Phosphorus and water quality

Introduction

How can we protect waterbodies from phosphorus pollution?

The pollution of fresh and coastal waters with nutrients including phosphorus and nitrogen is one of the most conspicuous impacts the human population has on the planet. That we continue to pollute the very water that we rely on for survival is a shocking level of self-harm. The rate of biodiversity loss in fresh waters is higher than in any other planetary domain; over 25% of all freshwater species are currently threatened with extinction globally, and freshwater fauna declined globally by 83% from 1970 to 2014, compared to 60% for all habitat types. While a wide range of emerging and persistent stressors are driving these losses, climate change and increasing nutrient delivery from food production and consumption are ubiquitous. They combine to generate a globally increasing incidence of eutrophication, the process whereby excess input of nutrients drives the formation of harmful algal blooms. This can lead to coastal dead zones, mass mortalities of fish, closure of economically important fisheries and shellfisheries, high rates of biodiversity loss, high rates of greenhouse gas emissions, and the loss of economic, societal, and cultural value associated with high-quality ecosystems. Under very high phosphorus concentrations, lakes can be turned into near monocultures of harmful cyanobacteria. Some cyanobacteria produce ‘cyanotoxins’ which are harmful to mammal; they have been know to cause the deaths of livestock and dogs, and represent a risk to human health through the consumption of contaminated water and food and potentially through the dispersal of aerosols. Algal blooms can significantly increase filtration costs in potable water treatment works, and in some cases make water unsuitable for drinking. Phosphorus management and control of access to waterbodies can be used to reduce the likelihood of exposure in fresh waters, but this brings with it the consequences of reducing amenity and recreational use of waters. We also know that phosphorus, with nitrogen, contributes to the creation of marine ‘dead zones’. Beyond lakes and fresh water, in only a few thousand years, the contribution of phosphorus towards long-term ocean anoxia has potentially highly damaging consequences to the Earth’s biogeochemistry.

Phosphorus concentrations in aquatic ecosystems have been elevated worldwide by human activities. For example, in the USA, phosphorus concentrations in 72% of rivers and 79% of lakes exceeded background levels because of human activity in the last decades. In the European Union (EU), ~32,000 km2 of lake surface area (about 40% of monitored lakes by number) is deemed to fail ecological quality targets under the EU Water Framework Directive. Over 83% of freshwater habitats in the EU were classed as being in an unfavourable condition in 2015, higher than any other habitat type, many of which are impacted by eutrophication. Over 50% of the phosphorus mass input (load) to 23 of the world’s largest lakes originates from human activities, representing a threat to current and future water security. Population growth and economic development whilst increasing water demand will - without intervention - simultaneously cause greater water pollution, effectively reducing the availability of clean water. The effects of climate change are likely to exacerbate ecological responses to phosphorus enrichment within lakes and coastal zones. Phosphorus loss to water is expected to increase under climate change, without any change in land use and management, and the impacts of eutrophication, in terms of the release of greenhouse gases from highly enriched systems, is likely to exacerbate climate change globally. The outlook for aquatic ecosystems is bleak.

The reduction of phosphorus concentrations in aquatic ecosystems as part of an integrated nutrient management strategy lies at the core of control of freshwater eutrophication globally. To achieve this, it is often first necessary to identify anthropogenic sources of phosphorus within the catchment and to understand how they are transported and transformed within the water system, alongside nitrogen sources and fluxes. However, phosphorus stored within catchments, aquifers, and bed sediments during decades of enrichment (termed ‘legacy phosphorus’), can be released back to the water, delaying recovery for many years following a reduction in catchment phosphorus loading.

Phosphorus is one of the key drivers of the global nutrient challenge and the biodiversity loss emergency with respect to freshwater and marine ecosystems. To relieve the effects of eutrophication we must balance phosphorus losses alongside nitrogen losses, predominantly from food systems and human waste. Whilst restoring ecosystems is a notoriously difficult task, it is within reach; however, the economic and cultural costs of large-scale environmental management are likely high and must be equitably shared. Indeed, the process of eutrophication in lakes through phosphorus pollution has become a central exemplar of the links between ecological behaviour and natural capital and economics. It is, therefore, important to raise awareness of ecosystems under threat and to work across governments to ensure their long-term integrity, as well as to identify short-term disaster response plans where trends of degradation are deemed unacceptable. We call for a greater focus on preventative nutrient management in order to safeguard global biodiversity in freshwater and coastal ecosystems and meet long-term sustainability goals.

We argue that to meet the growing global demands for clean water and food, we must first meet the overarching goal of delivering more sustainable phosphorus management. This should be framed within the context of scale-appropriate interventions that have an additive impact towards global-scale ambitions. Next, we introduce the key challenges and solutions associated with this overarching goal.