Unlikely neighbours …

One of the lessons I learned from writing “A tale of two diatoms …” is that we can often learn more about the ecology of a species by contrasting its behaviour with that of another species rather than by just relating the distribution of that species to features of its environment.  I came across another example of this when I was writing up the results of the latest “ring-test” that UK diatom analysts undertake to maintain their competence.

The sample came from a stream in east Devon (the one that had a walk-on part in “The challenging ecology of a freshwater diatom”).  This stream receives effluent from a small sewage works but our sample comes from just upstream of this works.   We know that the stream downstream of the sewage works is quite polluted but were also interested in the condition of the stream above the works.   This has proved to be challenging and, it seems, there are some pollution sources, including septic tanks and runoff from fields, that mean that the stream already shows signs of impact before it reaches the sewage works.   There are, however, mixed messages when we look at the aquatic flora, and some of the diatoms that are abundant are characteristic of low or only slight enrichment.

One feature of the stream that was quite unusual was a relatively large number of cells of Reimeria uniseriata, a relative of Reimeria sinuata which is quite common.  Both of these are illustrated below: note that R. uniseriata tends to be slightly larger and has distinctly punctate striae.  However, when I looked at the distribution of these species in response to water chemistry, I could see few differences, with most of the records suggesting a preference for water with low or slightly elevated phosphorus concentrations.   Reimeria sinuata is more common than R. uniseriata and when the latter is found, the former is usually present too.  They seem to be able to share their habitat quite comfortably.

Reimeria sinuata from Polly Brook, Devon, December 2016.   a. – f.: valve views; g.: girdle view focussed on ventral side.  Scale bar: 10 micrometres (= 1/100th of millimetre).  Photos: Lydia King.

Reimeria uniseriata from Polly Brook, December 2016.  h., i.: valve views; j.: girdle view focussed on dorsal side; k., l.: girdle views focussed on ventral side.   Scale bar: 10 µm (= 1/100th of millimetre).  Photos: Lydia King.

In other words, we cannot learn very much from looking at differences in the distribution of these two species of Reimeria, given our current state of knowledge.  There is, however, one other “compare and contrast” within the data that I collected from Polly Brook that is more intriguing.   If Reimeria sinuata, in particular, usually indicates a healthy stream, possibly with a little nutrient enrichment, Rhoicosphenia abbreviata is more often associated with enriched conditions.   We have met this diatom before (see “Cladophora and friends” amongst other posts) and I have explained that it is often found growing as an epiphyte on other algae.  We rarely see situations where both species are abundant at the same time, as the graph below shows.

The relative distribution of Reimeria sinuata and Rhoicosphenia abbreviata in the 6500 UK stream and river samples in the DARES dataset.   The horizontal and vertical lines indicate 10% relative abundance of each species.

When I started looking at stream algae there was a prevailing assumption that there were strong causal relationships between the species of diatom that were found at a site and the level of chemical pressures.  In the case of phosphorus, in particular, I am now not convinced that the evidence supports this whilst, at the same time, am more convinced that we should be able to, at the very least, describe what a healthy stream algal community looks like and give reasons.  I use the word “describe” because I think that many of us have been preoccupied with counting and measuring, often at the expense of a qualitative understanding.  These two species illustrate my point as when I look down a microscope and see Reimeria sinuata, I can usually assume that the stream where it was growing was reasonably healthy, even if the nutrients are a little higher than would be ideal.  On the other hand, seeing lots of cells of Rhoicosphenia makes me suspect that there has been a breakdown in the functioning of the healthy community.  These conclusions would be irrespective of what the chemistry or the values that biological indices told me.

Two species is barely enough to base a credible assessment upon but we could stir more into the mix: I often find Reimeria sinuata with Achnanthidium minutissimum, and that, in in summer especially, suggests strong top-down control by grazers, which means that pathways of energy flow have not been disrupted.   And Rhoicosphenia, as I have already mentioned, is associated with Cladophora which, in abundance, suggests a breakdown in these pathways, as shown by Michael Sturt and colleagues from University College, Cork, a few years ago.   That Polly Brook has both Reimeria and Rhoicosphenia in abundance suggests that it might just be at the tipping point between these two states.

The naïve answer to making sure that the upper stretches of Polly Brook do not cross this threshold would be to manage the nutrients.  However that is not quite as easy as it sounds in an agricultural catchment.   It could be that managing other aspects of the riparian environment are equally effective at keeping the stream in a healthy condition but that takes us into areas where the evidence is still accumulating.  It could be that the simplistic determinism that drove much of the development of biological assessment methods actually held back the gathering of that evidence for a long time.  Reimeria sinuata – and it’s cousin, R. uniseriata – stand as two reminders that there is more to the management of aquatic ecosystems than strong correlations.

Reference

Sturt, M.M., Jansen, M.A.K. & Harrison, S.A.C. (2011).  Invertebrate grazing and riparian shade as controllers of nuisance algae in a eutrophic river.  Freshwater Biology 56: 2580-2593.

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