How does an alga move upstream? I’m curious because, I am now seeing populations of Lemanea fluviatilisabout four kilometres further upstream in the River Ehen than when I first started my regular visits in 2013. I can explain the presence of the organism partly through changes in the hydrology of the river: a small tributary, Ben Gill, that had been diverted into the lake in Victorian times was reconnected to the river in 2014 and this introduced periodic pulses of intense energy to the river that had immediate effects on the substrate composition. Lemanea fluviatilisis a species that thrives in the fastest-flowing sections of streams so I am quite prepared to believe that even a small shift in the hydrology of this very regulated river might make the habitat more conducive.
But that does not explain how it got there in the first place. If the alga was occurring a few kilometres further downstream we would not have any such problems: the upstream populations would provide innocula and, if the habitat conditions changed at the downstream location, then some of those propagules might be able to establish at the downstream locations. But what about movement in the other direction?
There has been relatively little published on this topic in recent years. I have a review by Jørgen Kristiansen from 1996 that considers the dispersal of algae but most of the references that he cites are quite a lot older than this and I have not seen much published subsequently. He lists our options: dispersal by water, by organisms, by air currents and by human activity. Let’s consider each in the context of Lemaneain the River Ehen. Lemanea, like most red algae, has a complicated life cycle with the potential for dispersal in both the haploid and diploid phases, but that is probably more detail than we need right now. We’ll just outline the options in broad terms:
Water:the linear flow of the river means that it is almost impossible for the downstream population to provide inocula for the new upstream locations. It may be possible for populations from further upstream in the catchment to seed the new locations. I have not seen Lemaneain any of the streams that flow into Ennerdale Water (from which the Ehen emerges) but my knowledge of the catchment is not exhaustive. Likelihood: very low to low.
Young shoots of Lemanea fluviatillis(bottom right) growing on a submerged boulder in the River Ehen at a location where I have not previously seen it. These are growing alongside thick growths of diatoms (yellow-brown in colour) and patches of green filamentous algae.
Organisms:much of the older literature is concerned with the possibility of living algae or their propagules being transported in mud attached to bird’s feet or feathers and this cannot be ruled out. There is also a recent study showing how mink may act as a vector for Didymosphenia geminata in Chile. The Ehen also has aquatic mammals (such as otters) that could be acting as vectors for Lemanea, as well as migratory fish such as salmon and trout that could move propagules upstream. There is also some evidence that some algae can survive passage through mammalian and invertebrate guts, and this, too, may provide a means for Lemaneato spread upstream. Likelihood: low to medium.
Air currents / wind:quite a lot has been written about airborne dispersal of algae, with even Darwin making a contribution (see reference in Kristiansen). The key hazard in airborne dispersal is desiccation so, in the case of Lemanea, the most likely lifecycle stages that could be dispersed in this way would be the diploid carpospores or haploid monospores. This, however, would assume that there were times during the year when the relevant life-cycle stages were exposed and, as Lemaneais a species that I usually find in the Ehen only fully-submerged, this is not very feasible. Likelihood: low.
Human activity:there is evidence that Didymosphenia geminatacan be transported between sites attached to waders and new records often correspond with patterns of recreational use (references in Bergey & Spaulding – see below). When we work in the Ehen we prefer to move downstream in order to minimise the risk of moving organisms on our kit, and we also clean our kit before we start. However, a lot of people work in this part of the Ehen and it only takes one dirty wader to introduce a propagule. Likelihood: low to medium.
We’ll almost certainly never know for sure why Lemanea fluviatilisis now thriving four kilometres further upstream than it was five years ago. It is, however, worth bearing in mind that, given enough time, even a low probability may yield a positive result. So none of the four hypotheses can be ruled out for sure. Three of the possibilities are entirely natural, with one – movement by the stream itself – being constrained by the direction of flow. Biological vectors look like a very plausible means of moving algal propagules around catchments but, for this to work, we need wildlife-friendly corridors around the river to support the animals and birds. The upper Ehen has these, but many other rivers do not.
Actually, having a number of options all with a relatively low likelihood adds to the sense of mystery that every ecologist should have when they approach the natural world. When cause and effect are too predictable, we tend to focus on engineering the right “solution”. The truth, in our muddled and unpredictable world, is often that nudging several factors in the right direction will give us a more resilient outcome, even though we may have to wait longer for it to happen.
Bergey, E.A. & Spaulding, S.A. (2015). Didymosphenia: it’s more complicated. BioScience65: 225.
Kristiansen, J. (1996). Dispersal of freshwater algae – a review. Hydrobiologia336: 151-157.
Leone, P.B., Cerda, J., Sala, S. & Reid, B. (2014). Mink (Neovision vision) as a natural vector in the dispersal of the diatom Didymosphenia geminata. Diatom Research29: 259-266.
Raven, J.A. (2009). The roles of the Chantransia phase of Lemanea (Lemaneaceae, Batrachospermales, Rhodophyta) and of the ‘Mushroom’ phase of Himanthalia (Himanthaliaceae, Fucales, Phaeophyta). Botanical Journal of Scotland46: 477-485.