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.
Ponds and lakes may account for upwards of 16% of all methane (CH4) produced by natural ecosystems. That compares with 1% in all the world’s oceans! One reason for this difference is that the microbes living in the oxygen-depleted parts of ponds and lakes receive much more detritus to feed upon. Yet until now we have known relatively little about what explains the large amount of variation in methane production within individual lakes. Field studies have suggested that areas covered by aquatic plants, such as cattails, can be particularly productive.
In our paper, we added different types of plant debris to natural lake sediments in a laboratory experiment. We were shocked to find the sediments producing at least 400-times more methane when we added litter from cattails as opposed to forests. The forest litter actually did not produce more methane than where no plant material was added, suggesting that the methane-producing microbes were being inhibited.
To further test the idea of inhibition, we added a known community of methane-producing microbes to the sediments. We again found a large increase in methane production in the sediment amended with cattail debris, as well as the controls. Methane production remained inhibited in the presence of forest litter. The abundance of the microbes themselves seemed to mirror these patterns.
So what was going on? Well it turns out that litter varies greatly in polyphenolics – natural chemicals that plants produce such as for defense and signalling. These polyphenolics prove particularly effective at inhibiting methane production and methane-producing microbes. A similar process occurs in peatlands, where chemicals in the soil help act as a ‘latch’ that traps carbon dioxide. But this is the first time this mechanism has been shown with methane and in lakes.
The implications of our research are somewhat scary – hence the sensationalized titles of various news articles covering the story. Namely, aquatic plants are predicted to migrate northwards in the near future with a warmer climate where they will encounter more suitable lakeside habitat. Any increase in the distribution of aquatic plants poses a major risk of increasing methane production from northern lakes. In the crudest sense, this could just act like a positive feedback loop to further increase temperatures, and further increase the area of suitable habitat for aquatic plants, and so on and so on.
Of course, there are many other factors that will influence future methane production, such as changes in forest cover around lakes and air temperatures that influence the activity of microbes. What our study does emphasize is that models of the global carbon cycle need to pay much closer attention to the many ways that ponds, lakes, and rivers interact with the world’s land surface and atmosphere.