News of a “rust tide” in Buzzards Bay made headlines earlier this week. Reading a handful of the stories left me with an incomplete and jumbled picture of what’s going on. Is "rust tide" just another way of saying red tide? Is it or isn't it "dangerous," as some headlines proclaimed? To get some answers, I got in touch with Dr. Don Anderson, a harmful algae expert at Woods Hole Oceanographic Institution. Here’s the skinny:
WHO
Terms like "rust tide" or "red tide" aren't scientists' favorites. At best, they're rather uninformative. At worst, they can be misleading. After all, there are plenty of red-tinted algae out there that are completely benign ("red tide" tends to connote toxicity of some variety) and plenty of highly toxic algae whose blooms are completely invisible. To figure out what (or rather, who) you're dealing with, you need a species name. In this case it's Cochlodinium polykrikoides, a photosynthetic dinoflagellate – a single-celled alga that makes a living on sunlight. It’s not a plant, but it acts a lot like one.
WHAT
Blooms of Cochlodinium polykrikoides are known for causing mass fish mortality (a.k.a. fish kills) that can have major economic impacts in areas supporting commercial fisheries. Shellfish can also be affected. How Cochlodinium polykrikoides works its dark magic is still a matter of some debate and study.
Deadly oxygen depletion, known as hypoxia (low oxygen) or anoxia (no oxygen), can be a secondary effect of any dense algal bloom. When the algal cells die and sink to the bottom, they become food for other microbes (mainly bacteria) who consume oxygen, along with the algae. Cochlodinium polykrikoides puts an added twist on things by producing copious quantities of mucus which also has to be broken down, causing the scavengers to hoard even more precious oxygen.
That mucus has also been suggested as a more direct culprit in fish mortality. Some think it clogs the gills of fish, thereby suffocating them.
But that’s not all. Recent studies suggest that Cochlodinium polykrikoides produces reactive oxygen species. Hydrogen peroxide is an example of a reactive oxygen species, so that gives you an idea of what they can do. In this case, it appears not to be hydrogen peroxide, per se, but another as-yet-unspecified reactive oxygen species that contributes to fish and shellfish mortality.
What Cochlodinium polykrikoides does not appear to do is poison humans. So that’s the good news.
WHEN and WHERE
Cochlodinium polykrikoides is found all over the world in tropical and warm-temperate waters. Some local observers have suggested that Cochlodinium polykrikoidesblooms are a new or increasing phenomenon here in southeastern Massachusetts. But Dr. Don Anderson, a harmful algae expert at Woods Hole Oceanographic Institution, says that such blooms have been common occurrences for decades and possibly longer, and there’s no hard data to support the idea that todays blooms are worse than they used to be. That said, on a global scale, there is some evidence that the pestilent species has been expanding its range and/or blooming more intensely in recent years.
WHY
What could drive such expansion or intensification?
Nutrient pollution is the explanation that has been most widely invoked locally. There’s no denying that the Cape and Islands have a serious nitrogen problem that stems from our heavy reliance on septic systems. And, as any gardener knows, nitrogen is plant food – a fertilizer. So it’s a logical place to go, and global increases in nutrient inputs to coastal waters is one possible factor contributing to the rise of Cochlodinium polykrikoides.But there’s currently no data that directly links nutrient levels to blooms here in Massachusetts.
Climate change is another possibility. As previously noted, Cochlodinium polykrikoidesis a warm-water species, so increasing ocean temperatures could favor it. Other climate-related changes in ocean conditions (currents, salinity, etc.) could also come into play. Again, there’s no data. This is just speculation.
Finally, Cochlodinium polykrikoidesmay be simply taking advantage of the opportunities afforded by global shipping. Many cargo ships carry water as ballast. That water is taken on in one port and dumped out in or near another. Ballast water has been the mode of transport for countless introduced species; Cochlodinium polykrikoidesmay be an example.
