Cause of 2015 Toxic Algal Bloom in Monterey Bay Identified
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SEAFOODNEWS.COM [Monterey Herald] by Tommy Wright – June 7, 2017
Monterey – Upwelling caused the toxic algal bloom that poisoned large numbers of marine animals and led to the closure of commercial fisheries in Monterey Bay in 2015, but a research paper published Monday shows an imbalance between two nutrients may have caused high toxicity levels.
The bloom, considered the most toxic ever observed in the bay, happened in late spring 2015, when scientists from Monterey Bay Aquarium Research Institute, UC Santa Cruz, Moss Landing Marine Laboratories and the National Oceanic and Atmospheric Administration were conducting a large-scale biology experiment in the bay called Ecology and Oceanography of Harmful Algal Blooms.
“It was a great coincidence and what it allowed us to do was to use the technologies that we love to use and to apply to see what’s going on,” said John Ryan, an MBARI oceanographer and lead author of the research paper. “That included these autonomous underwater vehicles that can not only map the environment and the distributions of the toxic algae at very high resolution, they can use their onboard sensors to take samples from the most dense bloom patches so we really know the what the extent of the most toxic populations is.”
The scientists also used environmental sample processors, which Ryan called “basically a laboratory in a can,” anchored at the northern and southern end of the bay. The processors take water samples, break open the algae cells and look at the DNA.
“That identified that one species (Pseudo-nitzschia australis) almost completely dominated this bloom,” Ryan said. “There are some 40 species of Pseudo-nitzschia, but it was this one that’s particularly toxic that dominated the bloom completely.”
While researchers considered unusually warm surface water in the Pacific Ocean a factor in the bloom, which stretched from Central California to the Alaska Peninsula, water in the Monterey Bay wasn’t unusually warm. Upwelling takes place when strong northwest winds move surface water away from the shore, allowing cold water, rich in nitrate, silicate and other nutrients, from deep in the ocean to rise to the surface.
“What was surprising is the warm anomaly had persisted already over a year, this ‘warm blob,’ was completely eliminated locally because of that upwelling of cold, nutrient-rich water,” Ryan said.
The warm water allowed the algae to bloom farther north than normal, which helped cause what Ryan called “the largest spacial scale over which marine mammals had ever been observed to be poisoned by this type of bloom” in the Northeast Pacific. Strong upwelling in Monterey Bay initiated the bloom locally and several milder events allowed the algae to persist and accumulate.
Pseudo-nitzschia australis is a regular inhabitant of Monterey Bay, but the bloom in 2015 contained especially high levels of domoic acid, which led to closures of anchovy, sardine, shellfish and crab fisheries. Algae diatoms need nitrate for biochemical processes, including the production of domoic acid. The diatoms need silicate to grow and reproduce. The researchers concluded the extremely high levels of domoic acid were caused by a low ratio of silicate to nitrate in Monterey Bay.
“This wasn’t a sudden occurrence in 2015, it accompanied the warm blob,” Ryan said. “So even though the temperature itself might not have had a direct effect, it may have had an indirect effect through its influence on ocean chemistry.”
According to Ryan, one of the key questions in understanding, predicting and preventing algal blooms is figuring out if humans have a role in the frequency or the severity of these events.
“We know that when we affect the nutrient chemistry of the coastal ocean, we can influence what types of microscopic algae bloom,” he said. “This very same species can be made more toxic when it’s exposed to urea, which can enter the coastal ocean from wastewater outflow. So we know that’s one example where we have to be careful about how we affect coastal ocean chemistry because it can come back and bite us. But in these case, it appears to be of much more natural, large-scale change in ocean chemistry that lasted at least as long as the warm blob. And we need a little more time of observation to determine if it’s fully returned to normal, or if this is part of a longer-term change in ocean chemistry that could promote more frequent or more severe toxic events.”
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