Center for Integrated Marine Technologies: Harmful Algal Blooms
- Mary Silver
University of California, Santa Cruz
End Date: July 31, 2008
Single-celled toxic algae periodically undergo an extreme period of growth, creating what is called a harmful algal bloom (HAB). In Monterey Bay and other coastal waters, the toxins produced by these tiny organisms are transferred up the food chain, sometimes poisoning seabirds and marine mammals or closing the take of commercial species (e.g., shellfish). However, not all blooms are toxic, and even species that can produce toxins do not always do so, making the timing and location of HABs variable.
In the last decade one particular species has received a great deal of attention in Monterey Bay. Pseudo-nitzschia australis is a chain-forming diatom which had not been recognized on the west coast until the early 1990s. It blooms in Monterey Bay from late spring to early fall and can produce domoic acid, a potent neurotoxin that can cause neural damage, disorientation, short-term memory loss and even seizures and brain damage in vertebrates.
Domoic acid is readily passed up the food chain. The toxic phytoplankton is consumed by small fish and zooplankton, which are in turn eaten by larger fish, seabirds, mammals, and even humans. Domoic acid can become concentrated in filter feeding animals such as mussels. Contaminated mussels can cause serious illness and even death among humans. Domoic acid also killed 100 brown pelicans and cormorants in Monterey Bay in 1991 and 50 sea lions that washed up along the coast of California from San Luis Obispo to Santa Cruz in 1998.
Dr. Mary Silver and other CIMT scientists are in the process of characterizing the food base of the Monterey Bay ecosystem. By studying the population structure of these potentially toxic species, the researchers hope to gain a better understanding of the mechanisms that control the distribution and toxicity of these HABs.
Summary to DateDomoic acid was produced in 1991 by a species not previously known to the west coast. The toxin is vectored through the food chain to birds and mammals, which often become sick and can die in large numbers. The species responsible for the 1991 bloom was eventually identified as Pseudo-nitzschia australis, one of the few diatoms to produce a toxin. This species forms large chains making it available directly to herbivores, even birds and mammals.
There are 15 species in the Pseudo-nitzschia complex, and it is very difficult to distinguish species, requiring the examination of microstructures with a powerful scanning electron microscope. However, recent advances in gene probe technology now allow scientists to recognize species within one day.
Two of the 15 species are known to produce domoic acid, but toxicity is highly variable and may be dependent on nutrient conditions. Anchovies and krill that feed directly on the diatoms have high levels of domoic acid. Seizuring sea lions, which often feed on anchovies, are indicative of toxic blooms. And while whales consume krill and have high toxin levels in their feces, it is not clear how they are affected by the toxin.
- During 7 of 9 CIMT cruises (up to October 2003) Pseudo-nitzschia australis densities were measured at potentially dangerous levels.
- Spatially the HABs are variable, with no focal point of populations of the toxin-producing species. However, in the one month where a full data set is available (March 2003) there did seem to be a gradient in cell toxicity, with the northern and offshore portions of the bay having cells that contained higher domoic acid concentrations.
- There are multiple toxic species present in Monterey Bay. In addition to the diatoms that can produce domoic acid, there are also dinoflagellates that cause Paralytic Shellfish Poisoning (PSP) and another group of dinoflagellates that cause Diarrhetic Shellfish Poisoning (DSP), which promotes tumor growth in some laboratory animals.
- It remains unclear why the toxins are produced by the algal cells, although it may inhibit grazing by herbivores, be a stress-induced response, or serve some metabolic function that has not yet been determined.
- Habitat association
- Migration/movement patterns
- Other pollutants
Figures and Images
Figure 1. The chain-forming diatom Pseudo-nitzschia australis.
Figure 2. Identifying the toxic species is difficult and until recently relied on SEM images that reveal cell wall fine-structure. Methods have now been developed to use genetic probes to rapidly identify species. (Source: P.E. Miller)
Figure 3. Spatial distribution and relative density (circle diameter) of Pseudo-nitzschia blooms in November 2002.
Figure 4. In March 2003, scientists re-sampled the same stations from November 2002. Note the differences in density of Pseudo-nitzschia.