The abnormal proliferation of Ulva in the Yellow Sea has instigated the notorious green tide phenomenon. Mitigating this ecological challenge necessitates a holistic comprehension of Ulva’s nitrogen and phosphorus uptake behaviors. Investigating the mechanisms governing nutrient absorption, encompassing factors like concentration, form, and input dynamics, has unveiled their profound influence on nutrient assimilation rates. The nutrient absorption characteristics of Ulva prolifera, including its preference for abundant nutrients, a high nitrogen-to-phosphorus (N/P) ratio, and its ability to efficiently absorb nutrients during pulse nutrient input events, determine its dominant role in the green tide events in the Yellow Sea. Although source control and preemptive salvaging are effective methods for managing green tides, addressing the root causes of these coastal ecological disasters requires the implementation of long-term pollution control strategies that align with sustainable development goals, with a priority on reducing marine eutrophication. This is crucial for the effective management and restoration of the coastal ecosystem in the Yellow Sea.

Calcareous tropical green macroalgae of the genus Halimeda are key reef-builders, yet the drivers of their diversification and population dynamics remain poorly understood. This study analyzed the species diversity of Halimeda in the Xisha (Paracel) Islands based on tufA gene sequences, focusing on evaluating the genetic diversity, population structure, and historical dynamics of two widespread species—Halimeda discoidea and Halimeda macroloba. The results indicate new records of Halimeda cylindracea and Halimeda cf. stuposa in the Xisha (Paracel) Islands. More importantly, H. discoidea and H. macroloba exhibited significantly different evolutionary histories. Specifically, H. discoidea showed a highly fragmented population structure, restricted gene flow, and a multimodal mismatch distribution, suggesting a complex historical process or long-term stability. In contrast, H. macroloba exhibited lower population differentiation, extensive gene flow, and non-significant neutrality test results, indicating long-term demographic stability without recent, drastic population events. Further validation based on gene flow analysis and divergence time estimation revealed that the lineage divergence of H. discoidea is older, while H. macroloba represents a lineage with a relatively younger evolutionary origin restricted to the Indo-Pacific region. This striking dichotomy clearly illustrates the interplay between intrinsic species-specific traits (e.g., dispersal capacity) and extrinsic historical factors (e.g., paleo-oceanographic events), leading to contrasting evolutionary outcomes among widespread marine taxa. By elucidating how differing evolutionary histories influence patterns of genetic diversity, this study provides a predictive framework for evaluating the resilience and guiding conservation priorities for critical marine calcifiers in the context of rapid environmental change.