# Advances in the Monitoring of Algal Blooms by Remote Sensing: A Bibliometric Analysis
## 4 Discussion  
Aquatic ecosystems are very dynamic, especially coastal ecosystems, which implies great variability. Phytoplankton is a variable that is subject to changes that can be influenced by anthropogenic or climatic variables. Among climatic phenomenon or variables that can have an effect on phytoplankton abundance and composition, we have found studies of dust storms [33], medicanes [34], typhoon-induced upwellings [35], El Nino [36; 37; 38], or Indian Ocean Dipole [39], among others, but the variation scale is different, and it can range from seasonal to decadal [20]. However, most studies analyse point situations or compare two-point situations [40]. According to several indices, the El Nino and La Nina phase of the 1997-1998 ENSO was one of the strongest on record and several studies analysed its influence on phytoplankton blooms [34; 35; 36]. However, most studies analysed short time series, such a period of 3 to 4 years (including the year before and the year after) [39; 40] or a year of coverage (October 1997 to September 1998) [23]. There is growing interest regarding the impacts of future climate change on blooms, either in marine waters [41; 42] or in inland waters [43; 44; 45; 46]. However, few studies have assessed decadal trends in variables such as surface temperature that may be altering the growth of blooming species [41; 42]. Several studies analysed two decades. [PERSON] et al. [41] analysed a time series over the 1997-2010 period in the warm oceans (region with average SST \(>\) 15 \({}^{\circ}\)C that includes tropical and subtropical biomes). [PERSON] et al. [42] studied satellite-based sea surface temperature records since 1982. [PERSON] et al. [43] studied the Western Basin of Lake Erie for the 20-year period from 1998 to 2017. [PERSON] et al. [44] studied cyanobacterial blooms from 1990 to 2016 in 30 large lakes and 10 reservoirs. However, publications analysing three decades are only 1% of the articles included in this study and are very scarce. For instance, [PERSON] et al. [45] used three decades of high-resolution Landsat 5 satellite imagery to investigate long-term trends in summertime near-surface phytoplankton blooms for 71 large lakes. These studies and others suggest that warming temperatures have and will support an expansion of some types of blooms. However, other variables must be taken into account from an historical perspective, such as fertilizer use, increased wastewater treatment, etc., which are sources of nutrients that are another important driver of algal blooms. Nevertheless, [PERSON] et al. [45] found that lakes with a decrease in bloom intensity warmed less compared to lakes with an increase in blooms, suggesting that lake warming could counteract management efforts to ameliorate eutrophication. The increasing frequency and severity of blooms in recreational waters (lakes and coastal waters) and water supply reservoirs is a concern to public health and a significant threat to the environment, \begin{table}
\begin{tabular}{c c} \hline \hline
**Study Period (Years)** & **Percentage of Publications (\%)** \\ \hline \(\leq\)1 & 43 \\ \hline \(\leq\)5 & 19 \\ \hline \(>\)5 & 18 \\ \hline \(\leq\)10 & 19 \\ \hline \(>\)30 & 1 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Duration of study period. and bloom monitoring is necessary for early warning and treatment [47; 48]. The major concern is with cyanobacteria-dominated HABs [49]. We observed in Figure 5 an increasing trend in inland water studies since 2010. This trend can be due to the increasing awareness on eutrophication problems of these water bodies, but it has been made possible thanks to the recent advancements in high spatio-temporal resolution sensors [47]. The greatest efforts of monitoring programs go to the freshwater resource, that is in inland waters, recognizing freshwater as a first-class ecosystem service [23].