Limnology and Oceanography最新文献

Rivers transform and transport much of the organic input they receive from terrestrial ecosystems. This carbon sustains stream food webs and fuels the production and release of carbon dioxide and methane to the atmosphere. Warming water temperatures and intensification of the hydrologic cycle due to climate change are likely to affect these carbon transformations and downstream transport in streams. Here, we examine the natural variability and long-term shifts in the metabolism of New Hope Creek, North Carolina, USA, site of the earliest published estimates of a stream's annual metabolic regime in 1969. We estimated annual ecosystem metabolism over 3 yr (2017–2020) and used the variability observed in the modern dataset to provide context for interpreting long-term change in response to climate drivers. We found that New Hope Creek was heterotrophic in all years, with highly seasonal carbon cycling. Much of the modern variability can be explained by water temperature and flow conditions. Warmer temperatures and longer periods of low flow conditions led to faster carbon cycling and increased heterotrophy, while autumn floods suppressed annual ecosystem respiration by reducing river carbon stocks. Comparing modern estimates to those from 50 yr ago, we find that New Hope Creek is now substantially warmer and has higher metabolic fluxes. Despite the limitations of inferring trends between two distant time points, we use modern data to hindcast metabolism and show how climate change has likely accelerated carbon cycling and shortened carbon residence time in New Hope Creek.

Bacterial communities associated with animals show complex spatial and temporal variation. The main driving forces behind this variation are still to be deciphered. Differences in microbiome composition could be caused by stochastic changes, such as random gain and loss of microbiome components, as well as deterministic factors, such as variation in temperature (or other abiotic factors), diet, or the availability of microbes with the potential to colonize the hosts in the surrounding environment. Here we investigated seasonal variation in the microbiome of Hydra polyps and the bacterioplankton surrounding them to test the hypothesis that the contribution of environmental microorganisms to host-associated microbial communities varies seasonally. Sampling was performed for two consecutive years in three distinct temperate water bodies in Eastern Hungary: a shallow lake, a deep lake, and a river. We found that the microbiomes of polyps differed from their surrounding environment and varied seasonally. The similarity of polyp and water microbiomes changed seasonally in a population-specific way: microbial communities associated with polyps became markedly more similar to that of their surrounding environment during the summer in the shallow lake habitat, but not in the other populations. Our results suggest that environmental and host-associated microbiomes change independently during most of the year, but high temperature increases the impact of environmental microbiome on host-associated microbial communities.

Recent experimental and modeling work predicted salt fingers, known in saline water bodies, would form under ice in freshwater lakes with specific conductance (SC) as low as 50 μS cm⁻¹. To test this prediction, Toolik Lake, Alaska (summer SC 60–90 μS cm⁻¹) was instrumented with temperature-conductivity arrays. Calculations of solutes excluded with ice formation and a mass balance of changes in concentration of solutes within the lake indicated 90% to 100% of increase in solutes for several months following ice-on was from cryoconcentration. Two metrics based on the ratio of density gradients of temperature and solutes, R_ρ and the Turner angle (Tu), obtained by conductivity, temperature, depth (CTD) and microstructure profiling, and Ɍ, ratio of solute and heat fluxes at the ice-water interface, had values indicative of salt fingers below ice. R_ρ and Tu were in the range for salt fingers and the diffusive mode of double diffusion in intrusive-features in lower water column. Step-like changes in temperature and SC provide further evidence for double diffusion. Rates of dissipation of turbulent kinetic energy below ice and in intrusions were between 10⁻¹² and 10⁻¹⁰ m² s⁻³. Increases in SC above the sediments following ice-on at sites 4, 10 and 15 m deep in the 24 m deep lake imply that salt fluxes created localized increases in density conducive for intrusive flows. These results are the first for freshwater lakes illustrating formation of salt fingers and complex intrusive flows and indicate the need to revise models of under-ice circulation.

The biological carbon pump mediates the export of particulate organic carbon from the euphotic zone to the deep ocean, where it provides the base of the food web. Although deep-sea microbial metabolism is considered to be largely associated with macroscopic particles, such as marine snow, the specific contribution of particle-associated microorganisms to the utilization of bulk organic matter has rarely been directly quantified. We used in situ pumps to collect particles larger than 3 μm from mesopelagic and bathypelagic waters along a latitudinal transect in the North Atlantic. Prokaryotic abundance, respiration, heterotrophic biomass production, and community composition were determined and compared to the bulk prokaryotic community collected by Niskin bottles. Although particle-associated prokaryotes represented less than 1% of bulk prokaryotic abundance, they contributed on average 28% to bulk prokaryotic respiration and 12% to biomass production. The organic carbon turnover time of particles mediated by prokaryotes was 0.5–1.5 months, while it was up to 3 yr for the total organic carbon fraction. Thus, particles represent hotspots of organic carbon remineralization in the mesopelagic and bathypelagic ocean. Furthermore, metagenomic analyses revealed clear differences in taxonomy and diversity between the free-living (0.2–0.8 μm) and particle-associated (> 3 μm) prokaryotic communities. Our results emphasize the significant role of particle-associated prokaryotes in driving organic matter utilization in the dark ocean.

The Yellow Sea (YS) and East China Sea (ECS) are marginal seas in the Northwestern Pacific that receive large amounts of aged, terrestrial organic matter. In this study, we measured dissolved organic carbon (DOC) concentrations and radiocarbon contents (Δ¹⁴C) in these seas during summer and autumn, extending a previous winter study to provide a more comprehensive understanding of the DOC cycle, including its sources and removal. The significant negative correlation between DOC concentrations or Δ¹⁴C values and salinity shows that vertical and horizontal water mass mixing between coastal waters and the water intruding to the site from the Northwestern Pacific is the primary control on the distribution of DOC. The Δ¹⁴C values and the inverse of DOC concentrations show significant negative correlation, suggesting that marine primary production is the dominant DOC source in this region. However, deviations from this correlation imply inputs of aged DOC. Although freshwater input is highest in summer, the effects of aged DOC are greater in autumn and winter. Terrestrial organic matter delivered by rivers is rapidly degraded, and this process likely stimulates marine primary production. In addition, large seasonal differences in Δ¹⁴C values in Kuroshio-derived waters indicate significant removal of marine refractory DOC on the continental shelf. The results show that continental shelves have a key role in the removal of terrestrial and marine refractory DOC.

Climate change-induced glacier retreat leads to the reorganization of fluvial landscapes in proglacial terrains and transitions between streams fed predominantly by glacial meltwater and groundwater. To explore the effects of such ecosystem transitions on benthic biofilm communities, we gradually mixed water from a glacier-fed stream (GFS) and a groundwater-fed stream (GWS) in 30 stream-side flume mesocosms. Over 70 days, we studied how microbial biomass and community composition responded to changes in water sources compared to the respective controls. Biofilms responded readily to shifting water sources, with increased algal and bacterial biomass as GFS influence diminished, supporting previous reports of GFS “greening” as glacial influence is reduced. Bacterial community composition exhibited rapid and sensitive responses to the gradual transition between GFS and GWS, with an observed convergence between communities receiving the same water mixture. Partitioning temporal changes in bacterial communities revealed that increases in taxa abundance primarily underly compositional responses, indicating that taxa present in both stream types respond to changes. Piecewise Structural Equation Models suggest that changes in water source directly (through changes in nutrient availability) and indirectly (through benthic algal biomass) drive the observed compositional responses. Our experimental insights provide evidence for the “greening” of proglacial streams and shed new light on the sensitivity of benthic microbial communities to ecosystem transitions in proglacial floodplains.

Mangroves offer substantial carbon sequestration, acting as nature-based climate solutions. Yet, soil carbon dioxide (CO₂) and methane (CH₄) emissions partially offset these benefits. Despite many studies on emission patterns and drivers, lacking source-specific partitioning hinders deeper mechanistic insights. Here, a 1-yr in situ experiment using deep collar insertion in Kandelia obovata and Avicennia marina forests partitioned soil CO₂ and CH₄ fluxes into heterotrophic and root-affected sources, examining seasonal dynamics, temperature sensitivity (Q₁₀), and soil properties controls. Soil–air carbon fluxes, except for root-affected CH₄, were lowest in winter and peaked in summer or autumn. Soil and root-affected CH₄ fluxes were significantly higher in A. marina forests than in K. obovata forests annually or seasonally. The annual flux ratio of root-affected CO₂ to soil CO₂ averaged 39%, and was relatively 19% higher in A. marina forests than in K. obovata forests. Soil properties collectively explained 64%, 62%, and 36% of variation in soil, heterotrophic, and root-affected CO₂ fluxes, respectively, but only 3%, 22%, and −9% for corresponding CH₄ fluxes. The Q₁₀ of soil CH₄ fluxes was significantly higher in A. marina forests than in K. obovata forests, and root-affected CO₂ fluxes had a higher Q₁₀ than heterotrophic CO₂ fluxes only in A. marina forests. These findings reveal mangrove species-specific differences in the magnitude and Q₁₀ of soil–air carbon fluxes, underscoring mangrove species as key to assessing climate benefits and guiding restoration. We also emphasize the role of soil conditions and flux partitioning in predicting soil CO₂ and CH₄ fluxes, respectively.

Benthic microalgal (BMA) communities contribute significantly to food webs, nutrient cycling, and carbon flows in intertidal habitats. However, the contribution of BMA to saltmarsh carbon stocks (“blue carbon”) is unclear. BMA and sediment total organic carbon (TOC) stocks were measured in an east coast American Atlantic saltmarsh, revealing key relationships between biofilm biomass, carbohydrate, and carbon content. BMA biomass (chlorophyll a) was highest in Sporobolus stands and mudflat habitats, with diatoms the dominant algal group, and cyanobacteria more important in upper saltmarsh sites. Habitat-specific differences in biofilm properties (biomass, carbohydrates, photopigments, near-infrared spectra) corresponded to differences in overall contributions to sediment TOC. Carbohydrates contributed between 8% and 23% of sediment TOC, with the highest levels in Sporobolus and mudflat habitats. BMA biomass and colloidal carbohydrate were significantly correlated, except on lower shore sandflats. The greatest relative contribution of colloidal carbohydrate to %TOC was in upper marsh and tidal channel habitats (1%). Mudflats had the highest %TOC (up to 5% dry weight), but TOC stocks (2000 g C m⁻² to a depth of 10 cm) were highest in Sporobolus habitats. A modeling approach, based on LIDAR and sediment measures, determined a BMA carbon contribution of 1.3–8% of sediment TOC, with the lowest values in Sporobolus and mudflat habitats. Upscaling from m², incorporating habitat heterogeneity, gave median values of 14–16 t TOC ha⁻¹ for the North Inlet Estuary saltmarshes, of which BMA contributed 0.06–0.08 t C ha⁻¹. This approach could permit BMA contributions to blue carbon to be estimated across other saltmarshes.

Marine dissolved organic matter (DOM) represents a primary reservoir in the biogeochemical cycle, and marine microorganisms are essential to the transformation and long-term sequestration of DOM as recalcitrant dissolved organic matter (RDOM). In China's marginal seas, DOM levels are affected by coastal productivity and terrestrial inputs, yet the molecular mechanisms driving the DOM to RDOM transformation remain insufficiently characterized. This study aimed to elucidate the mechanisms behind the DOM transformation mediated by marine microorganisms in the Bohai and Yellow Seas, particularly focusing on molecular-level characterizations of microbial carbon cycling processes. Here, using 16S rDNA amplicon sequencing, we analyzed the bacterial communities across the surface and deep layers. Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) was used to molecularly characterize the DOM. Our findings revealed distinct bacterial diversity and functional profiles between the surface and deep layers, with deep layers exhibiting higher microbial diversity. Furthermore, the deep layers were characterized by higher proportions of RDOM, with molecular indicators such as carboxyl-rich alicyclic molecules (CRAM) suggesting enhanced carbon stability. This study highlights the role of microbial processes in shaping the molecular characteristics of DOM across depths, supporting the microbial carbon pump (MCP) framework and characterizing the Bohai and Yellow Seas as significant carbon sinks in the coastal region. These findings advance our mechanistic understanding of oceanic carbon sequestration, particularly in coastal marginal seas.

Lagoons are recognized as significant CO₂ sources in the global carbon cycle. However, the lack of comprehensive measurements capturing simultaneous spatiotemporal variations in partial pressure of CO₂ (pCO₂) limits our understanding of mechanisms driving CO₂ dynamics in lagoons. In this study, autonomous buoys were deployed at five stations across Chiku Lagoon (Taiwan) during the wet season, continuously measuring temperature, salinity, and pCO₂ for over 24 h. Discrete water samples of total alkalinity and dissolved inorganic carbon were collected to establish a freshwater-to-seawater mixing model. Our analysis revealed that biological activity accounted for most of the pCO₂ variability (59%), followed by mixing processes (36%) and the temperature variations effect (5%). These effects contributed to spatial heterogeneity, with the upper lagoon exhibiting stronger emissions (4.8 ± 2.5 mmol m⁻² h⁻¹; mean ± standard deviation) than the middle and lower lagoon (0.6 ± 0.8 to 1.4 ± 1.3 mmol m⁻² h⁻¹). Meanwhile, tidal influences on CO₂ fluxes were evident, with emissions at low tide (1.4 ± 0.5 mmol m⁻² h⁻¹) nearly double those at high tide (0.6 ± 0.3 mmol m⁻² h⁻¹). On average, all stations acted as net sources of atmospheric CO₂ over the sampling period (1.2 ± 1.2 mmol m⁻² h⁻¹). A resampling sensitivity test of the high-resolution buoy data suggests a 3-h interval is optimal in biologically and tidally driven lagoons such as Chiku. These results provide a framework for understanding spatiotemporal CO₂ dynamics and serve as a guide for future monitoring and carbon management strategies in coastal environments.