Zach and I have been camped out in Kangerlussuaq for almost 5 weeks now, on a quest. Our quest is of a limnological nature. We are trying to better understand one of the more ubiquitous yet perplexing features of these Arctic lakes – the sea tomato.
And in case you need a little sea tomato refresher, these are a type of cyanobacteria of the genus Nostoc, which form spherical, gelatinous colonies in the water. The colonies can range from quite small (<5 mm) to very large (softball size). They typically lie on the bottom of the lake, and can occur in densities of hundreds, to hundreds of thousands per lake.
As cyanobacteria, sea tomatoes play important role in the ecosystem — they aren’t plants (they are bacteria), but they do contain special photosynthetic pigments in their cytoplasm that enable them to photosynthesize. They also have the remarkable ability to fix atmospheric nitrogen into a useable form, and this combination of metabolic features can be crucial to thriving in a nutrient-limited environment like an oligotrophic (low nutrient) Arctic lake.
BUT…WHY do sea tomatoes occur so abundantly in some lakes, and barely grow in others? Some lakes don’t have any sea tomatoes, while lakes located less than 100 meters away may have 100,000’s…
To investigate this phenomenon, we built a research vessel…specifically designed to fit our limnological needs.
We’ve installed an underwater sonar device to the back of the boats and we paddle transects across each lake in order to map the shape and size of the lake basins. We also measure a variety of water chemistry parameters (pH, conductivity, dissolved oxygen, temperature), and take biological samples including zooplankton, phytoplankton, and sea tomatoes, which we will use to construct species composition for each lake, and run a variety of nutrient and toxin analyses (did I mention the sea tomatoes are also producing toxins? That may call for a separate entry…).
We have now surveyed 19 lakes ranging from the mouth of the fjord to the edge of the ice sheet:
One of our most interesting discoveries has been that there are a few different species of sea tomato here!
We believe the smooth, spherical species in all the photos above is Nostoc pruniforme. It is the most abundant and widespread species of sea tomato in these lakes.
This next one, however, is bumpy, irregular in shape, contains dark patches of greenish-black, and doesn’t occur in nearly as dense quantities as N. pruniforme. We are not sure yet, but we think it may be a very rare species called Nostoc zetterstedtii.
There are even occasions where the two are found attached…
And finally, we also came across a very dark colony of tiny spheres and much rougher texture, which we have not yet been able to identify…
* I’d be very interested to hear if anyone has come across something similar to this one before.
And now in our final week, we are now focused on finishing some underwater photography, with an effort to more accurately estimate sea tomato densities across lakes.