Bollihope Burn does not disappear dramatically down a single swallow hole in the way that Gaping Gill swallows up Fell Beck on the slopes of Ingleborough. Rather, there is a gradual diminishment of flow, as the river percolates through the joints in the limestone, before the remnants of the stream swirl down a final sinkhole (see “Co. Durham’s secret Karst landscape”). I was intrigued to see how the organisms that inhabited Bollihope Burn reacted to these stresses so got down on my knees close to this final sinkhole to get a closer look.
My waterproof Olympus TG2 (see “Getting close to pearl mussels with my underwater camera”) set to super-macro mode is equivalent to putting my head under the surface of the water and then peering at the rock through a magnifying glass … but gets fewer odd looks from passers-by. Fortunately, this is an isolated corner of Weardale and passers-by were limited to a few rabbits, because sticking a camera into a stream to take a photograph of a stone is, itself, odd enough to attract stares from most people.
These close-up views of freshwater algae in their natural habitat continue to surprise me. It is only in the last few years that waterproof digital cameras with macro facilities have fallen to an affordable price. Before this, underwater photography required special kit that few freshwater biologists could afford. Yet, removing a stone to photograph the algal growths meant that the algae were never photographed in their natural habitat, and were deprived of the buoyancy that the water afforded them. I have plenty of photographs of green or brown gunk composed of different algae but, with the algae removed from their context, these photographs offer few insights into the biology of the stream bed. The photograph below, however, shows a community with a distinct structure – a “turf” of near-vertical filaments waving in the gentle eddies of the stream as it swirls around before disappearing down the swallow hole.
A cobble in Bollihope Burn, close to the swallow hole, covered by a short “turf” of algae, April 2016. Scale bar: approximately two centimetres.
Under the microscope, the structure of this “turf” starts to reveal itself. The filaments appear to be aggregations of diatoms around dying filaments of the green alga Ulothrix zonata. This is an alga that is common in Pennine streams in the winter and early Spring but which disappears as the weather starts to warm up. It often forms very conspicuous green patches on the river bed for a short period of time, as in the following picture, which I took a few kilometres away from my current location, in the River Wear at Wolsingham. The difference in appearance between the alga in the two photographs is mostly due to the Bollihope population being smothered with diatoms whilst the Wolsingham population was virtually a pure growth of Ulothrix. This may be partly due to the Bollihope picture being taken taken two months later than the Wolsingham image. Ulothrix zonata produces copious quantities of mucilage and the Wolsingham population was slimy to the touch. I rarely see epiphytes on this or any other slime-producing algae in their healthy state. However, Ulothrix is a species that thrives in cold water. Indeed, a study has shown that when the water starts to warm up and the day length increases, the Ulothrix filaments switch into their dispersal and reproductive modes and that is what may be happening here. As the rate of photosynthesis declines, so there is less carbohydrate from which the slime molecules can be made and, as a result, less of a deterrence to any diatom looking for a perch. From now until next winter, Ulothrix zonata will not be very obvious in the streams that I visit. This is because the zygotes which are produced by sexual reproduction lie dormant until day length decreases and temperature drops. At this point, they germinate and divide to produce zoospores which, in turn, grow into new Ulothrix zonata filaments.
Growths of Ulothrix zonata on cobbles in the River Wear at Wolsingham, February 2009.
The photographs taken under the microscope illustrate this well. On the left hand side there is one of the few healthy looking Ulothrix filaments that I found, with a chloroplast wrapped around the inside of the cell wall On the right hand side you can see that the chloroplasts have gone, replaced by dark green blobs which are (I think) bundles of gametes awaiting release. More significantly, you can also see several diatoms around the Ulothrix filament, taking advantage of it to lift themselves up above the rock surface.
The paradox is that these algae are entering their senescent phase just as most of the plant life in Weardale is flourishing. This is probably not a coincidence: life in cold water means fewer grazing invertebrates and less shade to intercept the precious winter sunlight. I suspect that algae, once masters of the planet, have gradually adapted and evolved to live a subordinate life, flourishing in those periods of the year when most of us are content to stay indoors.
Ulothrix zonata from Bollihope Burn, April 2016. The left hand image shows a healthy vegetative filament; the right hand image shows zoospore production and colonisation by diatom epiphytes.
Graham, J.M., Graham, L.E. & Kranzfelder, J.A. (1985). Light, temperature and photoperiod as factors controlling reproduction in Ulothrix zonata (Ulvophyceae). Journal of Phycology 21: 235-239.
van den Hoek, C., Mann, D.G. & Jahns, H.M. (1995). Algae: an Introduction to Phycology. Cambridge University Press, Cambridge.