Lakes turning to jelly

Our latest paper in the Proceedings of the Royal Society on the “jellification” of temperate lakes has gotten an impressive amount of on-line attention.  At the time of writing this blog, Altmetric scores it as the 22nd highest ranking paper ever published in the journal.  You can read summaries from the Washington Post, New York Times, Daily Mail, CBC, CBC Radio, and Yahoo, among others.  I’ve also done four separate interviews this week with BBC radio stations (BBC Radio 5, BBC Wales, BBC Cambridgeshire).  You can catch the latest, with the BBC World Service from the 26th of Nov, below:

 

The main finding of the paper is that a small planktonic animal named Holopedium glacialis has been dramatically increasing in two very different lake regions of Canada as the keystone grazer in these lakes, the water flea (Daphnia spp.), has been disappearing.  Our results show that this is mainly driven by declines in lake water [Ca].  Daphnia need large amounts of Ca to build their body shell, while Holopedium surround themselves in a gelatinous polysaccharide “bubble”:

An individual Holopedium with the jelly capsule clearly visible. (Photo: Ian Gardiner / E-Fauna BC)

This jelly also protects Holopedium from predators.  By contrast, Daphnia are increasingly susceptible to predators at low [Ca] because their ability to induce evolved defences is also impaired. Our analyses show how vanishing Daphnia have now left more algae uneaten for their competitors to exploit, allowing them to multiply in number.  Many media reports have picked up on this as Holopedium liking ‘pollution’, with low [Ca] somehow being the result of this.  But it is more in fact a legacy of pollution.  While we have curbed industrial emissions and reduced acid rain, the historical depletion of base cations from the thin soils of the boreal shield, have left behind much lower [Ca] than present prior to industrial activity.  Ca concentrations have consequently been falling across much of North America and Europe.

I won’t summarize the implications of the study further as I think many of the media links and interviews do that well.  But I will give a bit of the background behind the story as I think it is a really nice example of different disciplines collaborating to produce work that is greater than the sum of their parts.

I first got involved with this story about three years ago.  At the time, I was working with Norman Yan at the Dorset Environmental Science Centre, a beautiful research institute in the woods of central Ontario.  Norm had been collaborating with the research group of John Smol on the biological consequences of falling lake water Ca for several years, most notably producing a 2008 Science paper that reported the vast scale of this issue.  One of the real strengths of this paper was the use of sediment cores to show that Ca-rich Daphnia species were becoming near extirpated from boreal lakes as compared with pre-industrial times.  The one big uncertainty, however, was what would take Daphnia‘s place.  That is until Ron Ingram from the Ontario Ministry of the Environment found this in one of his monthly hauls of lake water from Plastic Lake:

A handful of Holopedium

A handful of Holopedium (Photo: Ron Ingram)

Analysis of ca. 35 years of data in 8 intensively studied lakes soon revealed that Holopedium had steadily been increasing with falling lake water [Ca] .  Similar trends also emerged in 31 lakes that were originally surveyed between 1981 and 1990, and again from 2004 to 2005.  As in the Science paper, this was linked to human activities.  Adam Jeziorski, working with John Smol and his lab, found clear evidence that Holopedium were more abundant in modern core ‘tops’ than in older lake sediments.

The mechanism for this pattern, however, was seemingly much more complicated.  This is where I came in.  I had recently been wrapping up work on using structural equation models to infer causal mechanisms in complicated observational datasets.  Norm suggested that understanding the mechanisms behind the rise of Holopedium could be tractable with this approach and a good application of the method that I was trying to sell.  By mining Norm’s 40 years of understanding of zooplankton biology and lake food webs, and synthesizing different long-term datasets, we soon formulated some plausible models that I then spent about two years trying to fit.  The result is that we’ve gone from a pattern to strong mechanistic inference, testing multiple mutually-inclusive hypotheses.

Beyond the jelly, I think the study is a really nice example of how ‘neo-ecologists’ can contextualize global change and its underlying mechanisms by exploiting the power of palaeo-ecology.  It also shows how neo-ecology and modelling can be used to understand the processes generating the biological records deposited in sediments.

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