Limnology and Oceanography最新文献

Stratification and turbulent mixing have an immense impact on biogeochemical regimes and ecosystem dynamics in stratified large lakes. This study investigates the physical properties of the epi- and metalimnion of Lake Biwa (Japan), focusing on the persistent vertical diffusive fluxes due to turbulent mixing in the lower metalimnion and their effects on upward nitrate transport. We conducted 24-h observations at an offshore station in the summer for three consecutive years to examine the temporal and vertical variations of stratification and turbulence. A mooring system provided long-term data on stratification and current velocity, while thermal profiles along two offshore transects were collected to investigate lateral changes. Our findings revealed strong stratification in the upper metalimnion, where vertical diffusive fluxes were essentially suppressed. Distinct turbulent mixing layers were identified near the water surface and within the moderately stratified lower metalimnion. Field data and numerical model suggested that the turbulence in the lower metalimnion was driven by shear instability, straining, and/or wave–wave interactions linked with the persistent internal waves that occur even during weak winds. Vertical eddy diffusivity of > 10⁻⁵ m² s⁻¹ in the lower metalimnion was associated with rather strong turbulence, which has not been reported in other large lakes. The elevated turbulence resulted in an upward nitrate flux of 0.2 mmol N m⁻² d⁻¹ across the lower metalimnion, indicating that the upward nutrient transport could support primary productivity and play an important ecological role in deep stratified lakes.

Marine environments are highly heterogeneous, varying across scales of a few meters to entire ocean basins. Understanding the relationship between environmental variability and species distribution is essential for area-based management and conservation. However, this requires a precise alignment of seabed mapping with environmental and biological sampling, which is often difficult to achieve in the deep sea. There is thus an urgent need to tackle this challenge to effectively manage high-diversity habitats such as deep-sea coral and sponge aggregations. Relying on multiple subsea platforms, seafloor mapping, and imaging techniques, we mapped the distribution of megafaunal communities at Sur Ridge (780–1525-m depth; off central California) across multiple spatial scales. First, remotely operated vehicle video transects were conducted to characterize community distribution along the ridge in relation to substratum type, environmental conditions, and 1-m resolution bathymetry. Five distinct communities, located in specific areas of the ridge, were identified. These communities were primarily structured by depth, availability of hard substrata, and terrain complexity (slope and rugosity). Indicator taxa were identified for each community and their distributions were characterized at the centimeter scale from coregistered 5-mm resolution photomosaic and 5-cm lateral resolution bathymetry produced during low altitude remotely operated vehicle surveys. High-resolution mapping allowed the identification of associations between deep-sea coral and sponge and other benthic taxa and showed that, even at these small scales, different taxa associate with distinct microhabitats. These results highlight the importance of accounting for habitat heterogeneity, and its role in supporting biodiversity when designing management and conservation strategies.

Heterotrophic prokaryotes play a vital role in organic matter cycling in the ocean and have been observed to undergo substrate-controlled successions during phytoplankton blooms. However, there is limited understanding of the succession patterns during blooms triggered by upwelling events of different characteristics. Here we simulated eight upwelling scenarios of varying intensity and duration (single vs. recurring pulses) by adding nutrient-rich mesopelagic waters into large-scale mesocosms containing oligotrophic surface waters from the subtropical North Atlantic. Over a monitoring period of nearly 6 weeks, we observed that phytoplankton blooms displayed diverging outcomes depending on the upwelling mode: treatments with single upwelling pulses presented a unique, short-lived bloom, whereas recurring upwelling resulted in blooms that were sustained over time. Prokaryotic abundances were positively related to upwelling intensity and presented three similar abundance cycles in all treatments, whereas heterotrophic activity differed between the two upwelling modes. The successional dynamics of free-living and particle-associated communities were consistent regardless of upwelling intensity and mode, with four or five prokaryotic assemblages sequentially proliferating during the experiment. Yet, some differences were observed in the taxa that formed the assemblages in both upwelling modes. Together, our results suggest that, despite differences in activity, prokaryotes seemed to be more influenced by processes taking place within the community than by phytoplankton bloom patterns, with similar succession dynamics even under widely distinct blooms. These findings can help advance our understanding on prokaryotic ecology and its relation to organic matter cycling across different upwelling scenarios.