The Effects of Upwelling on Hermatypic Coral Reefs by Mikayla Taylor

 Introduction

The process of upwelling is essentially a natural cooling system for the surface of the world’s oceans. It is defined as the rising of deep, cold, nutrient rich waters to the surface as wind pushes warmer surface waters offshore (NOAA 2024). The changes in temperature and nutrient content brought about by this process can be both beneficial and detrimental to marine ecosystems.

Graphic demonstrating the replacement of windblown surface waters by colder, nutrient-rich waters from below. (NOAA 2024)

 

One crucial ecosystem affected by upwelling is coral reefs, more specifically hermatypic (reef-building) corals. These species require very specific temperature, pH, and turbidity conditions to survive and become highly susceptible to life-threatening stress if oceanic conditions are outside of their preferences even slightly. Their ideal temperature range is between 23-29°C (though this varies slightly among species) (NOAA 2024), their ideal pH range is between 8.0-8.3 (the standard pH of seawater) (Living Oceans Foundation n.d), and they require very clear waters with maximum light penetration (low turbidity) (NOAA 2024). Outside of these conditions, corals are at risk of losing their symbiotic algae called Zooxanthellae that are their main source of energy through photosynthesis. This is because the zooxanthellae become stressed and begin releasing a harmful chemical that can damage the host coral’s tissues, so they expel the algae and become bleached and vulnerable to disease (NOAA n.d.). These unideal parameters also create a lack of the calcium carbonate (CaCO₃) ions required to build their hard exoskeletons due to higher dissolution rates and a reduced ability to perform photosynthesis caused by inadequate sunlight.

As shown by numerous studies, upwelling can provide support and reprieve for reefs in a variety of ways but also worsen matters depending on case-to-case variation.

The Benefits of Upwelling

The highest threat to coral reefs today is climate change, with warming sea surface temperatures (SSTs) causing coral bleaching and reef health decline. Since upwelling brings cooler water from the depths to the surface, it aids in keeping bleaching events at bay. In regions like the Quiong Dong Upwelling Zone (QDU) in the South China Sea where upwelling is a regular seasonal occurrence, SSTs remain safe for corals even in the peak summer months, staying around 27°C and significantly preventing bleaching when its most necessary. On the other hand, nearby non-upwelling regions reach 31°C and bleaching is much higher, especially in the hottest months of the year (Zhu et al. 2022).

This cooling ability is especially valuable during extreme climate events like the El Nino Southern Oscillation (ENSO), when SSTs warm 0.5°C or more for months during the El Nino phase causing mass global bleaching events (NOAA 2016; Claar et al. 2018). During the 1997-98 El Nino, upwelling kept the Gulf of Panama below 30°C and prevented any bleaching, while the non-upwelling zone of the Gulf of Chiriqui experienced a mass bleaching after temperatures rose above 30°C (D’Croz et al. 2001). Upwelling has shown to decrease the frequency, duration, and severity of thermal disturbances by 55%, 24% and 4% respectively (Chollett et al. 2010).

Map of the Pacific demonstrating the SST differences between the upwelling Gulf of Panama and non-upwelling Gulf of Chiriqui. (Gravinese et al. 2018)

 

Upwelling also protects corals from fast, thick growing turf and macroalgae that may compete with them for resources or smother them. Cooler water temperatures have been shown to reduce the metabolic activity of these organisms. In the Pacific, before the seasonal upwelling period, turf algae were found to have the highest primary productivity rates on the reef, but as the upwelling season progresses, their primary productivity remained stagnant as coral primary productivity skyrocketed. Corals were shown to have made the largest contribution to reef productivity by the end of the upwelling period (Stuhldreier et al. 2015).

For already stressed or bleached corals, aid lies in the increased phytoplankton density created by the upwelling of more nutrients. Since a lack of zooxanthellae means decreased photosynthesis capabilities, corals can rely on suspension feeding while bleached and the phytoplankton become a critical food source (Sawall et al. 2020). Increased turbidity from higher phytoplankton density also shields corals from excess UV rays that could increase their stress, aiding in their recovery after bleaching events (Chollett et al. 2010).

Upwelling as a Tool for Climate Mitigation

The thermal refugia provided by upwelling is being mimicked through artificial upwelling (AU), where scientists have been testing the effect of short pulses of cool water from different ocean depths on different corals species in a controlled laboratory environment or through simulation technology. Corals exposed to AU exhibited significant indicators of improved health such as increased photosynthesis rates, zooxanthellae density, and chlorophyll-a levels. Less than two hours a day of upwelling conditions decreased the overall period of thermal stress experienced by 1.6°C-weeks (Sawall et al. 2020).

Coral tank system used in Sawall et al. 2020 AU experiment. (Coxworth 2020)

 

Simulations show AU reducing and delaying bleaching events for decades, but there are risks to the method. Downwelling began to occur at the AU site, which warmed the deep waters and reduced the temperature gradient that drives upwelling, therefore weakening its cooling effect. Overly extreme AU practices can overcool surface waters, increase surface acidity, and cause an excess nutrient influx. Ocean circulation patterns can even be disrupted causing changes in ocean chemistry and heat budget (Feng et al. 2020). For these reasons, AU requires further research before we can utilize it in our fight to save the reefs.

The Disadvantages of Upwelling and Stipulations of its Abilities

The drawbacks of upwelling are usually scenario dependent. Non-stressed corals are harmed by increased nutrients rather than helped because increased phytoplankton density decreases their light availability (which is needed in high amounts for photosynthesis), increased algae growth smothers them, and harmful microbes can grow supported by the organic matter release of dying phytoplankton (Sawall et al. 2020).

Dead coral smothered by overgrown algae. (Wolcott 2010)

 

Upwelling can cause an imbalance in nutrients like nitrogen and phosphate. When nitrogen is high, but phosphate is limited, coral growth rate is higher but unsupported by the necessary nutrients for cell maintenance, adding to coral stress (Buckingham et al. 2022).

Corals may be harmed simply by naturally living in an upwelling region and being accustomed to it. In one study, corals from the Gulf of Panama (upwelling zone) bleached faster and more severely when exposed to heat compared to Gulf of Chiriqui (non-upwelling) corals, likely because they were already adapted to the cooler temperatures in the area (D’Croz et al. 2001). This lack of tolerance could cause a disaster in the face of an extreme climate event or an interruption in the natural upwelling process.

Geographical variability in upwelling makes it an inconsistent thermal refuge. During the 1982-83 ENSO, upwelling failed to cool waters enough to prevent bleaching in Panama – both upwelling and non-upwelling zones had equal SSTs and bleaching, with coral mortality even reaching almost 85% in the upwelling zone. Upwelling only works as a thermal refuge when it occurs at the same time as warming seasons and has a significant effect on SSTs, but this overlap can be hard to accomplish in oceans like the Pacific where extreme thermal events often occur throughout the entire year. In these regions, upwelling delays the warming period but doesn’t reduce its length or severity (Chollett et al. 2010).

Considerations of Anthropogenic Influence

It is important to consider how human activity plays a role in the effects of upwelling. Our greenhouse gas emissions have caused global warming and pollution from coastal development and agricultural runoff contribute to excess nutrients and competitive organism overgrowth. In Zhu et al. 2022’s study, land use overshadowed the effects of upwelling because nutrient content was extremely elevated. Results that were unusual for an upwelling zone were produced, like lower turbidity and less acidic water (as compared to the non-upwelling zone), showing that nearby anthropogenic influences disrupted the data. So, not only does human activity cause the factors that stress corals, but it also prevents us from properly measuring the impact we have, and which reefs need the most help. Moving forward, we must work to alleviate the stressors we have the most control over (like pollution) to give corals their best fighting chance and to gain clearer understanding of how we can work alongside upwelling rather than against it.

Conclusion and Recommendations

Upwelling, as both a natural and artificial process, shows great promise in providing refuge for corals from detrimental climate changes by cooling SSTs, increasing food supply, reducing UV radiation, and preventing overgrowth of other competitive organisms. However, its success is highly dependent on seasonality, location, and anthropogenic activity. To make the most of this potential refugia, future studies should explore AU techniques more representative of natural upwelling by introducing longer cooling periods rather than short bursts to determine which duration of upwelling produces greater coral health over time. It is important to critically analyze our unsustainable practices as humans as well to be the better advocates for reef survival, especially as global warming and extreme climate events continue to worsen. As time goes on and climate changes worsens, it becomes more critical than ever that we analyze our unsustainable practices to be better advocates and protectors of our beloved coral reefs!

References

B Coxworth (2020) "Artificial upwelling" could help save coral reefs. Environment. https://newatlas.com/environment/artificial-upwelling-coral-bleaching/

DC Claar, L Szostek, JM Irwin-McDevitt, JJ Schanze, JK Baum (2018) Global patterns and impacts of El Nino events on coral reefs: A meta-analysis. PLoS One 13(2): e0190957. https://doi. org/10.1371/journal.pone.0190957

H Wolcott (2010) Dead Coral Overgrown with Algae. Ocean. https://ocean.si.edu/ecosystems/coral-reefs/dead-coral-overgrown-algae

I Chollett, PJ Mumby, J Cortes (2010) Upwelling areas do not guarantee refuge for coral reefs in a warming ocean. Marine ecology Progress Series 416: 47-56. https://doi.org/10.3354/meps08775

I Stuhldreier, C Sanchez-Noguera, F Roth, J Cortes, T Rixen, C Wild (2015) Upwelling Increases Net Primary Production of Corals and Reef-Wide Gross Primary Production Along the Pacific Coast of Costa Rica. Frontiers in Marine Science 2: 00113. https://doi.org/10.3389/fmars.2015.00113

L D'Croz, JL Mate, JE Oke (2001) Responses to elevated sea water temperature and UV radiation in the coral Porites lobata from upwelling and non-upwelling environments on the Pacific coast of Panama. Bulletin of Marine Science 69: 203-214. https://www.ingentaconnect.com/content/umrsmas/bullmar/2001/00000069/00000001/art00013#

Living Oceans Foundation (n.d.) Environmental Conditions. Khaled bin Sultan Living Oceans Foundation. https://www.livingoceansfoundation.org/education/portal/course/environmental-conditions/#abiotic-factors

MC Buckingham, C D’Angelo, TB Chalk, GL Foster, KG Johnson, Z Connelly, C Olla, M Saeed, J Wiedenmann (2022) Impact of nitrogen (N) and phosphorus (P) enrichment and skewed N:P stoichiometry on the skeletal formation and microstructure of symbiotic reef corals. Coral Reefs 41: 1147-1159. https://doi.org/10.1007/s00338-022-02223-0

NOAA (2016) Understanding El Nino. National Oceanic and Atmospheric Administration. https://www.noaa.gov/understanding-el-nino

NOAA (2024) In what types of water do corals live? National Oceanic and Atmospheric Administration. https://oceanservice.noaa.gov/facts/coralwaters.html

NOAA (2024) What is upwelling? National Oceanic and Atmospheric Administration. https://oceanservice.noaa.gov/facts/upwelling.html

NOAA (n.d.). Satellites & bleaching. NOAA Coral Reef Watch Tutorial. https://coralreefwatch.noaa.gov/product/5km/tutorial/crw02b_more_bleaching.php#:~:text=When%20the%20water%20gets%20too,to%20themselves%20and%20to%20Polly

P Gravinese, CJ Randall, RB Aronson, L Toth (2018) How Do Upwelling and El Niño Impact Coral Reef Growth? A Guided, Inquiry-Based Lesson. Oceanography 31: 184-188. 10.5670/oceanog.2018.424

W Zhu, J Xia, X Li, Y Ren, M Zhu, X Liu, H Yin, D Huang, R Chen (2022) The impact of coastal upwelling on coral reef ecosystem under anthropogenic influence: Coral reef community and its response to environmental factors. Frontiers in Marine Science 9: 888888. https://doi.org/10.3389/fmars.2022.888888

Y Sawall, M Harris, M Lebrato, M Wall, EY Feng (2020) Discrete pulses of Cooler Deep Water Can Decelerate Coral Bleaching During Thermal Stress: Implications for Artificial Upwelling During Heat Stress Events. Frontiers in Marine Science 7: 00720.  https://doi.org/10.3389/fmars.2020.00720