I’ve been visiting the River Ehen now for three years and this is my 32nd post on this small Cumbrian river. You might think that, after about 40 visits to each of four locations, I should be beginning to understand the biology of the Ehen. I sometimes find myself being lulled into this false sense of complacency myself: I have a fairly good idea of where in each reach particular algae can be found, and when, in each year, they will be most abundant. It was no great surprise, last week, for example, when Maria finished measuring the biomass with her BenthoTorch and announced that it was higher than the measurements she made last month. We’ve seen this trend of a sharp increase in the quantities of algae at this time in previous years. Similarly, we were expecting (and therefore looked for) the pink blushes of the red alga Audouinella appearing on the rocks at the lowermost site. Again, these had been a common sight in the river during the cooler months throughout the period of our visits.
Yet, in some other ways, the River Ehen has changed over the course of our visits. Most noticeably, the uppermost site, just below the outfall from Ennerdale Water, had some lush growths of the alga Nitella flexilis when we visited last week. I’ve recorded this from the Ehen before (see “Finding the missing link in plant evolution”) but not from this particular location, and not in such quantities. Growths of the submerged angiosperm Myriophyllum alterniflorum were also more conspicuous at this site than on previous occasions.
A few kilometres below the outfall, at the lowermost site that we visit routinely, I also noted that the mats of Phormidium autumnale, a cyanobacterium (blue-green alga) that is an ever-present at this site also seemed to be more conspicuous than on any of our previous visits. The mats of this alga were generally most obvious at the stream margins, where they were periodically exposed to air (see “In which the spirit of Jeremy Clarkson is evoked …”). However, last week, they were also prolific at some permanently submerged locations.
Nitella flexilis growing in the River Ehen, about 400 metres downstream from the outfall from Ennerdale Water, October 2015. The clump is approximately 25 cm in length.
Are these changes in the distribution of alga in the Ehen telling us something about how the river itself is changing or are they within the range of “natural variation”? My suspicion is that, even after three years close observation we have not seen every nuance of the river’s behaviour. Yet three years is a long time by the standards of many ecological studies. We also know that the river has changed over the period that we have been visiting. In particular, Ben Gill, a tributary stream that had been redirected to flow into Ennerdale Water in the 19th century, has recently been reconnected to the River Ehen. This should mean that the flow regime in the river is now more natural but we’ve noticed a lot more fine sediment in the river on occasions since this happened. The lake acts as a huge sediment trap so maybe this sediment itself is a ‘natural’ feature of the stream? There is a temptation to define ‘natural’ in terms of our own experience, which can make it hard to evaluate the significance of events that happened more than a lifetime ago.
A mat of Phormidium autumnale on the bed of the River Ehen, about five kilometres from the outfall from Ennerdale Water. The scale bar indicates approximately one centimetre.
Indeed, I would not even be writing about the Ehen today had I not been called in to look at the river due to the excessive growth of algae. This brings in another facet to the story: my colleague in this study, Ian Killeen, is an expert on the ecology and conservation of pearl mussels. He had seen enough rivers with healthy pearl mussel populations to know that the quantities of algae that he could see in the Ehen were unusual, and he then got in touch with me. We have to be aware, as applied ecologists, that we often do not start collecting data until someone perceives a problem. That, too, creates difficulties when trying to understand the ‘natural’ or ‘baseline’ condition of a river.
These are not new themes for this blog. I’ve talked about the need for broadly-skilled, observant field-based biologists as the foundation for any effective environmental management process (see “Slow science and streamcraft”) and pointed out the limitations of pursuing highly-specialised tech-based approaches (see “Replaced by a robot?”). On the other hand, I am also realistic enough to recognise that the luxury of having a biologist look at the same stream on a monthly basis is way beyond the means of most regulators. So we need to find a compromise and, in the process, we need to agree on the non-negotiable elements of any compromise. Suffice it to say, ensuring that experienced biologists have enough time to visit the sites that they are expected to assess would have to be part of any package. However, don’t take that for granted: the line has, I am afraid, already been crossed by some environmental regulators.