Having talked about diversity on a microscale in the previous post, I thought it would be interesting to place this in context by looking at the variations that I have observed in the River Wear at Wolsingham over the past decade or so. The River Wear has seen some significant improvements in water quality over this period, but those have mainly affected sections of the river downstream from Wolsingham. Most of the changes at Wolsingham are, therefore, giving us some insights into the range of natural variation that we should expect to see in a river.
I’ve got 31 samples from the River Wear at Wolsingham on my database, collected since 2005. Over this period, nine different diatom species have dominated my counts: Achnanthidium minutissimum on 21 occasions, Nitzschia dissipata twice and Cocconeis euglypta, Encyonema silesiacum, Gomphonema calcifugum, Navicula lanceolata, Nitzshia archibaldii, N. paleacea and Reimeria sinuata once each. I also have records for non-diatoms during 2009, during which time the green alga Ulothrix zonata, and two Cyanobacteria, Phormidium retzii and Homeothrix varians were the dominant alga on one occasion each. In total, I have recorded 131 species of diatom from this one reach, although only I’ve only found 91 of them more than once, and only 59 have ever formed more than one percent of the total. I’ve also got records of 22 species other than diatoms.
This – along with my comments in “The mystery of the alga that wasn’t there …” raises questions about just how effective a single sample is at capturing the diversity of algae present at a site. . In 2009 I collected a sample every month from Wolsingham and the graph below shows how the total number of species recorded increased over that period. Typically, I find between 20 and 30 species in a single sample, and each subsequent month revealed a few that I had not seen in earlier samples. Importantly, no single sample contained more than 40 per cent of the total diversity I observed over the course of the year. Part of this high diversity is because of the greater effort invested but there is also a seasonal element, as I’ve already discussed. The latter, in particular, means that we need to be very careful about making comments about alpha diversity of microalgae if we only have a single sample from a site.
Increase in the number of diatom taxa recorded in successive samples from the River Wear at Wolsingham. In 2009 samples were collected monthly between January and December whilst in 2014 samples were collected quarterly.
This seasonal pattern in the algal community also translates into variation in the Trophic Diatom Index, the measure we use to evaluate the condition of streams and rivers. The trend is weak, for reasons that I have discussed in earlier posts, but it is there, nonetheless. Not every river has such a seasonal trend and, in some cases, the community dynamics results in the opposite pattern: higher values in the summer and lower values in the winter. It is, however, something that we have to keep in mind when evaluating ecological status.
Variation in the Trophic Diatom Index in the River Wear at Wolsingham between 2005 and 2015, with samples organised by month, from January (1) to December (12). The blue line shows a LOESS regression and the grey band is the 95% confidence limits around this line.
All of these factors translate into uncertainty when evaluating ecological status. In the case of the River Wear at Wolsingham, this is not particularly serious as most of the samples indicate “high status” and all are to the right of the key regulatory boundary of “good status”. However, imagine if the histogram of EQRs was slid a little to the left, so that it straddled the good and moderate boundaries, and then put yourself in the position of the people who have to decide whether or not to make a water company invest a million pounds to improve the wastewater coming from one of their sewage treatment plants.
At this point, having a long-term perspective and knowing about the ecology of individual species may allow you to explain why an apparent dip into moderate status may not be a cause for concern. Having a general sense of the ecology of the river – particularly those aspects not measured during formal status assessments – should help too. It is quite common for the range of diatom results from a site to encompass an entire status class or more so the interpretative skills of the biologists play an important role in decision-making. Unfortunately, if anything the trend is in the opposite direction: fewer samples being collected per site due to financial pressures, more automation in sample and data analysis leading to ecologists spending more time peering at spreadsheets than peering at stream beds.
I’ve never been in the invidious position of having to make hard decisions about how scarce public sector resources are used. However, it does strike me that the time that ecologists used to spend in the field and laboratory, though deemed “inefficient” by middle managers trying to find cost savings, was the time that they learned to understand the rivers for which they were responsible. The great irony is that, in a time when politicians trumpet the virtues of evidence-led policy, there is often barely enough ecological data being collected, and not enough time spent developing interpretative skills, for sensible decisions to be made. Gathering ecological information takes time. But if that leads to better decisions, then that is not time wasted …
Ecological Quality Ratio (EQR: observed TDI / expected TDI) of phytobenthos (diatoms) at the River Wear, Wolsingham) between 2005 and 2015. Blue, green, orange and red lines show the positions of high, good, moderate and poor status class boundaries respectively.
* the title is borrowed from the late Linda Smith’s BBC Radio 4 comedy series