Search

Carbon turnover in aquatic environments is dependent on biochemical properties of organic matter (OM) and its degradability by the surrounding microbial community. Non-additive interactive effects represent a mechanism where the degradation of biochemically persistent OM is stimulated by the provision of bioavailable OM to the degrading microbial community. Whilst this is well established in terrestrial systems, whether it occurs in aquatic ecosystems remains subject to debate. We hypothesised that OM from zooplankton carcasses can stimulate the degradation of biochemically persistent leaf material, and that this effect is influenced by the daphnia:leaf OM ratio and the complexity of the degrading microbial community. Fresh Daphnia magna carcasses and 13 C-labelled maize leaves (Zea mays) were incubated at different ratios (1:1, 1:3 and 1:5) alongside either a complex microbial community (<50 µm) or solely bacteria (<0.8 µm). 13 C stable-isotope measurements of CO 2 analyses were combined with phospholipid fatty acids (PLFA) analysis and DNA sequencing to link metabolic activities, biomass and taxonomic composition of the microbial community. Our experiments indicated a significantly higher respiration of leaf-derived C when daphnia-derived OM was most abundant (i.e. daphnia:leaf OM ratio of 1:1). This process was stronger in a complex microbial community, including eukaryotic microorganisms, than a solely bacterial community. We concluded that non-additive interactive effects were a function of increased C-N chemodiversity and microbial complexity, with the highest net respiration to be expected when chemodiversity is high and the degrading community complex. This study indicates that identifying the interactions and processes of OM degradation is one important key for a deeper understanding of aquatic and thus global carbon cycle.

Non-predatory mortality of zooplankton provides an abundant, yet, little studied source of high quality labile organic matter (LOM) in aquatic ecosystems. Using laboratory microcosms, we followed the decomposition of organic carbon of fresh 13 C-labelled Daphnia carcasses by natural bacterioplankton. The experimental setup comprised blank microcosms, that is, artificial lake water without any organic matter additions (B), and microcosms either amended with natural humic matter (H), fresh Daphnia carcasses (D) or both, that is, humic matter and Daphnia carcasses (HD). Most of the carcass carbon was consumed and respired by the bacterial community within 15 days of incubation. A shift in the bacterial community composition shaped by labile carcass carbon and by humic matter was observed. Nevertheless, we did not observe a quantitative change in humic matter degradation by heterotrophic bacteria in the presence of LOM derived from carcasses. However, carcasses were the main factor driving the bacterial community composition suggesting that the presence of large quantities of dead zooplankton might affect the carbon cycling in aquatic ecosystems. Our results imply that organic matter derived from zooplankton carcasses is efficiently remineralized by a highly specific bacterial community, but does not interfere with the bacterial turnover of more refractory humic matter., (© 2018 Society for Applied Microbiology and John Wiley & Sons Ltd.)

Most studies of aquatic plankton focus on either macroscopic or microbial communities, and on either eukaryotes or prokaryotes. This separation is primarily for methodological reasons, but can overlook potential interactions among groups. Here we tested whether DNA metabarcoding of unfractionated water samples with universal primers could be used to qualitatively and quantitatively study the temporal dynamics of the total plankton community in a shallow temperate lake. Significant changes in the relative proportions of normalized sequence reads of eukaryotic and prokaryotic plankton communities over a 3-month period in spring were found. Patterns followed the same trend as plankton estimates measured using traditional microscopic methods. The bloom of a conditionally rare bacterial taxon belonging to Arcicella was characterized, which rapidly came to dominate the whole lake ecosystem and would have remained unnoticed without metabarcoding. The data demonstrate the potential of universal DNA metabarcoding applied to unfractionated samples for providing a more holistic view of plankton communities., (© 2017 Society for Applied Microbiology and John Wiley & Sons Ltd.)

Background: Mortality is a main driver in zooplankton population biology but it is poorly constrained in models that describe zooplankton population dynamics, food web interactions and nutrient dynamics. Mortality due to non-predation factors is often ignored even though anecdotal evidence of non-predation mass mortality of zooplankton has been reported repeatedly. One way to estimate non-predation mortality rate is to measure the removal rate of carcasses, for which sinking is the primary removal mechanism especially in quiescent shallow water bodies., Objectives and Results: We used sediment traps to quantify in situ carcass sinking velocity and non-predation mortality rate on eight consecutive days in 2013 for the cladoceran Bosmina longirostris in the oligo-mesotrophic Lake Stechlin; the outcomes were compared against estimates derived from in vitro carcass sinking velocity measurements and an empirical model correcting in vitro sinking velocity for turbulence resuspension and microbial decomposition of carcasses. Our results show that the latter two approaches produced unrealistically high mortality rates of 0.58-1.04 d(-1), whereas the sediment trap approach, when used properly, yielded a mortality rate estimate of 0.015 d(-1), which is more consistent with concurrent population abundance data and comparable to physiological death rate from the literature., Ecological Implications: Zooplankton carcasses may be exposed to water column microbes for days before entering the benthos; therefore, non-predation mortality affects not only zooplankton population dynamics but also microbial and benthic food webs. This would be particularly important for carbon and nitrogen cycles in systems where recurring mid-summer decline of zooplankton population due to non-predation mortality is observed.

Seasonal changes in environmental conditions have a strong impact on microbial community structure and dynamics in aquatic habitats. To better elucidate the response of bacterial communities to environmental changes, we have measured a large variety of limnetic variables and investigated bacterial community composition (BCC) and dynamics over seven consecutive years between 2003 and 2009 in mesotrophic Lake Tiefwaren (NE Germany). We separated between free-living (FL, >0.2, <5.0 μm) and particle-associated (PA, >5.0 μm) bacteria to account for different bacterial lifestyles and to obtain a higher resolution of the microbial diversity. Changes in BCC were studied by DGGE based on PCR-amplified 16S rRNA gene fragments. Sequencing of DGGE bands revealed that ca. 70 % of all FL bacteria belonged to the Actinobacteria, whereas PA bacteria were dominated by Cyanobacteria (43 %). FL communities were generally less diverse and rather stable over time compared to their PA counterpart. Annual changes in reoccurring seasonal patterns of dominant freshwater bacteria were supported by statistical analyses, which revealed several significant correlations between DGGE profiles and various environmental variables, e.g. temperature and nutrients. Overall, FL bacteria were generally less affected by environmental changes than members of the PA fraction. Close association of PA bacteria with phytoplankton and zooplankton suggests a tight coupling of PA bacteria to organisms of higher trophic levels. Our results indicate substantial differences in bacterial lifestyle of pelagic freshwater bacteria, which are reflected by contrasting seasonal dynamics and relationships to a number of environmental variables.

Aggregates and zooplankton may provide refuge for aquatic bacteria against external hazards. The ability of attached bacteria to survive and recover from stressors commonly used for water treatment was tested in the laboratory. Without zooplankton or aggregates, both UV and ozone significantly reduced abundance of free-living bacteria in both freshwater and marine medium. The presence of zooplankton carcasses and aggregates, however, allowed some of the attached bacteria to survive and recover quickly within 3 days. Heat exposure was the least effective as both free-living and attached bacteria were able to recover quickly. Selective survival of bacterial phylotypes led to large changes in bacterial community composition after stress exposures, and some of the bacteria that recovered belonged to groups with known pathogens. This study demonstrates that zooplankton and aggregates protected various aquatic bacteria from external stressors, and organic remains generated from zooplankton and aggregates after stress exposure even enabled the surviving bacteria to quickly regrow and subsequently be released into the surrounding water. Hence, water disinfection treatments that overlooked the potential persistence of bacteria associated with organisms and aggregates may not be effective in preventing the spread of undesirable bacteria.

Microorganisms and zooplankton are both important components of aquatic food webs. Although both inhabit the same environment, they are often regarded as separate functional units that are indirectly connected through nutrient cycling and trophic cascade. However, research on pathogenic and nonpathogenic bacteria has shown that direct association with zooplankton has significant influences on the bacteria's physiology and ecology. We used stratified migration columns to study vertical dispersal of hitchhiking bacteria through migrating zooplankton across a density gradient that was otherwise impenetrable for bacteria in both upward and downward directions (conveyor-belt hypothesis). The strength of our experiments is to permit quantitative estimation of transport and release of associated bacteria: vertical migration of Daphnia magna yielded an average dispersal rate of 1.3 x 10(5) x cells x Daphnia(-1) x migration cycle(-1) for the lake bacterium Brevundimonas sp. Bidirectional vertical dispersal by migrating D. magna was also shown for two other bacterial species, albeit at lower rates. The prediction that diurnally migrating zooplankton acquire different attached bacterial communities from hypolimnion and epilimnion between day and night was subsequently confirmed in our field study. In mesotrophic Lake Nehmitz, D. hyalina showed pronounced diel vertical migration along with significant diurnal changes in attached bacterial community composition. These results confirm that hitchhiking on migrating animals can be an important mechanism for rapidly relocating microorganisms, including pathogens, allowing them to access otherwise inaccessible resources.

Parasites form an integral part of food webs, however, they are often ignored in classic food web theory or limited to the investigation of trophic transmission pathways. Specifically, direct consumption of parasites by nonhost predators is rarely considered, while it can contribute substantially to energy flow in food webs. In aquatic systems, chytrids constitute a major group of fungal parasites whose free‐living infective stages (zoospores) form a highly nutritional food source to zooplankton. Thereby, the consumption of zoospores can create an energy pathway from otherwise inedible phytoplankton to zooplankton ("mycoloop"). This parasite‐mediated energy pathway might be of special importance during phytoplankton blooms dominated by inedible or toxic primary producers like cyanobacteria, which are on the rise with eutrophication and global warming. We theoretically investigated community dynamics and energy transfer in a food web consisting of an edible nonhost and an inedible host phytoplankton species, a parasitic fungus, and a zooplankton species grazing on edible phytoplankton and fungi. Food web dynamics were investigated along a nutrient gradient contrasting nonadaptive zooplankton species representative for filter feeders like cladocerans and zooplankton with the ability to actively adapt their feeding preferences like many copepod species. Overall, the importance of the mycoloop for zooplankton increases with nutrient availability. This increase is smooth for nonadaptive consumers. For adaptive consumers, we observe an abrupt shift from an almost exclusive preference for edible phytoplankton at low nutrient levels to a strong preference for parasitic fungi at high nutrient levels. The model predicts that parasitic fungi could contribute up to 50% of the zooplankton diet in nutrient‐rich environments, which agrees with empirical observations on zooplankton gut content from eutrophic systems during blooms of inedible diatoms or cyanobacteria. Our findings highlight the role of parasite‐mediated energy pathways for predictions of energy flow and community composition under current and future environmental change. [ABSTRACT FROM AUTHOR]

The contribution of dead zooplankton biomass to carbon cycle in aquatic ecosystems is practically unknown. Using abundance data of zooplankton in water column and dead zooplankton in sediment traps in Lake Stechlin, an ecological-mathematical model was developed to simulate the abundance and sinking of zooplankton carcasses and predict the related release of labile organic matter (LOM) into the water column. We found species-specific differences in mortality rate of the dominant zooplankton: Daphnia cucullata, Bosmina coregoni and Diaphanosoma brachyurum (0.008, 0.129 and 0.020 day−1, respectively) and differences in their carcass sinking velocities in metalimnion (and hypolimnion): 2.1 (7.64), 14.0 (19.5) and 1.1 (5.9) m day−1, respectively. Our model simulating formation and degradation processes of dead zooplankton predicted a bimodal distribution of the released LOM: epilimnic and metalimnic peaks of comparable intensity, ca. 1 mg DW m−3 day−1. Maximum degradation of carcasses up to ca. 1.7 mg DW m−3 day−1 occurred in the density gradient zone of metalimnion. LOM released from zooplankton carcasses into the surrounding water may stimulate microbial activity and facilitate microbial degradation of more refractory organic matter; therefore, dead zooplankton are expected to be an integral part of water column carbon source/sink dynamics in stratified lakes. [ABSTRACT FROM AUTHOR]

Mixotrophy is increasingly recognized as an important trophic pathway among phytoplankton, yet its underlying drivers remain largely unknown and unexplored. Here, we present a study utilizing 69 lake samplings in boreal Quebec, Canada, identifying variables driving the success of phytoplankton that have a capacity for mixotrophy and pointing to the underlying mechanisms. We found that the success of mixotrophs (% of total biomass) was positively influenced by both colored dissolved organic matter (cDOM) and dissolved CO2 concentration but limited by the abundance of crustacean zooplankton. The effect of cDOM manifested as a consequence of limited autotrophic phytoplankton biomass in lakes with reduced light penetration. We observed a nonlinear (u‐shaped) relation between CO2 and mixotrophs, with biomass favored at both low and high CO2 concentrations. A reduced fitness of mixotrophs at near‐atmospheric CO2 concentrations is likely owing to the costs of rapidly switching between or maintaining multiple trophic strategies. The abundance of zooplankton had a negative effect on mixotroph biomass but a positive effect on autotrophic phytoplankton. We also found that while the community composition of potentially mixotrophic phytoplankton was to some degree likely influenced by zooplankton biomass, composition was unaffected by the CO2 and cDOM gradients. Overall, this study highlights mixotrophy in boreal lake systems as a strategy of persistence, with the maintenance of a moderate but constant presence across a changing gradient of light and trophic conditions. The results of our study support the hypothesis that phytoplankton with a capacity for mixotrophy provide a superior and stable stoichiometric food source for zooplankton, implicating mixotrophs as a vital component of boreal lake food webs. [ABSTRACT FROM AUTHOR]

The oligotrophic subtropical gyre covers a vast area of the Atlantic Ocean. Decades of time-series monitoring have generated detailed temporal information about zooplankton species and abundances at fixed locations within the gyre, but their live/dead status is often omitted, especially in the dynamic subtropical convergence zone (STCZ) where the water column stratification pattern can change considerably across the front as warm and cold water masses converge. We conducted a detailed survey in the North Atlantic STCZ and showed that over 85% of the copepods were typically concentrated in the upper 200 m. Copepod carcasses were present in all samples and their proportional numerical abundances increased with depth, reaching up to 91% at 300–400 m. Overall, 14–19% of the copepods within the upper 200 m were carcasses. Shipboard experiments showed that during carcass decomposition, microbial respiration increased, and the bacterial community associated with the carcasses diverged from that in the ambient water. Combining field and experimental data, we estimated that decomposing copepod carcasses constitute a negligible oxygen sink in the STCZ, but sinking carcasses may represent an overlooked portion of the passive carbon sinking flux and should be incorporated in future studies of carbon flux in this area. [ABSTRACT FROM AUTHOR]

The sum of benthic autotrophic and bacterial production often exceeds the sum of pelagic autotrophic and bacterial production, and hence may contribute substantially to whole-lake carbon fluxes, especially in shallow lakes. Furthermore, both benthic and pelagic autotrophic and bacterial production are highly edible and of sufficient nutritional quality for animal consumers. We thus hypothesised that pelagic and benthic transfer efficiencies (ratios of production at adjacent trophic levels) in shallow lakes should be similar., We performed whole ecosystem studies in two shallow lakes (3.5 ha, mean depth 2 m), one with and one without submerged macrophytes, and quantified pelagic and benthic biomass, production and transfer efficiencies for bacteria, phytoplankton, epipelon, epiphyton, macrophytes, zooplankton, macrozoobenthos and fish. We expected higher transfer efficiencies in the lake with macrophytes, because these provide shelter and food for macrozoobenthos and may thus enable a more efficient conversion of basal production to consumer production., In both lakes, the majority of the whole-lake autotrophic and bacterial production was provided by benthic organisms, but whole-lake primary consumer production mostly relied on pelagic autotrophic and bacterial production. Consequently, transfer efficiency of benthic autotrophic and bacterial production to macrozoobenthos production was an order of magnitude lower than the transfer efficiency of pelagic autotrophic and bacterial production to rotifer and crustacean production. Between-lake differences in transfer efficiencies were minor., We discuss several aspects potentially causing the unexpectedly low benthic transfer efficiencies, such as the food quality of producers, pelagic-benthic links, oxygen concentrations in the deeper lake areas and additional unaccounted consumer production by pelagic and benthic protozoa and meiobenthos at intermediate or top trophic levels. None of these processes convincingly explain the large differences between benthic and pelagic transfer efficiencies., Our data indicate that shallow eutrophic lakes, even with a major share of autotrophic and bacterial production in the benthic zone, can function as pelagic systems with respect to primary consumer production. We suggest that the benthic autotrophic production was mostly transferred to benthic bacterial production, which remained in the sediments, potentially cycling internally in a similar way to what has previously been described for the microbial loop in pelagic habitats. Understanding the energetics of whole-lake food webs, including the fate of the substantial benthic bacterial production, which is either mineralised at the sediment surface or permanently buried, has important implications for regional and global carbon cycling. [ABSTRACT FROM AUTHOR]

Zooplankton carcasses are ubiquitous in marine and freshwater systems, implicating the importance of non-predatory mortality, but both are often overlooked in ecological studies compared with predatory mortality. The development of several microscopic methods allows the distinction between live and dead zooplankton in field samples, and the reported percentages of dead zooplankton average 11.6 (minimum) to 59.8 (maximum) in marine environments, and 7.4 (minimum) to 47.6 (maximum) in fresh and inland waters. Common causes of non-predatory mortality among zooplankton include senescence, temperature change, physical and chemical stresses, parasitism and food-related factors. Carcasses resulting from non-predatory mortality may undergo decomposition leading to an increase in microbial production and a shift in microbial composition in the water column. Alternatively, sinking carcasses may contribute significantly to vertical carbon flux especially outside the phytoplankton growth seasons, and become a food source for the benthos. Global climate change is already altering freshwater ecosystems on multiple levels, and likely will have significant positive or negative effects on zooplankton non-predatory mortality. Better spatial and temporal studies of zooplankton carcasses and non-predatory mortality rates will improve our understanding of this important but under-appreciated topic. [ABSTRACT FROM AUTHOR]

Bacterial community compositions (BCC) associated with the cladoceran Bosmina coregoni and the cyclopoid copepod Thermocyclops oithonoides in oligotrophic Lake Stechlin versus eutrophic Lake Dagow (northeastern Germany) were compared using molecular techniques. We also transplanted the zooplankton from their native lake to the other lake, and studied changes in their associated BCC as a result of the modified ambient environment. Bacterial community composition associated with B. coregoni was quite conservative between the oligotrophic and eutrophic lakes, and most of the bacteria belonged to Alpha- and Betaproteobacteria. In contrast, BCC associated with T. oithonoides was highly variable and dependent on the environment, and Betaproteobacteria and Bacteroidetes were the most represented among the bacteria. Thermocyclops oithonoides from the oligotrophic lake was more able to retain its bacteria after being transplanted to the eutrophic lake than vice versa. This suggests that bacteria in oligotrophic water were more firmly attached to the copepod and better in resisting environmental fluctuations than those in eutrophic water. [ABSTRACT FROM AUTHOR]

A considerable fraction of freshwater zooplankton was recently found to consist of dead specimens that sink to the lake bottom. Such carcasses host intense microbial activities that may promote oxygen depletion at the microscale. Therefore, we tested the hypothesis that sinking zooplankton carcasses are microsites of anaerobic nitrogen cycling that contribute to pelagic fixed-nitrogen loss even in the presence of ambient oxygen. Incubation experiments were performed with the ubiquitous copepods Eudiaptomus sp. and Megacyclops gigas at different ambient oxygen levels that sinking carcasses encounter during their descent in stratified lakes. 15N-stable-isotope incubations revealed intense carcass-associated anaerobic nitrogen cycling only at low ambient oxygen levels (<25% air saturation). Dissimilatory nitrate reduction to ammonium (DNRA) dominated over denitrification and thus the potential for fixed-nitrogen loss was low. Consistent with this partitioning of anaerobic nitrogen cycling, the relative abundance of the carcass-associated marker gene for DNRA (nrfA) was ∼20–400 times higher than that for denitrification (nirS). Additionally, the relative nrfA and nirS abundances were ∼90–180 times higher on copepod carcasses than in lake water. This functional distinctiveness of carcass-associated bacterial communities was further substantiated by 16S rDNA-based fingerprinting. We conclude that the unique bacterial communities and microenvironments provided by zooplankton carcasses influence pelagic nitrogen cycling in lakes, but mainly at seasonally low ambient O2 levels in the bottom water. [ABSTRACT FROM AUTHOR]

Using the carcass sinking rate and density determined in laboratory for several freshwater zooplankton species, we developed a model of zooplankton carcass sinking as affected by turbulence and stratification. The model was subsequently used to estimate the residence time of zooplankton carcasses in the water column of Lake Stechlin, a typical temperate dimictic lake in northeastern Germany. The residence time varied among the different species and was strongly affected by thermal stratification. At the peak of summer stratification, the carcasses stayed up to 5d in the 70m-deep water column before reaching the lake bottom. Residence time was long enough that zooplankton carcasses could serve as an important matter and energy source for bacteria in the lake's pelagic zone and hence have the potential to significantly affect aquatic carbon and nutrient cycling. The proposed model of sinking rates, based on physically sound relationships, can be easily applied to other passively sinking particles, and be integrated into large ecosystem models. [ABSTRACT FROM AUTHOR]

Understanding the fate of organic carbon (OC) inputs in aquatic ecosystems is of primaryimportance for comprehending their functioning, in particular in relation to their carboncycle. A large part of terrigenous OC, coming from the watershed, enters aquatic ecosystemsin the form of refractory organic matter (ROM), which are difficult to oxidise. Recent studiesin the literature have suggested, however, that ROM mineralisation rates may be enhanced inpresence of labile organic matter (LOM), making ROM compounds easier to degrade, with aconsequent alteration of the carbon budget in those systems. This process refers to as primingeffect. In lakes, zooplankton carcasses represent an abundant and high quality LOM substrate formicroorganisms that has the potential of priming ROM mineralisation in water column andcreating new and overlooked pathways in the lake carbon cycle. To investigate this, it iscrucial to measure the in-situ sedimentation rate and residence time of dead zooplankton, as afunction of hydrodynamic conditions, microbial decomposition and organisms&#x2019;characteristics (such as size and density). Current velocimetry acoustic methods cannot measure, however, velocities of singlemillimetre-sized carcasses because particles are too small and measurements are not accuratedue to the low correlation of the acoustic pings. To overcome this, we propose an inexpensiveparticle tracking velocimetry (PTV) system to estimate the instantaneous 3D velocity ofparticles in the field. From the data it is possible to measure sinking rates of deadzooplankton and potentially infer carbon fluxes in lakes in relation to the primingeffect. The PTV rig consists of two GoPro cameras arranged in a frontal parallel arrangementand installed in a waterproof case. The case is mounted on a stainless-steel frame with alaminate blank screen placed in front of the cameras to restrict the system sampling volume.A torch was also attached on the frame to enhance lighting underwater conditions. Thecameras are set up to acquire stereoscopic films of sinking particles at 48 frames per secondand calibrated in a calibration tank to estimate their distortion and rectification parameters. Asoftware package was also developed with new stereo algorithms to sync the videos,detect and track carcasses, as they move in the water, and measure their velocity andlength. In this study, we also present early results of sedimentation rates from the PTV system,estimated from 2 deployments in summer 2018 in the mesocosm of Lake Stechlin (Germany)at 7m. Finally, the settling rates are compared with (1) those inferred from the Stokes&#x2019; lawand (2) the daily settling rates estimated from sedimentation traps installed in the mesocosmat the same depth. [ABSTRACT FROM AUTHOR]