Phytoplankton as Indicators of Climate Change: History
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Rael Adhiambo
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Paul Kojo Mensah
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Emmanuel Acheampong

Phytoplankton represent a taxonomically diverse group of photosynthetic, mostly single-celled aquatic organisms that drift with the current. 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.

  • climate change
  • phytoplankton
  • plankton communities

1. Introduction

Aquatic ecosystems cover about 71% of the Earth’s surface and comprise diverse habitats ranging from freshwater to brackish and marine environments [1]. These ecosystems provide critical functions important for achieving the sustainable development goals related to biodiversity conservation and livelihoods [2]. Now, it is common knowledge that these aquatic ecosystems and habitats in freshwater, brackish, and marine environments are under pressure from multiple stressors and factors related to pollution, global climate change, and other human activities [3][4]. Only 13% of the world’s marine environments are currently considered pristine or untouched by human impacts [5].
For coastal ecosystems in particular, estimates suggest that about 50% of salt marshes, 35% of mangroves, 30% of coral reefs, and 29% of seagrasses are already lost or degraded worldwide due to human activities [6]. This degradation is expected to be even more accelerated with the increasing urbanization of riparian/coastal communities and changes in land use, particularly for agriculture and other industrial activities [7]. This threatens the objectives of the global ocean decade as well as the sustainable development goals (SDG) to achieve food security (SDG 2) and decent economic growth (SDG 8) through sustainable management of life below water (SDG 14) [8]. There is therefore an urgent call for concerted, global action to understand and possibly predict the response(s) of aquatic ecosystems and organisms to the combination of multiple environmental stressor factors.

2. Climate Change and Its Impacts on Aquatic Ecosystems

Climate change represents one of the most important sources of environmental change in aquatic ecosystems [9][10]. Climate change has resulted in five critical global environmental changes: the warming temperature of the Earth’s surface and the oceans, changes in the global water cycle (“hydrologic” cycle), declining glaciers and snowpack, sea level rise, and ocean acidification. These changes in the environment are primarily driven by the emissions of greenhouse gases in the atmosphere from anthropogenic sources, such as the combustion of fossil fuels, coupled with the resultant warming of the Earth’s surface. Reports from the Intergovernmental Panel for Climate Change (IPCC) have shown that the world has been warming gradually, with some studies estimating that these temperature increases have doubled over the last 50 years alone [9][10][11]. This warming will potentially have disproportional catastrophic effects on various ecosystems by altering their structures and functions, particularly those adapted to specific climatic conditions [10][11][12][13].
Many aquatic organisms, particularly within the tropics and polar regions, have evolved to survive within specific ranges of environmental conditions [14][15]. Climate change impacts may create physical and biological conditions that the majority of these organisms have previously not experienced within their evolutionary history [16], thereby pushing them towards extinction [13][15]. In particular, research has shown that many of the species found in tropical marine and coastal ecosystems are already living on the edge of their thermal tolerances [13][17]. In addition, climate change impacts such as heatwaves are also more prevalent in tropical regions, thereby altering the biodiversity, productivity, and potential for aquatic ecosystems to provide functions and services [13]. There is therefore a need to increase our understanding of how climate change impacts aquatic biodiversity and productivity, particularly in tropical ecosystems [15][18].
Regarding the impacts of global climate change in particular, phytoplankton play a key role in regulating the balance of carbon in the atmosphere. With a turnover rate of about one week [19], phytoplankton account for half of the carbon fixation in the global carbon cycle; they release a large fraction (>90%) of the organic matter that fuels ocean carbon sequestration and burial [20][21]. The sum of these processes controls more carbon than the amount available in the atmosphere [22][23]. Phytoplankton research is therefore considered imperative for ocean science [24] and requires insight into the evolution of phytoplankton communities in different climate regions of the world [25]. This is because phytoplankton species diversity, function (e.g., carbon fixation), and climate sensitivity differ among species from different climate zones [19][25][26]. Anthropogenic (e.g., pollution) and natural forces (e.g., upwelling) that impact phytoplankton biology are also not equally distributed around the world [25].

3. Phytoplankton as Indicators of Climate Change

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].

This entry is adapted from the peer-reviewed paper 10.3390/w15244288

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