Bioactive trace metals, including iron (Fe), manganese (Mn), copper (Cu), and zinc (Zn), are essential micronutrients that regulate marine microbial metabolism and biogeochemical cycling. Although their role on phytoplankton productivity is established, their effects on mixed oligotrophic microbial assemblages comprising heterotrophic bacteria and picocyanobacteria remain poorly understood. Here, we investigated how short-term enrichment with Fe, Mn, Cu, Zn, and a mixed-metal treatment influenced microbial community composition and functional potential across six stations in the Indian Ocean. Shipboard microcosm experiments were conducted using prefiltered seawater amended with 10 nM trace metals, followed by 16S rRNA gene amplicon sequencing and metatranscriptomic analysis. Exploratory PERMANOVA indicated that station identity accounted for a larger proportion of compositional variation (R2 = 0.578) than metal treatment (R2 = 0.112). Metal additions stimulated picocyanobacteria, particularly Prochlorococcus, and enhanced the relative abundance of hydrocarbon-associated taxa, although responses were strongly site-specific. Metatranscriptomic analyses revealed contrasting transcriptional patterns: negative shifts in core metabolic pathways at NM9 and NM11, modest or treatment-dependent responses at NM10 and NM12, and predominantly positive transcriptional shifts at NM14. Across stations, functional responses were frequently decoupled from taxonomic shifts, indicating that community composition alone does not predict functional gene-expression patterns. Because the incubations contained both heterotrophic bacteria and photosynthetic picocyanobacteria, indirect effects mediated through photosynthetic microorganisms and associated microbial interactions cannot be excluded. Overall, these results show that short-term trace-metal enrichment was associated with station-specific shifts in community composition and directional changes in transcriptional profiles.
Ocean Thermal Energy Conversion (OTEC) uses the temperature gradient between the warm surface and the cold deep water. Existing thermal resource screening studies rely only on short observational campaigns or multi-decadal climatologies that characterize seasonal conditions but not year-to-year continuity. This study screens the eastern coast of Sorsogon Province, Philippines, integrating a 20-year (2005–2024) CMEMS GLORYS12V1 reanalysis with GEBCO bathymetry, interpolated to 10–1000 m and analyzed across rainy, cool-dry, and hot-dry seasons. Sea surface temperature ranged 27.5–29.5 °C, deep water below 500 m remained 4–7 °C, and the vertical gradient exceeded the 20 °C OTEC threshold up to 21.5–25.3 °C. The interannual coefficient of variation, computed from 20 discrete annual fields, stayed below 1.7% at all depths and decreased with depth. Shoreline-to-1000 m contour distances ranged from 8.3 km (Prieto Diaz) to 53.9 km (Matnog). CMEMS was evaluated through spot-check against 20 independent World Ocean Database profiles drawn from only 6 of the 20 years of record and concentrated in the northern and central study domain, yielding RMSE of 0.21–0.32 °C below 500 m, comparable to the products' documented global accuracy. An Entropy-TOPSIS multi-criteria framework, combining thermal persistence, interannual stability, and bathymetric accessibility, identified Prieto Diaz as the most favorable site, and the ranking was robust across four weighting schemes, though entropy weighting assigned negligible weight to the thermal criteria, indicating the ranking is effectively dominated by bathymetric accessibility at the resolution of the available reanalysis. These findings indicate the preliminary screening-level OTEC potential and provide a basis for further hydrodynamic, environmental, and techno-economic assessment; they do not constitute an engineering feasibility or net power assessment.
Global warming has transformed Arctic sea-ice conditions from thick to predominantly thin ice, altering the availability of underwater solar irradiance and influencing phytoplankton growth. Concurrently, enhanced glacial melt can increase sediment inputs and reduce light penetration, thereby influencing phytoplankton productivity. Recent studies have reported more frequent summer phytoplankton blooms beneath sea ice, which may contribute to uncertainties in satellite-based estimates of primary productivity. These findings underscore the need for depth-resolved optical measurements and improved models to characterize Arctic waters. The diffuse attenuation coefficient, Kd(λ), is closely linked to light availability and the optical properties of optically active constituents (OACs) in the water column. Here, we developed a bio-optical model for Kdλ in Arctic fjords that explicitly incorporates inherent optical properties and solar zenith angle. The low solar elevation and strong seasonal variation in solar geometry at high latitudes were considered in the model formulation. Validation demonstrated strong agreement between in-situ measurements and model estimates of Kd(λ), with high correlations (R2 = 0.80 in Kongsfjorden; R2 = 0.73 in Krossfjorden). The results demonstrate the potential of the framework for estimating underwater light attenuation in optically complex Arctic fjord waters.
Future coastal water-level assessments commonly focus on mean sea-level rise, although climate-driven changes in the seasonal sea-level range may also alter coastal conditions. We compare projected annual sea-level-cycle (ASLC) range change and mean sea-level rise (MSL) along global coastal margins under SSP3–7.0. Both components are first scaled by local tidal range, and the analysis is then repeated using absolute changes to determine how strongly the results depend on tidal normalization. CMIP6-derived ASLC projections, AR6 regional sea-level projections, and EOT20 tidal range are evaluated within 100 km of land. Most coastal margins (66.4%) are below the high-value cutoff for both tide-normalized components, whereas 18.4% rank in the highest quarter for both ASLC change and MSL rise relative to local tides. Coastal margins with only one elevated component occupy smaller but similar shares, and the most extreme ASLC and MSL locations overlap only partly. Without tidal normalization, the global fraction with both absolute changes in the highest quarter decreases to 6.1%. The contrast is strongest in the Mediterranean–Black Sea, where this fraction decreases from 96.6% to 0.8%, showing that its normalized concentration primarily reflects small local tides rather than two extreme absolute changes. Historical seasonal variability in this region is only modestly higher than the global median. Future ASLC change therefore adds information not captured by MSL alone, while considering both relative and absolute changes provides a clearer interpretation of future coastal water-level regimes.
The northern waters of Liaodong Bay (LDB) frequently exhibit low water quality due to the region's semi-enclosed hydrological conditions, high terrestrial pollutant inputs, and seasonal sea ice cover. Existing studies have not systematically elucidated the comprehensive regulatory mechanisms of sea ice on the marine environment. A three-dimensional (3D) hydrodynamic module was coupled with the ECO Lab module to reveal the spatiotemporal evolution of water quality parameters in the study area under ice-covered conditions, and subsequently a comprehensive water quality index (WQI) assessment was conducted. Sea ice exerts both thermal insulation and flow-suppressing effects: it elevates sub-ice water temperature, weakens flow velocity and vertical mixing, and reduces the extent of low-salinity zones around river estuaries. Dissolved oxygen (DO), nutrients, and chlorophyll a (Chl-a) all exhibit higher concentrations near estuaries and lower levels offshore, with their seasonal peak values appearing at different times. The overall water quality of LDB is rated as good, with an annual average WQI of 86.32, although substantial spatial differences and seasonal fluctuations are observed. Offshore water quality is superior to that near estuaries; seasonally, summer exhibits the most severe pollution and poorest water quality, whereas ice-covered winter shows the best. This study clarifies the integrated mechanisms of seasonal sea ice on hydrodynamic, thermohaline, and ecological variables within semi-enclosed bays, and fills the research gap regarding coupled water environment simulation for cold-region ice-covered bays.
