Abstract Chlorophyll-a (Chl-a) concentration is an important indicator of the state of marine ecosystems. In the current study, the interannual-decadal variation in the Bohai Sea's surface Chl-a concentration and the underlying dynamics are investigated based on 16 years (August 2002–February 2019) of satellite ocean colour observations. The Chl-a concentration shows a spatially coherent increasing trend from 2003 to 2011 and a decreasing trend from 2012 to 2018 in both the Bohai and North Yellow Seas. Both decadal trends are clearer in the Bohai Sea than in the Yellow Sea and in coastal waters than in offshore waters. The decreasing trend is more obvious than the increasing trend. The low-frequency variations are mainly controlled by sea surface temperature (SST) and rainfall variations and eventually related to the Pacific Decadal Oscillation (PDO). During positive PDO phases, warm SST anomalies exist in the Bohai and Yellow Seas and negative rainfall anomalies exist over the sea and surrounding lands. These anomalies would decrease the surface layer dissolved inorganic nitrogen concentration through suppressing the upward turbulent nutrient diffusion and reducing land-sourced nutrient fluxes into the ocean, leading to negative Chl-a concentration anomalies. During negative PDO phases, the opposite situation occurs. Human activities may make a secondary contribution through enhancing/reducing the anthropogenic nitrogen emission before/after 2011. This study emphasizes the natural climate variability in modulating low-frequency variations in surface Chl-a concentrations in the Bohai and Yellow Seas in recent decades and is useful for marine resource planning and marine ecological management in the Bohai Sea in the future.

中文翻译：

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过去16年渤海卫星表面叶绿素a浓度的年际变化

摘要 叶绿素a（Chl-a）浓度是反映海洋生态系统状况的重要指标。在目前的研究中，基于16年（2002年8月-2019年2月）的卫星海洋颜色观测，研究了渤海表面Chl-a浓度的年际-年代际变化及其潜在动态。渤海和北黄海的 Chl-a 浓度从 2003 年到 2011 年呈现出空间一致的增加趋势，从 2012 年到 2018 年呈下降趋势。渤海比黄海和近海比近海更清楚这两个年代际趋势。下降趋势比上升趋势更明显。低频变化主要受海面温度（SST）和降雨变化控制，最终与太平洋年代际涛动（PDO）有关。在正 PDO 阶段，渤海和黄海存在暖海温异常，海洋和周围陆地存在负降雨异常。这些异常将通过抑制向上湍流养分扩散和减少陆源养分流入海洋来降低表层溶解无机氮浓度，从而导致 Chl-a 浓度负异常。在负 PDO 阶段，会发生相反的情况。2011年前/后人类活动可能通过增加/减少人为氮排放做出次要贡献。