ecosystems and global change

The summer of student interns (part II)

We were fortunate this year to host 6 Cambridge undergraduate students in our research group on a diverse mix of projects.  Although they’re now back in class, here are their stories about what they got up to.  This is Part 2 of 2.  You can read about Part 1 of 2 here.

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The invisible world beneath our waters

I recently had the pleasure of writing for The Conversation. This article is republished from them under a Creative Commons license. Read the original article.


A lake in Banff National Park, Alberta, Canada.
Sergey Pesterev/Unsplash, CC BY-SA


Lakes and ponds are the final resting place for many of the Earth’s plants. Rivers collect much of the planet’s dead organic matter, transporting it to rest in calmer waters.

But on a microscopic scale, lakes are anything but calm. An invisible metropolis of microbes feeds on these logs and leaves, producing greenhouse gases as a byproduct.

As a result, lakes may be responsible for as much as a quarter of the carbon in the atmosphere – and rising. New research conducted with my colleagues in Cambridge, Germany and Canada suggests that emissions from freshwater lakes could double in the coming decades because of climate change.

All known life on Earth is made of carbon. When plants and animals reach the end of their lives, microorganisms such as bacteria and fungi come to feast. They feed on the carbon-based remains of other organisms and their waste products — collectively known as organic matter.

As a byproduct of this never-ending feast, microbes release gases such as carbon dioxide and methane into the environment. While each individual microbe releases a minuscule amount of gas, they are the most abundant organisms on Earth, so it adds up. Energy from sunlight can also break the chemical bonds between molecules of organic matter, releasing smaller molecules, such as carbon dioxide, into the environment.

Lakes and land are not isolated systems.
Aaron Burden/Unsplash, CC BY-SA


Some of this degradation happens on the forest floor. But much of the organic matter that falls to the ground ends up in the water. Winds, rain and snow transport it into lakes, or more often into the rivers that feed them.

The amount of greenhouse gases released from lakes by microbes and sunlight is huge. Initial estimates were about 9% of the net carbon released from the Earth’s surface to the atmosphere – that is, the amount released over and above the Earth’s carbon-storing processes.

But, thanks to improved measurements, recent research has revised the figure to as high as 25%. These numbers are substantial given that that lakes only comprise about 4% of the global land surface.

In the coming years, lakes will receive more and more organic matter for microbes to digest. A warming climate will bring more forest cover around lakes and a greater proportion of broad-leaved trees, such as maples and oaks, as compared to needle-leaved trees, such as pines.

Carbon in a thousand forms

To understand how changes to forests will alter the role that lakes play in the carbon cycle, we performed an experiment in two Canadian lakes.

We filled plastic containers with rocks, sand, clay and different amounts and types of organic matter from nearby forests. This was intended to mimic the change in forest cover and composition expected from climate change.

We then submerged the containers in shallow lake waters where organic matter is most likely to accumulate and monitored them for three years.

Using new techniques to analyse the carbon chemistry of water, we found that those containers simulating a level of forest growth expected in the next few decades led to between 1.5 and 2.7 times more greenhouse gases in the water than conditions simulating today’s forest conditions.

The invisible diversity of organic compounds in the water was the most important factor causing this rise – even more important than the diversity of microbes and the overall amount of organic matter.

The likely explanation for this result is that the same microbes can feed on many different types of molecule. So as the number of carbon-based compounds in the water increases, there are more ways for microbes to feed and release greenhouse gases.

They’ll find their way to a lake sooner or later.
Aaron Burden/Unsplash, CC BY-SA


The increase in diversity of organic matter alone was enough to raise greenhouse gas concentrations by about 50%. But the size of this effect nearly doubled in containers with darker overlying waters – a scenario expected in most lakes as climate change brings increased tree cover.

Accurately tracing how carbon makes its journey from land to atmosphere is vital to predict the pace of climate change and mitigate its effects. By better understanding how the vegetation around lakes controls greenhouse gas concentrations in waters, our research can inform whether changing the way we manage land near lakes could help reduce carbon emissions.

For example, we might want to plant fewer aquatic plants such as cattails in lakeside areas, because they produce much higher concentrations of greenhouse gases than organic matter from forests.

Work also remains to understand fully the role lakes play in the carbon cycle. Not all organic matter that reaches lakes is digested by microbes. Some sinks to the lake floor to form muddy sediment, locking away carbon. The amount of sediment formed will also increase with climate change, but we don’t yet know by how much – and so to what degree this increase in stored carbon will offset the increased greenhouse gas emissions from lakes.

Answering this question will be crucial in improving the accuracy of carbon accounts – and assessing how much time humanity has to balance them.

Invasive species with The Naked Scientists

We recently caught up with BBC-syndicated talk show The Naked Scientists about invasive species and their impacts on natural ecosystems.  You can listen in below:


And, in case you missed it, we were on the weekly science and technology radio show Pythagoras’ Trousers last year to discuss our work on methane at the 15:40 mark:

Why are some places so species-rich?

Our newest paper is out in Science Advances – the culmination of 3 years of hard work on our biodiversity hotspots project.

The idea of biodiversity hotspots (you can see ours mapped below) came into the mainstream with Norman Myers’ now classic paper published in 2000.  He reported that nearly half of all vascular plant species and a little more than one-third of all mammals, birds, reptiles, and amphibians are confined to 25 “regions” that comprise only 1.4% of the Earth’s land surface.  Recent estimates have raised this number to about 20% of the Earth’s land surface holding more than 50% of all vertebrate species.  While Myers’ paper has been tremendously influential for conservationists in guiding their interventions, much less thought has been given to how this remarkable pattern came to be in the first place.

hotspot_map (1)

Biodiversity hotspots for mammals estimated in 100 km by 100 km grid cells. Red cells are hotspots.

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Would elephants really replace the woolly mammoth?

Rewilding involves restoring nature at large scales, typically by reintroducing species that have gone extinct and have had important interactions with other organisms.  It has received a lot of popular attention lately, helped in part by George Mobiot’s 2013 book Feral and its growing number of success stories.  But the idea of rewilding remains highly controversial, particularly when it involves adding apex predators like wolves into places with people.  One the reasons for the controversy is the lack of empirical data to assess the effectiveness of its outcomes.  Conservationists are often relying on a handful of well-known examples, such as from Yellowstone National Park.

rewildcoverIn a new paper published last week in the Philosophical Transactions of the Royal Society B, we now summarise the numerical data around whether rewilding works and identify the biases in experimental study.

The paper is part of a special issue on rewilding, organised by Elisabeth Bakker and Jens-Christian Svenning, to which we were kindly asked to contribute towards.  And we were even interviewed in last week’s issue of Science about the special issue and importance of trophic rewilding for the important task of keeping the Arctic cool.


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Digging into lake microbiomes

Two new papers have just been published from our RELATED project.  The work shows how future changes in forest cover around lakes will influence the contributions of inland waters to global carbon cycles.

The first paper published in ISME finds that the positive effects of microbial diversity on CO2 production depends on present and past environmental gradients.  Using a space-for-time substitution for forest greening, the study also finds that a doubling in the tree cover around lakes can increase CO2 production by five-times.  More broadly, the work highlights how widely reported biodiversity-ecosystem functioning relationships need to be contextualised with other ecosystem properties.

A second paper published in Global Change Biology sheds light on the mechanisms underpinning the decomposition of terrestrial organic matter in lake sediments.  Using the RELATED experimental platform, the study finds that identical organic matter additions to sediments have contrasting outcomes for carbon cycling depending on lake-specific characteristics.  In lakes with clear waters, future increases in terrestrial organic matter inputs can stimulate CO2 production because of photo-oxidation.  By contrast, bacteria in darker waters may possess functional genes for degrading organic matter, thereby priming their productivity.  I’m particularly proud of the teamwork on this one, which involved almost the entire group!

Farting lakes warm the world

Lakes release gas too.  In the latest paper freely available from our RELATED project, we show that the vegetation in and around lakes can play an important role in influencing how much of the potent greenhouse gas methane is produced by microbes.  The story, led by our former postdoc Erik Emilson, has been covered by BBC News.

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