A global ocean study shows that nutrient ratios in plankton depend primarily on deep-sea processes rather than surface conditions, significantly reshaping how scientists model marine ecosystems and climate change.
As the world warms, sweeping changes in marine nutrients seem like an expected consequence of increased ocean temperatures. However, the reality is more complicated. New research suggests that processes below the ocean surface may be controlling what is happening above.
Plankton are some of the most numerous and important organisms in the ocean. The balance of chemical elements inside them varies and is critical to shaping many marine processes, including the food web and the global carbon cycle. Temperature has been traditionally thought to control the ratio of these elements. However, a new study suggests this balance is largely dependent on activity in the subsurface ocean, from depths of over 300 feet. The work, led by scientists at Bigelow Laboratory for Ocean Sciences, was recently published in Communications Earth and Environment.
Subsurface Nutrient Ratios Influence Marine Life
The team looked at samples from eight locations in oceans around the world. They found that the ratio of nitrogen and phosphorus introduced from the subsurface ocean controls the balance of those nutrients in the marine microorganisms that form the foundation of ocean health. This discovery could allow scientists to more accurately explore complex ocean processes.
“This is the first time that we’ve looked across a broad range of ocean regions and directly measured the balance of nutrients in ocean microorganisms, which is really exciting,” said Mike Lomas, lead author on the paper. “Now we can apply more realistic parameters based on what is actually driving marine dynamics to the computer models used to forecast ocean change.”
For decades, researchers have been using a fixed ratio to estimate the balance of carbon, nitrogen, and phosphorus in marine environments. Scientists and groups like the International Panel on Climate Change use this ratio in computer simulations to make predictions about the future of the planet.
However, it does not necessarily represent the wide diversity of chemical balances in the ocean or the significant role that organisms play in cycling nutrients.
“The problem is that the fixed ratios are safe estimates that do not actually represent how biology works,” Lomas said. “More realistic, but risky and complicated, approaches are not yet widely utilized.”
To develop a more accurate understanding of these ratios, Lomas directly measured them in phytoplankton – some of the most critical marine organisms worldwide. The elements in these organisms’ cells reflect the available nutrients in their habitat and shine light on the role of biodiversity in how the nutrients cycle.
Flow Cytometry Reveals True Nutrient Ratios
This is not the first time phytoplankton have been examined to understand nutrient levels in the ocean, but it is the most advanced and comprehensive. The team examined phytoplankton around the world to create a snapshot of three critical nutrient elements across broad environmental conditions. Traditionally, researchers have used physical filters to sort out plankton from seawater before examining them. However, this approach can also capture bacteria and tiny particles, leading to errors.
This study used a technique called flow cytometry, which allows researchers to examine and sort hundreds to thousands of cells per second. This enabled the researchers to isolate and examine only the cells they were interested in. It not only gave them a more accurate understanding of the diverse ratios of elements in the ocean, but also what processes are controlling them.
Subsurface Processes Dominate Nutrient Dynamics
The team found that, contrary to the most common hypothesis, the ratio of carbon, nitrogen and phosphorus in cells was primarily dependent on the ratio of nitrogen and phosphorus supplied from the subsurface ocean to the sunlit waters where phytoplankton are active. This was true across all locations, regardless of the kind of phytoplankton or their environmental conditions.
Lomas hopes that this improved understanding of nutrients can be used to better picture how oceans will respond to climate change.
“We can’t examine the nutrients in every single cell in every ocean, but we need to be sure all the controlling factors are included in computer models,” Lomas said. “As we blend these results with other advanced disciplines, we will really advance our understanding of ocean dynamics and ability to forecast future conditions.”
Reference: “Varying influence of phytoplankton biodiversity and stoichiometric plasticity on bulk particulate stoichiometry across ocean basins” by Michael W. Lomas, Steven E. Baer, Celine Mouginot, Kristina X. Terpis, Debra A. Lomas, Mark A. Altabet and Adam C. Martiny, 14 July 2021, Communications Earth & Environment.
DOI: 10.1038/s43247-021-00212-9
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1 Comment
“The team found that, contrary to the most common hypothesis, the ratio of carbon, nitrogen and phosphorus in cells was primarily dependent on the ratio of nitrogen and phosphorus supplied from the subsurface ocean to the sunlit waters where phytoplankton are active.”
Should anyone be surprised that the most productive fisheries throughout the world are were there is upwelling from the ocean deeps? Upwelling presents some challenges to organisms, such as lowered pH, and sometimes lowered oxygen levels. However, that is more than made up for by the addition of re-cycled nutrients for the bottom of the food-chain.
One of the problems with holding a particular hypothesis with religious fervor is that adherents are prone to suffering from the Hammer Syndrome. That is, to the handyman whose only tool is a hammer, all problems look like a nail. Thus, it becomes difficult (particularly for young, inexperienced specialists) to look outside the box that is nailed together.