What is phosphorus and where does it come from?
Phosphorus (P) is a naturally occurring element that is essential for life. It originates from minerals in rocks and helps all living things to grow, repair tissues and produce DNA. In healthy, natural river systems phosphorus concentrations are fairly low – typically below 0.03 mg/L PO4-P. Plants and algae access phosphorus and other nutrients to grow – when they have used the available supply they stop growing.
Human activity introduces additional sources of phosphorus to rivers. Important human sources of phosphorus are sewage discharges (both treated and untreated via storm overflows), industry and runoff from agriculture. It is estimated that 60 – 70% of the phosphorus in English rivers comes from sewage treatment works. Most of that (around 70%) is from the urine and faeces with the rest coming from food additives and phosphorus dosing of drinking water. Phosphorus is added to drinking water in order to stop lead from leaching from pipes, and much of that enters the wastewater system. Phosphates are now largely banned from detergents, so their contribution to the overall load is considered minimal. Some, but not all sewage treatment works have phosphorus removal systems in place.
Agricultural inputs of phosphorus are largely due to the urine and faeces generated by livestock, phosphorus rich fertilisers applied to arable crops and soil runoff. Whilst fertiliser inputs have reduced over recent decades, in many catchments there is now a legacy of phosphorus enriched soil that is released into rivers each time there is heavy rain.
Why is it important?
The role of phosphorus in aiding plant and algae growth is what makes it an important factor in the health of our river systems – phosphorus itself is not toxic to aquatic life. Still, it contributes to a process known as ‘eutrophication’.
Eutrophication occurs when there is an excess of nutrients in rivers, lakes or reservoirs. When conditions are right, this can cause rapid growth of aquatic plants and algae. In freshwater systems, there is often plenty of nitrogen (mostly as nitrate) to go around – so it is the amount of phosphorus that makes the difference. When an abundance of phosphorus is made available, there is a sudden explosion of plant and algae growth; species that can grow quickly are best able to take advantage of the extra nutrition, and this leads to other slower-growing species being deprived of light. During the day, photosynthesis means that these plants produce large amounts of oxygen, but by night, that process reverses, and large amounts of oxygen are consumed – leading to dangerously low levels in the early hours of the morning. As plants and algae then start to die off (when starved of light or nutrient levels drop) the aerobic bacteria that decompose them also consume oxygen. Eutrophication is more likely to lead to mass fish mortality in slow-flowing rivers in the warmer summer months.
So what is ‘orthophosphate’?
Phosphorus is not encountered in its elemental form in water but as phosphate ions (e.g. orthophosphate, PO43-). Orthophosphate, sometimes referred to as just ‘phosphate’ consists of one P atom bonded to four oxygen atoms. Three of those four oxygen atoms are negatively charged which is why you will sometimes see orthophosphate accompanied by the notation PO43- – sometimes the ‘3-‘ is left off but PO4 is the same thing.
Most testing techniques actually measure orthophosphate (PO4) – but because it is the amount of phosphorus (P) that is important for environmental processes, we often see phosphate expressed as its phosphorus equivalent, i.e. 1.5 mg/L (PO4 as P). Since we know the molecular weight of the P and O atoms, we can calculate phosphorus (P) from measured orthophosphate (PO4) accurately using the following conversion:
Orthophosphate (PO4) = Phosphorus (P) x 3.066
How much is too much?
Naturally occurring phosphorus levels are generally very low. They vary according to the underlying geology and soil types but will typically be below 0.03 mg/L PO4-P. In general terms, if phosphorus levels are frequently at 0.2 mg/L PO4-P or above then there is potential for problems caused by nutrient enrichment. Whether or not these problems actually occur will depend on other factors – such as the temperature, sunlight, river flow levels and the availability of other nutrients. It is particularly useful if, in addition to recording a phosphate concentration, citizen scientists are able to record indicators of eutrophication where present – such as dense plant growth, algal blooms or fish at the surface gasping for air.
The targets or thresholds that rivers need to meet under the Water Framework Directive classification system (which is the relevant legislative framework underpinning our understanding of river health) vary according to both the alkalinity and altitude of the site in question. They are calculated using a very complicated formula. This is intended to better reflect the likely naturally occurring levels of phosphorus in different types of river, but does make it difficult to know what the standards are for any one waterbody. Prior to 2015, the standards were fixed for four different categories of sites based on thresholds for altitude (above or below 80 m above mean sea level) and alkalinity (greater or less than 50 mg/L of calcium carbonate (CaCO3)). We can use these categories to show indicative values for the standards required to achieve a WFD status of high, good, moderate or poor. For example, upland sites where rivers flow over granite geology would be expected to have very low natural concentrations of phosphorus – so you would need to have an annual mean concentration of around 0.03 mg/L PO4-P to achieve ‘good’ status for phosphate. Lowland rivers flowing over more soluble rock types are likely to contain higher levels of naturally occurring phosphorus and would therefore achieve ‘good’ status with an annual mean concentration of around 0.07 mg/L PO4-P. These numbers refer to an average of annual sampling results; it is not possible to classify a river from a single measurement, and a statistically significant number of samples should be taken over the whole year. Monthly or fortnightly sampling is ideal.
These standards are important because the Environment Agency and water companies use them as a target – all waterbodies are supposed to be at ‘good’ ecological status by 2027. Ecological status is assessed using other water quality parameters (ammonia, dissolved oxygen, pH and temperature) and morphology and biological indicators (fish, invertebrates and plants). Waterbodies that are classed as ‘moderate’ or worse are more likely to be a focus for monitoring, whilst water company discharges, farming practices and/or new developments in those areas are more likely to be restricted or receive funding for improvement.
| Type | Annual mean of reactive phosphorus (mg/L PO4-P) | |||
| High | Good | Moderate | Poor | |
| Lowland, high alkalinity | 0.036 | 0.069 | 0.173 | 1.003 |
| Upland, high alkalinity | 0.024 | 0.048 | 0.132 | 0.898 |
| Lowland, low alkalinity | 0.019 | 0.040 | 0.114 | 0.842 |
| Upland, low alkalinity | 0.013 | 0.028 | 0.870 | 0.752 |
| Notes: The figures are the median values for sites that fall into the four categories, where “Lowland” sites are at or below 80 metres above mean sea level “Upland” sites are more than 80 metres above mean sea level, “Low alkalinity” sites have a concentration of calcium carbonate of less than 50 mg/L, and “High alkalinity” sites have a concentration of calcium carbonate greater than or equal to 50 mg/L | ||||
TABLE: Indicative values for WFD phosphorus thresholds (from UK WFD TAG – Updated recommendations on phosphorus standards for rivers. Final report. 2013.)
Standards for rivers that are designated as Special Areas of Conservation (SAC) or Sites of Special Scientific Interest (SSSI) are derived differently and are more stringent. They are still variable according to the upland/lowland high/low alkalinity categories and whether the site is a headwater, river or large river. For example, the target phosphorus concentration (as an annual mean and a growing season (March – September) mean) in a headwater reach of an SAC river (upland / low alkalinity) would be 0.005 mg/L PO4-P – compared with 0.013 mg/L PO4-P required to gain ‘high’ WFD status for phosphate. Even some laboratory analytical methods struggle to achieve such a low limit of detection.
Different methods for testing phosphorus
Our long list of methods includes over 30 different types of testing for phosphorus. The methods being used and tested by most of the Demonstrator catchments are Hanna Checker (audited), Kyoritsu Packtest low-range (unaudited), and LaMotte Insta-Test Low Range Phosphate Test Strips (unaudited).