From time to time, huge carpets of algae spread out in the oceans of the world. Toxic algae species are often involved, which pose a health risk to humans and marine animals. A global analysis over 33 years now shows: Even if harmful algal blooms do not generally occur more frequently than in the past, they now cause greater damage. The main reason for this is that aquaculture has gained in importance and the poison of the algae makes whole crops of seafood inedible and can kill thousands of cultivated fish. However, the researchers were unable to confirm the widespread assumption that algal blooms are increasing due to climate change. Increased reports of harmful events are more likely to be due to increased monitoring.
The poison from toxic algae species can kill fish and marine mammals, store itself in seafood and cause economic and health damage to humans. The poisoned fish and seafood cause considerable financial losses and waters polluted by algal blooms no longer attract bathing tourists. Again and again there are reports of people who have become seriously ill or even died from specialties from the oceans that have become poisonous. So far, many experts have assumed that algal blooms have become more frequent within the last few decades, as climate change is heating up the water and creating ideal growth conditions. Most of the studies that saw an increase, however, relied on local observations, which often only revolved around individual species.
More reports, but not a general increase
A team led by Gustaaf Hallegraeff from the University of Tasmania in Australia has now presented the first global analysis of harmful algal blooms (HAB). To do this, the researchers analyzed over 9,500 reports on events in which algal blooms had negative effects on humans. They also used the database of the Ocean Biodiversity Information System (OBIS), which contains seven million microalgae observations, including almost 300,000 occurrences of toxic algae species. In order to be able to estimate the extent to which the frequency of algal blooms has changed, Hallegraeff and his colleagues put the number of annual reports in relation to the monitoring effort.
“There is a widespread claim that HABs are increasing in prevalence, frequency, or intensity around the world, so a quantitative global assessment was long overdue,” says Hallegraeff. “Compared to previous studies, our new big data approach shows a much more nuanced trend. Our study concludes that the health and economic damage caused by harmful microalgae – such as seafood poisoning, water discoloration that affects tourism, and the death of fish on aquaculture farms – varies from region to region. ” While harmful algal blooms have increased in South and Central America, for example, they have become rarer on the west coast of America as well as off Australia and New Zealand. There were no significant changes between 1985 and 2018 in Europe, Southeast Asia and the East Coast of America.
Significant economic consequences
Almost half of the harmful events recorded were toxins in seafood. In humans, these can lead to diarrhea, nausea, neurological deficits, breathing problems and even death. In most cases, however, food monitoring prevents contaminated seafood from ending up on the plate. But such lockdowns have serious economic consequences. Since 1985, seafood aquaculture production has increased from 11.35 million tons annually to 178.5 million tons in 2018. But at times of strong algal blooms, entire mussel harvesting areas are often closed. There is no profit for the people who depend on it.
The more intensive the aquaculture production in a region, the more often harmful algal bloom events were reported. According to Hallegraeff, this is mainly due to the fact that algal blooms are being monitored more closely in these regions. In addition, aquaculture ensures that the surrounding waters are increasingly polluted with nutrients, which promotes the growth of algae. “But the data on nutrient pollution are insufficient,” says Hallegraeff. “The relationship between aquaculture-related nutrients and HABs is therefore an important direction for further research.”
Understanding important for future management
According to the study, algal blooms had an even more serious economic impact on fish farmed in aquaculture. While wild marine fish can avoid the algae carpets, fish from aquaculture farms die en masse when their areas are infested with algal blooms. In Chile, algal blooms caused salmon deaths in 2016, with losses of US $ 800 million. In 2019, fish worth $ 100 million died in Norway. However, the existence of potentially dangerous algae species does not always indicate major economic losses. Apparently the location and size of the aquaculture farms also play an important role.
“It is expected that the occurrence of harmful species and their effects will change locally, regionally and globally over time, along with the effects that climate, hydrography and human impacts have on the coastal environment,” says co-author Henrik Enevoldsen from the University of Copenhagen in Denmark. “Understanding the trends and distribution patterns of noxious species and events on different spatial and temporal scales helps predict if, where and when HABs can be expected. This knowledge is fundamental for effective management of HABs and for optimizing the use and value of maritime space in coastal areas. “
Source: Gustaaf Hallegraeff (University of Tasmania, Australia) et al., Nature Communications Earth & Environment, doi: 10.1038 / s43247-021-00178-8