Limnology and Oceanography Letters最新文献

The Rowley Shoals, off northwestern Australia, are renowned as a biodiversity hotspot. This remote system comprises three atolls, Clerke, Imperieuse and Mermaid, which in recent years have been increasingly targeted by illegal fishing, a pressure rising across the Indo-Pacific. The objective of this study was to quantify recent changes in sea cucumber assemblages across the Rowley Shoals. We surveyed nine sea cucumber species at monitoring sites in 2018 and 2023 to assess changes in their abundance. Total assemblage densities declined significantly across all atolls and habitats. Documented seizures of illegally harvested holothurians by the Australian Fisheries Management Authority over this period suggest that illegal fishing pressure likely contributed to the observed declines. Notably, IUCN-listed species Holothuria whitmaei and Actinopyga miliaris declined severely, raising concerns about their recovery potential. These findings provide the first quantitative evidence of widespread declines within this protected reef system and bring renewed attention to its conservation.

Autumnal hypoxia in temperate estuaries is often overlooked due to its smaller extent, weaker intensity, and sparse observations compared to summer. However, climate variability may alter its seasonality. Using 40 yr (1984–2023) of hypoxic volume data from the Chesapeake Bay, combined with numerical simulations, we examined interannual drivers of autumnal hypoxia. September wind speeds were negatively correlated with hypoxic volume (r = −0.49, p < 0.01), reflecting wind-driven destratification. Conversely, September river discharge showed a strong positive correlation with October hypoxia (r = 0.81, p < 0.01), indicating that lagged freshwater inputs from late-season cyclones enhance stratification and re-establish hypoxia. Model simulations of a representative tropical cyclone confirmed this dual effect that storm winds temporarily alleviated hypoxia, but subsequent river inflows prolonged it into October. Model results highlight that physical processes largely regulate autumnal hypoxia dynamics, and suggest that more frequent intense cyclones under climate change may increase its persistence.

The presence of vegetation in aquatic environments alters hydrodynamics and sediment resuspension. A recent paradigm has suggested that turbulent kinetic energy (TKE) serves as a better predictor of sediment transport in aquatic canopies than bed shear stress. This observation has led to the development of formulations to predict TKE for vegetated flows in the laboratory. However, model validation from natural heterogeneous field environments is lacking. Here, we explore the application of laboratory-based formulas in a real environment, characterized by multiple vegetation length scales. We measured turbulence within a sparse canopy of mangrove pneumatophores and saplings during an experimental period with negligible wind-wave activity. The existing formulations for TKE performed well in the field, but only when using the measured values for horizontal eddy length scales. These length scales accounted for the generation of additional turbulence from the surrounding sapling canopy, leading to notably larger TKE values than in similar laboratory experiments.

Dissolved silicate (DSi) export from rivers is shaped by both natural processes and human activities. Using long-term observations at Lijin station combined with chemical weathering and reservoir silicon cycling models, we reconstructed annual DSi fluxes and source-sink dynamics in the Yellow River Basin since the 1980s. Chemical weathering inputs and terrestrial vegetation uptake have increased, whereas soil erosion has declined and reservoir retention has remained relatively stable in the 21^st century. Riverine DSi concentrations at Lijin have decreased significantly since 2000. This decline is primarily driven by enhanced vegetation uptake and reduced soil erosion, with increased precipitation and discharge likely contributing to dilution. In contrast, annual DSi fluxes exhibit strong interannual variability mainly controlled by river discharge. Overall, our results reveal a transition from predominantly natural regulation toward increasing human control of riverine silicon cycling.

Green tides caused by Ulva species have become one of the most serious marine ecological disasters, now impacting many coastal nations around the world. Although climatic and environmental drivers of these macroalgal blooms are well recognized, growing evidence identifies Ulva-associated microbiota as potential pivotal regulators of bloom initiation, development, and demise. Here, we synthesize current advances that reframe green tides as emergent outcomes of the Ulva–microbiota holobiont. During bloom initiation, microbiota produce signaling molecules and phytohormone-like compounds that regulate spore colonization, germination, and morphogenesis. At the bloom stage, microbial partners sustain rapid macroalgal proliferation by mediating nitrogen fixation, phosphorus mobilization, and iron acquisition, enabling large-scale biomass accumulation. Conversely, during bloom decline, algicidal and decomposer taxa accelerate Ulva mortality and biomass breakdown, recycling nutrients and driving carbon, nitrogen, phosphorus, and sulfur cycling. Collectively, these findings highlight that microbiota exert far more intricate and influential roles in green tide dynamics than previously recognized. Deciphering these holobiont interactions not only advances our understanding of the ecology of green tide but also offers microbiome-informed strategies to mitigate future green tide outbreaks.

Understanding marine ecosystem responses to climate change is crucial for developing ecosystem-based adaptation strategies. We applied the StrathE2E model to assess climate change impacts on the food web of the St Helena marine protected area (SHMPA). The model was parameterized using two Earth System models (GFDL, CNRM) and two future climate scenarios (SSP1-2.6, SSP3-7.0) from the NEMO-ERSEM model for a baseline period (2010–2019) and future decades up to the 2060s. The SHMPA will become warmer and more oligotrophic, leading to declines in primary producers, fish, and top predators. Despite quantified uncertainty, the direction of change was consistent, with larger declines in CNRM than GFDL. Net primary production is highly sensitive to upwelling and downwelling, with greater stratification under SSP1-2.6 than SSP3-7.0, causing stronger productivity losses. This study presents the first food web model with ecosystem-level assessment of climate change on SHMPA. The projections suggest potential for profound ecosystem-wide transformations posing management challenges.

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.