Limnology and Oceanography Letters最新文献

Coral reefs are being degraded globally, largely due to coral bleaching from rising ocean temperatures. Internal bores, generated by nonlinear internal waves, can help mitigate this stress by delivering cooler, nutrient-rich water to shallow reefs (< 20 m). Between October 2023 and April 2024, a field experiment at Mermaid Reef Atoll (Australia's North West) examined how these bores influence reef temperature. Moored instruments on the windward side recorded tidally driven bores advecting cooler offshore waters onto the forereef slope (17–40 m), causing rapid cooling of up to 6.9°C in 30 min. Some cold-water pulses crossed the reef crest, cooling the shallow reef flat (< 8 m) by up to 4.4°C for minutes to hours. A heat budget analysis indicates that without internal bores, shallow reefs experience up to three times more heat advection, increasing bleaching risk. These findings highlight internal bores' critical role in reducing thermal stress and enhancing coral reef resilience under climate change.

Atmospheric nitric oxide (NO) is a pollutant and climate-relevant trace gas. Dissolved NO accumulates in the surface ocean and therefore the ocean is a natural source of atmospheric NO. It is generally assumed that NO in the surface ocean is produced photochemically from nitrite (NO₂⁻). Here, we present the first dissolved NO measurements in the oligotrophic North Atlantic Ocean during an east-to-west transect (December 2021/January 2022). NO concentrations and mean saturations ranged from 9.4 to 48.2 pM and 10,355% to 19,484%, respectively, indicating that the study site was indeed a significant source of atmospheric NO despite extremely low NO₂⁻ concentrations. NO concentrations correlated significantly with phytoplankton pigment concentrations, indicating that NO variability may be influenced by phytoplankton-related processes rather than photochemical pathways alone. These findings highlight the need to further investigate the production mechanism of NO in nutrient-poor oceanic environments.

The surface ocean is an important source of methane to the atmosphere, contributing 4% of natural global production. The degradation of methylphosphonate (MPn) is a key methane production pathway in the aerobic euphotic zone. Methylphosphonate is an important component of dissolved organic phosphorus (DOP), and it serves as a source of phosphorus (P) for microorganisms in oligotrophic regions of the ocean. Methylphosphonate is metabolized by bacteria into labile phosphorus and methane by the C-P lyase enzymatic pathway. This pathway is regulated by the concentration of ambient inorganic phosphate (Pi) and is utilized to access alternative forms of P when bacteria experience P limitation. However, the concentration threshold of Pi, as it relates to the onset of C-P lyase activity, is unknown. In this study, an isolate of the methane-producing marine bacterium Stutzerimonas frequens (Pseudomonadaceae) was cultured under a range of Pi concentrations in the presence of MPn. In treatments where the estimated Pi concentration dropped below 92 nM, methane production was observed. Isotopic mass balance confirmed the methane was derived from MPn. The drawdown of MPn and the production of methane occurred concomitantly with the uptake of other organic P compounds, demonstrating that C-P lyase is utilized in conjunction with other P acquisition pathways to combat Pi limitation. As the global climate shifts and increased warming strengthens stratification, increased P limitation may lead to higher rates of MPn utilization and methane production in the euphotic open ocean.

Diel rhythms structure microbial activity in the ocean, yet the role of viral lysis in driving short-term community dynamics remains poorly understood. Here, we combined dilution experiments, high-resolution microbiome sampling and extracellular ribosome sequencing to investigate diel virus–host interactions in the South China Sea. Viral lysis accounted for an average of 41% of Synechococcus and 24% of heterotrophic bacterial mortality at night. Overall, 74% of amplicon sequence variants (ASVs) showed lysis signals, with many displaying significant diel fluctuations. Notably, rare taxa comprised a large fraction of diel-responsive ASVs, indicating high turnover among low-abundance populations. Synechococcales exhibited tightly coupled nighttime peaks in abundance and lysis, while SAR11 and Flavobacteriales showed delayed lysis after late-day growth. In addition, virus-mediated mortality had a stronger influence on metabolically active and diel-responsive ASVs. These findings highlight viral lysis as a temporally structured and taxon-specific force sustaining microbial turnover and diversity in oligotrophic waters.

Tidal wetland reclamation is a widespread anthropogenic disturbance that alters sediment biogeochemistry; however, its impacts on biological nitrogen fixation (BNF) and diazotrophic communities remain poorly understood. Here, we integrated ¹⁵N isotope labeling, metagenomic sequencing, and network analysis across a bare tidal flat (B), moderate-density (M), and high-density (H) aquaculture ponds to examine these effects. Biological nitrogen fixation rates declined by 53.8% ± 29.1% (M) and 70.0% ± 59.2% (H) relative to B (0.99 ± 0.06 μmol kg⁻¹ h⁻¹), driven more by reclamation than by seasonal variation. Microbial communities shifted from diazotroph dominance (e.g., Pseudomonadota) to an increased contribution of lineages without detected nif genes (e.g., Thermodesulfobacteriota), accompanied by reduced diversity and functional gene expression. nifH phylogeny showed a shift toward aerobic/facultative bacterial nitrogenases, highlighting archaeal sensitivity to aquaculture. This study reveals key mechanisms by which aquaculture suppresses sediment BNF and suggests that anfD may serve as a microbial indicator for wetland restoration monitoring.

Rapid warming in the Arctic have influenced the ecological and biogeochemical systems in the Arctic Ocean, particularly the phytoplankton community that plays a fundamental role in food webs and carbon cycle. Our 10-yr study found an abrupt change in the phytoplankton community in the western Arctic Ocean basin between 2008 and 2018. This shift involved the replacement of nanoplanktonic prymnesiophytes with picoplanktonic prasinophytes. This change was closely associated with a sudden deepening of both the nitracline (from an average depth of 37 ± 15 to 45 ± 8 m) and the chlorophyll maximum layer (from an average depth of 47 ± 16 to 57 ± 10 m) after 2012, driven by thickening of the relatively fresh surface layer (S < 31) in the western Arctic Ocean basin. The northwestward movement and intensification of the Beaufort Gyre may have exacerbated this surface freshening of the western Arctic Ocean by regulating freshwater redistribution, thereby driving changes in the phytoplankton community.

Phytoplankton community size structure is a key attribute that influences pelagic trophic energy transfer and the vertical flux of organic matter to benthic food webs. To capture the ephemeral scale of phytoplankton population size spectra we developed an approach, combining long-term survey datasets, machine learning modeling and 3 yr of high-resolution moored vertical profiler measurements. We show that larger phytoplankton (> 10 μm cell diameter) dominated the spring bloom throughout the water column, however, after the bloom, smaller cells (< 10 μm cell diameter) comprise 70–80% of the phytoplankton community in the surface mixed layer. In contrast, larger cells are predominantly present at depths near the mixed layer interface. However, intermittent wind mixing events results in enhanced pulses of net community production and larger phytoplankton in the upper ocean. Our findings highlight how transient seasonal and vertical shifts in phytoplankton size structure occur in response to variable oceanographic conditions.

The Benguela upwelling system (BUS) exhibits strong seasonal variability in dissolved oxygen ([O₂]), driven by upwelling and remotely advected water masses, and primary productivity. In contrast, its interannual variability remains poorly understood due to limited long-term observations. In this study, we applied an interpretable machine learning (ML) approach to investigate the interannual variability of surface [O₂] in the BUS, highlighting a deoxygenation trend. This trend is driven by widespread warming which reduces [O₂] solubility in the adjacent South Atlantic basin, along with interannual variability in upwelling water masses and declining coastal primary productivity. Conversely, an oxygenation trend is observed off Lüderitz, attributed to perennial upwelling that sustains primary productivity throughout the year. The deoxygenation trend in the BUS is significant and may intensify low-[O₂] events, highlighting the need for improved monitoring and assessment of impacts on marine ecosystems.

Methane (CH₄) ebullition is a major emission pathway in lakes. However, ebullition remains difficult to assess, and data from large lakes are scarce. This study investigates ebullition in the littoral zone of a large lake, demonstrating that ship waves, even after traveling long distances, regularly trigger gas bubble releases from the sediments in shallow waters. Although these ebullition events occur only during the short time intervals of ship-wave passages, they lead to substantial CH₄ emissions. Comparing several littoral sites, we assess the frequency and extent of ship-wave-induced ebullition events and contrast them with ebullition caused by a storm event. Our findings reveal a strong cross-correlation between pressure drops due to ship waves and ebullition fluxes, highlighting ship-wave-induced pressure changes as a significant yet understudied driver of CH₄ emissions in lakes. Considering the prevalence of ship traffic on lakes, understanding ship-wave-induced ebullition is essential for improving estimates of lake-wide CH₄ emissions.