Review of Fronts and Its Ecological Effects in the Shelf Sea of China

Ocean fronts exist at the boundaries of two or more water masses with distinct characteristics. The secondary circulation and convergence on the front can significantly affect marine matter transport and biological production. In the marginal seas of China (MSC), there are fourteen typical fronts (including Bohai Strait Front, Shandong Peninsula Coastal Front, Jiangsu Coastal Front, Seohan Bay Front, Kyunggi Bay Front, Western Chejudo Front, Yangtze Bank Ring Front, Min-Zhe Coastal Front, Kuroshio Front, Taiwan Bank Front, Min-Yue Coastal Front, Estuarine Front of Pear River, Coastal Front of Eastern Hainan Island, and Coastal Front of Beibu Gulf). Meanwhile, sub-mesoscale and small-scale processes such as double fronts, penetrating fronts, and frontal waves were observed in these frontal zones. Together with shelf circulation and other meso- and sub-mesoscale processes (such as eddies, internal waves, etc.), fronts control the transport and exchange of matters and energy in the MSC. Previous studies have found that the transport of matters along and across the front played a crucial role in the seasonal biogeochemical cycle and biological process in the MSC. From winter to spring, the frontal relax can enhance the transport of nutrients from the nearshore to the shelf, and modulate the timing and magnitude of the spring phytoplankton blooms. From summer to autumn, the convergence effect and the secondary circulation from fronts (e.g., estuarine fronts and the fronts formed around the Yellow Sea Cold Water Mass) can significantly increase the nutrient and light levels and play an important role in promoting the growth and aggregation of phytoplankton. In the eutrophic estuaries and coastal waters, the frontal zone can be a hot-pot of red tide or hypoxia. In addition, the physical barrier effect of the front maintains relatively independent physical and biogeochemical characteristics for the water masses of both sides, and front thus plays an important role in the biogeographical zonation, and the biodiversity gradients in the MSC. Based on our understanding to frontal effects on the biogeochemical cycling and biological production in the MSC, it is necessary to carry out the ocean observations for the fine structure and variability of the fronts, the frontal sub-mesoscale and small-scale processes, combined with high-resolution satellite data and numerical models. The ecological effects from sub-mesoscale and small-scale processes deserved a deep exploration via an interdisciplinary method.