By taking up carbon dioxide (CO2) from the air, phytoplankton play an influential role in the natural carbon cycle, helping to regulate the amount of CO2 in the atmosphere and to keep the Earth’s climate in balance.
Climate models suggest that, as rising temperatures alter oceanic currents and the stability of the upper ocean, deep ocean nutrient supply is likely to be reduced, resulting in less resources for phytoplankton to multiply. In turn, this would slow down the biological pump, leaving more CO2 in the atmosphere - contributing to further climate change, creating a self-perpetuating cycle of climate change.
The Redfield ratio
SPOTLIGHT ON PHYTOPLANKTON
➤ What are phytoplankton?
➤ Phytoplankton dynamics
➤ Phytoplankton and climate change
➤ Studying phytoplankton
Early clues to the global importance of phytoplankton appeared in the 1930s. During research voyages of the time, samples of sea water from the deep ocean were measured to analyse the relative amounts of carbon, nitrogen and phosphorus — elements needed to construct essential cellular molecules — in both phytoplankton and the sea water.
Remarkably, in every region of the ocean sampled, the ratio of nitrogen atoms to phosphorus atoms in the deep ocean was 16 to 1 — the same ratio as in phytoplankton.
For more than 20 years, the scientific community puzzled over why these ratios were identical. Harvard scientist Redfield eventually made a crucial conceptual leap, proposing in 1958 that phytoplankton not only reflected the chemical composition of the deep ocean, but in fact created it.
His conclusion that as phytoplankton and the animals that ate them died and sank to the bottom, along with those animals’ faecal matter, microorganisms in the deep sea broke that material down into its chemical constituents, creating sea water with the same proportions of nitrogen and phosphorus, became known as The Redfield Ratio or Redfield stoichiometry, and is one of the foundations of ocean science today.
Phytoplankton underestimated impact on earth’s climate
As climate change came to the fore, in the 1990s science came to realise that it had vastly underestimated the global influence of the ocean’s phytoplankton. Although they account for less than 1% of the photosynthetic biomass on Earth, phytoplankton contribute almost half of the world’s total primary production, making them as important in changing the planet’s cycle of carbon (C) and CO2 as all the world’s land plants combined.
Phytoplankton photosynthetic physiology can be investigated through single turnover active fluorometry (STAF) approaches, which carries the unique potential to autonomously collect data at high spatial and temporal resolutions. Chelsea Technologies’ LabSTAF is a new generation fluorometer that uses single turnover active fluorometry to measure very low levels of photosynthesis, ideal for oligotrophic zones
Given that phytoplankton are so important to the planet’s carbon cycle, the fate of dead phytoplankton needed consideration. With 45 billion tonnes of new phytoplankton regenerated each year (in contrast, the world’s land plants have a total biomass of 500 billion tonnes, regenerated every ten years), as they die and sink slowly below the top 500 metres of the ocean, this is one of earth’s fundamental cycles of deep-ocean circulation. A biological pump on a global scale, sending CO2 to the deep sea for hundreds of years.
In practise, those dead phytoplankton are turned into oil over a period of several million years, but now each year, humanity consumes the same oil that took a million years to produce. This has pushed the atmospheric level of CO2 to more than 390 parts per million (ppm), 40% higher than before the industrial revolution, and is one of the main drivers of global warming. However: if the phytoplankton in the upper ocean stopped pumping carbon down to the deep sea tomorrow, it is believed that atmospheric levels of CO2 may eventually rise by another 200 -400 ppm and global warming would accelerate further.
“We cannot cheat on DNA. We cannot get round photosynthesis. We cannot say I am not going to give a damn about phytoplankton. All these tiny mechanisms provide the preconditions of our planetary life. To say we do not care is to say in the most literal sense that “we choose death.” (Barbara Ward, Baroness Jackson of Lodsworth)
Scientists around the world have concluded that global warming has begun to slow down this phytoplankton-driven pump. Between 1999 and 2005, the upper layer of the ocean became warmer. As this happens, water becomes less dense, and is more likely to float without mixing into the cold, nutrient-rich waters below. The warm top layer of these stratified waters therefore gains less nutrients, which reduces phytoplankton growth, and which in turn diminishes pumping to the deep sea.
Globally, this biological carbon pump transfers about 10 gigatonnes of carbon from the atmosphere to the deep ocean each year – even the smallest of changes in the growth of phytoplankton therefore has the potential to affect atmospheric CO2 concentrations, which would feed back to global surface temperatures. The more we learn, the more questions we have.
Phytoplankton science is complex
The overall impact of increased temperature on phytoplankton is complex and requires further study. For example, increasing temperatures can lead to more stratified waters, especially in summer months, and prevent nutrient replenishment at the ocean surface. The warming of surface waters can result in lower phytoplankton production, particularly concerning at low latitudes. Reduced frequency of cold winters and unusual types of phytoplankton succession have also been reported in some regions. Changing weather patterns, and nutrient imbalances are thought to be driving shifts in phytoplankton community such as a decrease in the ratio of dinoflagellate to diatom abundance at high latitudes.
The case for more phytoplankton research around their climate effect is clear. Although studies show that overall global oceanic phytoplankton density has decreased in the past century, there is limited availability of long-term phytoplankton data with methodological differences in data generation and a large annual and decadal variability in phytoplankton production. Other studies suggest a global increase in oceanic phytoplankton production and changes in specific regions or specific phytoplankton groups. For example, the global Sea Ice Index is declining, leading to higher light penetration and potentially more primary production; however, there are also conflicting predictions for the effects of variable mixing patterns and changes in nutrient supply and for productivity trends in polar zones. Further studies with instrumentation such as LabSTAF can only serve to add clarity to this uncertain scenario.
If greenhouse gas emissions continue rising to elevated levels by 2100, some phytoplankton models predict an increase in species richness, or the number of distinct species within a given area. This increase in plankton diversity is traced to warming ocean temperatures. In addition to species richness changes, the locations where they are distributed are expected to shift towards the Earth’s poles. Such movement may disrupt ecosystems, because phytoplankton are consumed by zooplankton, which in turn sustain fisheries. These shifts in phytoplankton location may also diminish the ability of phytoplankton to store carbon that was emitted by human activities, again, adding to the pressing case for further research.
Phytoplankton’s fundamental importance to climate studies
Because phytoplankton are so crucial to ocean biology and climate, any change in phytoplankton productivity could have a considerable influence on the pace of global warming. Numerous models of ocean chemistry and biology predict that as the ocean surface warms in response to increasing atmospheric greenhouse gases, phytoplankton productivity will decline. Productivity is expected to drop because as the surface waters warm, the water column becomes increasingly stratified; there is less vertical mixing to recycle nutrients from deep waters back to the surface.
“The ocean makes up roughly 70% of the planet; living on land, we fail to recognize the importance of our ocean and the marine life that inhabit it. Ocean acidification is making phytoplankton toxic, which is bad news for the organisms that depend on them as a source of food and oxygen. phytoplankton generates a large portion of the world’s O2. If they’re out of balance, the rest of life on Earth is going to be out of balance.” (Joseph P. Kauffman)
Over the past decade, scientists have begun looking for this trend in satellite observations, and early studies suggest there has been a small decrease in global phytoplankton productivity. For example, ocean scientists have documented an increase in the number of oligotrophic zones – the least productive ocean areas -over the past decade. These low nutrient zones appear to be expanding due to rising ocean surface temperatures.
However, satellite data comes with a set of challenges, as will be explored in the last article in this series. Satellites, which have been scanning the oceans with their sensors for the past three decades, are an imperfect solution at best: although they can be used to gauge the amount of the plant pigment chlorophyll-a in the water – as an indicator of how high the general concentration of phytoplankton is, distinguishing between different types of phytoplankton remains extremely challenging. Moreover, there is no way to use satellite data to show phytoplankton at anything other than the surface, and slow laborious spot verification using photosynthetron-based measurement of 14C fixation is still necessary.
“The key scientific direction for STAF Technologies is to develop widely accessible instrumentation that science can use on significantly wider spatial and temporal scales for climate studies than 14C fixation, at comparable levels of accuracy and precision”
Photosynthetron-based measurement of 14C fixation, the standard method, is slow, manual, and gives a spotlight on one particular sample, as opposed to wide geospatial scales that science requires for climate studies, and scientists are well aware of its limitation. The key scientific direction of STAF technologies is to do exactly this, to make measurements over much wider spatial and temporal scales than are possible with 14C fixation but with continuing reference to parallel 14C fixation data.
Phytoplankton and climate monitoring options from Chelsea Technologies
LabSTAF is the world’s leading portable instrumentation option for Phytoplankton primary productivity.
- Benchtop instrument to measure primary productivity using fluorescence, giving data for over 50 useful parameters within 15 minutes
- LabSTAF includes a peristaltic pump, solenoid unit and flow-through stirrer unit to provide for mixing, sample exchange and a periodic cleaning cycle.
- Data from LabSTAF is interpreted and analysed internally in the included Surface Go
With three different parameter combinations and depth ratings to 2000m, TriLux Algae is suitable for a wide variety of water environments:
- Chlorophyll a, Phycocyanin, Turbidity for freshwater deployments
- Chlorophyll a, Phycoerythrin, Phycocyanin for coastal deployments
- Chlorophyll a, Phycoerythrin, Turbidity for marine deployments
TriLux is compact and easy to integrate in wider systems and platforms including CTDs, data buoys, monitoring stations, AUVs and gliders.