivity (NEP) of the entire water column was also increased. These protracted increases in ecosystem metabolism and autotrophy were driven by a proliferation of inedible filamentous cyanobacteria released from light and nutrient limitation after they were entrained from below the thermocline into the surface water. Thus, thermocline deepening by a single severe storm can induce prolonged responses of lake ecosystem metabolism independent of other storm-induced effects, such as inputs of terrestrial materials by increased catchment run-off. This highlights that future shifts in frequency, severity or timing of storms are an important component of climate change, whose impacts on lake thermal structure will superimpose upon climate trends to influence algal dynamics and organic matter cycling in clear-water lakes. Keywords: climate variability, ecosystem productivity, extreme events, gross primary production, mesocosm, respiration stratified lakes

Quantification of grazing losses of marine heterotrophic bacteria is critical for understanding nutrient and carbon pathways in aquatic systems. The dilution method is a commonly used experimental approach for quantifying bacterivory. However, valid estimates of grazing rates obtained using this method depend on several methodological assumptions including that the method does not influence specific growth rates of bacteria. Here, we hypothesize that filtration during the set-up of a dilution experiment has the potential to release allelochemicals from phytoplankton cells and thereby stimulate or inhibit bacterial growth with the consequence of biased grazing estimates. We tested this hypothesis during a natural Phaeocystis pouchetii bloom at two different locations within an Arctic fjord. Results from the dilution experiments suggest higher gross growth rate and grazing impact for bacteria in the outer fjord compared with the inner fjord. However, specific growth rates estimated by bacterial production cell−1 were significantly elevated in dilutions of water from the outer fjord but not the inner fjord. The analysis of dissolved metabolites in the seawater from both experiments prior and after filtration revealed altered metabolic profiles after filtration at both stations. As unaffected specific growth of prey on dilution is one of three fundamental assumptions of the dilution method, we conclude that it is important that empirically estimated bacterial specific growth rates be routinely included when using the dilution method to quantify bacterivory. publishedVersion

We studied the effect of variable phytoplankton biomass and dominance of the diatom Skeletonema marinoi on the planktonic community respiratory carbon requirement over a period of 14 d (14 to 28 April 2008) in 3 different mesocosms filled with natural water at Espegrend marine biological field station in Raunefjord, Norway. The carbon requirement was measured on mesozooplankton (the calanoid copepod Calanus finmarchicus) and 3 other size fractions of plank- ton— 50% of the primary production, while bac- terioplankton showed an intermediate and variable contribution (ca.

Gelatinous zooplankton, such as ctenophores and jellyfish, are important components of marine and brackish ecosystems and play critical roles in aquatic biogeochemistry. As voracious predators of plankton, ctenophores have key positions in aquatic food webs and are often successful invaders when introduced to new areas. Gelatinous zooplankton have strong impacts on ecosystem services, particularly in coastal environments. However, little is known about the factors responsible for regulating population dynamics of gelatinous organisms, including biological interactions that may contribute to bloom demise. Ctenophores are known to contain specific bacterial communities and a variety of invertebrate parasites and symbionts; however, no previous studies have examined the presence of viruses in these organisms. Building upon recent studies demonstrating a diversity of single-stranded DNA viruses that encode a replication initiator protein (Rep) in aquatic invertebrates, this study explored the presence of circular, Rep-encoding single-stranded DNA (CRESS-DNA) viruses in the ctenophores Mnemiopsis leidyi and Beroe ovata collected from the Skidaway River Estuary and Savannah River in Georgia, USA. Using rolling circle amplification followed by restriction enzyme digestion, this study provides the first evidence of viruses in ctenophores. Investigation of four CRESS-DNA viruses over an eight-month period using PCR demonstrated temporal trends in viral prevalence and indicated that some of the viruses may persist in ctenophore populations throughout the year. Although future work needs to examine the ecological roles of these ctenophore-associated viruses, this study indicates that viral infection may play a role in population dynamics of gelatinous zooplankton.

We studied the effect of a developing Skeletonema marinoi/Phaeocystis spp. bloom on Calanus finmarchicus hatching success, early naupliar survival and metabolism. Our focus was (1) on the development of reproductive rates during a bloom initiation, peak and decline in relation to the production of potentially toxic algal metabolites and (2) on the proportional importance of female nutrition versus naupliar food environment for the production of viable nauplii. Despite polyunsaturated aldehyde (PUA) production by both S. marinoi and Phaeocystis sp., we did not observe any harmful effects on hatching success or naupliar survival and condition in any stages of the short-term (

Uptake of inorganic and organic nitrogen (N) by phytoplankton and bacteria was inves- tigated during a mesocosm study conducted in Raunefjord, Norway in April 2005. One mesocosm was fertilized with nitrate and phosphate at a ratio of 16:1 and maintained in the light, while one un- amended light mesocosm served as a control. Dissolved nutrients, phytoplankton and bacterial bio- mass, and phytoplankton community composition were monitored throughout the 26 d experiment. Uptake of 15 N-labeled ammonium and nitrate, and dual-labeled ( 15 N and 13 C) urea and dissolved free amino acids (DFAA) was measured for phytoplankton and bacteria using 2 methods: size fractiona- tion into 0.2-0.8 and >0.8 μm size classes, and flow cytometric sorting based on chlorophyll autoflu- orescence. Prior to fertilization, dissolved inorganic N concentrations were low and comprised ~5% of total dissolved N. Added nitrate was completely utilized in the amended mesocosm within 10 d, stimulating a large bloom of colonial Phaeocystis pouchetii. Ammonium contributed over half of total measured N uptake by phytoplankton and bacteria in both enclosures, while nitrate and urea each supplied roughly 10 to 25%. Overall, DFAA were a negligible N source to phytoplankton but con- tributed 11% to total bacterial N uptake. Bacterial uptake represented a significant portion of total uptake of all N forms, especially urea and DFAA. Comparison of the 2 methods for measuring phyto- plankton versus bacterial uptake demonstrates how the use of 0.8 μm filters can lead to significant overestimation of phytoplankton N uptake due to the retention of bacterial biomass.

Copepods dominate the biomass of marine zooplankton and through their prey selection they act as top-down regulators of planktonic communities. We investigated feeding preference of copepods in the presence of the diatom Skeletonema marinoi at different time points throughout the development of a bloom and a culture. Quantitative PCR gut content assessment revealed that the food uptake of the copepod Calanus spp. on mixed diets and on artificially induced mesocosm blooms was selective. Uptake of S. marinoi was highest during the post-bloom phase in the mesocosms even if the abundance of this alga was already low. In laboratory assays, copepods showed a greater preference for S. marinoi in the late stationary phase than for cultures of the same strain under exponentially growing culture conditions. The copepods thus discriminate between different growth phases of a single algal species in both laboratory and field settings. In parallel, we monitored cellular metabolites of the diatom using a metabolomic approach. Complex changes in the metabolic profile occur during development of a culture. Since no obvious effect of nutrient quality and cell size was involved, we suggest that changes in (info)chemicals within or surrounding S. marinoi regulate selective feeding by zooplankton.

Molecular methods are becoming increasingly common for taxonomic and ecological studies of marine and freshwater plankton. Recently, nucleic acids have been used as target molecules for identification and quantification of prey species in studies of trophic interactions. A critical step in the quantification of mesozooplankton feeding by molecular analysis is the isolation of microalgal DNA from predator guts and in the food environment. It is essential that total genomic DNA extraction provides maximum quantitative yield suitable for downstream analysis. In this study, we compared the efficacy and experimental variability of eight different protocols for total genomic DNA extraction from free-living microalgae and microalgae within the gut of copepods. We also developed and evaluated different sampling procedures for copepods prior to genomic extraction. The optimal protocol was evaluated using real time quantitative polymerase chain reaction (qPCR) and the integrity of the genomic DNA was determined by amplifying PCR targets of increasing size. Considerable variability was observed between purification protocols. Qagen DNeasy® Blood & Tissue kit was the most efficient of the tested methods for genomic extraction from both free-living microalgae and microalgae inside copepod guts. Furthermore, the appropriate handling of predator copepods prior to genomic extraction was essential for quantitative gut content estimates.

Quantification of feeding rates and selectivity of zooplankton is vital for understanding the mechanisms structuring marine ecosystems. However, methodological limitations have made many of these studies difficult. Recently, molecular based methods have demonstrated that DNA from prey species can be used to identify zooplankton gut contents, and further, quantitative gut content estimates by quantitative PCR (qPCR) assays targeted to the 18S rRNA gene have been used to estimate feeding rates in appendicularians and copepods. However, while standard single primer based qPCR assays were quantitative for the filter feeding appendicularian Oikopleura dioica, feeding rates were consistently underestimated in the copepod Calanus finmarchicus. In this study, we test the hypothesis that prey DNA is rapidly digested after ingestion by copepods and describe a qPCR-based assay, differential length amplification qPCR (dla-qPCR), to account for DNA digestion. The assay utilizes multiple primer sets that amplify different sized fragments of the prey 18S rRNA gene and, based on the differential amplification of these fragments, the degree of digestion is estimated and corrected for. Application of this approach to C. finmarchicus fed Rhodomonas marina significantly improved quantitative feeding estimates compared to standard qPCR. The development of dla-qPCR represents a significant advancement towards a quantitative method for assessing in situ copepod feeding rates without involving cultivation-based manipulation.

A mesocosm experiment was conducted during the spring of 2003 in Raunefjord, west Norway. Inorganic nutrients (16 µmol l -1 nitrate, 1 µmol l -1 phosphate) were added to 2 enclosures with 10% of those concentrations (1.6 µmol l -1 nitrate, 0.1 µmol l -1 phosphate) added daily thereafter; a third unamended mesocosm was used as a control. Nitrogen (N) (ammonium, nitrate, urea and amino acid) uptake rates for >0.8 µm (largely composed of phytoplankton) and 0.2 to 0.8 µm (largely composed of bacteria) size classes were measured, as well as nutrient, chlorophyll, phytoplankton and microzooplankton concentrations. The nutrient additions initiated a process resulting in a large bloom of Phaeocystis pouchetii colonies. There was a 2.5 wk lag, during which a smaller bloom of phototrophic flagellates, followed by diatoms, formed in all mesocosms; diatoms increased until sili- cic acid was depleted. After the flagellate and diatom bloom dissipated, the mesocosms were depleted of inorganic N. Dissolved organic N (DON) remained constant in all mesocosms during the flagellate and diatom bloom but increased with the onset of the large P. pouchetii bloom, largely due to release of urea and amino acids, which made up >93% of the DON pool at the end of the experi- ment. Uptake rates of urea often accounted for the largest percentage of total N uptake in all meso- cosms for both the >0.8 µm and 0.2 to 0.8 µm size classes, generally providing the largest percentage (up to 88%) of bacterial N demand of any substrate. This was particularly true during the Phaeocys- tis bloom when urea uptake rates were up to 65 times greater than the other N substrates. We observed a switch from new production, when nitrate uptake dominated during the diatom bloom, to regenerated production during the Phaeocystis bloom. The present study adds to the growing body of evidence that organic N is a substantial source of N for both phytoplankton and bacteria.

The marine phytoplankton species Phaeocystis pouchetii often forms large blooms of gelatinous colonies during spring in north temperate, boreal, and Arctic waters. The macroscopic colonies in such blooms develop originally from free-swimming solitary cells that shed their flagella, secrete a gelatinous membrane, and undergo repeated cell divisions within an enlarging colony membrane. However, the essential first steps in this process have not been described in situ, and hence the factors controlling initiation of colony blooms are poorly understood. Studies were con- ducted in mesocosms in a western Norway fjord to document the development of small young colonies under simulated and ambient conditions, resulting in 2 novel perspectives: (1) The critical early stages of colony bloom formation may occur in a relatively brief window in time. The rate of for- mation of young colonies from solitary cells was faster in nutrient-fertilized compared to unfertilized mesocosms, but this only happened after a period of diatom growth in the mesocosms and subse- quent increases in P. pouchetii colony abundance. Then, small percentages of solitary cells were mathematically necessary to account for observed colony numbers. (2) Initial formation of new young colonies occurred during the dark period of the ambient springtime diel light cycle, and patterns of colony accumulation implied rapid multiple cell divisions within colonies during the dark period, a process termed phased ultradian growth. Such an evolutionary strategy of rapid change from single cells to multi-cell colonies would be advantageous as a means of minimizing grazing losses during the critical period of size transition from microscopic free-swimming cells that are susceptible to microzooplankton and viruses, to larger colonies that are susceptible to metazoan zooplankton but relatively impervious to viruses and microzooplankton. The combined results offer a new insight into colony proliferation: colony blooms do not necessarily require a long period of constant conversion of solitary cells into colonies, but rather may be the product of punctuated, rapid life cycle transforma- tions and ultradian growth of young colonies.

Chemical defence is a potential mechanism contributing to the success of Phaeocystis species that repeatedly dominate the phytoplankton in coastal areas. Species within the genus Phaeocystis have long been suspected of imposing negative effects on co-occurring organisms. Recently a number of toxins have been extracted and identified from Phaeocystis samples, but it is not clear if they do enhance the competitive advantage of Phaeocystis species.In the present study the cytotoxic impact of live Phaeocystis pouchetii to human blood cells in close proximity, regardless of the nature of the responsible mechanism, was quantified using a bioassay. Haemolytic activity was measured during blooms of P. pouchetii in mesocosms. These environments were chosen to mimic natural conditions including chemically mediated interactions that could trigger defensive and/or allelopathic responses of Phaeocystis.Haemolytic activity correlated with P. pouchetii numbers and was absent during the preceding diatom bloom. Samples containing live P. pouchetii cells showed the highest activity, while filtered sea water and cell extracts were less haemolytic or without effect. Dose-response curves were linear up to 70% lysis, and haemolysis in samples containing live P. pouchetii cells reached EC50 values comparable to known toxic prymnesiophytes (1.9 * 10(7) cells l(-1)). Haemolytic activity was enhanced by increased temperature and light. The results indicate that unprotected and thus presumably vulnerable cells present in a P. pouchetii bloom may lyse within days.

Knowledge of the complex life cycle of Phaeocystis is a key to understanding its role in marine ecosystems and global biogeochemistry. An existing life cycle model was modified and used to integrate understanding of the Phaeocystis life cycle. In model-driven research, models expose gaps in our understanding, empirical studies ensue, and feedback improves understanding. Following this scheme, three facets of the life cycle model were examined here. With four exceptions, the empirical studies described have been presented in other literature citations. The first facet involved testing for the existence of a process or producing its description. These studies included: demonstration of in vitro colony division in Phaeocystis pouchetii, description of in vitro change in colony shape for P. pouchetii associated with senescence, determining which P. pouchetii life stage is vulnerable to viral infection and lysis, and an experiment designed to determine whether the sediment could be a source of new Phaeocystis colonies to overlying waters; results suggested that more-detailed investigation of benthic particles as a physical substrate for colony formation is warranted. The second facet involved investigation of process rate quantification or process control parameters. Process rate quantification included measurements of colony division rate and growth rate using mesocosm-derived colonies. Process control experiments included testing diatom frustule enhancement of P. pouchetii colony formation from solitary cells, and investigation of mesozooplanktonic suppression and microzooplanktonic enhancement of Phaeocystis globosa colony formation by planktonic grazer infochemicals. The third facet pertained to the molecular identification of genetic differences between single cells and colonies of P. globosa. These studies were designed to provide insight to the question of control factors involved in the transition between single cell and colonial life stages. The life cycle model provided a ready place to incorporate new insights and understanding from empirical studies into an existing model, and can be used to improve simulation models of the direct and indirect effects of Phaeocystis on global biogeochemistry.

Phaeocystis pouchetii is an ecologically important colony-forming marine phytoplankton species in northern hemisphere cold waters. It plays a central role in the processing of biogeochemically important elements in the upper ocean during spring blooms. Here, we report highly significant quantitative relationships among colony size, cell number and particulate carbon and nitrogen contents of field populations of P. pouchetii, which provide the means to quantitatively convert its biological expression into units of biogeochemical significance. Populations were sampled from mesocosms incubated in situ in western Norway and either fertilized with nitrate and phosphate or left unamended. Phaeocystis colony blooms developed in both scenarios, but were much greater in fertilized treatments. Colonies from the latter treatments were larger, contained higher concentrations of cells and more particulate carbon and nitrogen than those in the unfertilized mesocosm. Considering all data, log cell number increased linearly with log colony volume with a slope of 0.54. Log carbon and nitrogen increased with log colony volume, with respective slopes of 0.92 and 1.22, indicating a significant component of non-cellular carbon and nitrogen within the colonies. Carbon and nitrogen contents of colonies were linearly related, and fertilized colonies contained more nitrogen relative to carbon than unfertilized colonies. These equations are particularly applicable to contemporary studies of P. pouchetii because they represent colonies growing in environments that mimic a continuum from natural to eutrophicated ecosystems.