My reflections on the effects of the heatwave on freshwater algae continued with the latest of my regular visits to the River Wear at Wolsingham. A comparison of the picture above with that at the head of “Spring comes slowly up this way …” says it all: the sun was shining and the gravel berms that I usually use to enter the river were occupied by families with barbeques whilst their children splashed around in the water. At times such as this, a grown man picking up stones and then vigorously brushing their tops with a toothbrush would have invited too many questions, so I slunk off 100 metres or so downstream and found a quieter spot to explore.
The biofilm in the main channel of the River Wear at Wolsingham, July 2018.
The first thing I noticed was that the biofilm coating the submerged stones at the bottom of the river had a greenish tinge, rather than its usual chocolate brown appearance. It also was more crusty and less slimy to the touch than I usually see in this river. When I got a specimen under the microscope, I could see that the composition was completely different to that which I had observed in previous months. Most samples from this location that I’ve looked at in the past have been dominated by diatoms, with occasional spring flourishes of filamentous green algae. Today, however, the sample was dominated by small green algae – a group that I am not very confident at identifying. My rough estimate is that these formed about three quarters of all the algae that I could see, with diatoms and cyanobacteria each accounting for about half of the remainder. The most abundant greens were a tiny single-celled alga that I tentatively identified as Keratococcus bicaudatus, along with a species of Scenedesmus and Desmococcus communis. There were also a number of cells of Monoraphidium arcuatum and some of Ankistrodesmus sp.
Two views of biofilms from the River Wear, Wolsingham in January 2018. Left: from the main channel; right: from pools at the edge of the channel.
Green algae from the River Wear at Wolsingham, July 2018: a. Desmococcus communis; b. Monoraphidium arcuatum; c. Scenedesmus sp.; d. unidentified, possibly Keratococcus bicaudatus. Scale bar: 10 micrometres (= 1/100th of a millimetre).
However, there were also pools at the side of the channel, away from the main current but not so cut off that they were isolated from the river itself. These were dominated by dense, brown filamentous growths, very similar in appearance to the Melosira varians flocs I described in “Some like it hot …”. The filaments, however, felt coarser to the touch and, in close-up, could be seen to be branched, even without recourse to a microscope. Once I got these under the microscope, I could see that they were filaments of Cladophora glomerata, another green alga, but so smothered with epiphytic diatoms (mostly Cocconeis pediculus) that they appeared brown in colour.
This combination of Cladophora glomerata and Cocconeis pediculus in the backwaters were as much of a surprise as the green-algae-dominated biofilms in the main channel. These are species usually associated with enriched rivers (see “Cladophora and friends”) and, whilst I have seen Cladophora in the upper Wear before, it is an unusual occurrence. Just as for the prolific growths of Melosira varians described in “Some like it hot …” it is tempting to leap to the conclusion that this must be a sign that the river is nutrient-rich. However, the same conditions will apply here as there: “nutrients” are not the only resource that can limit plant growth and a steady trickle of phosphorus combined with warm, sunny conditions is just as likely to lead to prolific growths as a more conventionally “polluted” river.
Cladophora filaments smothered by the diatom Cocconeis pediculus in a pool beside the River Wear at Wolsingham, July 2017. The frame width of the upper image is about 1 cm; the scale bar on the lower images is 20 micrometres (= 1/50th of a millimetre).
Another way to think of these situations is that, just as even healthy people are occasionally ill, so healthy streams can go through short periods when, based on a quick examination of plants and animals present, they exhibit symptoms associated with polluted conditions or simply (as for the first sample I described) different to what we usually expect to find. A pulse of pollution might have passed downstream or, as seems to be happening at the moment, an unusual set of conditions lad to different organisms thriving. Just as the ability to fight off infection forms part of a doctor’s understanding of “health”, so I expect that the River Wear will, in a few weeks time, be back to its usual state. Healthy ecosystems, just like healthy humans, show “resilience”. The irony is that, in this case, the “symptoms” are most obvious at a time when we are enjoying a summer better than any we’ve had in recent years.