Phosphate: to those like me who answered their Chemistry O level multiple choice by rolling a pencil down the desk because that way they tended to get higher marks, phosphate is just another chemical. But in terms of river ecology in general and chalk stream ecology in particular, phosphate is very, very important.

That’s why I’ve spent the past few weeks adding info to the Abstraction Sensitivity Band table I published in the previous post in December. The Chalk Stream Table now includes the Water Framework Directive (WFD) status for Flow and Phosphate.

Flow is a “supporting element” and so it is adjudged either to support or not to support Good Ecological Status. In WFD lingo these are the acronyms SG or DNSG. The column for the WFD ‘element’ Phosphate (P), is classified as either High (which means High status and therefore low phosphate concentrations), Good, Moderate, Poor or Bad (Bad means a high phosphate concentration). Phosphate is a pass or fail element: if P readings are Moderate, Poor or Bad the waterbody can’t be deemed to meet Good Ecological Status. There are no absolute readings used to assess status: it is somewhat tailored to each stream. Generally however, P needs to be lower than 0.036 mg/l for the chalk stream to achieve High status.

(Note: I have indicated which Sewage works remove Phosphate as best I can with the information available. The table may include some errors in that regard. Also I have included the actual P readings from the Dorset streams but not the rest (not enough time in the day), but I will in due course. There is no one source of information on this important issue of Phosphate and sewage treatment standards, which is something I hope the new chalk stream hub we are developing will resolve.)

I’ve taken P here as something of a totem for water quality in general, although that is a massive oversimplification. Nevertheless P has a huge impact on the ecology of our chalk streams. If you want to read an authoritative explanation from the expert on this subject I refer you to Phosphorous and River Ecology by Chris Mainstone. This is my Ladybird Book simple version:

Phosphorous is the key chemical that drives nutrient enrichment of chalk streams. That enrichment has a number of deleterious effects on a river’s ecology which increase in line with increasing P enrichment. All plants need P to grow, but different plants and plants communities either thrive or conversely suffer and are out-competed at differing levels of P concentration.

Higher order and important chalk stream plants like Ranunculus thrive at very low, background natural P concentrations. The first effect of P enrichment is actually an increase in the growth-rate of the higher order plants, but with commensurate weakening in root growth – making the plants vulnerable in high flows. As P levels increase further the river’s ecology shifts towards a dominance of the higher order plants that are most tolerant of nutrient enrichment, and that leads to a reduction in the overall bio-diversity of the plant community. 

Finally, if P concentrations keep on rising, the river’s ecology will switch over to an algal-dominated plant community. Benthic algae smothers the river bed and the interstices in the gravel in which many insect species live and epiphytic algae cloaks the leaves and stems of the higher order plants, reducing their ability to photosynthesise. The prevalence of algae will also cause extreme diurnal variations in dissolved oxygen levels, with really low oxygen levels at night and in the early morning, which stresses fish and insects alike. There comes a point where, if the P is very concentrated, the river turns into an anoxic soup and nothing much survives.

P is very limited in a natural chalk stream system. But P is contained in human sewage (treated and raw) and animal slurry, as well as in agricultural fertilisers. P is used in cress farms and there is P in the food used in fish farms and in the poo from those fish. There are other diverse sources of P: our drinking water is dosed with P, for example. Consequently there is much more P in our anthropogenically impacted river systems than might be considered natural. Relatively modest increases in P can cause the ecological changes outlined above. Any reduction in P will benefit the ecology of a chalk stream, but if P levels are high, or there’s loads of it still washing around the system, you might have to reduce P by an awful lot before you start noticing the difference.

There are also complex relationships between different states of P, which might be dissolved within the water column (Soluble Reactive P) or bound to organic and inorganic particles which accrete on the river bed: P can move between these two during its journey through a river system, meaning that P locked away within the sediments on the river bed can be reanimated in high flows when the river bed is disturbed.

P gets into a chalk stream through “Point Source” and “Diffuse” pathways.

The main Point Source supply of P is through the human sewage system, but fish farms and cress farms are also Point Source suppliers: a large fish farm (40 tonnes annual production), for example, can generate as much P as a secondary Sewage Treatment Works (STW) serving 1000 people.

Diffuse Source P, on the other hand, flows in multiple pathways from the wider landscape, and particularly from farmland. The majority of Diffuse P gets to the river by surface or shallow sub-surface flow during the wet winter months, when soil is saturated. 

A clear pattern you will see in this table is the correlation between chalk streams where there are no sewage treatment works (STWs) or chalk streams where the STWs includes a tertiary P stripping phase which tend to be of Good or even High status for P.

And conversely between chalk streams which have one or several STWs which do not remove P and which tend to be of Poor, Moderate or Bad status for P.

On some rivers I have noted in the r/h column where the P readings were markedly different upstream and downstream of a STW. One stark example is the Misbourne and I will publish a revealing chart of that in the next post.

There are other clear patterns: the relatively higher status for P on the larger systems as you move in a downstream direction. This is because the Urban Waste Water Treatment Directive which has driven investment in P removal over the last two decades applies to larger sewage catchments of 10,000 people or more. Many to most chalk streams do not have towns of that size on them, or if they do it will be in the middle to lower reaches of the larger river systems. Although its a good news story for P removal, it has tended to benefit the lower reaches of larger systems leaving the headwaters and smaller rivers behind.

Another clear pattern is the difference in % of STWs on a given river or in a given catchment that feature P removal and the designated status of the river. All chalk streams are a Priority Habitat, but some are also SSSIs (Sites of Special Scientific Interest) – the Frome, Test, Kennet, Nar and Driffield Beck and some are SACs (Special Areas of Conservation) – the Itchen, Avon, Lambourne and Wensum. These two designations are much more powerful. SSSI and even more so SAC protected status has driven an investment in P removal in smaller-scale sewage treatment works which is clearly reflected in the WFD assessments.

That is not to say that our SSSIs and SACs are perfect. They clearly still suffer phosphate issues as some charts I will publish in due course show. But it does make the point very clearly that if we want to effect real change for all our chalk streams, right across the map from Dorset to Yorkshire, we need to look at the protected status of all chalk streams, not just the shining examples. Much of the anger and frustration felt by people who care about chalk streams is driven by the condition of the Cinderella streams that are still bedecked with sewage works (there are 175) which do not remove Phosphate. Priority Habitat as it stands is clearly not a sufficient imperative to action. Protected status is symbolically important, but more than that it drives investment. And investment is what these rivers so desperately need.