How green is my river?

Atma_River_July13

I wrote recently about the problems of knowing whether changes we observe in the biology of streams and rivers are signs of long-term shifts caused by human activities or just the result of short-term variation (see “’signal’ or ‘noise’?”). An interesting paper has just been published that allows us to see our observations on the River Ehen into a broader perspective.   The paper was written by my friend Susi Schneider, of the Norwegian Institute for Water Research, and is based on long-term observations by herself and colleagues on the Atna River in Norway (illustrated above; see “A brief excursion to Norway” and subsequent posts).

First of all, here is a graph summarising our observations of biomass in the River Ehen over three years. You can see a fairly regular pattern emerging of low biomass in the summer (when grazing activity is most intense) and high biomass in the winter. But you can also see strong differences between years. There were much greater quantities of algae in winter 2013/14, for example, compared with winter 2014/15 and we are puzzling over why this may have happened.

Ehen_summary_graph

Trends in algal biomasss in the River Ehen, Cumbria between October 2012 and November 2015.   Values are the means of measurements made at four separate sites in a 5 km stretch of the river below the outflow of Ennerdale Water. Arrows indicate the approximate time of year when surveys of the Atma River were performed.

Though the Atna is about 1000 km to the north-east of the Ehen, there are similarities: both rivers have soft water and low levels of human impact and, furthermore, one of the two sites Susi writes about is immediately below a lake, just as our sites on the Ehen are downstream of Ennerdale Water.   The difference is that we visit the Ehen monthly, whereas Susi only visits the Atna once a year, although she takes care to visit at the same time each year.   I’ve indicated the time of year of her visits on the graph of the River Ehen, to aid comparisons between the two datasets.

One of the problems we have in the Ehen is that there is anecdotal evidence of lower quantities of biomass in the past.   The large quantities of algae was the trigger for our study; as is often the case, we generally do not start monitoring until a problem is perceived, which means that we then don’t have the baseline data that we need to understand the causes.   One of the interesting points that arise from Susi’s study is that there have been recent increases in algal cover at both the sites she studied.   Note that the pattern was different at the two sites (one just below a lake, one not). To put the two studies in perspective, the green box on the graphs from the Atma shows the length of time of our study covers, compared to Susi’s.

The reasons for the high algal cover in the Atna include cool, wet summers, driven by the North Atlantic Oscillation, and high discharges in August (i.e. the month before Susi’s regular visits). The former is a natural cyclical phenomenon; high August discharges are, in turn, a consequence of the cool, wet summers, and probably exert their effect by removing the grazers that would naturally control the biomass.   High discharges in the spring (i.e. 10 times the average discharge) also seem to have a major effect on the quantities of algae recorded later in the year.

Atma_long_term_algae_trends

Long term trends in algal cover at two locations in the upper catchment of the Atna River, Norway. The green box indicates the length of time covered by our observations on the River Ehen (2012-2015).  Graph from Schneider (2015).

What should we learn from this comparison?

  • Lesson 1: start monitoring at least 10 years before the problem arises, so that you have a strong baseline.   There is a serious point here, as environmental monitoring is likely to be one major casualty of the cuts in current spending. When problems do arise, the availability of historic data from the site is inevitably very useful but the quid pro quo is that you may need to invest in data collection even when there is no obvious short-term justification for that monitoring.
  • Lesson 2: following on from this, regard environmental monitoring is an insurance policy, insofar as you may not need to make a “claim” on every single site where you monitor.   In effect, this means accepting that some monitoring data that you collect may be redundant. The problem is that you don’t know which data will be redundant until at least a decade or so after you have collected it.   However, the complex nature of many of the problems that we face, particularly where there are interactions with climate (as in the Atna), you will not be able to make evidence-based decisions without long runs of reliable data.
  • Lesson 3: when dealing with algal growth in rivers (which reflects interactions between the physical, chemical and biological environment), do not try to draw any conclusions until you have measurements from years when the North Atlantic Oscillation is in each of its positive and negative phases.   Susi’s paper shows the problems of unravelling the complexities of biological interactions with climate. We need to think in “decades”, not “years” if we are to truly understand environmental change.
  • Lesson 4: simple measurements of criteria that can be easily understood by non-technical stakeholders aid communication. In both the Atna and the Ehen, measurements relate directly to public perceptions of healthy versus unhealthy rivers.   We have all the nerdy details of what algae are found at each site in both the Ehen and Atna, but the take-home message can be put across in terms of “how green is my river?”
  • Lesson 5: if you have to sample at widely-spaced intervals (i.e. yearly, as in Susi’s study), make sure that you sample at the same time every year.
  • Lesson 6: all of these lessons can be ignored if you are a politician with ambitions to create a leaner public sector. The sad truth is that the consequence of failing to invest in monitoring networks is not likely to be apparent for several years (well beyond the next General Election, to be blunt). Almost any aspect of public spending can be hacked away by a skilled political operator, so long as the effects of these decisions are chronic and slow to manifest themselves …

Reference

Schneider S. (2015). Greener rivers in a changing climate? – Effects of climate and hydrological regime on benthic algal assemblages in pristine streams. Limnologica 55: 21-32.

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2 thoughts on “How green is my river?

  1. Pingback: And the waters prevailed on the earth … | microscopesandmonsters

  2. Pingback: The water had receded from the earth … | microscopesandmonsters

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