Phytoplankton - The Foundation of the Oceanic Food Chain
The Southern Ocean, also known as the Antarctic Ocean, is an ocean like no other. Encompassing the southernmost reaches of the global ocean and encircling the Antarctic continent, it is crucial in shaping the planet's environmental dynamics. These expansive waters play an important role in regulating the Earth’s climate, as well as maintaining the carbon and nutrients cycle.
The Southern Ocean, also known as the Antarctic Ocean, is an ocean like no other. Encompassing the southernmost reaches of the global ocean and encircling the Antarctic continent, it is crucial in shaping the planet's environmental dynamics. These expansive waters play an important role in regulating the Earth’s climate, as well as maintaining the carbon and nutrients cycle.
The ongoing global climate change is anticipated to cause notable changes in the Southern Ocean. These include elevation in water temperature, decrease in sea ice and stronger westerly winds, consequently triggering stronger divergence and changes in the nutrient distribution. Such changes are predicted to affect the phytoplankton composition. This is significant since phytoplankton (the photosynthesising minute marine plants) are primary producers, forming the base of several aquatic food webs. Not only that, phytoplankton account for roughly half of the planet’s photosynthetic activity, making them one of the world’s most important contributors of oxygen production. They also contribute substantially to the oceanic carbon sequestration process. Oceanic carbon sequestration is a process of capturing, removing, and storing atmospheric carbon dioxide from the surface of the ocean and exporting it to deeper depths. It is a vital process in reducing carbon dioxide emissions. Therefore, any changes to the phytoplankton population will affect the surrounding marine ecosystem and, by extension, the global ecological balance of the planet.
A collaboration between Malaysian and Australian scientists is trying to understand better how climate change is affecting the phytoplankton composition in the Southern Ocean – headed by Dr Cheah Wee, a Senior Lecturer in the field of Oceanography and Earth Observation at the Institute of Ocean and Earth Sciences (IOES). This is a joint effort between IOES and the National Antarctic Research Centre (NARC) in Universiti Malaya, the Commonwealth Scientific and Industrial Research Organisation (CSIRO), the Australian Antarctic Division (AAD) as well as the University of Tasmania in Australia.
The sampling location, showing the subtropical zone (STZ), subantarctic zone (SAZ), polar frontal zone (PFZ), Antarctic zone (AZ) and southern zone (SZ); subtropical front (STF), subantarctic front – north, middle, south (SAF-N,M,S), polar front – north, middle, south (PF-N,M,S), southern Antarctic circumpolar current front – north, south (SACC-N,S)
This research project involves seawater sampling between Tasmania, Australia and Dumont d’Urville station in Antarctica, covering a distance of about 2500 km. The sampling was conducted between 2002 to 2011, 3 times a year – between October and November, December and January, and February and March. The vessel used was the French icebreaker l’Astrolabe, where the surface seawater samples were collected daily from an underway pump. Then, the samples were tested for their temperature and salinity, as well as for their nitrate, nitrite, phosphate, silicate and phytoplankton pigments content.
Different sizes of phytoplankton
Phytoplankton can be categorised based on their size – very small ones called picophytoplankton (0.2 - 2µm), medium-sized nanophytoplankton (2-20 µm) and larger microphytoplankton (20 – 200 µm). The current composition of phytoplankton in the Southern Ocean mainly consists of microphytoplankton and nanophytoplankton. However, the warmer waters heading south have caused changes in the phytoplankton composition. There is an increasing trend in pico- and nanophytoplankton, accompanied by a corresponding decrease in microphytoplankton, suggesting that the increase in temperature favours smaller phytoplankton. This poses a concern for organisms that prefer larger phytoplankton, e.g., Antarctic krill, which feeds mostly on diatoms (a subset of microphytoplankton). This will, in turn, impact the food web at a higher trophic level as Antarctic krill are the primary food source for baleen whales, such as the blue whale, humpback whale and minke whale, along with various penguins and fishes. On top of that, changes in phytoplankton composition will also affect nutrient cycling as different sizes of phytoplankton have different nutrient requirements as well as different capacities of carbon sequestration. Consequently, this will affect nutrient distribution and carbon cycles, influencing the ocean’s ability to absorb carbon dioxide from the Earth’s atmosphere.
The trend in phytoplankton composition. Note the increasing trend in picophytoplankton (blue), and a decreasing trend in microphytoplankton (red)
In conclusion, though covering only 6% of the global oceans, the Southern Ocean plays a disproportionately critical role, accounting for about 25% of the oceanic carbon dioxide uptake. This underscores the importance of studying this unique ecosystem. Investigating these phytoplankton communities provides a window into the dynamic and intricate marine food web of the Southern Ocean, offering valuable insights into its ever-evolving nature. In addition, the Southern Ocean is connected to the Pacific Ocean, Indian Ocean and the Atlantic Ocean. Hence, changes in the Southern Ocean, especially the carbon, nutrient and heat cycles, will affect other parts of the seas, including Malaysian waters. Therefore, a better understanding of the Southern Ocean will help us better prepare for potential disruption to our marine resources.
Researcher featured:
Dr Cheah Wee
Institute of Ocean & Earth Sciences (IOES)
Universiti Malaya
Author:
