Coral reefs are native to tropical environments which are known to have relatively stable water parameters year round. A key characteristic of reefs are their low concentrations of nitrogen and phosphorus. Carbon is generally not limited in coral reef environments. Despite this, an increase in carbon levels can still significantly impact the ocean by decreasing pH, and providing substrate to phytoplankton. Carbon, nitrogen and phosphorus enter the ocean through various pathways, including the atmosphere, geosphere, and hydrosphere. While quantifying the exact amount of anthropogenic nitrogen, phosphorus, and carbon entering coral reef ecosystems is difficult, it is undeniable that the impact is substantial enough to disrupt the delicate balance of the reefs. This paper will briefly express the importance of coral reefs and examine the direct and indirect effects which carbon, nitrogen, and phosphorus have on coral biology and reef ecology. It will also briefly mention some solutions.
Coral Reefs Importance for the Ocean, Public Health and Global Economy
Nutrient pollution is one of the greatest threats facing coral reefs, and it is with great urgency that this issue must be resolved. A quarter of all marine life depends on coral reefs, although they occupy less than a percent of the ocean floor. Reefs provide a great percentage of ocean primary production and 60% of primary production can be attributed to the corals themselves (Cortés et al., 2015). Reef-building corals provide essential habitats for all trophic levels and they provide nurseries for juvenile fish. With coral reefs on the decline worldwide, it is important to mention that without the habitat provided by coral reefs, many marine species could face severe population decline and even extinction. Coral reefs are incredibly beneficial to the ocean, public health, and the global economy.
Corals are used to discover new medicinal compounds and they play a huge role in coastal protection. Protecting coral by limiting nutrient pollution means saving human life. It is through research that corals indirectly save human lives with over 20,000 pharmaceutically active compounds that have been derived from corals, including treatments for various cancers and cardiovascular diseases (Clarke, 2017). More directly, corals also protect over 5.3 million people from storm surges each decade (Burke, 2022). As such, reefs must not be exploited or put in harm by any human action because it endangers precious life.
Human life is more valuable than any dollar value that could be placed on corals; however, corals are believed to have a yearly value of $29.8 billion dollars (EPA). The global economy is greatly dependent on coral reefs as they provide economic growth and income in tropical nations through tourism, coastal protection, and by providing optimal habitat for fisheries. The reefs protect nearly 109 billion dollars of human infrastructure each decade (Burke, 2022). Yet this is just a fraction of what they provide.
In sum, reefs are essential for human life and have great value both monetarily and scientifically. It is of utmost importance that these fragile but incredibly diverse ecosystems receive the protection which they deserve. This includes reducing nutrient pollution through better resource and waste management, and reducing carbon emissions.
What is nutrient pollution?
Nutrient pollution occurs due to improper management of biologically essential elements such as carbon, nitrogen and phosphorus. These elements are introduced to foreign environments due to human activities such as agriculture, wastewater runoff, and industrial discharges. These nutrients run off into the ocean and cause an ecological imbalance by increasing primary production of the ecosystem. An uncontrolled increase in primary production can have many detrimental impacts on a surrounding ecosystem including harmful algal blooms, oxygen depletion, and the degradation of water quality. The effects of nutrient pollution are expansive and can damage marine biodiversity as well as impact public health and the economy.
Brief Coral Anatomy
Corals are animals which undergo cellular respiration, producing CO2 in the process. In the ocean, CO2 can transform into the reactive form of a carbonate ion, which the coral will then combine with calcium to create a stable calcium carbonate skeletal structure (Scheid, 28-53). Various coral species, both soft and hard, exhibit distinct calcium carbonate structures. Zooxanthellae is the coral endosymbiont responsible for photosynthesis and provides a significant portion of coral nutrition. Zooxanthellae is integrated into coral tissue, and it blankets the calcium carbonate structure. The relationship between these organisms requires a very specific balance of zooxanthellae population and coral colony size. (Wilkinson et al., 2000).
Coral Dependence on Nutrient Levels
Consistent nutrient levels are of key importance in coral reefs due to the symbiotic relationship between corals and algae as well as coral competition with macroalgaes and phytoplankton. The competition for space and nutrients between corals and algae is a central aspect of the reef. When nutrient levels are excessive, algae seize the enhanced photosynthetic potential. This increase in algal production disrupts the internal equilibrium of corals and also increases the photosynthetic competition on the reef. The intricate balance between coral and algae is typically maintained in low-nutrient environments in which corals evolved.
Nutrient pollution poses a significant threat to corals because zooxanthellae, macroalgaes and phytoplankton are adept at handling elevated nutrient levels whereas corals are more complex and less adept at handling the change in environment. An increase in these key nutrients will lead to an unhealthy increase in all algal populations. This unchecked growth disrupts the delicate balance of the reef.
Corals are capable of regulating zooxanthellae populations within them to maintain optimal levels through various mechanisms. As zooxanthellae populations rise, so does the production of reactive oxygen species (ROS) during photosynthesis. While corals possess genetics and antioxidants to cope with normal ROS levels, surpassing a certain threshold subjects corals to intense stress from their endosymbionts, and thus they expel zooxanthellae in an event widely recognized as "coral bleaching." Greater concentrations of algal biomass in the coral has also been shown to compete with corals for carbon and thus hinder coral calcification. Zooxanthellae also retain a greater percentage of their photosynthates when their populations are increased (Stambler, 316-317). This means that as algal biomass increases in the host organism, corals receive a lower percentage of the energy produced by photosynthesis, at this point the endosymbiosis no longer exists. If corals are placed in an environment with elevated nutrients, the zooxanthellae will outcompete every time and it will eventually lead to the coral’s demise.
Reef Ecology
The first effect of nutrient pollution is observed in the ocean’s primary producers. Phytoplankton blooms have adverse effects on coral reefs by limiting the amount of sunlight and carbon, as well as increasing the bioavailability of nitrogen and phosphorus. Heterotrophic organisms’ only source of nitrogen, phosphorus and carbon is from their diet. Phytoplankton production of oxygen during the day has pH boosting effects, however their respiration during the night decreases ocean pH. Pelagic phytoplankton play a massive role in nutrient distribution and thus directly and indirectly affect coral reefs.
Phytoplankton compete with corals for sunlight in nutrient rich conditions. Phytoplankton blooms will tinge the water green and reduce the total amount of sunlight which reaches the corals. The phytoplankton, being pelagic and close to surface waters, quickly uptake the nutrients and sunlight present in the water column. This factor limits the photosynthetic potential of sessile invertebrates such as corals.
Phytoplankton can disrupt major biological processes of corals by altering pH, carbon, and oxygen makeup of the water. Phytoplankton night respiration will drastically reduce oxygen content at night and increase CO2 content. The increase in CO2 will also increase the amount of hydronium ions in the water and thus lower pH during nighttime. The depletion of oxygen can also lead to anoxic conditions in the ocean, although rare in reefs. Phytoplankton, like all living organisms, need carbon. They uptake carbon in order to facilitate photosynthesis and create sugars. These sugars, as a primary energy source for phytoplankton, provide energy for other biological processes such as protein synthesis. The primary production of proteins is an essential component of ocean nutrition.
Coral calcification is greatly impacted by the pH and alkalinity of ocean water. Alkalinity is the measure of the ocean's buffering capacity and is measured as the amount of carbonate and bicarbonate in the water. Both carbonate and bicarbonate can bind with acidic molecules and neutralize them, thus buffering the ocean. However, when carbonate and bicarbonate bind with acids they release hydronium ions which lower ocean pH. Lower pH is indicative of higher concentrations of carbonic acid and lower concentration of calcium carbonate. Phytoplanktons’ capability of lowering pH cannot be taken lightly as pH is one of the most important factors for calcifying organisms such as corals.
The majority of photosynthetic reactants utilized by zooxanthellae come from coral diet and not direct uptake from the water. This means that the coral controls the amount of nutrients which reach the zooxanthellae and thus gives them the ability to monitor their growth. Nitrogen being present in the water is not enough to disrupt the algal symbiosis within the coral. It requires digestion of zooplankton and bacteria which are enriched in these nutrients to damage the coral health. Phosphorus however can directly be extracted from the water by coral polyps. The uptake of phosphorus affects algal cell density and skeletal structure density. Corals in captivity have been observed to grow at faster rates in slightly elevated phosphate, however this affects the calcium carbonate density of the skeleton. The skeleton most likely grows in order to keep up with elevated algal cell density.
Corals evolved almost entirely around capturing prey, suggestive of feeding being an essential part of coral biology. A peculiar trait considering most coral energy comes from their photosynthetic endosymbionts. Feeding allows assimilation of key nutrients. Primary producers assimilate nutrients, which are consumed by bacteria and animals such as zooplankton. Corals primarily feed on zooplankton and bacteria and they invest the acquired energy to culture their own zooxanthellae. The zooxanthellae reward the coral with excess sugars and proteins which can then be incorporated into the growth of the colony. Corals are unique because they feed on phytoplankton predators and use the nutrients which these predators bioaccumulate in their lifetime in order to grow their own phytoplankton, instead of just ingesting and utilizing the original prey or instead of evolving to capture phytoplankton as a main source of food.
The Reef as a Warzone
The reef is the most tranquil war zone that exists. Corals must battle everyday for space on the reef. Fierce competition exists between corals and macroalgaes. Macroalgaes are observable plant structures that are often referred to as seaweed. These algae are aggressive in their growth and reproduction, and have the ability to quickly spread throughout a reef. It is thanks to fish such as tangs, damselfish and gobies which feed on macroalgaes in order to survive and they also intentionally protect their coral habitats against algal overgrowth. It is also thanks to specifically evolved coral traits which allow them to wage active and constant warfare on algaes present in the reef environment. Algae encroaching on corals is an irritant and can result in coral recession. There is significant documentation on corals’ ability to kill macroalgaes through stinging. The competition between macroalgae and corals is often mediated by herbivorous fish. Pictures are often seen of coral reef bleaching events, and it is within days that these dead coral skeletons are covered in macroalgae. Corals also have the ability to sting nearby corals. The damage caused by a coral sting can often be extensive and result in localized death of the coral. Algae is also quick to grow in these wounds and further irritate coral flesh. This competition is fueled mostly by the increase in nutrients in the water, which has more drastic effects on coral biology when compared to macroalgae and allows the proliferation of macroalgae.
Corals have evolved specific forms of combat to aid in their competition with macroalgaes. Corals possess specialized stinging cells passed down from their jellyfish ancestors. These stinging cells inject venom to all nearby threats including macroalgaes (Nugues, 2004). Other corals use broadcast chemical warfare to evade predation and even limit growth of antagonist species nearby. While these evolutionary traits have allowed corals to thrive, corals are losing their edge due to multifactor stressors which are directly linked to nutrient pollution. Nutrient pollution greatly hinders the health of corals but increases the growth potential for macroalgae and phytoplankton, making this a multifaceted issue. Nitrogen, phosphorus and carbon are some of the most abundant forms of nutrient pollution and they are all key reactants in photosynthesis. Thus these big three are of particular interest in coral reef ecology and conservation.
Nitrogen in the Ocean and Major Pathways
Nitrogen rarely exists as N2 in the ocean but rather as ammonium and nitrate, with nitrate being the most abundant. Nitrogen is essential for all biology as it is present in DNA, amino acids and chlorophyll. Phytoplankton have the ability to uptake nitrate and ammonium and incorporate these elements into chlorophyll or other necessary biomolecules. Nitrogen is the usual limiting reactant in the oceans photosynthetic capabilities, thereby placing this element among the most detrimental forms of nutrient pollution. Excess nitrogen will lead to massive coral reef die-offs due to increased algal populations and their aforementioned effects.
There are multiple pathways by which the ocean becomes enriched in nitrogenous waste. Most anthropogenic nitrogen enters the ocean through human wastewater runoff and agricultural runoff. Gaseous nitrogen compounds can also be fixated by diatoms and enter the marine environment. Nitrous oxide and other nitrogen gasses are released into the atmosphere by the combustion of fossil fuels.
Phosphorus in the Ocean and Major Pathways
Phosphorus exists in many forms in the ocean but the most common is as a phosphate ion, PO4-. Phosphorus is present in many biological molecules including DNA and ATP meaning it has essential functions within photosynthesis. Anthropogenic phosphorus enters the ocean through wastewater runoff, agricultural runoff, mining and combustion of fossil fuels. Excessive phosphate can cause eutrophication of coastal waters and changes in phosphate levels can also affect phytoplankton species composition. Diatoms and Cyanobacteria are known to utilize phosphates at a greater rate than other species of phytoplankton. Consistently elevated levels of phosphates will impede upon coral calcification in two ways: coral skeletons will grow rather quickly but brittle, or it will completely halt all growth. The skeleton will either grow brittle in order to manage algal cell density or the coral will stop growing and eventually due to the stress caused by increased algal cell density it will suffer mortality(Dunn et al., 2011).
Carbon in the Ocean and Major Pathways
Carbon has many pathways into the ocean through atmospheric CO2 and terrestrial runoff of organic molecules. Carbon plays an essential role in coral reef ecology as it affects primary production, and is a part of all living systems within the ocean. An increase in carbon concentration will create more substrate for phytoplankton to multiply, which has inverse impacts on the health of the reef. Most carbon in the ocean exists as bicarbonate ions, which can neutralize acidic compounds. When CO2 is dissolved in seawater, it forms carbonic acid (H2CO3) which is then buffered by the oceans alkalinity, and forms hydronium ions and bicarbonate. The hydronium ions that are released in this process decrease ocean pH and also decrease the availability of carbonate ions that are needed by corals and other calcifying marine organisms.
Combined sources of carbon, phosphorus and nitrogen and Effects
Human wastewater runoff is a common source of carbon, phosphorus, and nitrogen entering the ocean. In the United States, wastewater treatment removes only about 30% of organic waste products. The demand for agriculture, driven by human consumption, contributes to almost all nutrient pollution. Current farming practices involve the use of man-made fertilizers, leading to increased levels of organically active nitrogen and phosphorous compounds which have not been present in the environment for millennia. Excessive fertilizer use, results in nutrients being present in the wind and wastewater discharge. Humans consume plants and animals rich in carbohydrates, fats, and proteins, which all contain carbon, phosphorus, and nitrogen as a result of increased primary production due to the discovery of man-made fertilizers. Human consumption levels are unsustainable with current practices as it requires constant fixation of nitrogen and phosphorus. Besides wastewater, nitrous oxide, carbon dioxide, and airborne phosphorus can enter the ocean through atmospheric means. Nitrous oxide and carbon dioxide are present in fossil fuels. However, most nitrous oxide enters the atmosphere through microbial breakdown of human and animal waste products.
In recent decades, the substantial increase in human and animal waste products can be primarily attributed to the widespread use of fertilizers. Unfortunately, current practices treat both animal and human feces as waste, often disposing of it in local waterways. However, there exists a tendency to overlook the potential utility of these byproducts. Just a century ago, our civilization experienced far less plant production and consequently generated significantly less "waste." It is imperative for humanity to recognize the valuable potential of these byproducts; failure to do so may result in a significant population decline.
Moreover, the continued consumption of fossil fuels exacerbates environmental issues by introducing more carbon and other contaminants into the environment. Interestingly, waste products share similar molecular structures with fossil fuels. By converting these byproducts into biofuels, we can simultaneously address two critical marine issues – nutrient pollution and ocean warming due to greenhouse gasses. Fertilizers, which once revolutionized human civilization and addressed population limitations, now possess even greater potential as biofuels.
Current Occurrences of Nutrient Pollution on Reefs
The first mass coral bleaching event occurred in 1998 and six subsequent mass events have occurred since. Coral reefs have been consistently regressing year by year, however mass bleaching events are giant steps in the wrong direction. Mass bleaching is present during the summer months when water temperatures rise. Higher temperatures allow greater proliferation of coral algal cells. Nutrient pollution is an issue corals deal with year round however it is when the temperatures rise that the algal production becomes unbearable and creates these horrific scenes of coral bleaching. Multifactor stressors such as increased nutrients, increased coral food supply and increased water temperature creates a zooxanthellae frenzy which is unsustainable for a healthy coral. Reefs are dying due to increased human consumption and demand of nutrients, combined with improper waste treatment and global warming.
Nitrogen, phosphorus, and carbon allow increased algal cell density. This increased algal cell density has direct impacts on the organism's bodily functions and calcification. Increases in these essential nutrients allows proliferation of competitive phytoplankton, macroalgae and the endosymbiotic zooxanthellae. Phytoplankton is able to outcompete corals for every essential element. Macroalgae is capable of competing with corals for space on the reef, by limiting coral growth and preventing suitable substrate for coral larvae. Zooxanthellae are essential to coral survival however in elevated nutrients and temperatures, they are more detrimental than beneficial. Coral reefs are dependent on stable nutrient levels consistent with the environment in which they evolved. Corals are not suited to adapt quick enough to elevated nutrient levels with other stressors such as pathogens and warming environment. Humans must find better ways to remove and reuse the biologically active nitrogen, phosphorus and carbon that they produce yearly if this issue is to be stopped.
References
Burke, Lauretta. “Shoreline protection by the world’s coral reefs: Mapping the benefits to people, assets, and infrastructure.” Science Direct, 16 June 2023, https://www.sciencedirect.com/science/article/pii/S0308597X2200358X.
Accessed 6 December 2023.
Clarke, Chris. “We're Finding Amazing, Life-Saving Natural Medicines in Coral Reefs.” PBS SoCal, 23 March 2017, https://www.pbssocal.org/shows/earth-focus/discovering-life-saving-medicines-in-coral-reefs.
Accessed 6 December 2023.
Cortés, Jorge et al. “Upwelling Increases Net Primary Production of Corals and Reef-Wide Gross Primary Production Along the Pacific Coast of Costa Rica.” Frontiers, 23 December 2015, https://www.frontiersin.org/articles/10.3389/fmars.2015.00113/full.
Accessed 6 December 2023.
Dunn, Jeremy G. “Effects of phosphate on growth and skeletal density in the scleractinian coral Acropora muricata: A controlled experimental approach.” Science Direct, 16 June 2023, https://www.sciencedirect.com/science/article/pii/S0022098111004588.%20.
Accessed 6 December 2023.
Wilkinson., et al. “Corals.” British Geological Survey, 2000, https://www.bgs.ac.uk/discovering-geology/fossils-and-geological-time/corals/.
Accessed 6 December 2023.
Nugues, M. M. “Coral defence against macroalgae: Differential effects of mesenterial filaments on the green alga Halimeda opuntia.” ResearchGate, 2004, https://www.researchgate.net/publication/250218140_Coral_defence_against_macroalgae_Differential_effects_of_mesenterial_filaments_on_the_green_alga_Halimeda_opuntia. Accessed 6 December 2023.
Scheid, Walther. “How climate change alters ocean chemistry.” World Ocean Review, 28-53, https://worldoceanreview.com/wp-content/downloads/wor1/WOR1_en_chapter_2.pdf. Accessed 6 December 2023.
Stambler, Noga. “Coral Reefs and Eutrophication.” OSTI.gov, 2000, https://www.osti.gov/etdeweb/servlets/purl/680529.
Accessed 6 December 2023.