Xiaogang Xing

Understanding the seasonal dynamics of phytoplankton biomass and the deep chlorophyll maximum in oligotrophic environments: A Bio‐Argo float investigation

We deployed four Bio-Argo profiling floats in various oligotrophic locations of the Pacific subtropical gyres and Mediterranean Sea to address the seasonal phytoplankton dynamics in the euphotic layer and explore its dependence on light regime dynamics. Results show that there is a similar phytoplankton biomass seasonal pattern in the four observed oceanic regions. In the lower part of the euphotic layer, the seasonal displacement of the deep chlorophyll maximum (DCM) is light driven. During winter, the chlorophyll a concentration ([Chl a]) always increases in the upper euphotic mixed layer. This increase always results from a photoacclimation to the reduced irradiance. Depending on the location, however, the concentration can also be associated with an actual increase in biomass. The winter increase in [Chl a] results in an increase in irradiance attenuation that impacts the position of the isolume (level where the daily integrated photon flux is constant) and DCM, which becomes shallower. In summer when the [Chl a] in the upper layer decreases along with light attenuation, the DCM deepens and becomes closer to (and sometimes reaches) the nitracline, which enhances the phytoplankton biomass at the DCM. The bio-optical mechanisms and their relationship to light regimes that are revealed by the time series appear to be generic and potentially characteristic of all of the areas where a DCM forms, which is 50% of the open ocean.

An overview of remote sensing of chlorophyll fluorescence

Besides empirical algorithms with the blue-green ratio, the algorithms based on fluorescence are also important and valid methods for retrieving chlorophyll-a concentration in the ocean waters, especially for Case II waters and the sea with algal blooming. This study reviews the history of initial cognitions, investigations and detailed approaches towards chlorophyll fluorescence, and then introduces the biological mechanism of fluorescence remote sensing and main spectral characteristics such as the positive correlation between fluorescence and chlorophyll concentration, the red shift phenomena. Meanwhile, there exist many influence factors that increase complexity of fluorescence remote sensing, such as fluorescence quantum yield, physiological status of various algae, substances with related optical property in the ocean, atmospheric absorption etc. Based on these cognitions, scientists have found two ways to calculate the amount of fluorescence detected by ocean color sensors: fluorescence line height and reflectance ratio. These two ways are currently the foundation for retrieval of chlorophyl l - a concentration in the ocean. As the in-situ measurements and synchronous satellite data are continuously being accumulated, the fluorescence remote sensing of chlorophyll-a concentration in Case II waters should be recognized more thoroughly and new algorithms could be expected.

Seasonal variations of bio‐optical properties and their interrelationships observed by Bio‐Argo floats in the subpolar North Atlantic

Based on in situ data sets collected using two Bio-Argo floats deployed in the subpolar North Atlantic from June 2008 to May 2010, the present study focuses on the seasonal variability of three bio-optical properties, i.e., chlorophyll-a concentration ([Chla]), particle backscattering coefficient at 532 nm (bbp(532)), and particle beam attenuation coefficient at 660 nm (cp(660)). In addition, the interrelationships among these properties are examined. Our results show that: (1) [Chla], bbp(532) and cp(660) are largely well coupled with each other in the upper layer, all being minimum in mid-winter (January) and maximum in summer; (2) the backscattering coefficient presents an abrupt increase in late summer in the Icelandic Basin, likely due to a large contribution of coccolithophores following the diatom spring bloom; (3) the intercorrelations between the three bio-optical properties are basically consistent with previous studies; (4) seasonal variation in the of [Chla] to cp(660) ratio exhibits a clear light-dependence, most likely due to the phytoplankton photoacclimation.

The relation of chlorophyll-a concentration with the reflectance peak near 700 nm in algae-dominated waters and sensitivity of fluorescence algorithms for detecting algal bloom

In order to investigate the relation of chlorophyll-a concentration with the reflectance peak near 700 nm, reflectance spectra of harmful algal bloom (HAB) species and non-HAB algae were obtained based on in situ measurements in the oceans and cultural tank data. It is found that the fluorescence line heights (FLH) of reflectance spectra and the concentrations of seawater chlorophyll-a have good correlation; their coefficients of determination are larger than 0.86, excepting Ceratium furca and Heterosigma akashiwo. It is specially noted that for some algae, such as Dicrateria zhanjiangensis Hu., Pyramimonas sp. and Nitzschia closterium, the corresponding coefficients of determination exceed 0.95. In addition, the various satellite fluorescence algorithms were compared, and the sensitivity of fluorescence algorithms was investigated. It is found that the designed bands of Medium Resolution Imaging Spectrometer (MERIS) are more reasonable than those of Moderate Resolution Imaging Spectroradiometer (MODIS) for detecting algal bloom.

A novel near real-time quality-control procedure for radiometric profiles measured by Bio-Argo floats: protocols and performances

AbstractAn array of Bio-Argo floats equipped with radiometric sensors has been recently deployed in various open ocean areas representative of the diversity of trophic and bio-optical conditions prevailing in the so-called case 1 waters. Around solar noon and almost every day, each float acquires 0–250-m vertical profiles of photosynthetically available radiation and downward irradiance at three wavelengths (380, 412, and 490 nm). Up until now, more than 6500 profiles for each radiometric channel have been acquired. As these radiometric data are collected out of an operator’s control and regardless of meteorological conditions, specific and automatic data processing protocols have to be developed. This paper presents a data quality-control procedure aimed at verifying profile shapes and providing near-real-time data distribution. This procedure is specifically developed to 1) identify main issues of measurements (i.e., dark signal, atmospheric clouds, spikes, and wave-focusing occurrences) and 2) validate...

In situ determination of sun-induced chlorophyll a fluorescence quantum yield in the North China Sea

Sun‐induced chlorophyll a fluorescence signals were measured in the North China Sea using an underwater hyperspectral spectroradiometer. We combined these signals with other apparent and inherent optical properties to derive the quantum yield of chlorophyll a fluorescence in vivo, and determined that the maximum was around 0.06 and the average value reached 0.017, with most values ranging from 0.001 to 0.02, consistent with results obtained in other sea areas. In addition, we validated some empirical relationships with in situ measurements, and compared them with the former results. We found that the correlations between inherent optical properties and chlorophyll a concentration in this area are very different from those in other seas; however, the relationship between the diffuse attenuation coefficient for the instantaneous photosynthetically available radiation (IPAR) and that at 490 nm is comparatively stable.