By Jeanne Yacoubou, MS

In a recent article, we calculated the carbon footprint of meat pizza to be approximately seven times greater than the carbon footprint of vegan pizza. In other words, there are significantly more carbon emissions from the production of meat and dairy ingredients on a pizza compared to those generated by growing vegetable toppings. Carbon emissions accelerate the climate and ecological crises.

Here, we turn the focus on the amount of water pollution resulting from the production of all the ingredients in the two types of pizza. We quantify water pollution using a eutrophication metric by asking: How do the eutrophication footprints of vegan vs. meat pizza compare? To answer this question, we begin with the concepts of cultural eutrophication and eutrophication potential. Understanding these notions will make it easier to see how eutrophication is about much more than water pollution.

What is cultural eutrophication?

Distinct from natural eutrophication which refers to the normal aging of waterways, cultural eutrophication is the process by which water bodies receive excessive amounts of nutrients, especially nitrogen or phosphorus, from human activities.

As a source of eutrophication, nitrogen usually exists in the following forms:

• Nitrate ion (NO₃^– )
• Ammonia (NH₃)
• Ammonium ion (NH₄⁺)

Phosphorus exists as a phosphate ion (PO₄³⁻ ) in aquatic ecosystems.

The nutrient influx leads to exponential algal and large plant growth called an algal bloom. As the algae and plants die, bacteria decompose them, using up the dissolved oxygen in the water. Lacking oxygen, fish and other aquatic organisms die. In extreme cases, a dead zone (devoid of all life) results from eutrophication. Harmful algal blooms caused by cyanobacteria (cyanoHAB) produce dangerous toxins.

What causes cultural eutrophication?

The major human activities which cause cultural eutrophication include:

• Runoff of synthetic (chemical) fertilizers applied to crops
• Runoff of manure or human sewage applied to crops
• Aquaculture (farmed fish, shrimp, prawns, etc.)
• Human wastewater from treatment plants released into natural waterways
• Industrial waste piped directly into water bodies

As we noted in our pizza carbon footprints article, 78% of freshwater pollution is due to agriculture. The first three ways in the list above are the mechanisms by which agriculture pollutes freshwater.

What is a eutrophication potential?

Eutrophication potential (EP) is a way to quantify how much eutrophication could result from a certain activity or input. To standardize the environmental impacts from various sources on freshwater systems, every effect is expressed in a metric called phosphate (PO₄) equivalents, (kg PO_4eq).

As an example: If one kilogram of ammonia has an EP of 0.35 kg PO_4eq, this means the eutrophication potential of that quantity of ammonia is the same as the EP of 0.35 kg of phosphate.

On the other hand, nitrogen equivalents (kg N_eq) are typically chosen to quantify marine eutrophication. Through the use of conversion factors, it is possible to convert kg PO_4eq into kg N_eq and vice versa.

Both phosphate and nitrogen equivalents are ways to compare the eutrophication potentials of different pollution sources similar to how the concept of carbon dioxide equivalents allows us to compare in a direct manner the global warming potentials (GWP) of various greenhouse gases. In this way, eutrophication footprints are analogous to carbon footprints.

How does cultural eutrophication relate to the climate crisis?

Increased cultural eutrophication of lakes and ponds leads to greater release of greenhouse gases (GHG), especially methane (CH₄). In fact, eutrophic shallow lakes emit nearly 50% more methane than non-eutrophic lakes. According to simulated modeling studies, eutrophication will increase CH₄ emissions from freshwater bodies by up to 90% over the next century.

Excessive nitrogen inputs specifically transform freshwater bodies from being nitrous oxide (N₂O) sinks to becoming net N2O emitters. Nitrous oxide is a GHG significantly more potent than methane at trapping heat in the atmosphere.

Researchers concluded that the contribution of aquatic greenhouse gas emissions is more than 30% that from fossil fuel emissions (all sources). That percentage will increase with the growing human population demand for more food unless agricultural runoff is significantly reduced. Eliminating animal agriculture, including all the heavily fertilized feed crops as well as its manure, will reduce the eutrophic contribution of GHGs significantly.

The climate Crisis also leads to more eutrophying emissions. Here are some of the ways global heating and extreme climate events affect eutrophication:

• Greater release of nutrients from lake bottom sediments
• Increased nutrient losses from land as agricultural runoff
• Accelerated methane production by the speedy decomposition of aquatic plants and animals

Eutrophication potentials of food ingredients

The chemical fertilizer used to grow foods and the manure produced by animals are two of the major ways agriculture contributes to eutrophication. When the fertilizers and manure run off into water bodies, the potential for eutrophication increases dramatically.

Here’s a select sampling of the EPs of one kilogram of select foods based on the 2018 work by Poore & Nemecek and the 2021 work by Clark et al as calculated by Our World in Data.

It should come as no surprise that the eutrophication potentials (EP) of meat and dairy foods are considerably higher than those of plant foods. All other things being equal, animals produce manure while plants don’t. Manure is fuel for eutrophication.

Eutrophication potentials of vegan pizza vs. meat pizza

Borrowing and modifying the tables from our pizza carbon footprints article, here are the eutrophication potentials of pizza ingredients:

Note: Values are approximate due to rounding.

Vegan Pizza Ingredients

Total: 23.86 g PO_4eq 

Meat Pizza Ingredients

Total: 166.19 g PO_4eq

Conclusions on Pizza’s Eutrophication Potential

Based on our calculations, the eutrophication potential of meat pizza is nearly seven times greater than the eutrophication potential of vegan pizza. This means that producing the ingredients for meat pizza is seven times more polluting of freshwater than producing the ingredients for vegan pizza. Because of the connections between eutrophication and the climate crisis, meat pizza contributes seven times more to the climate crisis via eutrophication compared to vegan pizza.

Readers who wish to limit their contribution to water pollution and the climate crisis would do better by choosing a vegan pizza over a meat pizza.

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