At the time of writing, four of my five working days are given over to ecology whilst the fifth is spent volunteering for the local Foodbank, which is gearing itself for a huge run on the stocks built up from generous donations over the Christmas period. It occurred to me last week that I spend four days extolling hunger in an ecological context whilst spending the fifth trying to alleviate it in a human one.
“Hunger” in an ecological context is a them to which I have returned several times over the years. We set the threshold for “good ecological status” for attached algae at a point that we thought coincided with the algal community switching from species that were adapted to “stressed” (i.e. nutrient-poor) conditions to those adapted to compete when nutrients were not in short supply (see “What does it all mean?” and references therein). I’ve also talked, in some of my posts, about the adaptations some algae have to scavenge scarce nutrients (“A day out in Weardale”). We’ve then gone on to try to work out what that means in terms of nutrient concentrations in UK and European rivers (references at the end of the post).
So I was pleased to see a paper appear last week that confirms some of these hunches. Broadly speaking, Eleanor Mackay and colleagues have shown, using in situ bioassays, that as the concentration of inorganic nutrients decreases so the algae make more use of phosphorus and nitrogen bound into organic complexes. As the algae get more “hungry”, in other words, they become more adept at scavenging for the resources that they need.
The graph at the top of this post is the graphical abstract from the paper which summarises this, whilst the one below shows the response to organic sources of phosphorus as a function of the concentration of “soluble reactive phosphorus” (the standard measure of “inorganic” phosphorus). I’ve added an arrow to the right-hand side of this which shows roughly the current UK threshold, based on the work mentioned above. Ellie’s graph seems to be confirming that, once that limit is exceeded, the algae are no longer “hungry”, meaning that they no longer need the nutrients bound into organic complexes. Because organic phosphorus utilisation depends upon production of phosphatase enzymes to break down the organic complexes to releasee the phosphorus, there must be a greater energetic cost to the organism than if there was a ready supply of inorganic phosphorus that they can access. I have, I must admit, never seen any figures that quantify this cost.
Fig. 5c from Mackay et al. (2020): The relationship between “soluble reactive phosphorus” and dissolved organic phosphorus use by algae in in situ bioassays. The “response ratio” is the natural logarithm of the ratio between the chlorophyll concentration of the treatment and the chlorophyll concentration of the corresponding control. The arrow on the right-hand side indicates the approximate position of the regulatory threshold for phosphorus (see note at end of post). The figure at the top of the post is the graphical abstract from Mackay et al. (2020).
Part of me, then, is reassured that the regulatory threshold for phosphorus is roughly in the right place. The Environment Agency’s reliance on a single measure of inorganic phosphorus, measured infrequently, is often criticised by hydrochemists but we can take some comfort from knowing that other forms of phosphorus (more difficult to analyse and quantify) only become important at concentrations lower than the current UK targets. There is still part of me, however, that sees room for improvement. That there are relationships between algae and other plants and phosphorus is not in doubt, and I am sure that a shift in strategies for nutrient acquisition help to define this relationship, particularly at low concentrations. However, the relationships are not very strong and predictions about the ecological benefits of lowering phosphorus concentrations are imprecise.
Adding another strand of evidence to the current decision-making process makes scientific sense, and looking at how organisms respond to nutrients, rather than just measuring chemistry and describing community structure, seems like a sensible way of doing this. In situ bioassays clearly have potential, as this paper shows; however, they are time consuming. An alternative would be to measure phosphatase activity directly. The Environment Agency did, in fact, fund research on this in the late 1990s and David Harper used these assays in a DEFRA-funded project in the early 2000s, but they have never become routine. That’s a shame because, particularly for catchment-level investigations, they could add a useful additional insight.
The downfall of all these methods is not science, but the “more-with-less” ethos that has prevailed in the UK public sector for the past decade. Everyone recognises that diffuse nutrient pollution offers a challenge that current monitoring and decision-making processes struggle to address. However, most of the serious research effectively concludes with “if you spend a lot more money, you’ll discover that the problem is more complicated than you initially thought”. That’s a difficult message to pass up through managerial hierarchies trying to keep a cash-starved regulatory agency moving forward.
Mackay, E. B., Feuchtmayr, H., De Ville, M. M., Thackeray, S. J., Callaghan, N., Marshall, M., Rhodes, G., Yates, C.A., Johnes, P.J. & Maberly, S. C. (2020). Dissolved organic nutrient uptake by riverine phytoplankton varies along a gradient of nutrient enrichment. Science of the Total Environment 722: 137837. https://doi.org/10.1016/j.scitotenv.2020.137837
Poikane, S., Kelly, M. G., Salas Herrero, F., Pitt, J. A., Jarvie, H. P., Claussen, U., Leujak, W., Solheim, A.S., Teixeira, H. & Phillips, G. (2019). Nutrient criteria for surface waters under the European Water Framework Directive: Current state-of-the-art, challenges and future outlook. Science of the Total Environment 695: 133888. https://doi.org/10.1016/j.scitotenv.2019.133888
Note: regulatory threshold for inorganic phosphorus
The arrow indicating the approximate position of the regulatory threshold for phosphorus uses the current UK TAG phosphorus standard. This is site specific, using altitude and alkalinity as predictor variables. This means that a range of thresholds is possible and the position of the arrow reflects the average alkalinity (50 mg L-1 CaCO3) and altitude (75 m) in a database of river samples collected as part of DARES project. Note, too, that P standards are based on the Environment Agency’s standard measure, which is unfiltered molybdate reactive P. This approximates to “soluble reactive P” or “orthophosphate-P” in most circumstances but the reagents will react with phosphorus attached to particles that would have been removed by membrane filtration.
Some other highlights from this week:
Wrote this whilst listening to: My lockdown project of listening to all Bob Dylan’s albums in sequence has brought me up to Bringing It All Back Home and Highway 61 Revisited.
Cultural highlights: Bait, a low-key black and white British film from 2019. Definitely sits in the “sub hero” genre that I much prefer to the crash, bang, wallop of most Hollywood blockbusters.
Currently reading: About three-quarters of the way through Hilary Mantel’s The Mirror and The Light now. Jane Seymour is gone; Anne of Cleeves coming up next.
Culinary highlight: Grilled mackerel with sautéed potatoes, probably. A close second was home-made tortellini filled with mushroom paté and served with garlic mustard (Alliaria petiola) butter. Mrs K is forager-in-chief hereabouts.