Marine pollution bulletin最新文献

Coastal deoxygenation impacts phytoplankton communities crucial for marine productivity. The inter- and intra-annual variability in phytoplankton communities at a shallow (27 m) station over the Western Indian Shelf (CaTS site, off Goa) during deoxygenation events of the late southwest monsoon (LSWM September-October) were studied from 2020 to 2023. The water column (0-27 m depth) experienced seasonal hypoxia/anoxia at subsurface depths (0-1.4 mL L^-1 O₂, 18-27 m) with the presence of hydrogen sulfide in the bottom waters during LSWM of all years. The stratified, deoxygenated, near-bottom waters were cold, highly saline, and nitrate-depleted, whereas the upper layers of the water column (<18 m) retained warmer temperatures and well‑oxygenated conditions, with relatively lower salinity. The phytoplankton abundance and species composition showed inter and intra-annual variations. Interestingly, higher phytoplankton abundance found in the anoxic waters (>18 m) was dominated by diatoms, especially pennate forms, in the dark zone (i.e., photosynthetically active radiation (PAR) <1 μE m^-2 s^-1). Experiments on the effect of dark, anoxic conditions on phytoplankton further revealed that certain pennate diatoms, such as Fragilaria sp., Pleurosigma sp., and Navicula sp., exhibit greater resilience and survival under dark, anoxic conditions compared to other species. The oxic surface waters were chiefly dominated by centric diatoms. At times, oxic surface waters were also dominated by dinoflagellates, while they were relatively less abundant in low-oxygen waters in the study area. In the climate change scenario, future studies are required to understand the expansion and magnitude of the deoxygenation zones in the coastal waters and its effect on phytoplankton in the marine ecosystems.

Wind-induced currents are the major forces responsible for sediment resuspension and transport in micro-tidal bays. To reveal the impact of wind-induced residual currents on the sediment flux, in-situ measurements using acoustic Doppler current profilers (ADCPs) were conducted at two mooring stations in a heavily contaminated, micro-tidal Onsan Bay. During the mooring period, the suspended sediments at both stations were transported seaward (landward) at the surface (bottom) layer mainly through the residual currents (mean-flow flux F_mean: > 70 % of the total flux). Under northerly winds, the landward bottom residual currents at both stations strengthened, resulting in the "intensification" of landward F_mean. This suggests that the northerly winds might be a primary factor intensifying the landward sediment fluxes, potentially resulting in the increased sediment deposition into the bay. The findings provide insights into managing sedimentation in contaminated coastal bays and highlight the importance of wind effects on sediment transport in micro-tidal bays.

Xiangshan Bay, one of China's most eutrophic semi-enclosed bays, was studied to examine the seasonal distributions of salinity, temperature, nutrients, and nitrate isotopes (δ¹⁵N and δ¹⁸O) to elucidate seasonal variations in nitrate sources and the key factors driving nitrogen level fluctuations. Based on nitrate δ¹⁵N (6.1-8.9 ‰) and δ¹⁸O (0-5.7 ‰) values, the primary nitrogen sources in Xiangshan Bay were identified as domestic sewage and agricultural runoff, influenced by seasonal agricultural activities and monsoon-driven Changjiang Diluted Water (CDW) dynamics. Nitrate levels peaked in autumn (53.1 ± 1.6 μmol/L), due to intensive agricultural activity and strong CDW intrusion driven by the northeast monsoon. Conversely, the southwest monsoon in summer, coupled with higher nitrogen uptake due to warmer temperatures and increased light availability, resulted in the lowest nitrate levels (35.2 ± 5.4 μmol/L). In winter, reduced nitrate inputs were observed, reflecting minimal agricultural activity and weaker Changjiang discharge. Differences in nitrate levels between the upper and lower bay were attributed to weaker mixing and stronger local inputs in the upper bay.

The rapid rise in temperature due to climate change has raised concerns about its impact on the keystone species Crangon hakodatei in the East Sea of Korea. However, no studies have examined the population characteristics of C. hakodatei or its response to temperature change along the coast. In this study, we investigated these aspects using year-round sampling of C. hakodatei and environmental data collected along the east coast of South Korea. A positive correlation was observed between shrimp abundance and temperature. Specifically, both the growth and reproductive characteristics of C. hakodatei were affected by temperature variations. Females and males grew more slowly during the spawning season and cold periods, respectively. Notably, primary breeding, with the highest participation of females, occurred during the coldest periods in the East Sea. Overall, these results warn of a climate crisis leading to a reduced abundance of keystone species in Korean coastal waters.