Can stable isotopes reveal an impact of small scale river restorations?

Remember Charlotte Pike, previously blogging for us re her MSc project with Ribble RT and part supervised by Jonny? We’re delighted she has taken a bit of time out from her job now as Marine & Freshwater Project Administrator at the prestigious Zoological Society of London to summarise her findings.

It’s almost a year now since I completed and handed in my MSc dissertation, looking at the effect of river restoration on food web architecture using stable isotope analysis (SIA). Rewinding back to our previous post, Abbie and I were just about to embark on a trip to North Yorkshire, to conduct my field sampling and join the Wild Trout Trust at their annual get-together. We had an amazing time and were extremely grateful to the WTT for allowing us to be honorary members for the weekend. Following this, Abbie, Jon (Grey), Mike Forty from the Ribble Rivers Trust (RRT), and I conducted two days of in-situ kick sampling and electrofishing at the two field sites; Bashall Brook and Towneley Hall. This sampling produced a representative view of the invertebrate and fish assemblage occurring in the rivers post-restoration’: after weir removal at Towneley Hall (below) and the planting up of a riparian zone with trees at Bashall Brook.

Back at the Queen Mary University of London lab, the samples were prepared using the same methods we outlined before (drying, grinding and weighing), before being run through a mass spectrometer. Not everything went to plan — an unfortunate mishap involving a faulty drying oven resulted in 40 of my invertebrate samples becoming a very melted mess (below). A lab evacuation and a few laughs (cries) later, and I was back on the train to Yorkshire to resolve the situation. Very kindly, Jon had helped organise some time to return to Towneley Hall and do more kick sampling to replace the lost samples – and the gorgeous weather continued to make it an absolute pleasure.

Fast forward a month, and many, many hours using the statistical package R’, I had a dissertation. So, I wanted to share the results with you all. For context on the results; the mass spectrometer does some fancy calculations, and spits out a carbon value, and a nitrogen value for each sample analysed. These values I plotted onto a graph, where the x’ axis is carbon, and the y’ axis is nitrogen. This is referred to as an isotopic bi-plot’, essentially a map of a food web. Once this is created, I used an R’ package called SIBER (Stable Isotope Bayesian Ellipses in R) to statistically analyse the data. SIBER works by plotting an oval around 40% of the data points for a particular organism like trout as a focal species. This is recognised as the isotopic niche’ of the organism, a representation of its trophic niche. 

I wanted to compare species ellipses from before the restoration occurred with after the restoration, to identify any changes in isotopic niche and determine whether the intervention had been successful. We anticipated that restoration would cause an increase in available habitat and hence potentially more utilisable resources/​prey, which we would be able to visualise as an increase in isotopic niche at all trophic levels. 

Visual representation of all energy pathways of two generalised food webs. (a) A basic web with reliance on very few primary resources and depleted trophic links, expected of: © a disturbed unstable system. Vs. (b) a complex web with a reliance on multiple primary resources containing numerous trophic links, expected of: (d) a restored stable system. (c,d) Sabden Weir before and after fish passage work, part of the Ribble Life Together project (Ribble Life Together, 2018).

What we hypothesised would happen to the isotopic niche after restoration. The ellipse has increased in size, as a result of more utilisable habitat and therefore resources.

For invertebrates at both Towneley Hall and Bashall Brook, we found increases in isotopic niche breadth in the upper and lower river reaches sampled. The reasoning for increases in invertebrate niche may be due to improvements in plant / biofilm diversity. In Towneley Hall, as previously explained, a weir was in place spanning the width of the river. Weirs have been found to effect river flow, temperature and transport of sediment, which are also all things that alter the richness and diversity of plant species and retention of organic matter. The removal of the weir therefore, may have caused improvement in plant / biofilm diversity and quality that can be seen further up the food web in the invertebrates. At Bashall Brook, the introduction of a riparian buffer zone will have improved terrestrial-aquatic links, and increased the flow of organic matter, i.e. leaves and woody debris into the water, providing more usable food sources for the invertebrates. Overall, there was support that the increase in resource diversity after restoration had positive effects on the invertebrate communities, indicating the beginnings of recovery.

Example of increased isotopic niche observed in invertebrate samples from the lower river reach at Bashall Brook.

Unfortunately, there were no detectable changes in isotopic niche for the fish. One probable reason for this may have been that the spatial scale of change, at least at Towneley Weir, was too small and that fish, being much more mobile, were using resources from outside the sphere of influence of the restoration. Furthermore, we all remember the lovely warm weather last summer… well this was not such great new for the rivers as I’m sure you’ll appreciate. Interestingly, increases in temperature have been linked to slower metabolic rates in fish such as brown trout which could have led to variations in nitrogen and carbon uptake, and as a result affected the values I observed. 

Overall, the results produced using the statistical metrics (wizardry) were extremely useful in witnessing changes in isotopic niche of some of the studied organisms. Invertebrates displayed increasing isotopic niche after the restoration, and so given the time-period, show the early stages of recovery. It may be therefore be ideal to conduct further study to evaluate long-term effects of these interventions and produce clearer conclusions on fish recovery.

So that’s it, I hope you have enjoyed reading about my work as much as I have enjoyed conducting it! I am massively thankful to my supervisors; Jon Grey and Chris Eizaguirre, as well as Mike Forty, the Wild Trout Trust and Ribble Rivers Trust, for all their help, guidance and allowing me the opportunity to conduct work on these restorations. 

Charlotte (@Charlotte_Pike1)