Phytoplankton represent a taxonomically diverse group of photosynthetic, mostly single-celled aquatic organisms that drift with the current. There are approximately 20,000 species of phytoplankton that are distributed among eight phyla. Phytoplankton can be divided into three distinct groups: diatoms, dinoflagellates, and coccolithophorids and microflagellates. Unlike terrestrial plants, which have more than 250,000 recorded species, phytoplankton are poor in species diversity but are phylogenetically diverse [27].

Apart from bacteria, phytoplankton are the most abundant life form in pelagic ecosystems [28]. They have short life cycles and are amenable to subtle variations in the environment [29,30,31]. Diatoms, for example, can adapt to warming over a period of about three weeks after 300 generations [19,32]. Dinoflagellates, including species (Prorocentrum micans) common in cold temperate to tropical waters, are also able to adapt to elevated pH levels over short cycles of 34–126 generations [33]. The calcification of coccolithophorids such as Emiliania huxleyi is also adaptable to elevated temperatures and carbon dioxide levels after seven generations [34]. This ability to respond quickly to changes in the environment qualifies microalgae as good reference indicators for assessing the impact of global climate change [30,35].

Current efforts to evaluate the response of aquatic ecosystems to climate change and other anthropogenic factors involve the use of water quality criteria, usually derived from studies testing the response of sensitive organisms [36]. These analyses typically consider the response of keystone organisms with large populations distributed across different ecosystem zones [29,37]. Microalgae are a good example of these keystone organisms because of the significant role they play in aquatic ecosystems [30]. These organisms modulate the efficiency of aquatic food webs with consequences for the global carbon cycle, food security, and livelihood opportunities (e.g., fisheries) in many communities [21,38,39].

In addition to primary production, phytoplankton also play a significant role in the carbon (C), nitrogen (N), phosphorus (P), iron (Fe), and silicon (Si) biochemical cycles. They utilise these compounds for their vital processes, and in these processes, they reintroduce them back to the environment as either particulate or dissolved organic matter, which is either remineralised by heterotrophs and transferred to higher trophic levels or sinks to form elemental compositions in deeper waters. Studies have highlighted that understanding how phytoplankton take up these inorganic nutrients and allocate their resources to undertake physiological processes is important in understanding the present, past, and future linkages to these important biogeochemical cycles [40].

Phytoplankton also play an important role in influencing the Earth’s climate and the functioning of the biological carbon pump. The biological carbon pump is a major component of the global carbon cycle and refers to the process by which atmospheric CO₂ is transferred by primary producers (mostly phytoplankton) from the eutrophic zone of the ocean to the underlying sediments [41]. Marine ecosystems provide the major sink for atmospheric CO₂ and account for the removal of approximately one-third of all anthropogenic CO₂ emissions from the atmosphere [42]. The conversion of atmospheric CO₂ to ocean sediment is a direct result of the combined effect of solubility and the biological pump [41]. In addition, the effectiveness of the biological pump depends highly on phytoplankton physiology and community structure. Phytoplankton primarily drive the biological pump through primary production, where they convert inorganic carbon into organic matter. As a result, after their consumers and bacteria that feed on their waste die, they are transported down into sediments, where they are locked out of circulation for centuries.

Despite their considerable importance, some phytoplankton species may have direct, devastating impacts on humans and animals through the production of toxic algal blooms. Algal blooms refer to the above-average outbreak of phytoplankton cells within a given body of water, which occur during peaks in the annual cycle of phytoplankton biomass and chlorophyll concentrations [43]. These blooms occur when the rate of phytoplankton assemblage exceeds their normal mortality rates, often facilitated by the occurrence of excess growth-limiting nutrients such as nitrogen and phosphorus in the environment. As a result, the phytoplankton assemblages accumulate in the water column until the limiting nutrients are depleted. Out of the thousands of phytoplankton species, only less than 5% can form algal blooms [43]. When they occur, HABs pose significant effects to human and biodiversity health, recreation, and aquaculture. It is projected that stressors such as pollution and climate change will influence aquatic planktonic systems, thereby increasing the frequency and intensity of harmful algal blooms [44].

The diversity, growth, and development of plankton communities are greatly tied to environmental parameters such as nutrient availability, light regimes, temperature, alkalinity, and pH [21,45]. Climate change and its associated impacts on aquatic environments alter these environmental parameters and, subsequently, phytoplankton community structures [20]. As a result, studies in phytoplankton ecology have always sought to understand how these organisms respond to environmental stressors emanating from anthropogenic activities [19,46,47,48,49,50,51].