Limnology and Oceanography最新文献

Variable chlorophyll a (Chl a) fluorescence, a standard tool for assessing phytoplankton physiology, offers a wealth of information on photosynthetic performance and primary productivity through noninvasive means. However, a better understanding of diurnal patterns in photosynthesis is critically important as advanced fluorescence techniques are increasingly used to monitor coastal and oceanic primary productivity rates in situ. In this study, we coupled a custom-made, Fast Repetition Rate Chl a fluorometer to an algal culture system (photobioreactor) to monitor the photosynthetic response of two strains of the green alga, Micromonas commoda, under highly regulated light and CO₂ conditions. When comparing diel profiles, our results reveal notable differences in the quantum yield of PSII and reoxidation kinetics across light patterns despite exposure to the same integrated photon dose per day. Specifically, CCMP2709 experienced a larger reduction in the quantum yield of PSII during the dark period, likely reflecting elevated chlororespiratory activity. This suggests that diel light patterns with rapid shifts in irradiance can bias phytoplankton toward the use of alternative electron pathways; such shifts can alter energetic budgets thereby leading to physiological changes. Our results also indicate a light-profile dependent response to elevated CO₂ conditions, as reductions in cell size, relative Chl a fluorescence per cell, and side scatter are observed under the gradual, but not rapid changes in light intensity. These findings emphasize the utility of high-resolution tools for monitoring photosynthetic performance in algal research, and the need to account for diel light patterns when comparing physiology among species and/or experimental conditions.

The shelf-break front of the Mid-Atlantic Bight (MAB) is hypothesized to cause increased biological productivity but observations of such enhancement have been scarce. Additionally, most previous studies at the MAB are based on chlorophyll rather than on actual observed rates of production. Here we present rates of net community production (NCP) and concentrations of phytoplankton carbon, collected at high spatial resolution (< 1 to 3 km) on repeated crossings of the shelf-break front in spring and summer during 4 yr on the Northeastern US Shelf. We find a localized 50% increase in NCP within 5 km of the shelf-break front on a majority of transects in early spring; there is a smaller and less frequent increase during later spring and summer. Phytoplankton carbon usually increased very close to the shelf-break front in the same transects. These data provide an unprecedented look at spatial patterns in productivity rates at the shelf-break front and show that strong increases in productivity occur at the front but also that such responses are likely short-lived and occur over short spatial scales, explaining why they may have been missed in other studies.

Tidal hydrodynamics influence mud suspension, flocculation, and particle settling behaviors, which are essential to sediment transport, nutrient cycling and morphological evolution in estuarine systems. This study examines near-bottom floc dynamics in a microtidal estuary under contrasting spring and neap tides, integrating field observations with advanced analytical techniques. Data from multiple field-deployed instruments were analyzed using a multi-peak fitting method to characterize particle size distributions, suspended floc characteristics, and dynamics. Spring tides, characterized by strong turbulence and minimal salinity stratification, resulted in dynamic transitions between microflocs and macroflocs, producing loose floc structures (fractal dimension ~ 1.77), faster settling velocities (~ 1 mm s⁻¹), and elevated sedimentation flux (~ 0.092 g m⁻² s⁻¹). In contrast, neap tides exhibited weaker flows and pronounced salinity stratification, favoring gradual aggregation of flocculi and microflocs into compact flocs (fractal dimension ~ 2.25) with slower settling velocities (~ 0.57 mm s⁻¹) and reduced sedimentation flux (~ 0.0182 g m⁻² s⁻¹). Suspended sediment concentration, turbulent shear and salinity emerged as key drivers, influencing floc size, volume and aggregation processes. These findings refine the understanding of cohesive sediment transport in tidally influenced estuaries and provide process-based parameters for improved morphodynamic and biogeochemical modeling in nature.

The impact of climate change stressors on marine copepods, key organisms at the base of ocean food webs, remains understudied. This study examined how warming affects their functional and numerical responses, critical life history traits linked to fitness. We conducted laboratory experiments on the calanoid species Paracartia grani, exposing it to a +6°C increase at two temporal scales, short term (4 d) and long term (21 generations; ca. 1 yr), and compared with the control at 19°C. We observed distinct patterns between warming treatments. Short-term warming increased intake rates and satiation levels, causing a mismatch between the functional and the numerical responses. Additionally, egg size and phosphorus body content declined, collectively resulting in reduced reproductive efficiency at high food concentrations. Long-term warming shifted satiation to higher food concentrations, possibly due to decreased foraging capacity (i.e., maximum clearance rate) in the smaller, warm-reared copepods; in turn, maximum feeding rates indicated physiological compensation, and reproductive efficiency showed no consistent pattern. High food availability altered the copepod elemental content, which appeared to be linked to reproductive effort. Short-term warming raised C : P and N : P ratios, while C : N ratios remained stable across all treatments. Our findings reveal how warming and resource availability interact to shape copepod performance and elemental fluxes, and highlight the importance of incorporating time-scale dependent thermal effects into ecosystem models for better forecasting in a context of climate change.

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.