Nitrogen is one of the most commonly limiting nutrients throughout the world’s oceans and it has long been acknowledged that pelagic Sargassum reliably hosts nitrogen fixing epiphytes, yet very little is known about what controls nitrogen fixation rates within this community. Here, we provide an updated examination of this process by relating rates of nitrogen fixation by the pelagic Sargassum community within the Gulf Stream to a range of biotic and abiotic factors. Nitrogen fixation rates were found to be significantly and positively associated with epiphytic cyanobacteria abundances as well as that of epiphytic toxin-producing dinoflagellates. Canonical correspondence analyses and generalized additive models revealed that the cyanobacterial diazotrophs Dichothrix spp. and Oscillatoriales were significantly and positively related to nitrogen fixation rate and that no physicochemical variable measured had a statistically significant relationship with nitrogen fixation rate. Additionally, we show that rates of nitrogen fixation are consistently higher when bioassays are initiated immediately upon Sargassum collection compared to if bioassay start times are delayed until after transport to the laboratory. This finding not only has important implications for the interpretation of macrophyte studies conducted ex situ but also contextualizes reactive nitrogen input estimates previously reported by this community. Additionally, this study advances understanding of the pelagic Sargassum holobiont by documenting the first report of an epiphytic Trichodesmium sp. bloom associated with Sargassum and by characterizing the microbiomes of select epiphytic dinoflagellate taxa that were associated with Sargassum throughout this study. Knowledge of the controls on nitrogen fixation, and the degree to which they influence rates, help to inform biogeochemical models thereby improving our understanding of the marine nitrogen cycle and how it may be altered in the face of global environmental changes.
IntroductionThe Hangzhou Bay seabed is characterized by complex site conditions, including seafloor topography, laterally variable marine deposits, and bedrock undulation. This study investigates how these conditions affect nonlinear seismic response by comparing one-dimensional (1D) and two-dimensional (2D) numerical simulations.MethodsA large-scale 2D ABAQUS model was developed from borehole investigation data, and corresponding 1D soil-column models were extracted at three representative boreholes, ZK1, ZK3, and ZK6. Far-field bedrock motions from the KiK-net were applied as input motions, and nonlinear seismic response analyses were conducted in ABAQUS with the non-Masing DCZ hysteretic constitutive model.ResultsThe results show that the seismic response of the Hangzhou Bay seabed is strongly influenced by the combined site conditions. At ZK1, relatively large horizontal surface accelerations were observed, which may be associated with nearshore topographic variation and shallow wave propagation. At ZK3, where bedrock relief and lateral sedimentary variability are more evident, the 2D model showed pronounced acceleration amplification at depths of approximately 60–150 m. Under the investigated conditions, the vertical peak ground acceleration (PGA) ratio between the 2D and 1D models at ZK3 reached 2.15–2.92. Frequency-domain and response-spectrum analyses also indicated amplification of motions below 1 Hz, localized amplification around 1–2 Hz, shifts in predominant periods, and multiple peaks in the normalized acceleration response spectra.Discussion and conclusionThe observed 1D/2D differences are interpreted in relation to the combined site conditions; the individual contributions of topography, bedrock relief, and sedimentary variability are not independently isolated in this study. At ZK6, relatively similar normalized spectral shapes can coexist with differences in absolute response amplitude, because normalized spectra and PGA characterize different aspects of ground motion. Thus, 1D analysis may provide preliminary information on the frequency characteristics of horizontal seismic response at sites with relatively flat bedrock and approximately horizontal sediment layering, whereas absolute response amplitudes should be evaluated with possible 2D site effects in mind.
IntroductionHarmful algal blooms (HABs) represent recurring coastal ecological hazards that threaten aquaculture, marine ecosystems and near‑shore environmental safety. Satellite remote sensing provides essential observational capacity for HAB monitoring; nevertheless, systematic quantitative inter‑comparisons between geostationary GOCI‑II and polar‑orbiting Sentinel‑2 MSI / Sentinel‑3 OLCI for identical bloom events remain limited. This study addresses the complementary strengths and limitations of these three ocean‑colour sensors for East China Sea HAB observation.MethodsWe selected two well‑documented HAB episodes occurring in the East China Sea during 2021 (30 April and 22 September). GOCI‑II, Sentinel‑2 MSI and Sentinel‑3 OLCI datasets were processed using sensor‑matched atmospheric correction schemes (standard GOCI‑II ocean‑colour processing, ACOLITE‑DSF for Sentinel‑2, C2RCC for Sentinel‑3 OLCI). Six spectral indices including NDCI, RBD, NRTI, RAB, PDI and DI were applied for bloom detection. Spatial‑degradation experiments at 20 m, 300 m and 500 m were implemented to separate geometric scale‑related effects on bloom area and boundary representation. Near‑simultaneous overpass comparisons and hourly GOCI‑II time‑series tracking were performed to quantify inter‑sensor discrepancies and intra‑day bloom dynamics.ResultsUnder ~10‑minute near‑coincident overpass on 30 April 2021, GOCI‑II mapped a bloom area of 901.52 km² versus 571.52 km² from Sentinel‑2B; the difference originated jointly from spatial resolution, spectral configuration, atmospheric correction and detection thresholds, while bloom advection exerted minor influence. On 22 September 2021, hourly GOCI‑II observations captured a complete intra‑day bloom cycle: bloom optical‑anomaly area varied from 37.30 km² up to a peak of 760.20 km² and declined to 107.67 km², accompanied by a 45.74 km centroid displacement, a dynamic that single polar‑orbiting snapshots only partially captured. Spatial degradation analysis demonstrated that boundary‑length loss exceeded 76 – 85 % at coarser resolutions, whereas total‑area deviations ranged from −14.8 % to +11.0 %, highlighting Sentinel‑2’s unique value for fine‑scale near‑shore boundary delineation. Even after resolution normalisation, substantial spatial mismatch persisted among sensor bloom products (IoU as low as 0.014).DiscussionIndividual sensors exhibit distinct trade‑offs: GOCI‑II delivers high‑frequency temporal screening, Sentinel‑2 provides fine‑scale spatial detail, and Sentinel‑3 OLCI supplies rich spectral information for regional contextual analysis. Accordingly, we propose a conditional three‑stage multi‑source synergistic workflow: GOCI‑II for rapid hourly screening and drift tracking, Sentinel‑2 MSI for priority‑zone fine‑scale inspection, and Sentinel‑3 OLCI for regional spectral validation and cross‑sensor consistency checking. Discrepancies across sensors stem from combined effects of spectral band configuration, atmospheric correction, index thresholds and sampling timing. Limitations include only two analysed bloom events and absence of dense pixel‑level in‑situ ground truth. Further multi‑event validation with harmonised atmospheric correction is required to generalise this conditional monitoring template for operational HAB early‑warning in optically‑complex East Asian coastal waters.
A hybrid framework named “TRR–LSTM–TRR” is developed to reconstruct and predict 137Cs activity concentrations in surface seawater of China’s coastal seas. The model integrates a bound-constrained Trust-Region Reflective (TRR) exponential baseline, an LSTM-based residual-learning network, and a physically constrained exponential refit. Historical observations from the Bohai Sea, Yellow Sea, East China Sea, and South China Sea during 1960–2017 were compiled (833 station records after data-quality corrections). The reconstructed environmentally effective half-lives were 10.52, 14.84, 15.05, and 16.40 a for the four seas, respectively, indicating the fastest attenuation in the Bohai Sea and the slowest in the South China Sea. The residual correction reduced absolute prediction errors in all basins; under the temporal holdout (training on 1960–2010, testing on 2011–2017) mean absolute errors were 0.30–0.61 Bq m−3. Because the recent records are few and low-valued, long-term projections are reported with 90% ensemble prediction intervals rather than as point estimates. Taking 0.2 Bq m−3 as the detection limit, the Bohai, East China, Yellow, and South China Seas are expected to fall below this threshold around 2036, 2049, 2053, and 2055, respectively.
