Feeding the ocean, an icy solution

Glaciers – immense rivers of ice, containing vast volumes of freshwater – are often considered barren environments, and the melting of them seen as an indicator of climate change. But glaciers are also a source of nutrients to our oceans, “fertilising” polar seas.  

Marine algae, or phytoplankton, are responsible for storing more carbon than all terrestrial plants, as well as forming the base of the marine food web. Like all plants, they need nutrients to thrive, and sources of nutrients like those from glaciers are vital to maintaining healthy ecosystems. 

So how do glaciers help fertilise our oceans – and how will this source of nutrients change in a warming world? 

Shaken or stirred? 

In the Antarctic summer of 2019/2020, Dr Rhiannon Jones joined a research expedition to Antarctica aboard the RRS James Clark Ross. She was part of the scientific team sampling the polar waters to understand the biological impact of glacial retreat. The sampling involved deploying small boats (Zodiacs) as close as possible to the glacier, where they sampled the water column to measure key biogeochemical variables.  

Sampling this close to a glacier can be an un-nerving experience, sitting on the sea surface beneath a wall of ice. In fact, once the expedition team was safely back aboard the ship, Rhiannon witnessed a calving event, where the front of the glacier collapsed, releasing ice bergs.   

The team aboard the ship opportunistically monitored the waters after the calving event and observed a new phenomenon – how the calving of ice into the ocean, caused internal waves, resulting in mixing of the water column. This has since lead to the launch of the project POLOMINTS, led by British Antarctic Survey’s Professor Mike Meredith, looking to understand the physics and impact of these calving events on the local marine system.  

But to biogeochemists like Professor Kate Hendry at BAS, the more important question is how these events mix nutrients from seafloor sediments into the surface ocean.  

Kate described glacial calving as: 

“Where you splash a great big chunk of ice into the ocean, and it stirs everything up. You resuspend sediments on the seafloor, and you mix up the water column, so you get those nutrients that were deep down, up to the surface where they’re needed.” 

The supply of nutrients to the surface ocean is key to providing life in our oceans. Phytoplankton are microscopic marine algae that need a variety of nutrients to survive. As photosynthetic organisms, they are crucial for taking in carbon dioxide and storing it within themselves, making them essential players in the ocean’s storage of carbon. They also form the base of the marine food chain. Sometimes marine algae don’t have enough of a particular nutrient or nutrients: if these missing nutrients can be supplied, then this leads to more phytoplankton growth and abundance, leading to more carbon storage, and more fisheries sustained. 

Glacial flour feeding our oceans 

However, physical mixing processes are not the only way glaciers provide nutrients to polar waters. Kate is the lead scientist on the project SiCLING which looks at how glacial flour feeds our oceans. Kate and her team have conducted multiple sampling expeditions to the Arctic, sampling both the polar oceans and terrestrial rivers fed by glacial melt.  

So, what is glacial flour? Kate explains: 

“You've got stuff dissolved in meltwater that's getting out into the ocean, but you've also got particulates, these really finely ground, little tiny pieces of rock, we call it glacial flour that is caused by the scraping of the ice over rock.” 

These tiny particles can react with water beneath glaciers and undergo chemical weathering before reaching their final destination in polar waters. This flour is full of nutrients that phytoplankton need, fuelling phytoplankton growth and production. 

Glacial flour reaches the ocean in two main ways. When meltwater flows from land-terminating glaciers into rivers, it travels long distances before reaching the sea, where its properties and usefulness for marine life may change. Once in the ocean, the fresh, less dense river water tends to float on top, carrying the flour to the surface ocean. 

But when glaciers melt straight into the ocean (marine-terminating), things happen differently. Here, the meltwater – and the flour it carries – flows out near the sea floor. Because the glacial water is lighter than seawater, it rises toward the surface in plumes, stirring up particles and mixing deep ocean water rich with nutrients. Some of those glacial particles settle on the seabed and might later be stirred up again or carried further out by deep ocean currents, spreading the nutrients far and wide. 

So, what nutrients?

Glaciers can help supply various nutrients via many methods, but what nutrients are they able to provide to the ocean?

“The meltwater itself is largely rich in iron and other metals and dissolved silica or particulate silica,” comments Rhiannon. She adds, “glaciers can drive that entrainment [or incorporation] of deep water, which will often be rich in nitrate, phosphate, silicate, potentially iron as well, and other nutrients.”

All these nutrients are essential for phytoplankton to thrive; some nutrients are in ample supply in some areas of the ocean but may be limited in other regions. The distribution of nutrients all comes down to their supply and ocean circulation.

Part of Rhiannon’s research focuses on iron from glacial meltwater. A recent paper published by Jones explained the process through which iron is incorporated into carbon rich particles. This iron carbon bonding allows the iron to remain in a form usable by marine algae for a longer period, increasing the abundance and growth of phytoplankton.

Antarctic vs. Arctic

The Arctic and Antarctic systems differ both in how glacial nutrients are delivered to the polar oceans, and in the role these nutrients play within their respective ecosystems. For one, Arctic waters are provided glacial flour by both land- and marine-terminating glaciers, while Antarctica has a predominantly marine-terminating glacial system.

What's more significant is the difference in the importance of this nutrient supply to the polar ecosystems. The Arctic is considered a low nutrient system, explains Kate:

“Things like nitrates and phosphate and dissolved silicon are really, really low. And glaciers in particular are really good at supplying things like silicon. So, you know you could actually be supporting biological productivity in the Arctic by supplying silicon from glaciers.”

Meanwhile, the waters around Antarctica are rich in most common nutrients, which results in thriving populations of phytoplankton and fisheries dependent on these marine algae. However, the Southern Ocean (the open ocean north of Antarctica) is critically limited by iron and sometimes other micronutrients such as manganese. Glacial melt water from marine-terminating glaciers can deliver these nutrients across the shelf, potentially alleviating nutrient limitation and increasing phytoplankton growth.

A difficult target to trace

Scientists know that nutrients from glacial sources extend beyond polar oceans. But how these nutrients are transported and how important they are to the open ocean, is poorly understood, and an area of research requiring more attention.

One way of observing the spread of nutrients is through tracers. This can be through stable isotopes which Kate explains as:

“It's a little bit like a fingerprint, all that glacial flour and detritus has a very isotopically light signal. You can follow it out and you can find the water that is typically lighter than they should be. And so, the obvious explanation is something isotopically light is dissolving into them.”

Scientists can therefore track these isotopically light waters and observe as they mix with other isotopically heavier waters in the open ocean, away from glacial influence.

Or radioactive isotopes can be used – one example is radium. Thorium occurs naturally in rocks, decaying to radium which is soluble in water, and the radium signal can be used to track the movement of dissolved material from the rock, like iron, across the shelf into open waters.

Alternatively, to understand the rate that silicon is taken up by marine algae, water samples can be spiked with silicon isotopes. In Greenland, Kate observes that:

“Phytoplankton take it all up and they're taking it up really quickly. And it shows that they're basically really hungry for silicon.”

Unknowns caused by a changing climate 

The melting of glaciers is a perfectly natural phenomenon that occurs every year, providing nutrients to the ocean. But human induced climate change is warming our polar regions at alarming rates; the surface waters of Antarctica are warming at a rate three times faster than the global average. Scientists are witnessing the impacts of this warming firsthand. 

While sailing aboard the RRS Sir David Attenborough this midwinter in Antarctica, Rhiannon witnessed a rare event. While it is usual for glaciers to calve in the summer due to seasonality, the team saw a glacial calving event in winter. This caused them to question, does climate change-driven glacier calving extend into winter? 

So, you may be thinking, is climate change creating a feedback loop in polar regions? Melt more ice, deliver more nutrients to the phytoplankton, more phytoplankton carbon storage and counteract climate change! Easy, right? 

However, there are still so many details of how glaciers provide nutrients to our ocean systems that are poorly understood, as well as how glaciers will respond to climate change. Kate confirms the challenge: 

“For glaciers in particular, we're showing increasingly that they're an important source of these nutrients, and they're a source that's changing incredibly rapidly. And the problem is that we don’t know enough about something that's changing so rapidly.” 

One hypothesised change to glacial nutrient supply is that as glaciers melt due to increased temperature, they will shift from marine to land-terminating glaciers. This has a whole range of potential consequences: the melt water will melt into rivers instead of directly to the sea; when the melt does reach the sea, it will be deposited directly on the surface, reducing the mixing of sediments and deep waters rich in nutrients to the surface.  

Phytoplankton not only store vast quantities of carbon, but also forms the foundation of the marine food chain. Altering how glaciers supply nutrients to the ocean could be detrimental to phytoplankton populations, having severe knock on effects. 

A race to predict the future 

With so little known about these glacial systems and how they will change in the future, efforts like those conducted at BAS are essential to helping us understand the potential impacts of climate change.  

Work done to understand the current state of these complex glacial systems is crucial for us to accurately predict how they may change in the future as a source of turbulent mixing, and nutrient input to our global oceans. Kate summarises this major gap: 

"None of the ocean climate interactions models we have - that include nutrients, that include phytoplankton - none of them have these sorts of processes in there. So, it's really, really hard to say what's going to happen with global warming.”