Assessment of the summer–autumn bloom in the Bohai Sea using satellite images to identify the roles of wind mixing and light conditions Feifei Liu a,b, ⁎, Jian Su b,c , Andreas Moll c , Hajo Krasemann b , Xueen Chen a , Thomas Pohlmann c , Kai Wirtz b a College of Physical and Environmental Oceanography, Ocean University of China, Songling Road, 238, 266100 Qingdao, China b Institute for Coastal Research, Helmholtz-Zentrum Geesthacht Centre for Materials and Coastal Research, Max-Planck-Str. 1, 21502 Geesthacht, Germany c Institute of Oceanography, University of Hamburg, Bundesstr. 53, 20146 Hamburg, Germany abstract article info Article history: Received 21 March 2013 Received in revised form 4 July 2013 Accepted 15 July 2013 Available online xxxx Keywords: Bohai Sea MERIS data Phytoplankton bloom Wind mixing Light condition In shallow coastal ecosystems, summer–autumn phytoplankton blooms require thorough investigation because of their large variability in space and time. This variability results primarily from the complex interplay of differ- ent governing factors, e.g., wind mixing and light availability. To identify the effects of these governing factors on the summer–autumn blooms in the Bohai Sea, we first established a statistical approach to define the biological variables that characterize summer–autumn blooms, i.e., the bloom onset time (denoted as bloom onset), the bloom magnitude, and the chlorophyll-a (Chl-a) increase rate during the bloom period (denoted as Chl-a in- crease rate). These variables were quantified using daily Medium Resolution Imaging Spectrometer (MERIS) sat- ellite scenes from 2003 to 2009. We observed a remarkable onshore–offshore gradient in all three biological variables. For example, bloom onsets were delayed in coastal areas compared to deep offshore waters. Spatially, the bloom onset exhibited a high correlation with wind mixing prior to the bloom period (R N 0.6), whereas the Chl-a increase rate and bloom magnitude were highly correlated to the distribution of the light attenuation coef- ficient (K d(min) ,R= -0.65 and R = 0.69, respectively). Furthermore, the empirical orthogonal function (EOF) analysis revealed similar interannual variability in the K d(min) , bloom onset and Chl-a increase rate. We also ob- served similar interannual trends in the bloom magnitude and wind mixing during the bloom period. From the analysis above, we believe that light is a crucial limiting factor for summer–autumn phytoplankton dynamics in the Bohai Sea. Wind influenced Chl-a dynamics in many ways and at various scales. Spatially, weak wind speeds prior to bloom events tended to provide favorable light conditions for the development of blooms. Over the long-term, however, enhanced wind mixing tended to supply the surface layer with nutrients more effective- ly and thus promoted a higher bloom intensity. © 2013 Elsevier B.V. All rights reserved. 1. Introduction The summer–autumn phytoplankton bloom dynamics in coastal waters have been poorly investigated in the past, mostly because of the lack of measurements with sufficiently high resolution in space and time (Cloern, 2001). In recent years, a unique means of studying the spatio-temporal distribution of coastal phytoplankton has been pro- vided by high-resolution ocean color sensors (e.g., Brickley and Thomas, 2004; D. Tang et al., 2003). Satellite-based studies have clearly identified a strong variability in chlorophyll-a (Chl-a) concentration over different spatio-temporal scales (Iida and Saitoh, 2007; Navarro and Ruiz, 2006; Sackmann et al., 2004). Although many studies have focused on spring blooms (e.g., Tian et al., 2009; Wiltshire et al., 2008), satellite-based data can also help to quantify variability in summer–autumn blooms and, thereby, start pinpointing the mechanisms behind that variability (Kim et al., 2007; Ueyama and Monger, 2005). In addition, not only can the growing number of satellite images constrain parameterizations of ecological models (Gohin et al., 2005), multi-scale correlations be- tween biological and environmental variables will help to improve functional representations within the models. The Bohai Sea is a semi-enclosed sea in northeast China (Fig. 1) with an area of approximately 77,000 km 2 . It is a typical shallow water body with a mean depth of 18.7 m and maximum depth of approximately 70 m located near the northern coast of the Bohai Strait. The Bohai Sea experiences low water exchange capacity because the Bohai Strait with a width of ~150 km is the only connection to the Yellow Sea (Zhao and Shi, 1993). The Yellow River, the second largest river in China, drains into the Bohai Sea. The run-off shows a very strong season- al variability reaching over 10,000 m 3 /s in summer with an annual mean value of only 1,400 m 3 /s. Connected to this freshwater inflow is a strongly variable input of sediments and nutrients (Martin et al., 1993). The wind field of the Bohai Sea is dominated by the East Asia Monsoon, which is characterized by strong northwesterly winds with a mean speed of 6–7 m/s in winter and weak southeasterly winds with a mean speed of less than 4–6 m/s in summer, both of which are Journal of Marine Systems xxx (2013) xxx–xxx ⁎ Corresponding author at: Institute of Oceanography, University of Hamburg, Bundesstr. 53, 20146 Hamburg, Germany. Tel.: +49 (0)40 42838 7486; fax: +49 (0)40 42838 2995. E-mail address: feifei.liu@zmaw.de (F. Liu). MARSYS-02404; No of Pages 15 0924-7963/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jmarsys.2013.07.007 Contents lists available at ScienceDirect Journal of Marine Systems journal homepage: www.elsevier.com/locate/jmarsys Please cite this article as: Liu, F., et al., Assessment of the summer–autumn bloom in the Bohai Sea using satellite images to identify the roles of wind mixing and light condi..., J. Mar. Syst. (2013), http://dx.doi.org/10.1016/j.jmarsys.2013.07.007