Limnology and Oceanography最新文献

Agulhas leakage, a key driver of global ocean circulation, funnels warm and salty water from the Indian Ocean to the Atlantic Ocean, significantly influencing climate. However, the mechanisms by which mesoscale eddies associated with Agulhas leakage heat transport remain incompletely understood. By analyzing eddy data and Argo profiles from 1993 to 2018, we showed the three-dimensional structures of mesoscale eddies, which are critical for understanding their role in heat and salt transfer to the Atlantic meridional overturning circulation. Our findings highlight the distinct roles of anticyclonic and cyclonic eddies in meridional heat transport—primarily via propagation (~ 58%) and stirring of isotherms (~ 25%), respectively. We also show that cyclonic stirring can modify the thermal environment surrounding anticyclones, highlighting its previously underappreciated role in heat transport. These findings enhance our understanding of how Agulhas leakage eddies contribute to interbasin heat flux and highlight the importance of accounting for both trapping and stirring processes when assessing their influence on the Atlantic meridional overturning circulation and broader climate variability.

Climate change significantly threatens coastal ecosystems, making effective conservation strategies essential and reliant on accurate data. This study employed the Sylt mesocosms facility to examine how coastal marine communities respond to both individual and combined stressors. Within the broader context of this research, environmental variables such as pH, temperature, and nutrient levels in the mesocosms were continuously monitored and compared with field data. In addition, growth rates of blue mussels (Mytilus edulis) and Pacific oysters (Magallana gigas) were assessed, along with the body condition of M. edulis, to evaluate the functionality of the facility. While the water temperature in the mesocosms remained closely regulated according to the programmed set points, the pH and nutrient levels varied in a manner similar to natural field conditions. The concentration of chlorophyll a, used as an indicator of food availability within the tanks, was lower than in the field but did not significantly influence the growth rates and conditions of the filter feeders. The facility demonstrated its effectiveness throughout the experimental period in controlling temperature, highlighting its potential for future research on the effects of single and multiple stressors in near-natural conditions. These findings highlight the importance of mesocosms as a research tool for understanding the complex dynamics of marine ecosystems facing climate change.

Floodplains are dynamic interfaces between aquatic and terrestrial ecosystems, where ecosystem functioning is strongly influenced by microbial communities. To investigate the composition of free-living and particle-associated prokaryotic and microbial eukaryotic communities, five interconnected study sites were sampled in one of the best-preserved Danube floodplains and subsequently analyzed using 16S and 18S rRNA gene amplicon sequencing. We compared community dynamics across low-water periods and minor to moderate floods and observed flooding to increase microbial diversity and promote gradual community shifts depending on flood intensity, whereas low-water conditions limited microbial exchange and reduced compositional connectivity across floodplain ecosystems. Dispersal effects were particularly pronounced in microbial eukaryotes, including Perkinsea and Fungi, pointing to the importance of hydrological connectivity in structuring micro-eukaryotic communities. Flooding also facilitated community mixing and more balanced interspecific interactions, while low-water periods led to more compartmentalized networks. Core microbial community size increased with flooding intensity, reflecting the influence of ecosystem mixing, allochthonous inputs, and increased nutrient availability in shaping floodplain communities. This study highlights the effects of flooding intensity on both prokaryotic and microbial eukaryotic communities, advancing our understanding of how hydrological variability shapes microbial dynamics in riverine floodplains.

Tidal freshwater wetlands are critical for removing or sequestering watershed-derived nitrogen loads before they reach the coast, where they can lead to eutrophication. However, rising seas and increasing climate variability will alter important physicochemical parameters that control nitrogen generation (e.g., nitrogen fixation) and removal processes (e.g., denitrification) in these habitats. Furthermore, the frequency and timing of these changes could vary from short, finite pulses during a storm or drought to long-term presses from sea level rise, which may differentially affect biogeochemical cycling. We used intact core mesocosms to examine how microbial community structure and nitrogen cycling changed in response to increased temperature and salinity under pulse and press disturbances. We found that net N₂ flux rates, defined as the balance between nitrogen fixation, which adds nitrogen, and denitrification, which removes it, did not directionally change in response to stressor pulse or press. Instead, it became more variable under both disturbance regimes, underscoring the importance of both denitrification and nitrogen fixation in these systems. Nitrous oxide production rates, however, decreased and became more stable over time in the press scenario but remained highly variable in the pulse scenario. Under both pulse and press disturbance, both the overall and the active component of the microbial community changed, particularly in response to the salinity treatment. Although there was an overall community shift, core members of the microbiome capable of denitrification and nitrogen fixation persisted. Both pulses and presses of temperature and salinity changed the microbial communities of tidal freshwater wetlands, but a combination of microbial resistance and functional redundancy appears to allow important N cycling processes to persist. These findings provide valuable knowledge on the functional and structural potential of the nitrogen cycling microbial communities in tidal freshwater wetlands when facing future climate variability.

Cable bacteria are filamentous microorganisms capable of centimeter-scale electron transport, which have great impacts on sediment biogeochemistry, especially oxygen consumption and sulfide depletion. While 16S rRNA sequences related to known cable bacteria have been identified in saline lakes, their genomic diversity, metabolic potentials, and evolution remain unknown. Eight cable bacteria genomes were retrieved from 23 sediment metagenomes across four saline lakes, representing five novel species adapted to different salinity niches. A deep-branching Electronema species, named Electronema qinghaiense, was found preferentially in brackish to saline environments, implying an ecological and evolutionary link between marine and freshwater lineages. Based on genome analysis, the three newly named cable bacteria species are likely mixotrophic diazotrophs capable of degrading diverse complex carbohydrates, while also participating in hydrogen metabolism via various groups 3 and 4 [NiFe]-hydrogenases. Genome streamlining and horizontal gene transfer likely drove ecophysiological differentiation among these Electrothrix and Electronema species, including an interphylum horizontal transfer of glycine/sarcosine N-methyltransferase (gsmt) and sarcosine/dimethylglycine N-methyltransferase (sdmt) genes into their common ancestor. Subsequent loss of these genes in some descendants led to adaptation to different salinity niches. Given the inferred ancestral physiological properties, phylogenomic analysis and the evidence that “freshwater” Electronema species experienced stronger purification selection than “saline” Electronema and “hypersaline” Electrothrix species, the evolutionary progression of cable bacteria occurred most likely in the saline-to-freshwater direction. Additionally, cable bacteria ecotypes adapted to specific salinity niches likely formed from selective sweeps with low homologous recombination. Collectively, these findings deepen our understanding of the ecophysiology and evolution of cable bacteria.

Inland waters release significant amounts of carbon into the atmosphere, with small ponds acting as hot spots. High variability and limited research make emissions from small waterbodies a major source of uncertainty, especially in underrepresented tropical ecosystems where unique drivers remain poorly understood. We evaluated the magnitude and sources of variability in emissions from small waterbodies of the páramo—a tropical ecoregion in the Andes mountains, characterized by carbon-rich soils. We measured partial pressure of carbon dioxide (pCO₂), methane (pCH₄) and CO₂ emissions from small (< 5000 m²) waterbodies, 11 ponds and 1 wetland, 3 times in the wet season and returned to 8 sites in the dry season. Sites were always supersaturated in pCH₄ (1096 ± 1482 μatm), but occasionally undersaturated in pCO₂ (1224 ± 1585 μatm). Variability between ponds was high and primarily driven by elevation and water temperature. Catchment soil-water connectivity was also predictive of pCO₂. Mean wet-season emission rates were 0.34 ± 0.54 g CO₂-C m⁻² d⁻¹ and 0.012 ± 0.018 g CH₄-C m⁻² d⁻¹ and surface area fluctuations were a large source of seasonal variability in some ponds. Though an open-water transect of the wetland site was similar to ponds, we measured very high pCH₄ (1678 ± 2629 μatm) and pCO₂ (5162 ± 3207 μatm) along the wetland perimeter. Our findings provide essential insights for incorporating a significant yet understudied tropical ecosystem into the global carbon budget by confirming previous observations that small ponds can emit a disproportionately large amount of carbon to the atmosphere, but also highlighting the importance of variables other than pond size in controlling emission hot spots.

This multi-year study highlights the ecological role of diversity and fatty acid availability in phytoplankton communities and their complex interactions with zooplankton. These findings, achieved through large-scale mesocosm experiments, provide novel evidence of the critical role of biochemical components in shaping zooplankton community composition in natural environments, which short-term and less complex laboratory studies cannot reveal. Over several years in a meso-oligotrophic lake, we investigated how phytoplankton diversity affects zooplankton growth and community composition via fatty acid availability. Results show that changes in phytoplankton fatty acid profiles influenced zooplankton abundance. Physiologically essential polyunsaturated fatty acids—arachidonic, eicosapentaenoic, and docosahexaenoic acids—were crucial for the presence of key zooplankton groups, including copepods and Daphnia. The study also revealed that different zooplankton species exhibited varied responses to fatty acids, highlighting the importance of dietary complementarity within zooplankton communities. While the findings largely support previous laboratory studies, they also reveal unique complexities in natural systems, where other factors may modulate the effects of fatty acids. This research underscores the significance of considering fatty acid profiles in understanding phytoplankton–zooplankton interactions and offers experimental evidence on the ecological consequences of changes in phytoplankton diversity in natural complex communities.

Marine copepods are the most abundant multicellular zooplankton in the global oceans. They imprint their surrounding waters with a unique bouquet of chemical compounds, including polar lipids such as copepodamides. Prey organisms can detect copepodamides and respond by inducing defensive traits including bioluminescence, toxin production, changes in colony size, and structural modifications. This mechanism has been suggested to contribute to harmful algal bloom formation, but to date only a limited number of species and strains have been experimentally exposed to copepodamides. Here, we quantify bioluminescence and toxin content in response to increasing concentrations of copepodamides in three harmful algal bloom-forming species of marine dinoflagellates: Alexandrium catenella, Protoceratium reticulatum, and Gymnodinium catenatum. All three species up-regulated their defensive traits in response to copepodamide exposure, including the first example of copepodamide-induced GC-toxin production. Neither bioluminescence nor toxin production was associated with measurable costs in terms of reduced growth rates. The results support the role of copepodamides as general alarm cues in marine phytoplankton. Moreover, the expression of simultaneous defensive traits may confound studies addressing the costs and benefits of these co-varying traits.

Coastal eutrophication results from increased riverine loads of inorganic nutrients, including phosphorus (P), which may co-occur with increased dissolved organic carbon (DOC) loading. These DOC molecules are often pigmented, causing water darkening, but they also contain dissolved organic P (DOP), which could exacerbate eutrophication. However, it is unclear how the bioavailable DOP (BDOP) pool responds to the individual and interactive effects of increased DOC, higher inorganic nutrient concentrations, and water darkening in coastal ecosystems. To explore these interactions, we conducted bioassays to estimate BDOP in a fully factorial mesocosm experiment manipulating the supply of labile DOC (glucose), inorganic nutrients and pigmented compounds that cause darkening. Whereas the evidence for labile DOC (glucose) effects on BDOP was weak, inorganic nutrient enrichment caused increases in BDOP concentrations in clear-water mesocosms. By contrast, in experimentally darkened waters, the addition of inorganic P did not contribute to BDOP but mainly persisted in its inorganic form. Our results suggest that water management efforts aimed at preventing or reversing coastal darkening could increase the removal of excess inorganic P from the water due to light-enhanced algal uptake. However, the total dissolved bioavailable P pool may not decrease but rather shift from dominance by inorganic to organic forms. Therefore, mitigating both coastal darkening and eutrophication in these ecosystems is essential for reducing total bioavailable P to a level that supports their ecological balance and functionality.