Abstract
Tides are the dominant hydrodynamic processes in most continental shelf seas and have been proven to have a significant impact on both marine ecosystem dynamics and biogeochemical cycles. In situ and satellite observations have suggested that the spring-neap tide results in fluctuations of chlorophyll-a concentrations (Chl-a) with a fortnightly period in some shelf waters. However, a large number of missing values and low observation frequency in satellite-observed Chl-a have been recognized as the major obstacle to investigating the regional pattern showing where and to what extent of the effects of spring-neap tide on Chl-a and the seasonal variations in the effects within a relatively large region. Taking Himawari-8 as an example, a simple algorithm appropriate for geostationary satellites was proposed in this study with the purpose of obtaining a tide-related daily climatological Chl-a dataset (TDCD) and to quantitatively estimate the effects of the spring-neap tide on Chl-a variations. Based on the Chl-a time series from TDCD, significant fortnightly signals of Chl-a fluctuations and high contribution together with high explanations of the fortnightly fluctuations for Chl-a variations were found in some specific inshore waters, especially in the East China Sea, Bay of Bengal, South China Sea, and northern Australian waters. The spring-neap tide was found able to induce the spatio-temporal fortnightly fluctuations of Chl-a with an annual amplitude of 12–33% of the mean in these inshore areas. Significant seasonal variations in the fortnightly fluctuation of Chl-a were observed in the temperate continental shelf regions, while levels remained relatively stable in the tropical waters. Further analysis implied that the spatio-temporal fortnightly fluctuations of Chl-a were closely associated with the tidal current differences between the spring and neap tides. Seasonal variations in the tidal current differences were found to be a key driving factor for seasonal fluctuations of the spring-neap tidal effects on Chl-a in the temperate continental shelf regions. This study provides a better understanding of tide-related marine ecosystem dynamics and biogeochemical cycles and is helpful in improving physical–biogeochemical models.
Highlights
The continuous global monitoring capability of satellite remote sensing led to its recognition for use in the studies on variations in chlorophyll-a concentrations (Chl-a) since the end of the last century, and our understanding of marine ecosystem dynamics from intraseasonal to interannual time scales with local, regional, or global perspectives has been advanced continually as a result of using this technology (Martinez et al, 2009; Dutkiewicz et al, 2019; Neil et al, 2019; O’Reilly and Werdell, 2019)
It has been reported that the growth of many fish species shows a fortnightly variation with the peak occurring just after the spring tide, and the bloom of plankton induced by the spring tide may be one of the most important events that account for these biological phenomena (Rahman and Cowx, 2006; Krumme et al, 2008; Li et al, 2021)
It should be pointed out that prominent fortnightly variations in Chl-a were mainly found in inshore waters, it is not always the case as there are some regions with prominent fortnightly variations in the time series of Chl-a in the open ocean, like the tropical waters
Summary
The continuous global monitoring capability of satellite remote sensing led to its recognition for use in the studies on variations in chlorophyll-a concentrations (Chl-a) since the end of the last century, and our understanding of marine ecosystem dynamics from intraseasonal to interannual time scales with local, regional, or global perspectives has been advanced continually as a result of using this technology (Martinez et al, 2009; Dutkiewicz et al, 2019; Neil et al, 2019; O’Reilly and Werdell, 2019). Most previous studies have focused only on the spatial and temporal variability of Chl-a during a single spring-neap tidal cycle within relatively small areas (e.g., bays and estuaries) due to the missing values in satellite-observed Chl-a (Shi et al, 2013; Gernez et al, 2017; Yang et al, 2020), while the regional pattern showing where and to what extent of this effect still remains unclear Both tide and Chl-a usually exhibit well-recognized seasonal variations, and it was littleknown that these seasonal variations exerted effects on the Chl-a fluctuation induced by spring-neap tides. This product has a spatial resolution of 1/30◦ and the assimilation of various altimetric and measured data has led to its application in many tide-related studies (e.g., Yu et al, 2015, 2017)
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