Shenzhen International Graduate School, Tsinghua University, Shenzhen 518055, PR China.
Shenzhen International Graduate School, Tsinghua University, Shenzhen 518055, PR China; Fujian Key Laboratory of Watershed Ecology, Key Laboratory of Urban Environment and Health, Institute of Urban Environment, Chinese Academy of Sciences, Xiamen 361021, PR China.
Sci Total Environ. 2022 Oct 20;844:157112. doi: 10.1016/j.scitotenv.2022.157112. Epub 2022 Jul 1.
The process and management of red tide in oligotrophic waters are poorly understood as most studies on red tide were focused on eutrophic areas. In this study, 404 red tide events together with the historical water quality dynamics during 1991-2020 were investigated in an anthropogenically influenced bay in China - Mirs Bay, whose most region is oligotrophic except small inshore areas. Red tides of oligotrophic offshore accounted for 20 % of all. With the effective governmental management on inshore areas, concentration of PO and DIN has been decreased to a low level (PO <0.01 mg/L while DIN <0.1 mg/L) in the bay since about 2000. However, the reduction of nutrients was still accompanied by the frequent outbreaks of red tides, as well as a shift of dominant algae from diatoms to dinoflagellates, which might be due to the unbalanced nutrient reduction, such as N:P ratio fluctuation and organic nutrient increase. This shift might trigger more red tide events and even some super ones (long-duration or large-scale) in oligotrophic areas. Detailed analysis on red tide events combined with model simulation proved that the outbreak of red tide in Mirs Bay was caused by the joint contribution of nutrients and hydrodynamics. Nutrients of inshore area supported the red tides there, and with the help of physical conditions, red tides inshore could be transferred to offshore areas and then were likely to bloom again or be preyed to support blooms of other organisms. This study acknowledged that the reduction of both N and P either inorganic or organic nutrients was essential to control red tides, even in oligotrophic waters, but a balanced strategy considering the dual reduction of both nitrogen and phosphorus was of pivotal role to restore the health of coastal water systems disturbed by human.
富营养化水域中的赤潮过程和管理尚未得到充分理解，因为大多数赤潮研究都集中在富营养化区域。本研究调查了中国人为影响的海湾——大鹏湾 1991-2020 年期间的 404 次赤潮事件及其历史水质动态，该湾大部分区域贫营养，仅小部分近岸区域为中营养。贫营养外海赤潮占所有赤潮的 20%。自 2000 年以来，由于对近岸区域的有效政府管理，海湾内 PO 和 DIN 的浓度已降低到较低水平（PO<0.01mg/L，DIN<0.1mg/L）。然而，营养物质的减少仍伴随着赤潮的频繁爆发，以及优势藻类从硅藻向甲藻的转变，这可能是由于营养物质的不平衡减少，如 N:P 比波动和有机营养物质增加。这种转变可能会在贫营养区域引发更多的赤潮事件，甚至是一些超级赤潮事件（持续时间长或规模大）。结合模型模拟对赤潮事件的详细分析证明，大鹏湾赤潮的爆发是由营养物质和水动力的共同作用引起的。近岸区域的营养物质支持了那里的赤潮，在物理条件的帮助下，近岸赤潮可以转移到外海，然后可能再次爆发或被捕食，以支持其他生物的繁殖。本研究认为，无论是无机营养物质还是有机营养物质，减少氮和磷的供应都对控制赤潮至关重要，即使在贫营养水域也是如此，但考虑到氮和磷的双重减少的平衡策略对于恢复受人类干扰的沿海水系统的健康具有关键作用。
