# Advances in the Monitoring of Algal Blooms by Remote Sensing: A Bibliometric Analysis
## 4 Discussion
A patchy distribution of blooms is quite common and sensors with a spatial resolution of 4, 9, and 25 km may not be enough [27]. The duration is quite variable, from days to weeks is the most common, but it can last even months [59]. The frequency of passage of the satellites depends on their orbit (polar or geostationary) and on the latitude of the area to be studied, and may vary between several daily passes and several days between passes (the lower the latitude, the lower the frequency). This added to the cloudy characteristics of some areas can prevent detection of short duration blooms. This disadvantage can be overcome using a multisensor approach. However, according to our research, the multisensor approach is still the minority [60; 61; 62]. It is important to highlight the effort undertaken by the European Space Agency (ESA) that launched its Climate Change Initiative (CCI) in 2010 [63]. This initiative was dedicated to producing climate-quality data for Essential Climate Variables (ECVs), and among them were ocean colour variables [63]. To be able to apply the multisensor approach, data from multiple sensors must be merged to create a single time series, and inter-sensor biases must be corrected to avoid introducing artefacts in the time series [19]. The CCI obtained a time series for the period 1997 to 2018 using data from the MERIS sensor of the European Space Agency, the SeaWiFS and MODIS-Aqua sensors from the National Aeronautics and Space Administration (USA), and VIIRS (Visible and Infrared Imaging Radiometer Suite) from the National Oceanic and Atmospheric Administration (USA) [19]. To achieve this, all radiometric products band-shifted to a common set of bands corresponding to SeaWiFS and corrected for inter-sensor bias [15]. The focus of the CCI is on open-ocean (case-1) waters [19]. Remote sensing reflectance (RRS) is the standard product provided by space agencies. To obtain chlorophyll-_a_ (Chl-_a_), which is the most common variable used to track changes in phytoplankton, several algorithms and quasi-analytical methods are applied to RRS [26; 64]. However, it has been proven that RRS in the blue-green spectrum is likely to have a stronger and earlier climate-change-driven signal than Chl-_a_[19]. This is because RRS integrates not only changes in-water Chl-_a_, but also in other optically important constituents (e.g., Coloured Dissolved Organic Matter (CDOM) [19]). This is key for optically complex waters (case-2). During events of HABs, water discolouration can be due to CDOM accumulated from decaying cells; then RRS will be more appropriate than Chl-_a_ algorithms. According to our results, approximately 70% of the reviewed studies focused on detecting high Chl-_a_ levels that can not necessarily be classified as HABs (e.g., do not cause mortality events) and do not report the causative species. The studies that identified at least the causative group or species are in the minority. Among the bloom forming species studied, we can cite the dinoflagellates _Karenia brevis_, _Karenia mikimotoi_, _Alexandrium fundyense_, _Cochodimium polykrikoides_, _Prorocentrum donghaiene_, _Prorocentrum minimum_, _Noctiluca miliaris_, _Alexandrium minutum_, _Alexandrium catenella_, _and Alexandrium taylori_; the diatoms _Pseudo-nitzschia_, _Rhizosolenia_ spp., and _Rhizosolenia mibricata_; the cyanobacteria _Microcytsis_ spp., _Trichdoesmium erythraeum_, _Trichdoesmium_ spp. _Synechococcus_, _Nodularis spumigena_, _Anabenenicinalis_, and _Aphanizomenon fos-aquie_ sp. ; and the haptohytes _Emiliania huxleyi_, _Phaeocystis globose_, and _Phaeocystis pouchetti_. Even if some of these species can be toxin producers, only a few studies analysed the toxin concentration in water samples [65; 66; 67; 68; 69; 70]. These studies involved inland waters or coastal waters with shellfish farming [70]. Regarding bloom depth, for a phytoplankton bloom to be detected by remote sensors it must be in the first optical depth [71].