Limnology and Oceanography Letters最新文献

While climate change affects the phytoplankton biodiversity at both local and global scales, predicting phytoplankton community responses to warming is impaired by their polyphyletic complexity. High mountain lakes are highly vulnerable systems, partly due to their limited biodiversity, and forecasting their ecological trajectories is a key challenge for scientists and conservation managers. We evaluated the phytoplankton's sensitivity to temperature in 24 high-altitude lakes over a multi-year (average 7-year) study. We detected assemblage-specific responses to warming, with different trends in biovolume and diversity observed among the diatom-dominant, mixed-mixotrophs dominant, and colonial-green dominant assemblages. The environmental settings partly governed assemblage responses, highlighting the role of the landscape filters in determining the response to warming. The biological stability of lakes, that is, their ability to resist shifts in their phytoplankton assemblage, is therefore determined both by the lake characteristics and warming intensity.

The daily cycle of solar radiation has a profound influence in structuring the physiology of microbes in the euphotic zone and subsequently setting the degree of coupling across trophic levels within ocean ecosystems. There has been an upsurge of interest in the biological role of the diel cycle and the ability to probe it using molecular approaches (i.e., “omics”), which now allow us to pinpoint the level of detail of the diel cycle that is required to better understand microbes' roles across multiple biogeochemical cycles. Although sampling the diel cycle requires additional resources, the payback is large. A better understanding of the diel cycle provides a holistic framework with which to align patterns and causal sequences across multi-omic layers, yielding consequent connections with metabolic processes to develop more robust mechanistic models. Such models provide the stepping stones to better understand how resource allocation in cells is driven by environmental forcing.

Understanding the impacts of multiple environmental stressors on phytoplankton biomass is crucial for predicting marine ecosystem responses under global climate change. This study employed a sequential modeling framework integrating principal component analysis, generalized additive models, and artificial neural networks to improve predictions of phytoplankton chlorophyll a concentrations in the Taiwan Strait. Analyzing a decadal dataset, we found that a 2°C rise in sea surface temperature and a 0.2 pH decline will each lead to an 11.3% reduction in chlorophyll a biomass, whereas nitrogen enrichment is expected to increase it by only 2.8%. The combined effects of these stressors will result in an 18.3% reduction, with the most significant declines occurring in high-chlorophyll areas during algal blooms. Compared to simpler models, our approach improved accuracy by reducing overestimation biases, particularly under acidification scenarios, highlighting the need for advanced, multivariate models in forecasting phytoplankton dynamics under global changes.

Litter decomposition is usually modeled with the negative exponential model, which assumes constant proportional mass loss. We assessed this assumption and its interpretive consequences using 145 stream litter mass loss time series and process-based simulations. Relatively simple (two to three parameters) models allowing time-varying decay rates produced more accurate predictions and were generally more parsimonious. Decomposition trajectories strongly deviated from constant decay for at least 50% of the time series, with the shape influenced by the degree of decomposition covered by a time series. Finally, simulations and empirical evidence suggested that the degree of decomposition covered can interact with time-varying decay rates and leachability to bias estimates of breakdown rates (k) from negative exponential models, obfuscating comparisons within and across studies. Considering alternative models could accelerate understanding and prediction of litter decomposition dynamics by enabling investigation of time-explicit decomposition dynamics and more precise modeling when warranted.

Freshwater systems are important sources of atmospheric methane (CH₄). However, estimated emissions are associated with high uncertainties due to limited knowledge about the temporal variability in emissions and their associated controls, such as air–water gas transfer velocity. Here, we determined the gas transfer velocity of CH₄ based on a novel measurement setup that combines simultaneous eddy covariance flux measurements with continuously monitored CH₄ water- and air-side concentrations. Measurements were conducted during a 10-d campaign in a freshwater lake in mid-Sweden. The gas transfer velocity fell within the range of existing wind-speed-based parameterizations derived for carbon dioxide in other lakes. For wind speeds below 4 m s⁻¹, the gas transfer velocity for CH₄ followed parameterizations predicting faster gas exchange, while for wind speeds above 5 m s⁻¹, it aligned with those predicting relatively lower gas exchange. This pattern can be explained by ebullition. Extending the wind speed range for such combined eddy covariance measurements with continuously monitored CH₄ water- and air-side concentrations would improve model reliability.

Cold thermal refuges may mitigate detrimental effects of future climate warming; yet, pond ecosystems have been largely omitted from thermal refuge research despite being globally numerous and providing critical ecosystem services. We create a formal definition for pond thermal refuge quality, then operationalize this definition by measuring the thermal characteristics and environmental attributes of ponds near Mount St. Helens (Washington, USA) to determine the environmental features that promote or hinder pond thermal refuges. Our results reveal substantial variation in thermal refuge quality between ponds and indicate that within-pond thermal refuges are a distinct metric from pond surface temperature. Denser floating surface vegetation promoted thermal refuges during summer conditions, while floating surface vegetation, water clarity, and canopy cover were associated with reduced mean pond temperatures during summer and heatwave conditions. These findings help identify ponds with high conservation value and suggest actionable steps for heightening the quality of pond thermal refuges.

Ultraviolet radiation (UV) is the most energetic waveband of incident solar radiation and has wide-ranging effects in the aquatic environment. Our analysis of an 18-year record of underwater irradiance and related limnological variables in sub-alpine, ultra-oligotrophic Lake Tahoe revealed orders of magnitude changes in UV transparency associated with interannual climate perturbations. The large-scale shifts between years were caused by pronounced changes in the loading of allochthonous particulate matter and colored dissolved organic matter associated with regional dry–wet cycles, while autochthonous factors explained the seasonal variations in UV under average weather conditions. Water clarity in the photosynthetically available radiation (PAR) waveband showed less variation, resulting in large interannual differences in the UV : PAR ratio. Clearwater lakes are likely to experience increasingly large fluctuations in underwater UV and spectral irradiance due to ongoing climate change and precipitation extremes, with potential impacts on their ecosystem structure and function.

Subterranean estuaries (i.e., seawater-fresh groundwater mixing zones at coastal aquifers) are highly reactive boundaries between continental groundwater and coastal surface seawater. Because particulate organic matter is retained in shallow sediments, internal microbial transformations rely on dissolved organic matter (DOM) supply and bioavailability. Here, we investigated DOM carbon content and optical characteristics in two nearby subterranean estuaries with contrasting oxygenation. Coastal organic carbon processing in the anoxic subterranean estuary resulted in the export of DOM enriched in recalcitrant compounds compared to the oxygenated one, which was a net sink of DOM. This contrasting behavior was not driven by opposite redox conditions but from the fast transfer of labile DOM and oxygen to the beach interior of the oxygenated subterranean estuary. There, heterotrophic processes, which rely almost exclusively on DOM, are enhanced, resulting in net DOM consumption prior discharge to surface waters.

Cobalt is a central component of cobalamins, which are nutrients essential for various metabolic processes in marine organisms. Dissolved cobalt in seawater is mostly bound to organic ligands, and the prevailing assumption to date is that these ligands are cobalamin-related compounds, yet the identity and impact of these ligands on cobalt bioavailability remain unknown. In this study, we examined cobalt ligand distributions and cobalamin cycling in surface waters across a North Pacific meridional transect. While we did not detect cobalamin derivatives in the dissolved cobalt ligand pool, the detection of transcripts associated with cobalamin synthesis and salvage pathways suggests that cobalamins may not be accumulating in seawater as cobalt-binding ligands and thus represent only a small fraction of the cobalt ligand pool in the North Pacific.