Myriads of differently shaped and microscopically small algae are to be found floating in the oceans and other waters on our planet. They form the basis of aquatic life because, under the influence of high-energy solar radiation they have the ability to convert inorganic carbon dioxide through photosynthesis into organic, biologically usable biomass. This process releases oxygen, which is essential for the respiration of almost all living organisms. A rich microalgae flora is therefore desirable and beneficial, but sometimes the algae colonies get out of control and multiply suddenly on a massive scale. Their number then increases so rapidly that the water becomes cloudy and the algae form thick carpets on the surface. These can be green or brown, sometimes even bluish in colur, depending on the predominant species of algae. The most impressive, however, are algae blooms of a deep red colour, which led to the phenomenon being called "red tide". The name has become so familiar that it is often used for all algae blooms, regardless of their actual colour.
Theoretically, all microalgae (phytoplankton) could "bloom", but as a rule only relatively few plankton species actually do so. Microalgae of the genera Karenia, Euglena and Oocystis, in particular, will bloom but also some blue algae (e.g. Microcystis, Oscillatoria, Anabaena), which strictly speaking are not algae but bacteria ("cyanobacteria"). Each algal bloom is unique with regard to its expansiveness, concentration, persistence and its effects on the environment and local communities. A small bloom with a high algae density can have more consequences than a large occurrence with a low algae concentration. Some plankton blooms can cause illness and even death because the algae release substances that are toxic to fish, shellfish, birds, marine mammals and humans. Such toxic algal blooms are also called "harmful algal blooms", HAB for short. Unfortunately, the term is often used today for all algal blooms, including "normal", non-toxic ones, thereby losing some of its linguistic force.
Algae blooms can be of either natural or anthropogenic origin, with human influence now clearly dominating. Natural blooms in the oceans can often be traced back to storm events or ocean currents that have carried nutrients from deeper water layers to the surface ("upwelling effect"). If the presence of such nutrients combines with other factors that favour the development of algae blooms, e.g. high water temperatures and strong solar radiation, mass development of phytoplankton can occur in the area without human intervention. A 16th century Spanish report mentions Florida Indians describing poisonous "red water that kills fish and birds". Today, humans play a major role in the development of algae blooms, because they introduce huge amounts of nutrients into the waters and thereby heavily over-fertilise them in many places ("eutrophication"). Poorly treated municipal wastewater and agricultural effluents flush so much nitrogen and phosphate into rivers, lakes, dammed lakes and shallow coastal areas that algae blooms occur quite regularly and in various degrees in warmer weather. Under these conditions even a small "initial spark" is often enough to set the development in motion…An increase in iron concentration in seawater, for example, which often triggers an explosive proliferation of cyanobacteria. Or standing water bodies with stagnant, thermally layered water, followed by persistent precipitation that lowers the salt content, and then several sunny days – scenarios like this almost inevitably lead to algae bloom in eutrophic waters.
Given the right conditions even dust from the Sahara can trigger a red tide. NASA satellites followed the path of a dust cloud which originated in Africa’s Sahara on 17 June 1999, drifted east and reached West Florida on 1 July. Because Sahara dust contains iron compounds the iron concentration in this area increased threefold, triggering a massive algae bloom covering almost 13,000 square kilometres.
Serious ecological and economic consequences
A quantitative assessment of the direct and indirect effects of algal blooms is extremely difficult. However, they can certainly have serious ecological, economic and often health consequences depending on the algae species concerned. In the overall balance, the recreation and tourism sectors are seen to suffer noticeably: hotels and restaurants located close to algal blooms are often affected by sudden reduction in guests. Who wants to go on holiday or for a walk on the beach when a thick, evil-smelling algae broth is spilling onto the shore? In addition, there is the cost of the routine checks that are necessary to warn of algal blooms. The algae’s presence in the water makes treatment of drinking water more expensive and many lake-side plots lose their value. In 2014, 500,000 people living in the area around Lake Erie (USA) were affected when drinking water supply had to be shut off completely for a few days due to a toxic algal bloom. The toxic substances from the algae in the water cannot be destroyed even by boiling.
In the case of toxic algae blooms, health effects are not rare. The alga Pseudo-nitzschia, for example, produces a toxin called domoic acid which can cause vomiting, diarrhea, confusion, cramps and loss of short-term memory and can even be fatal if consumed in very large quantities. Cyanobacteria of the genus Microcystis, which cause particularly frequent algal blooms, produce three groups of cyanotoxins: neurotoxins cause neurological damage, peptide hepatotoxins cause severe liver damage, and dermatotoxins cause skin irritation and respiratory diseases. Children, the elderly and people with low immunity are particularly sensitive to algal toxins. A US study puts the cost of treating marine pathogens and algal toxins at USD 900 million per year. If possible long-term consequences are also taken into account the figure is likely to be quite a lot higher.
Toxic algal blooms also cause considerable damage and loss of income to commercial fisheries and aquaculture. One example iof this is Ciguatera fish poisoning which occurs epidemically in warmer sea areas and is caused by metabolic products (ciguatoxin) from microalgae. Mussel and oyster cultures are particularly at risk because they accumulate toxins naturally ("bioaccumulation") when they filter algae out of the water. Consumption of such "poisoned" mussels or oysters can lead to severe illnesses, ranging from mild tingling in the limbs and various digestive problems to tachycardia, coordination problems and even suffocation due to respiratory paralysis. These diseases include Paralytic Shellfish Poisoning (PSP), Diarrhetic Shellfish Poisoning (DSP) and Amnesic Shellfish Poisoning (ASP). Because of these risks all the water bodies where mussel and oyster culture is carried out are strictly monitored by the authorities and, if algae concentrations exceed critical levels or toxins are detectable in the shellfish, the areas are closed for market supply. This guarantees consumer safety but doesn’t prevent the financial losses incurred by shellfish farmers.
In recent decades, global aquaculture has suffered considerable damage from algal blooms. In 1972, for example, a Chattonella antiqua algal bloom in the Seto Inland Sea (Japan) killed over 14 million farmed fish worth USD 60 million. On Norway's coast, over 500 tonnes of fish with a commercial value of USD 5 million were lost to Chrysochromulina algae in 1988. In 1998, hundreds of thousands of fish died in net enclosures off the coast of Guangdong, China, from toxic dinoflagellates (mainly Karenia mikimotoi and Karenia digitata). In the Persian Gulf and Gulf of Oman, the dinoflagellate Cochlodinium polykrikoides bloomed massively in 2009, damaging coral reefs, natural fish stocks and many fish farms, as well as causing considerable problems throughout the region in seawater desalination plants, thereby posing a serious threat to drinking water supply. However, this list of examples is by no means complete, especially as the frequency of such occurrences seems to be increasing. In May 2019, salmon farmers from northern Norway reported the loss of 11,600 tonnes of salmon with a market value of about EUR 102.5 million due to a persistent algal bloom. Chile's salmon industry was even more severely affected in 2016, when 14 per cent of the biomass, a total of 40,000 tonnes of fish, was lost to toxic algal blooms. At that time, AquaChile alone estimated its losses at 38.8 million euros. At two Grieg Seafood farms in British Columbia (Canada), salmon worth 2.6 million euros died in 2018 as a result of unexpectedly strong and sudden algae blooms that could not have been predicted despite regular inspections at the site.
Problems could be even more severe in the future
Some algal toxins do not only take effect in the water but can also cause severe damages via the atmosphere. Certain red tide algae emit neurotoxins which float above the red waves like a military poison gas attack and endanger airbreathers such as dolphins, whales, sea cows (manatees and dugongs) and humans. Several million fish and hundreds of manatees died in such an algae plague off the coast of Florida. The toxic gas clouds that were carried ashore by the wind caused severe mucosal irritations and breathing problems in many people.
Even non-toxic algal blooms can be a serious problem. The dense algae carpet on the surface of the water robs the macrophytes at the bottom of light and thus hinders their growth and existence. In the 1970s and 2000s large areas of seagrass meadows in Chesapeake Bay were destroyed. And even after the blooming period, when the dead algae sink to the bottom of the water body, the situation is no less dire. The dead organic matter, which forms a thick layer that covers all underwater structures, is slowly decomposed by oxygen-consuming bacteria. In some regions these bacteria consume so much oxygen that hypoxic "dead zones" are formed in the deep, emitting a horrible smell of hydrogen sulphide. Organisms that cannot escape from the hostile areas suffocate and die.
There is a strong suspicion that climate change, global warming and the associated rise in water temperature could lead to stronger and more frequent algal blooms. This would set in motion a fatal cycle that would further boost the process. Because the thicker algae mats on the surface absorb more sunlight, the water heats up more locally and accelerates algae growth. In fact, the incidence of harmful algal blooms has increased worldwide since the 1980s. This development is driven by the increase in long-range droughts that are often followed by heavy rainfall, which in turn leads to increased erosion and washes even more nutrients from the fields and surrounding landscape into the waters. The interaction of all these factors creates optimal conditions for algal blooms.
Microalgae could play a key role in the fight against global warming, however: they are, so to speak, a "secret weapon" when it comes to reducing carbon in the atmosphere. Worldwide, algae bind more carbon dioxide than trees because they cover a larger surface area in the ocean and grow faster. Climate researchers have calculated that – based on the same biomass – microalgae convert carbon dioxide from the atmosphere up to 400 times more efficiently than a tree. And, given the right know-how, many commercially sought-after products can be produced from the algae mass in a sustainable way… from biofuels, which offer an alternative to fossil fuels such as crude oil, to omega-3-rich algae oils, which can be used in fish feed for aquaculture, or plastics made from algae and even vegan burgers. Shampoo bottles, trash cans, dishes or sports shoes made from algae polymer would be an environmentally friendly alternative to products made from conventional raw materials, but algae-based production is often too expensive and is not yet profitable. Of course, the profitability calculations would look much better if the damages caused by algae blooms were to be taken into account.
Reduction of nutrient input is urgently needed
The appearance and impact of red tides is a global phenomenon that causes considerable financial losses, threatens the stability of aquatic ecosystems, hinders the use of important water resources, and often puts human health at risk. The enormous complexity of the causes of algal blooms makes it extremely difficult to predict such events with sufficient reliability. Another factor that contributes to the problem is the fact that experts have not yet been able to agree on an internationally binding threshold value above which algae concentrations are considered "blooms". Are a few hundred or thousand cells per millilitre sufficient for this – depending on the species of algae – or must it perhaps be millions? As in the case of volcanic eruptions or earthquakes we know the triggering factors but it is still not possible to predict exactly the time or the extent of devastation. Algae researchers have been searching for a long time for useful measurement methods and procedures to reliably detect and warn against the development of algae blooms. They monitor temperature trends in the ocean over large areas, regularly record nutrient concentrations, and determine algae densities. These data are fed into computer models, the perfection of which is constantly increasing. The researchers are even supported from space, because satellite images are particularly useful for estimating the extent of algae blooms.
But no matter how you look at it, predictions are only really reliable when the algae bloom is already in the making. At the Woods Hole Center for Oceans and Human Health (WHCOHH), for example, a robotic system called the Environmental Sample Processor was developed that tests the water for toxic substances directly in the ocean as soon as an algae bloom develops. The results are transmitted by radio to a central computer. It would also make sense to upgrade existing water treatment plants with analysis techniques that detect algae toxins in the water. This would probably require the development of new detection methods. The willingness to do so is not very pronounced, especially as the current control systems are normally sufficient to issue timely warnings for the affected areas, block coastal sections for visitors, prohibit bathing in the sea or to prohibit the consumption of mussels and oysters from local farms. More is hardly feasible at present. Apart from the spraying of algae-killing chemicals, which are often more harmful to the environment than the algal bloom itself, there is hardly any technology available to lessen the extent and intensity of plankton blooms. It would be particularly useful to develop a method that would "neutralize" the toxins of toxic red tides.
The situation is not hopeless for we know the triggers of microalgae blooms and should at least tackle the root causes that we can influence through our own actions. The reduction of nutrient inputs into water bodies is of crucial importance. This could be achieved through more effective sewage treatment plants, better agricultural practices, improved protection of wetlands that act as natural nutrient filters, and measures to prevent soil erosion. Precipitation must be "captured" where it accumulates so that it does not immediately enter the aquatic environment enriched with nutrients. Admittedly, this is difficult and expensive, but it would be feasible.