I’m very excited that our manuscript “Microbial community dynamics in two polar extremes: The lakes of the McMurdo Dry Valleys and the West Antarctic Peninsula Marine Ecosystem” has been published as an overview article in the journal BioScience. The article belongs to a special issue comparing different ecological aspects of the two NSF-funded Long Term Ecological Research (LTER) sites in Antarctica. I’m actually writing this post on my return trip from the first ever meeting of the International Long Term Ecological Research (ILTER) network at Kruger National Park in South Africa (an excellent place to ponder ecological questions).
This article had an odd genesis; the special issue was conceived by John Priscu, a PI with the McMurdo LTER project. I was ensnared in the project along with Trista Vick-Majors, a graduate student with John Priscu (now a postdoctoral scholar at McGill University), shortly after starting my postdoc with Hugh Ducklow, PI on the Palmer LTER project. The guidance we received was more or less “compare the McMurdo and Palmer LTERs”. How exactly we should compare perennially ice-covered lakes in a polar desert to one of the richest marine ecosystems on the planet was left up to us. Fortunately, microbial ecology lends itself to highly reductionist thinking. This isn’t always helpful, but we reasoned that on a basal level the two ecosystems must function more or less the same. Despite dramatically different physical settings, both environments host communities of phytoplankton (sometimes even similar taxonomic groups). These convert solar energy into chemical energy and CO2 into organic carbon, thereby supporting communities of heterotrophic bacteria and grazers.
To look at the details of this we stretched the bounds of what constitutes an “overview article” and aggregated nearly two decades of primary production and bacterial production data collected by the McMurdo LTER, and over a decade of the same from the Palmer LTER. By looking at the ratio of bacterial production to primary production we assessed how much carbon the heterotrophic bacterial community takes up relative to how much the phytoplankton community produces.
Typical marine values for this ratio are 1:10. At a value of around 1:5 the carbon demands of heterotrophic bacteria are probably not met by phytoplankton production (the majority of carbon taken up by bacteria is lost through respiration and is not accounted for in the bacterial production assay). Most of the lakes hover around 1:5, with values above this fairly common. Lake Fryxell however, an odd lake at the foot of Canada Glacier, has values that often exceed 1:1! Consistent with previous work on the lakes such high rates of bacterial production (relative to primary production) can only be met by a large external carbon subsidy.
Where does this external carbon come from? Each summer the McMurdo Dry Valleys warm up enough that the various glaciers at the valley peripheries begin to melt. This meltwater fuels chemoautotrophic bacterial communities where the glacier meets rock (the subglacial environment), and microbial mats in various streams and melt ponds. Like microbial communities everywhere these bleed a certain amount of dissolved carbon (and particulate; DOC and POC) into the surrounding water. Some of this carbon ends up in the lakes where it enhances bacterial production.
But external carbon subsidies aren’t the only part of the story. Nutrients, namely phosphate and nitrate, are washed into the lakes as well. During big melt years (such as the summer of 2001-2002 when a major positive SAM coupled to an El Nino caused unusually high temperatures) the lakes receives big pulses of relatively labile carbon but also inorganic nutrients and silt. This odd combination has the effect of suppressing primary production in the near term through lowered light levels (all that silt), enhancing it in the long term (all those nutrients), and giving heterotrophic bacteria some high quality external carbon to feed on during the period that primary production is suppressed. Or at least that’s how we read it.
Not a lake person? How do things work over in the Palmer LTER? One of the biggest ecological differences between Palmer and McMurdo is that the former has grazers (e.g. copepods, salps, and krill) and the latter does not, or at least not so many to speak off. Thus an argument can be made that carbon dynamics at Palmer are driven (at least partially) by top-down controls (i.e. grazers), while at McMurdo they are dependent almost exclusively on bottom-up (i.e. chemical and physical) controls.
At times the difference between bacterial production and primary production is pretty extreme at Palmer. In the summer of 2006 for example, bacterial production was only 3 % of primary production (see Fig. 4 in the publication), and the rate of primary production that summer was pretty high. The krill population was also pretty high that year; at the top of their 4-year abundance cycle (see Saba et al. 2014, Nature Communications). This is speculative, but I posit that bacterial production was low in part because a large amount of carbon was being transferred via krill to the higher trophic levels and away from bacteria. This is a complicated scenario because krill can be good for bacteria; sloppy feeding produces DOC and krill excrete large amounts of reduced nitrogen and DOC. Krill also build biomass and respire however, and their large fecal pellets sink quickly, these could be significant losses of carbon from the photic zone.
Antarctica is changing fast and in ways that are difficult to predict. Sea ice seems to be growing in the east Antarctic as it is lost from the west Antarctic, and anomalous years buck this trend in both regions. A major motivation for this special issue was to explore how the changing environment might drive ecological change. I have to say that after spending a good portion of the (boreal) summer and fall thinking about this, some of that time from the vantage point of Palmer Station, I have no idea. All of the McMurdo Lakes react differently to anomalous years, and Palmer as a region seems to react differently to each of abnormal year. I think the krill story is an important concept to keep in mind here; ecological responses are like superimposed waveforms. Picture a regularly occurring phenomenon like the El-Nino Southern Oscillation imposing a periodicity on sea ice cover, which we know has a strong influence on biology. Add a few more oscillating waves from other physical processes. Now start to add biological oscillations like the four-year krill abundance cycle. Can we deconvolute this mess to find a signal? Can we forecast it forward? Certainly not with 10 years of data at one site and 20 years at the other (and we’re so proud of these efforts!). Check back next century… if NSF funds these sites that long…
Many thanks to my co-authors for going the distance on this paper, particularly the lake people for many stimulating arguments. I think limnology and oceanography are, conceptually, much less similar than lakes and oceans.