A Toxic Ecosystem: Understanding the Drivers of Harmful Algal Blooms
In 2015, the west coast of North America experienced a widespread marine animal mortality event. Hundreds of seabirds, sea lions, seals, whales, and dolphins over a wide range were plagued with the often fatal symptoms of domoic acid poisoning. This occurence paralleled several past marine mortality events, including one in 2002 that led to the stranding of over 2,000 California sea lions and long-beaked common dolphins along the Southern California Bight. Marine mammal mortality events such as these have been reported in the California Current for decades and have led to extensive research into the causation of such widespread illness among animals in the ocean. Marine researchers have identified a commonality between these spikes in marine mammal death: all of these mortality events occur during extremely toxic blooms of the Pseudo-nitszchia.
What is Pseudo-nitzschia?
Pseudo-nitzschia is a genus of diatom, the most common type of phytoplankton. Several species of this diatom, including Pseudo-nitzschia australis and Pseudo-nitzschia multiseries in the California Current, are capable of producing domoic acid, the toxin that caused the mammal mortality events along the North American coast. Domoic acid is a neurotoxin that bioaccumulates up trophic levels of marine food webs. When humans eat shellfish or crustaceans contaminated with domoic acid, they can contract amnesic shellfish poisoning, a neurological and gastrointestinal illness.
When conditions are favorable for the diatom, Pseudo-nitzschia can proliferate into large harmful algal blooms (HABs) that rapidly produce domoic acid. The specific factors that drive Pseudo-nitzschia blooms, however, are poorly understood, as studies attempting to investigate the causes of domoic acid HABs have produced results that are often inconclusive, contradictory, or too narrow in scope to describe larger regional trends in Pseudo-nitzschia propagation. However, several studies have found links between upwelling events and Pseudo-nitzschia blooms as well as correlations between silicate and phosphate limitation and growth of the diatom.
Climate change and Pseudo-nitzschia
Throughout the past few decades, there has been an increase in the frequency and intensity of HABs associated with domoic acid. Many researchers have hypothesized that this increase in Pseudo-nitzschia blooms and domoic acid production may be related to the changes in the physical and chemical processes in the ocean that are occurring due to anthropogenic activity, including acidification, rising surface temperatures, and shifts in nutrient concentrations. However, there is little definitive research regarding the regional impacts of climate change on HABs in the California Current.
A research team led by K. Minden from the University of California, Los Angeles’ Atmospheric and Oceanic Science Department is attempting to fill this knowledge gap by investigating the responses of P. australis and P. multiseries to varying pH levels and temperatures. Minden et al. will take samples of strains of these Pseudo-nitzschia species from locations throughout the California Current, stretching from areas of the Southern California Bight to locations north of Point Reyes. By culturing these strains at different pH levels and temperatures, including conditions of low pH and high temperatures that represent projections for the end of the century assuming anthropogenic greenhouse gas emissions do not decrease, the team aims to understand the impact future ocean acidification and warming will have on Pseudo-nitzschia growth and domoic acid production.
Why should we care?
Pseudo-nitzschia blooms, and the amnesic shellfish poisoning that they can cause in humans, have been recognized as a public health issue by the California Department of Public Health. In addition to these severe human health implications, HABs have led to widespread closures of commercial and recreational shellfish, finfish, and crustacean fisheries. This has resulted in the loss of hundreds of millions of dollars in local economies, threatening thousands of livelihoods that depend on the fishing and marine tourism industries. Along with the human health and economic implications, increased HABs and higher concentrations of domoic acid in the California Current could severely disrupt marine ecosystems and heighten the already significant threat of climate change on ecological communities in the ocean.
In order to be able to effectively predict, and thus mitigate, the effects of oceanic changes on HABs, it is essential to research into the implications of acidification and ocean warming on Pseudo-nitzschia. In the face of intensifying changes in the ocean’s physical and chemical systems, it is crucial to develop a better understanding of the relationship between environmental conditions and toxigenic blooms, for the sake of human health, the economy, and marine ecosystems.
References:
Hallegraeff, G. A review of harmful algal blooms and their apparent global increase. Phycologia 32.2, 79–99 (1993).
Ryan, J. et al. Causality of an extreme harmful algal bloom in Monterey Bay, California, during the 2014–2016 northeast Pacific warm anomaly. Geophysical Research Letters 44.11, 5571–5579 (2017).
Zhu, Z. et al. Understanding the blob bloom: Warming increases toxicity and abundance of the harmful bloom diatom Pseudo-nitzschia in California coastal waters. Harmful Algae 67, 36–43 (2017).