Thread by @PearseJBuchanan

Models project that #ocean #phytoplankton will become increasingly starved of #nitrogen as #climatechange progresses.

If realised, this has implications for #fisheries.

But how can we monitor this change in the real ocean?

Nitrogen isotopes!
rdcu.be/cAlW2

What are nitrogen (N) isotopes?

There is a heavy isotope (15-N) and a light isotope (14-N).

Some processes prefer to use 14-N more than 15-N. When phytoplankton use nitrogen, they prefer 14-N.

This increases the ratio of 15-N to 14-N in seawater.

But when N is scarce, phytoplankton have less preference.

Like if there was only one kind of pizza at a party, you don't have the option of being picky (you have to have the pineapple).

In this case, phytoplankton no longer increase the ratio of 15-N to 14-N in seawater.

@altagliabue, @ProfCMahaffey, @laurent_bopp, Olivier Aumont and I show that increasing nitrogen limitation of phytoplankton causes widespread and detectable declines in the isotopic ratio of N.

Our model suggests that declines are strongest in the ocean's twilight zone.

But the N cycle is not only affected by #climatechange. Human's are dumping huge amounts of N into the ocean due to (1) fossil fuel burning and (2) fertiliser production.

This figure shows the increase in N deposition rates since preindustrial (Hauglustaine et al., 2014).

So which is more important for the N cycle and its isotopes? Climate change or deposition?

It turns out that climate change is the dominant perturber of the oceanic N cycle in the 21st century.

Both in terms of:
1 - The sources and sinks of N
2 - The N isotopes

While N deposition is important in the 20th century, the effects of climate change drive anomalous trends in the 21st century across much of the ocean.

Moreover, N isotopes (δ15N) are highly sensitive to these effects, more so than other properties related to the N cycle.

And what specifically about climate change is perturbing the N cycle?

A changing circulation.

The increase in stratification (Sallee et al., 2021) and slowdown of upper ocean overturning is causing nitrogen limitation, driving N-limitation and declines in marine productivity.

Overall:
1 - The 21st century N cycle is predominantly perturbed by climate change.
2 - This is specifically caused by a change in circulation that limits N supply to phytoplankton.
3 - Intensifying N-limitation of phytoplankton is fingerprinted by isotopic declines.