Limnology and Oceanography最新文献

Heterotrophic diazotrophs are potentially important to the nitrogen cycle in freshwater ecosystems, yet their abundance, N₂ fixation rates, diversity, and association with aggregates remain poorly understood. This study elucidates the contribution of freshwater heterotrophic diazotrophs as free-living or aggregate-associated cells to total N₂ fixation along the Jordan River-to-Lake Kinneret continuum. Heterotrophic diazotrophs ranged between 0.4 × 10⁷ and 6.4 × 10⁷ cells L⁻¹, accounting for 25–56% of the total unicellular diazotrophs. N₂ fixation rates by heterotrophic diazotrophs varied along the river (0.1–0.4 nmol N L⁻¹ d⁻¹), contributing between 38% and 100% of total N₂ fixation. The rates were mostly ascribed to free-living heterotrophic diazotrophs upstream while attributed to those associated with aggregates downstream. Diazotrophs diversity indicated that non-cyanobacterial diazotrophs dominated the free-living fraction along different river locations, while cyanobacteria were mostly identified in lake water. Compared to aggregates-associated N₂-fixers, the diversity of free-living diazotrophs was highly affected by environmental drivers, such as dissolved phosphorus, inorganic nitrogen, and water temperature. Our results highlight that freshwater heterotrophic diazotrophs are more ubiquitous than previously thought, can be found as free-living cells or associated with aggregates, significantly contributing to phytoplankton productivity.

Over the past two centuries, anthropogenic stress and climate change have blurred understanding of their individual and combined impacts on lake ecosystems. This study analyzed 477 ecological shifts documented in 224 paleolimnological records from lakes to trace their responses to climate change and anthropogenic stressors over time. By classifying ecological shifts according to their primary drivers (anthropogenic stress, climatic change, or their combined effects), this study characterized how lake ecosystems respond to these pressures. Stress response analysis revealed that climate-driven responses predominated during post-Little Ice Age warming, whereas anthropogenic stress became the dominant factor by the early 20^th century, accompanied by the onset of the 2^nd Industrial Revolution. While the Great Acceleration initiated widespread ecological shifts in lakes globally through synergistic interactions between anthropogenic activities and climate change, anthropogenic stress may still exert a greater impact on these shifts than climate change. Spatial analysis revealed divergent responses across lake ecosystems across the globe, though representation was limited from the Southern Hemisphere and tropical regions. Temperate lakes are highly susceptible to anthropogenic stressors; Arctic lakes have heightened sensitivity to climate change; and alpine lakes have coupled responses to both drivers. The cumulative response index developed in this study isolates individual stressors temporally, revealing substantial impacts of historical human development on lake ecosystems. These effects leave persistent signatures preserved in sedimentary archives, providing new perspectives on drivers of ecological trajectories across temporal scales.

This study presents the first multi-year assessment quantifying the contribution of primary production to vertical carbon flux in the ultra-oligotrophic southeastern Levantine basin of the Mediterranean Sea. Depth-integrated (0–180 m) daily primary productivity (PP) was 25% higher in the mixed winter period than in the stratified period (123 and 98 mg C m⁻² d⁻¹ respectively) and the predominant photoautotrophs contributed ~ 5%–11% of the bulk particulate organic carbon (POC). Time-resolved sediment trap data at 180 and 280 m from 2016 to 2020 showed POC fluxes ranging from 0.5 to 5.3 mg C m⁻² d⁻¹ (stratified-period) and 1.8 to 13.5 mg C m⁻² d⁻¹ (mixed-period), with primary producers potentially contributing 2.6%–7% of the POC flux at 180 m. Our calculated e-ratios are some of the lowest recorded in oligotrophic environments, reaching 0.061 during mixing and 0.026 under stratified conditions. High bacterial-to-primary production ratios and bacterial coupling to dissolved organic carbon (DOC) suggest that intense microbial recycling constrains the transformation of primary production to particulate export and reduces the biological pump efficiency. Our data show that applying generalized export models can overestimate export in the Levantine basin by overlooking microbial recycling and lateral carbon transport, underscoring the need for region-specific models that incorporate these processes under increasingly warm, stratified, and oligotrophic ocean conditions.

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.