On Eutrophication

Early in the pandemic shutdown, my family and I visited a local beach in Southern California on a spring night, lured by the possibility of witnessing a widely reported phenomenon known as bioluminescence. The phenomenon occurs when a particular light-emitting algae blooms. We drove to the closest beach and walked along a bluff overlooking the ocean. The water displayed a mystifying glow; blue streaks of light pulsed within the crashing waves along the entire beach.

About a year later, I encountered an article online describing the proliferation of “sea snot” in the coastal waters of Turkey. An image showed a layer of brown sludge around the docks and boats. This “sea snot” was an outbreak of marine microorganisms, mainly phytoplankton, that was growing into a dense layer of mucus-like slime. Scientists were afraid that the marine life underneath was in danger of being choked out. The government called it a “national crisis”.

Around this time, I read another article reporting on a disturbing record number of manatee deaths in Florida primarily from starvation due to a loss of seagrass, their food source. The article explained that this loss of seagrass was due to algal blooms blocking out sunlight in the water column. 

Dots started to connect. Perhaps these events were manifestations of the same underlying cause—signs of an unhealthy ecosystem.


In ecology, there is a process known as eutrophication. The word is from the Greek “eutrophos” meaning “well-nourished”. Eutrophication is the process in which bodies of water get enriched by excess nutrients over time. Anthropogenic activities have caused this otherwise naturally slow-occurring process to rapidly increase in many areas.

In coasts around the world, water pollution from constant agricultural and urban runoff causes eutrophication. Plant nutrients such as nitrogen and phosphorus from human activities drain through river systems into the ocean. Combined with warmer sea temperatures due to anthropogenic climate change, the influx of nutrients creates an environment that promotes excessive algae growth. These coastal algal blooms cause harmful effects such as algae-produced toxins, hypoxia, fish kills, ocean acidification, and decreased water transparency.

When algal blooms degrade, oxygen levels can fall below what most marine life can tolerate, forming areas of low biological activity called “dead zones”. According to the IPBES, a UN organization that reports on the state of global biodiversity, there are more than 400 “dead zones” around the world, totalling more than 245,000 square kilometers—an area bigger than the UK. These “dead zones” commonly occur at the mouths of major river systems. One of the largest dead zones in the world lies in the northern Gulf of Mexico, where the bulk of the US’s agricultural runoff in the midwest drains through the Mississippi river system. 


In the 1850s, large-scale livestock farming and grain production took off in California as farmers adopted new intensive technologies. Cattle ranching expanded to cover millions of acres, and the state soon became one of the largest wheat producers. As a consequence of this rapid intensification of agriculture, the soil degraded due to poor soil management. Years of intensive farming practices and overgrazing led to perfect conditions for soil erosion in California’s semi-arid climate. Massive amounts of soil washed into the ocean.

As a result of the sedimentation, California’s continental shelf biodiversity shifted. Once a rocky seafloor covered in filter feeders such as brachiopods and scallops, the shelf transformed into a mudscape. Detritivores (decomposers) and mobile predators, such as worms and crustaceans, moved in to replace the stationary filter feeders that were once abundant. Without the filter-feeders, the ecosystem’s ability to filter algae from the seawater was dramatically reduced. Today, small patches of California’s earlier shelf ecosystem survives around the offshore islands. 

Around a hundred years after California’s loss of seawater filtering capacity, the world underwent another agricultural transformation: the green revolution of the mid 1900s. Key to this revolution was the increased mass production of nitrogen, phosphorus, and potassium—three macronutrients in synthetic fertilizers. In 1914, German chemist Fritz Haber developed a new high pressure method of extracting nitrogen from the air. This method, called the Haber-Bosch process, enabled the industry to rapidly scale up production of nitrate fertilizers. In 2020, the process produced around 176 million tons of nitrogen-based ammonia globally, most of which was used to fertilize crops. Phosphorus and potassium, which are found in sedimentary mineral deposits, increased in production as well. The revolution in producing these synthetic fertilizers increased crop yields many times over; however, vast amounts of excess fertilizer made its way into coastal ecosystems.

As agriculture boomed, industrialization and growth pushed humans into cities and urban areas. All around the world, cities grew at unprecedented rates. As a result, mass urbanization began to compete with agriculture in its role in eutrophication. Landscaping fertilization, wastewater runoff, industrial discharge, and infrastructure development created new sources of pollutants and sedimentation that entered adjacent coastal marine habitats. 

At the same time, the global sea surface temperature began rising due to anthropogenic greenhouse gas emissions. Between 1971 and 2010, the temperature increased around 0.11°C per decade. In addition, marine heatwaves increased in duration and intensity. The heat fueled larger and faster growing algal blooms.

Over the past two centuries, human activities have produced conditions ripe for rampant eutrophication along the world’s coasts. Early livestock ranching caused soil erosion and sedimentation that extirpated delicate filter feeder ecosystems on coastal shelves. Then, the flow of synthetic fertilizers and urban runoff, coupled with anthropogenic sea surface warming, enabled algal blooms to proliferate.


In the ocean, phytoplankton form the foundation of the ocean’s food web and of life on the planet. Drifting along in the tides and currents, these marine plant organisms produce 50 to 80% of the planet’s oxygen, provide food for marine life ranging from mollusks to whales, and may even regulate the planet’s climate by contributing to the formation of clouds. Across the world’s oceans, natural phytoplankton blooms support areas of high productivity. There are over 4000 species of phytoplankton across the entire planet, and a single liter of seawater may contain over a million individual phytoplankton and 100 to 150 different species. 

In Southern California, Lingulodinium polyedra, a dinoflagellate phytoplankton that emits light using a chemical reaction, is responsible for the bioluminescent waves. In Turkey, various microbes and phytoplankton species exude sticky substances made up of polysaccharides. In Florida, numerous species rapidly grow from the nutrient runoff, reducing sunlight in the water column. These unique species represent just a few of the wide diversity of phytoplankton around the world.

We have only begun to discover how complex life on this planet is. Every ecosystem, from the expansive Amazon rainforest to the obscure armadillo gut microbiome, is composed of innumerable species, each performing unique behaviors in complex relationships. Despite the chaos and disorder of millions of individual organisms, these diverse ecosystems achieve a state of equilibrium, or homeostasis, over time. When a sudden alteration in the environment causes one element to get thrown off, the resulting population shifts impact many organisms and may even decrease total biodiversity. 

For example, in the human gut, gut dysbiosis (disruptions to microbial homeostasis) is associated with numerous diseases including inflammatory bowel disease, diabetes, and colorectal cancer. In coastal marine ecosystems, the unexpected influx of pollutants from land causes detrimental effects that reverberate through the food chain from seagrass to manatees, and ultimately, humans.


As we are increasingly becoming aware of, eutrophication is just one of innumerable changes occurring to coastal regions around the world and to the biosphere at large. All across the planet’s surface, there is a major upheaval of biological life. 

Many scientists today believe we are living in a new geological epoch called the anthropocene, which is marked by significant human impacts on Earth’s geology, biodiversity, and natural systems. 

The average global population of vertebrates such as mammals, birds, amphibians, reptiles, and fish has declined 68% between 1970 and 2016. A quarter of all plant and animal species that have been studied in detail are now threatened with extinction. International shipping, the wildlife trade, landscaping, and other activities are breaking down the barriers that led to speciation and diversity. Introduced species of animals, plants, fungi, and pathogens now affect practically every ecosystem on Earth. 

Humans have significantly reshaped the geomorphology of Earth’s surface through the building of roads, dams, and other human infrastructure. According to the IPBES, 75% of Earth’s land surface and 66% of the oceans have been significantly altered by human activities. In addition, human activities are changing the composition of the atmosphere. Carbon dioxide ppm has risen to 409 ppm, a third higher than in 1960—and significantly higher than at any point in the past hundreds of thousands of years. Methane concentrations have more than doubled since the industrial revolution. The implications of these higher concentrations of greenhouse gases are becoming apparent. These statistics describe just a handful of the innumerable changes that define the anthropocene.

What does all of this mean for life on this planet? And for future generations? 

Without a major change in the way we do things, the natural world, which we depend on, will continue to degrade from the cumulative effects of all our activities. Today, about half of the world’s habitable land is used for agriculture. With rising incomes and a global population projected to grow almost 50% from 7 billion to 10 billion humans by mid-century, agricultural land will have to expand by 3.3 billion hectares if there are no productivity gains. This would mean the complete elimination of the world’s forests and savannas. The effects of anthropogenic eutrophication would be even more severe. Unless humanity undergoes great change, there will be few reasons to believe eutrophication and dead zones will diminish anytime soon. 


Determined to get one last dive before an incoming storm turned the local waters into a toilet bowl, I ventured out to a nearby beach just as the initial drizzles of rain began. Alone, I dove into the water and past the waves. The visibility was muddled in the choppy waves. Swimming over the seafloor, I looked for signs of marine life. A large majestic bat ray glided along, disappearing through the cloudy water. At some point upon ascending to the surface to take a breath, I noticed an enormous silhouette of a sea creature rising from the depths of the water. Immediately, I felt a rush of fear as this sea animal, far larger than me, rose up underneath. The creature came into view: a giant sea bass, at least five hundred pounds and five feet long. It must have wandered over curiously. And just as it appeared, it was gone. My heart was pounding, and I could hardly believe I just encountered this giant sea dweller for the first time. 

As a regular ocean diver, I have the opportunity to witness the beauty of life in the ocean. However, it depends on the conditions of the water: most importantly, the water visibility. Unfortunately, for large portions of the year, the visibility is under five feet due to eutrophication, sedimentation, and other factors—either an algal bloom occupying the top layers of the sea, loose sediment kicked up by the surge, sewage runoff after a rain, or a combination of these causes. Sometimes it is so low I am unable to see my outstretched arm. Days with excellent visibility are the exception. 

In addition to diving, I forage and collect seafood. In the fall and winter, I hunt for spiny lobsters, clawless crustaceans that hide during the day and wander out after dusk. Recreational divers can catch them, using only their hands. Throughout the year, various seaweeds such as nori, kombu, and wakame can be collected from rocky intertidal zones. In colder months, in which the chances of algal blooms is lower, I can target mollusks such as giant pismo clams, rock scallops, and California mussels. However, as many foragers come to learn, the risk of dangerous toxins, that can accumulate from toxic red tides and algal blooms, persists.

One of these is called Paralytic Shellfish Poison or PSP, a marine biotoxin produced by some species of algae. When shellfish filter the algae, they can accumulate high amounts of the toxin which affects the human nervous system and can paralyze muscles, hence the name “paralytic”. Normally, populations of the algae are too low to cause problems, but blooms can increase the populations to dangerous levels. Another well known toxin, domoic acid, affects numerous species including dungeness crab, a major target species for fisheries in California. A severe domoic acid algal bloom that lasted for several months in the summer of 2015 affected the entire West Coast. As a result, the opening date of the dungeness crab fishery was postponed and area closures were implemented.


In the past few centuries, we’ve managed to change the physical world at an unprecedented scale to the benefit of some. We’ve revolutionized agriculture to support a population of billions, but neglected the harms it entailed on coastal ecosystems. We’ve experienced a transformation in scientific understanding of the natural world, but only begun to discover the extent that our old-fashioned processes clash with complex biological systems. Today, we face the greatest predicament threatening the survival of over a million species, including us. 

We have seen that the story of human existence has been too often a story of domination and exploitation, of pursuing power and wealth, knowingly or unknowingly at the expense of others: the natural world, animals, and other fellow human beings. And yet it is only us, who are able to change our ways. For if we do not, we will establish ourselves as the greatest failed species in the history of life on this planet. But if we do, we will be the catalyst for the next great transformation of life.


History of California Agriculture


California Continental Shelf




Green Revolution



Sea Surface Temperatures







Sea Snot


Biodiversity Loss



Projected Agricultural Land Usage


Paralytic Shellfish Poison


Domoic Acid