Back in 2016 I wrote a post entitled “The camera never lies?” which mused on the difficulties in interpreting photographs of stream beds taken with waterproof cameras. My point was that quantities of algae can be highly variable and interpretation of conditions from a single photograph is fraught with difficulties. Because waterproof cameras are now relatively cheap it is easy to take a photograph whilst out doing fieldwork and then include this in a report, making a point, perhaps, about the poor condition of a site. However, the frequency of visits to a site is often such that the person who took the photograph has little idea of whether that growth is persistent or not. The photograph at the top of the post (from the River Irt, Cumbria, just below Wastwater) shows just how variable the cover can be at a single site over the course of a year.
The question is still pertinent because, last year, an article appeared in FBA News suggesting that percent benthic algal cover, measured from bed photographs, was correlated with the level of phosphorus enrichment. The example from the River Irt challenges this: the cover recorded here (average: 17%, maximum: 36%, based on direct estimates of the percent of the stream bed covered with algae, rather than via photographs) suggests quite a high level of enrichment, whereas the Irt is, in fact, just downstream of one of the most oligotrophic lakes in the country. There are two plausible reasons for this: first, my experience of West Cumbrian rivers suggests that regulation can have a big effect on the quantity of filamentous algae present regardless of the amount of nutrients and, second, the article makes no differentiation between “good” algae and “bad” algae. Not all algae are indicators of nutrient enrichment, so you really need to know the identity of the algae before leaping to a conclusion (see “The democratisation of stream ecology?” for one way of addressing this.
A further issue is illustrated by the second image – this time, showing the bed of the River Ehen, about 15 kilometres north of the River Irt and, once again, downstream from an oligotrophic lake. Here, the total cover is even higher (average: 46%, maximum: 87%) but green algae represent a relatively small proportion of this (typically about a quarter). The FBA News article makes little reference to the type of algae, just referring to “benthic algal growth”. Again, identity is important: the Ehen site has a lot of red algae and Cyanobacteria as well as green algae and, again, these are not necessarily indicators of enrichment. Copious Lemanea fluviatilis in the Spring is, in fact, usually a sign of healthy conditions.
Seasonal variation in the appearance of the bed of the River Ehen, Cumbria. Photographs taken at approximately the same spot at two monthly intervals. The photograph at the top of the post shows variation in the River Irt, also in Cumbria.
One extra twist to the story comes when we compare visual estimates of cover with the biomass, as measured using a BenthoTorch (see “The complexities of measuring mass”). Both show a trend of increased measured biomass linked to increased observed cover, but the relationships are very noisy. Of particular interest is the row of points bouncing along the x axis, indicating visits where the eye saw lots of green algae but the BenthoTorch measured very little. What I think is happening is that the green algae can form bright but relatively thin layers on the rocks on these occasions. On other occasions, the measured biomass is much higher but if the green algae are covered with epiphytic diatoms, then they may not be so obvious to the naked eye, possibly leading to percent cover being under-estimated during surveys. A final possibility, certainly plausible in the Irt, is that the green algal growths are associated with boulders whereas our biomass measurements are made on moveable cobbles.
Measuring biomass with a BenthoTorch in West Cumbrian streams.
There are, in other words, good reasons for the mismatch between observed and measured quantities of algae. Neither is a perfect indicator of what is going on in a river. If we are interested in the effect of algae on ecosystems, then measured biomass would seem to be the better indicator of the extent to which other ecological processes in a stream are likely to be affected by the algae that are present. However, the general patchiness of algal growths, plus the relative ease with which visual estimates can be made probably outweighs their problems, at least from the point of view of basic assessments of ecosystem health.
The relationship between visual estimates of cover (x axis) and biomass measured using a BenthoTorch; a. all algae (Spearman’s rank correlation, r = 0.55); b. just green algae (r = 0.65). Data from West Cumbrian lakes and streams, 2019-20.
There are, in other words, good reasons for the mismatch between observed and measured quantities of algae. Neither is a perfect indicator of what is going on in a river. If we are interested in the effect of algae on ecosystems, then measured biomass would seem to be the better indicator of the extent to which other ecological processes in a stream are likely to be affected by the algae that are present. However, the general patchiness of algal growths, plus the relative ease with which visual estimates can be made probably outweighs their problems, at least from the point of view of basic assessments of ecosystem health. Following on from this, I doubt that a photograph offers any advantage over an inspection of the river bed for basic ecological health assessments. You need to explore a reasonable length of river, including variations in velocity, substratum type and shade, to get some idea both of the quantity of algae, and its variability.
After a quarter of a century spent contemplating the relationship between algae and nutrients in rivers, however, I’m reluctant to endorse any system that implies that a “green” riverbed equates to poor ecosystem health based on a single visit. These proliferations are, to use a human health analogy, the sniffles and colds of river ecosystems: most rivers suffer from them from time to time and most recover quite quickly. The concern is when rivers are persistently green, as this suggests a long-term breakdown in the links between trophic levels. The first response on seeing a green riverbed, in other words, should be to schedule a follow-up visit a month later to see if the situation persists rather than leaping to a rash conclusion. The second response should be to work out which alga is responsible for the bloom as this, too, can help with subsequent diagnoses.
On a more positive note, anything that makes people notice and record algae in rivers has got to be a good thing. Just don’t be seduced by any simplistic correlations that assume that algal growths are, by definition, a bad thing. Algae are key components of the ecological engine of rivers and if that is not part of the thinking behind any river health assessment, then treat it with extreme caution.
Everall, N.C., Johnson, M.F., Clarke, A. & Gray, J. (2019). The visual state of river beds and their associated invertebrate community biosignatures. FBA News 77: 11-15.
Some other highlights from this week:
Wrote this whilst listening to: early Fleetwood Mac, as I commemorated the death of Peter Green last weekend.
Cultural highlights: Once Upon A Time In Iraq, on the BBC iPlayer, is a gruelling exposé of Western policy blunders and ideological naivity that brought the Middle East to its present dire predicament.
Currently reading: Watching the English by Kate Fox. Cultural anthropology on our own doorstep.
Culinary highlight: Red Dragon (Aduki Bean) Pie – one of the best veggie alternatives to a shepherd’s pie that I have come across. Recommended by Julie Gething.