The Belgians have the culinary tradition of eating mussels with French fries; it is all a matter of taste. In Catalonia we serve them alone, in sauce, or in paellas or other equally delicious dishes. Mussels, apart from being a gastronomic delight, available to everyone, are incredibly efficient filtering machines. In their daily activity, they can filter and clear of plankton near 200 liters of seawater per day. And this is where the problems begin. Plankton are made up of a multitude of organisms either plants (phytoplankton) or animals (zooplankton) that rarely pose any danger to humans (I encourage you to consult the post a teaspoon of seawater, a miniature ecosystem). However, certain groups of phytoplankton are toxic. These include many dinoflagellated algae and a few species of diatoms and cyanobacteria. At the concentrations we find, nothing happens if we accidentally swallow a sip of seawater. However, if they are concentrated by a filter-feeder, such as mussels or oysters, or if they have been climbing the food web and bio-accumulated in fish, they can be harmful to humans, and even cause death.
What are the so-called red tides? They are nothing more than accumulations of unicellular algae, usually toxic, in a certain area, of a greater or lesser extent. Many of these algal blooms indeed have a reddish coloration, but there are several other colors, such as brown or green, which makes it strange to call them red tides when the water is, for example, green. Therefore, to be more precise, scientists have long adopted the term harmful algal blooms (HABs). These proliferations differ from the typical spring blooms (see post the four seasons of plankton) because they usually adversely affect the ecosystem or at least one or another organism, including us.
Why do HABs occur? Well, unfortunately, their origin is not clear. Whether by physical accumulation in semi-confined areas, such as ports or estuaries, promoted by sea currents, by exaggerated growth of some species that can suddenly take advantage of the opportunity of ideal conditions, or by the release of new cells from cysts accumulated in the sediment, the fact is that more or less every summer (and even some winter) we have on our shores some of these proliferations. What is certain is that human activity contributes to the increase of these phenomena. On the one hand, global warming is speeding up its growth rates. On the other hand, marinas act as centers of aggregation and initial seed for proliferations. The release into the sea of excessive nutrients from city sewage, agricultural activity, etc. gives the food to these algae and also promotes HABs. All in all, we are helping to create growing broths suitable for HABs. Fortunately, some of these issues (such as sewer efficiency and control over agricultural fertilizer) are currently being addressed and the results are promising.
How do they affect marine and non-marine life? There are many types of toxic algae and toxins. Some directly attack mollusks and fish (among other organisms), causing massive deaths in nature and aquaculture facilities. Others, such as the diatom Pseudonitzschia , which produces a potent neurotoxin (domoic acid), can drive animals crazy; Remember Alfred Hitchcock’s “The Birds?” It is believed that the famous director was based on a genuine case of seagull poisoning by this diatom after eating contaminated fish. There are also cases of sea lions affected by this toxin that act like zombies. Other HABs affect the food web from its base, acting on zooplankton and other algae species. Some are harmless, and their effects are simply aesthetic and harmful only to tourists looking for crystal clear water on the beaches. The worst, however, from a human point of view, are those that go more or less unnoticed through the food web and that, ultimately, reach us. From the well-known summer diarrhea, vomiting, convulsions, to even death, are the range of effects of these organisms on our species. A rather curious, and unfortunate, case happened in a laboratory in North Carolina in 1993 where two scientists who cultivated Pfiesteria, a toxic dinoflagellate attributed to fish mortality, were progressively showing symptoms of memory loss and disorientation. They never completed their study, and to date, they still have problems in their cognitive abilities.
As you can see, the risks from the proliferation of toxic algae are considerable. Fortunately, regular monitoring programs have been established in places that may be affected by these algal blooms, and especially near aquaculture facilities. Monitoring today comprises a handful of experts who look at water samples under a microscope and identify the usual suspects. Surely, soon this will be done automatically with machines that do real-time molecular or chemical analysis. But for now, we have to trust our experts.