# Advances in the Monitoring of Algal Blooms by Remote Sensing: A Bibliometric Analysis
## 1 Introduction  
Remote sensing of ocean colour began in 1978 with the launch of NASA's Coastal Zone Colour Scanner (CZCS). Since then, several new ocean-colour sensors have been launched and more are planned for the near future by various space agencies. Several water properties can be derived from remote sensing imagery, e.g., sea surface temperature (SST), chlorophyll-\(a\), turbidity, and optical properties. These properties are applied to study different water quality issues and marine environmental management [1; 2]. Among them, phytoplankton blooms, also known as algal blooms, are of significant relevance [3; 4]. An algal bloom can be defined as the rapid growth of one or more species leading to an increase in the species' biomass resulting from favourable environmental conditions [5; 6]. However, there is not a unique threshold that defines what can be considered a bloom [7]. To define a bloom, it iskey to know the standard values and seasonal patterns at a specific site, so a bloom can be defined as a deviation to higher values than those [8]. Unusual high phytoplankton biomass values can be very different between sites, from concentrations of a few hundreds to millions of cells per millilitre, depending on the causative species [6; 8]. Different adjectives have been used to characterize the degree of the negative impact of these blooms according to their characteristics and those of the causative species, such as toxic, noxious, or harmful [9], which can cause negative impacts to other aquatic organisms and on the ecosystem and human health, and they are often divided into toxic versus high-biomass blooms [10]. Harmful blooms causing mass mortality of organisms during a long lasting phytoplankton bloom may be a result of three main factors: (1) Asphyxia caused by saturation of the microalgae in the water and/or mechanical asphyxia caused by clogging of the fish gills. (2) Asphyxia caused by hyper-europhication of the system creating anoxic-hypoxic conditions in the water column. (3) Poisoning of organisms because of consuming or bioaccumulating toxins produced by certain phytoplankton species [11; 12; 13; 14]. The most common and repetitive blooms occurring at present cannot automatically be labelled as harmful algae blooms (HABs). Identifying phytoplankton blooms has been the target of extensive research [15]. Blooms in a strict sense are a natural phenomenon that has occurred throughout recorded history. However, due to anthropogenic activities, blooms appear to have increased in frequency, intensity, and geographic distribution [16]. Climatic variability plays a key role in factors like wind-driven upwelling and thermal stratification, which largely influence phytoplankton abundance and composition. Nowadays, it is hypothesised that climate change will have a direct effect on those factors and modify future scenarios for algal blooms [17]. Ocean colour is recognised as an Essential Climate Variable (ECV) by the Global Climate Observing System (GCOS) [18; 19]. To interpret the effect of different oceanographic factors, it is necessary to establish continuous monitoring programs for the variables that may be more sensitive. These programs must generate time series long enough to be able to differentiate natural variations from those of anthropogenic origin [20]. Data can be obtained from in situ monitoring, but, in recent years, remote sensing technologies have emerged as an adequate tool for providing a synoptic view of extensive ocean or coastal areas, effective in complementing in situ sampling programs [21; 22; 23; 24]. Time series derived from ocean-colour data, at the global scale and at high spatial resolution, are key to studying phytoplankton variability at seasonal and inter-annual scales [19]. But even satellite data will require decades of observations before the signal of climate change is noticed over large regions of the ocean [25].