Oceanic fronts are important physical boundaries separating adjacent water masses with distinct properties and play key roles in structuring marine ecosystems. To evaluate the influence of frontal systems on assemblage differentiation, we investigated spatial and interannual variability in fish and invertebrate assemblages across the Zhejiang–Fujian Coastal Front using autumn survey data collected in 2023 and 2024 from the Zhoushan Fishing Ground. Sampling stations were classified into front-inner (FI) and front-outer (FO) zones based on the frontal boundary identified using an improved histogram-based front detection algorithm. PERMANOVA revealed significant differences in assemblage structure between FI and FO, which were further visualized using PCoA. Biomass and abundance were markedly higher in 2023 than in 2024 and generally greater in FO. Dominant species composition also varied between FI and FO as well as between years, with only Portunus trituberculatus and Oratosquilla oratoria remaining consistently dominant. Distance-based redundancy analysis (db-RDA) showed that assemblage patterns were primarily structured by cross-shelf environmental gradients, particularly water depth, salinity, temperature, and chlorophyll-a concentration. Spatial db-RDA further revealed spatial structuring of assemblage composition associated with the FI and FO zones. Variation partitioning indicated that environmental and spatial factors jointly explained 35.2% of the variation in assemblage composition, with a large shared fraction suggesting that environmental gradients associated with assemblage variation were spatially structured. Collectively, these findings indicate that the Zhejiang–Fujian Coastal Front promotes distinct assemblage differentiation between FI and FO, providing new insights into ecosystem dynamics and supporting targeted conservation and management strategies.
Variability of mixed layer depth (MLD) in the Irminger Sea is critical to deep convection, deep water formation, and the Atlantic Meridional Overturning Circulation (MOC). Previous studies about MLD in the central Irminger Sea have shown that atmospheric surface forcing dominates interannual variability of MLD. However, the decadal-scale MLD variability in the East Greenland (EG) region (i.e., the western boundary of Irminger Sea) remains incompletely understood. Based on a multidecadal observation-based reconstructed dataset covering 1993–2022, this study identifies a robust three-decade MLD shoaling in the EG region, with an overall trend of −139.1 m·decade−1. This shoaling exhibits a clear regime shift around the late 2000 s: a rapid shoaling rate of −331.2 m·decade−1 in Period Ⅰ (first half, 1993–2007), followed by a marked slowdown to −76.5 m·decade−1 in Period Ⅱ (second half, 2008–2022). To identify the dominant driver of this MLD shoaling, we assess the relative contributions of five candidate processes: surface heat flux, surface freshwater flux, wind mixing, horizontal advection and sea-ice. Surface buoyancy fluxes exert a negligible net effect, as the shoaling impact of declining freshwater flux is compensated by heat flux. Sea-ice melt contributes only marginally due to its limited coverage. Instead, horizontal advection transports upstream water masses from the north and interior Irminger Sea, whose stratification favors mixed-layer shoaling at varying rates. Amplified by local coupled turbulent mixing processes, this drives the three-decade MLD shoaling and its late-2000 s regime shift. This long-term MLD shoaling in this key deep convection hotspot may have important implications for the long-term variability of the MOC, and hence for the climate system.
Understanding whether deep-sea trenches and island arc ridges function as barriers to dispersal or as drivers of genetic differentiation is essential for clarifying the processes that generate and maintain biodiversity in the deep ocean. Herein, this question was investigated across three hadal trench systems (Aleutian, Japan, and Kuril-Kamchatka) and two island arc systems (Aleutian Islands and Kuril Islands) in the North Pacific, focusing on the integrative biogeographical and population genetic patterns of two deep-sea isopod families, the epibenthic Haploniscidae and endobenthic Macrostylidae. Haploniscids exhibited greater dispersal flexibility than macrostylids, resulting in wider distributional and bathymetric ranges, also inhabiting marginal seas. Still, trenches and especially island arc ridges acted as major barriers limiting faunal exchange, though barrier-crossing species displayed only weak genetic differentiation across these barriers. Instead, most species exhibited increasing genetic differentiation with geographic distance, suggesting Isolation by Distance. Species turnover rates for both families hinted at a bathymetric ecocline extending from mid-abyssal to upper hadal depths, below which hadal endemic fauna took over and faunal turnover stagnated. Bathymetric structuring among closely related sister species within multiple species complexes indicated that depth might represent a major contributor to differentiation processes, detectable on an interspecific rather than an intraspecific level. Our findings showcase that areas of topographic complexity give rise to diverse biogeographic patterns for benthic isopod fauna in the deep sea. More elaborate population genetic investigations are necessary to disentangle the complex interplay of bathymetric gradients, geographic distances, topographic structures and isopod life histories that shapes genetic differentiation and speciation processes in the deep sea.
Seasonal variability has long been regarded as the dominant mode of phytoplankton biomass variability in temperate shelf seas. Growing observational evidence indicates that short-term synoptic events can trigger phytoplankton pulses, thereby introducing substantial subseasonal variability. However, the statistical role of subseasonal variability in explaining phytoplankton biomass fluctuations and its impact on annual-scale variation remain poorly understood. Here, using satellite-derived weekly chlorophyll-a (Chl-a) data in the Yellow and Bohai Seas (2003–2020), we decomposed the time series into subseasonal (<3 months), seasonal (∼3–12 months), and interannual (>12 months) components. Results show that the subseasonal variability contributed ∼ 60 % of the total variance of the Chl-a variability, substantially exceeding seasonal (∼30 %) and interannual (∼10 %) contributions, suggesting the dominant role of subseasonal forcing on the Chl-a variability. The subseasonal variability was mainly driven by small-scale processes with a mean spatial scale of ∼ 12 km. Spatially, it was stronger in the frontal region and the central Yellow Sea, which could be associated with frontal processes and high-frequency atmospheric forcing. Furthermore, the annual mean level of Chl-a is strongly correlated with the intensity of subseasonal variability but only weakly correlated with Pacific climate variabilities, suggesting the modulation of subseasonal forcing on the annual mean phytoplankton biomass. Mechanistically, the annual mean Chl-a level integrates intermittent, short-lived pulses and thus tends to be higher in years with more frequent or stronger subseasonal variability. The lack of link between subseasonal variability and climate may account for the weak modulation of climate on the shelf Chl-a level. This study highlights the contribution of high-frequency and small-scale processes to phytoplankton dynamics in shelf seas, with important implications for ecological observation and ecosystem modeling.
