Limnology and Oceanography最新文献

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.

Predator–prey interactions are important drivers of adaptation in aquatic communities, shaping the behavior of invertebrates with cascading effects on community dynamics. Behavioral responses, such as moving with the downstream current (drift) or altering the timing of emergence, are strategies that reduce the risk of predator encounters. These prey responses have been extensively studied over the last decades. However, much of the previous research was conducted in laboratory settings or focused on individual taxa, potentially overlooking the complexity of natural predator–prey dynamics. To address this, we here examined the effects of indirect and direct predation pressure on drift behavior and emergence patterns in a natural macroinvertebrate community using an outdoor mesocosm system (ExStream). This mesocosm system enables a more ecologically realistic assessment of these interactions. We analyzed how the number and size of drifting invertebrates and emerging insects changed with elevated levels of chemical fish cues (indirect predation, Gasterosteus aculeatus and Cottus rhenanus) and subsequent additional direct predation by C. rhenanus over 9 d. Within the framework of the ExStream system, we demonstrated that most invertebrate groups displayed increased drifting behavior to indirect, direct, or both forms of predation. Furthermore, our study revealed a significant predator impact on emergence patterns, reducing the number and size of emergent insects. Drift and emergence are key processes in invertebrate dispersal and contribute to the transfer of resources to downstream habitats and adjacent terrestrial systems. Our findings show that predation affects both processes, altering community dynamics and potentially reshaping resource availability across ecosystem boundaries.

Understanding groundwater–surface water connectivity in tidal freshwater zones is essential for managing ecosystems within the riverine–estuarine continuum. Groundwater discharge is a key source of dissolved nutrient loads that can lead to eutrophication, deoxygenation, and other ecosystem imbalances in coastal environments. However, groundwater is an often-overlooked nutrient delivery pathway, with limited studies quantifying groundwater inputs into freshwater transitional zones. Here, we quantify groundwater discharge and its associated nutrient fluxes along a ≈ 75 km section of a tidal freshwater zone within the Hawkesbury–Nepean River (New South Wales, Australia), which is surrounded by multiple land uses. High-resolution timeseries and spatial surveys were employed over three hydrologically distinct seasons using the naturally occurring radioisotope tracer, radon. Survey results revealed decreasing surface water radon activity and nutrients toward the downstream estuarine boundary. Estimated groundwater discharge rates were highest in the upstream reaches (6.0–12.5 cm d⁻¹), regardless of seasonal hydrological dynamics, and decreased downstream, aligning with a reduction in irrigation-dependent agricultural practices and a change in adjacent catchment morphology. During the dry period, groundwater accounted for up to ≈ 68% of upstream river inputs. Groundwater discharge was also a major source of dissolved inorganic nitrogen, contributing between 0.8 and 1.1 times the inputs from local wastewater treatment plants, and up to 15 times more phosphate than upstream river sources. These findings highlight the role of groundwater discharge as a nutrient driver for tidal freshwater zones, which could have significant implications for future management strategies in these environments.

The biogenic structure of foundation species provides critical habitat for coastal marine taxa. In temperate ecosystems, forests of large, canopy-forming macroalgae (Macrocystis pyrifera, Nereocystis luetkeana) support rich communities of fishes and macroinvertebrates. However, it is unclear how the biological assemblages of kelp forests vary with respect to the dominant canopy-forming species, structural attributes of the kelp forest (e.g., stipe density, frond length, forest biomass, forest area), and ambient environmental conditions. To identify the main influences on the composition of kelp-associated assemblages, we surveyed 21 kelp forests across an island archipelago in British Columbia. We found that the composition of fish and macroinvertebrate assemblages varied significantly with canopy kelp species. The assemblages of macroinvertebrates were significantly influenced by kelp species, frond length, and forest biomass, as well as some abiotic attributes (i.e., depth, hard substrate). In contrast, the pelagic–demersal and benthic fish assemblages were associated with single attributes of kelp forest structure (i.e., frond length and canopy kelp species, respectively). Our findings indicate that the biological assemblages that are associated with kelp forests, especially macroinvertebrates, may be closely tied to structural features of the canopy. By identifying the distinct assemblages fostered by kelp forests of different canopy species and structure, we can better interpret the response of marine communities to changes in coastal habitat.