Fieldwork in January is always a mixed blessing. On the one hand, you get a break from the tedium of office- and lab-based activities that dominate the ecologist’s winter agenda. On the other hand, there is the weather. It is only when the water is freezing and the wind is blowing that you realise just how reliant we are on our fingers. At each site I have to pick stones from the riverbed, then manipulate forceps to collect fragments of the different organisms present, and finally write some legible notes in my field book.
A year or so ago, I found a partial solution in the form of a box of veterinarian’s disposable gloves which extend right up my arm. A cyclist’s reflective ankle band then holds the top of the glove in place while I plunge my arm into the river. This means that I don’t need to remove outer layers of clothing before plunging my arm into the water, though the thin plastic of the gloves offers no insulation to my hand itself.
The second problem we face is that we are wholly at the mercy of the river flow. Life is easier now than in the past because you can get real-time readings of flow from the Environment Agency’s website. The knack is to translate the number that you read on the web page into a meaningful indication of risk. After visiting the Ehen for over a year, and comparing what I see with the hydrographs, I now know that a flow of about 100 MLD (mega-litres per day) means that the water comes no higher than my calves whilst 500 MLD is the limit for safe working. And today’s hydrograph reads … 500 MLD.
So here’s my question: if the river is flowing so fast that I can only just stand up, and if there have been a series of spates over the past couple of weeks, what is this going to do to all the algae that live on the stones at the bottom of the river? The answer will surprise everyone but me. The reason I am not surprised is that I am the one trying to stand up in the river and one of the problems I face is that the rocks beneath my feet are slippery with algae (remember “healthy streams are slippery streams …”?).
We’re using a device called a “Benthotorch” to measure the quantity of algae on the river bed. This is a portable fluorimeter, calibrated to measure the amount of chlorophyll on a given area of stone. The more algae there are on the stones the greater the concentration of chlorophyll that we record. When I collated all the evidence we’ve patiently collected over the past year, it becomes clear that there to be much more algae in the winter than in the summer. Substantially more. What is going on?
Maria using a benthotorch to measure chlorophyll concentrations on stones from the River Ehen, February 2013.
The summer values are, perhaps, the easier to explain: on most of our visits here during the summer we noticed lots of tiny midge larvae on the stone surfaces (see “A very hungry chironomid …“). These were munching their way through the algae and, in turn, were being eaten by larger invertebrates and fish. The low chlorophyll measurements, in other words, are a result of natural process in a healthy ecosystem. The larger quantities we find in the winter can partially be explained by the same mechanism: the cold weather means that the bugs are not so active, meaning that algae tend to accumulate rather than being converted into midge larvae.
Variation in chlorophyll concentrations on stones in the upper River Ehen. Bars are the average values of five replicate measurements from each of four sites. No data were collected in December 2012 or January 2013.
But if I have trouble keeping my feet in the fast current of the River Ehen, how come those algae that do accumulate are not washed away? Some of the algae will be removed by the current. Sand and gravel carried by the stream will abrade the surface of the stones and, at high velocities, the stones themselves will be rolled downstream. Yet it is easy to over-estimate the effect of the stream itself. The stream bed generates a huge amount of friction which slows the flow of water in the centimetre or so closest to the bottom to almost zero. The physics befuddle me but better brains than mine have worked out that life in this “boundary zone” can carry on whilst those of us who set out to study it are struggling to keep our feet.