CSIR - National Institute of Oceanography, Regional Centre, 31261, Kochi 682 018, India.
CSIR - National Institute of Oceanography, Regional Centre, 31261, Kochi 682 018, India.
Mar Pollut Bull. 2023 Dec;197:115696. doi: 10.1016/j.marpolbul.2023.115696. Epub 2023 Oct 26.
Seasonal upwelling and the associated incursion of hypoxic waters into the coastal zone is a widely studied topic over different upwelling zones. However, its persistence or variations over short time scales are poorly addressed. The present study, therefore, brings out a first report on hourly variations in the temperature, salinity and dissolved oxygen recorded by an environmental data buoy equipped with sensors, deployed in the nearshore waters of Alappuzha (southeastern Arabian Sea) from April to August 2022. The characteristic feature of the Alappuzha coast is the development of mud banks during the southwest monsoon, providing a tranquil environment suitable for continuous sensor-based measurements when the sea remains turbulent elsewhere. The results showed that despite an advance in the upwelling intensity, there is a significant variation in the oxygen concentration in the study domain on a diurnal scale. In general, the nearshore region was under hypoxia during the first half of the day (00:00 to 12:00 h), which increased steadily to reach normoxic and supersaturated levels during the rest of the day (12:00 to 24:00 h). Statistical analysis showed that winds significantly correlate to the coastal environment's subsurface oxygen concentration. During the morning hours, the wind was weak, and the water column remained stratified over the subsurface hypoxic water layer. The situation changed in the afternoon (12:00 h onwards), as there was a steady increase in the local wind speed (>5 m/s), which was sustained during the rest of the day. A local wind speed >5 m/s can disturb the stratification and enhance the mixing process from 12:00 to 24:00 h. The total kinetic energy of 11.5 J/m is the threshold for this oxygen supersaturation. These findings emphasize the role of wind-induced mixing in alleviating coastal hypoxia, highlighting the need for further biogeochemical and ecological investigations into the impacts of alternating oxic-hypoxic conditions in nearshore waters.
季节性上升流及其伴随的缺氧水入侵沿海区是不同上升流区广泛研究的课题。然而，其在短时间尺度上的持续存在或变化仍未得到充分解决。因此，本研究首次报告了 2022 年 4 月至 8 月期间在阿拉普扎近海（阿拉伯海东南部）部署的配备传感器的环境数据浮标记录的温度、盐度和溶解氧的每小时变化。阿拉普扎海岸的特征是在西南季风期间形成泥滩，为在其他地方海况动荡时提供适合连续基于传感器的测量的宁静环境。结果表明，尽管上升流强度有所增加，但在日尺度上，研究区域的氧浓度仍存在显著变化。一般来说，近岸区域在白天的前半段（00:00 至 12:00 小时）处于缺氧状态，随后逐渐增加，在其余时间（12:00 至 24:00 小时）达到正常氧和过饱和水平。统计分析表明，风与沿海环境的次表层氧浓度显著相关。在上午，风较弱，水柱状结构在次表层缺氧水层上方保持分层。下午情况发生变化（12:00 小时后），当地风速（>5 m/s）稳步增加，并在其余时间内持续。当地风速>5 m/s 可以扰乱分层并增强从 12:00 到 24:00 小时的混合过程。总动能 11.5 J/m 是达到这种氧气过饱和的阈值。这些发现强调了风引起的混合在缓解沿海缺氧中的作用，突出了需要进一步进行生物地球化学和生态研究，以了解近岸水域中交替的好氧-缺氧条件的影响。
