What Are the Red Tides?
The phrase “red tide” is a common vernacular term used to refer to harmful algal blooms along coastlines. Phytoplankton — microscopic, single celled, photosynthetic organisms — grow and reproduce rapidly, such that their presence can discolor the ocean to a red or orange tint. Dinoflagellates are the dominant phytoplankton responsible for the red tides and, when perturbed, produce an opulent glow.
Dinoflagellates: Dinoflagellates are a type of phytoplankton and are one of the major primary producers in our oceans [1]. They appear to live in low-nutrient, calm waters. Their blooms tend to occur in the warm waters of late summer and fall — although, blooms can happen at any time of year given proper conditions. Dinoflagellates are bioluminescent when disturbed, emitting a light molecule called luciferin to generate luminescence. Some dinoflagellates produce a harmful neurotoxin that affects muscle function and can kill fish, birds, and mammals.
Locations: The red tides are documented in every coastal U.S. state and occur along other coastlines, such as Australia, Canada, and Japan [2]. Common sites in the United States include southern California and Florida coasts.
Are the Red Tides Harmful?
Marine and Human Health: The algal blooms associated with the red tides can be harmful, whether the phytoplankton produce toxins or not [3]. Algal blooms harm marine ecosystems by depleting dissolved oxygen and blocking sunlight for photosynthesis. Various plant and animal marine life can die if oxygen levels reach critical lows, and photosynthetic organisms cannot survive with diluted sunlight. Some of the red tide blooms emit toxins that are uptaken by marine life. If ingested in high quantities, the toxin can kill organisms, and it is not uncommon to see vast amounts of dead marine life washed up on shores during and after the red tides. If humans ingest sea food, such as fish or shellfish, harvested from waters where there is a toxic algal bloom, they too ingest the marful toxin that can lead to adverse health effects.
Economic Impacts: The red tides lead to several economic faults [4]. During the red tide season, fishing is heavily restricted and many restaurants have to temporarily remove sea food from their menus under public health regulations. This curtailment economically hurts fisheries and restaurants by halting a valuable commodity in their industries. Similarly, the red tide is not favorable to beach and tourism industries (e.g. hotels and surf shops), as the red tides are associated with beach closures and lost revenue.
What Causes the Red Tides?
Causes: Much of the red tide remains unknown to the scientific community. The exact causes and conditions that give rise to coastal algal blooms is not well characterized. Part of this difficulty is that the open ocean is very dynamic and variable, and many prior attempts to model the algal blooms were unsuccessful [3]. One hypothesis is that a necessary precursor to the blooms includes calm waters and a nutrient depleted surface, conditions which are optimal for dinoflagellate growth and dominance against other algae. Other reasoning lies in eutrophic waters — waters extremely rich in nutrients to support plant life — that allow the algae to rapidly grow and reproduce. Eutrophic conditions can arise from oceanic upwelling and human waste that is dumped into the ocean [5]. Because the scientific community has yet to be able to accurately model algal blooms, scientists are unable to predict when a red tide will occur and cannot inform various officials and industries of the coming challenges associated with the red tides.
What We Know: There have been improvements in modeling and characterizing the mechanisms behind the red tides with the new modeling methods developed by Scripps Institution of Oceanography at the University of California, San Diego [3]. The team developed empirical dynamic models (EDM), which studies the ecosystem as a whole rather than separate pieces. The models were able to predict portions of algal blooms, although no model was able to replicate all the algal blooms. Nonetheless, the findings suggest nutrient concentrations and water stability were the driving factors of algal blooms.
So… What Now?
Current Challenges: The scientific community still does not yet have a working model to predict algal blooms. Fisheries, tourism, hotels, restaurants, and an array of industries and fields are still subjected to the unpredictable red tides and their negative effects. However, with the preliminary EDMs gaining accuracy in predicting algal blooms, the mysterious narrative behind the red tides is beginning to unfold and is opening the opportunity for further development and research into algal bloom modeling.
Future Challenges: Ocean temperatures are expected to warm significantly within the next century, and the southern California coast is expected to warm 6–9 °C by the years 2091–2100, with average temperatures ranging from between 24–29 °C [6]. Historically, dinoflagellates have grown in water temperatures from 15–20 °C but have optimum growth rates between 20–27 °C [6,7]. The predicted warming of the oceans will coincide with temperatures optimal for dinoflagellate growth. Moreover, warmer oceans are associated with more stratification (less mixing between layers) that suppress nutrient circulation. This trend increases the scale of nutrient depleted zones while reducing the size of nutrient rich zones. Together, warmer temperatures and nutrient depletion are favorable for dinoflagellates.
Next Steps: Because future conditions are predicted to be favorable for dinoflagellate growth, there is a risk that the range and duration of the red tides will increase. Consequently, the aforementioned industries will experience more negative impacts should this prediction be true. Accurate models can be used to inform public health officials to preemptively close beaches and regulate fisheries, and the fishing industry can take precautions to protect their farms. Hydraulic power and desalination plants will be able to shut off the equipment and protect from damage. Hence, it is vital for both public health and economic stability to improve our ability in forecasting the red tides.
Citations:
[1] Chan, W. Y. et al. (2021). Adaptive responses of free‐living and symbiotic microalgae to simulated future ocean conditions. Global Change Biology, 27(9), 1737–1754.
[2] NOAA. What is a red tide? National Ocean Service website, 02/26/21.
[3] McGowan, J. A. et al. (2017). Predicting coastal algal blooms in southern California. Ecology, 98(5), 1419–1433.
[4] Anderson, D. M. et al. (2000) Sea Grant: Woods Hole.
[5] Anderson, D. M., Glibert, P. M., & Burkholder, J. M. (2002). Harmful algal blooms and eutrophication: Nutrient sources, composition, and consequences. Estuaries, 25(4), 704–726.
[6] Thomas, M. K. et al. (2012). A Global Pattern of Thermal Adaptation in Marine Phytoplankton. Science, 338(6110), 1085–1088.
[7] López-Rosales, L. et al. (2014). Simultaneous Effect of Temperature and Irradiance on Growth and Okadaic Acid Production from the Marine Dinoflagellate Prorocentrum belizeanum. Toxins, 6(1), 229–253.
