NAAWAN, Misamis Oriental (MindaNews / 1 October) – A bright sun at daytime and an evening of heavy rain is a perfect weather, prevailing nowadays, to trigger red tides in some coastal waters.
In fact, red tide warnings have already been issued recently by the Bureau of Fisheries and Aquatic Resources (BFAR) in some waters in Negros Occidental, Bohol, Northern Leyte, Samar, and Surigao del Sur.
Red tide is a common name for algal bloom. The bloom is comprised of microscopic algae and other planktonic organisms that are so densely concentrated and spread far and wide on the water surface. This dense community of plankton drifts with the current and is so named as red tide for its rusty brown hue, which becomes more manifest especially when experiencing a mass die-off.
Planktons, that include both phytoplankton and zooplankton, are the primary producer in the marine food chain.
Red tide events actually occur from the interplay of factors that include a rain and shine weather, low salinity, a calm sea, warm ocean surface temperatures, the presence of red tide related algae in the waters, and a nutrient-rich landscape nearby.
This is how a red tide develops. There’s a downpour in the evening that creates nutrient-rich runoff, say, from settlements, industries and/or from crops and livestock farms. The heavy rain is followed the next day by a dry and hot weather. Under the bright sun, the plankton feeding on the nutrients from runoff replicate exceedingly or make a massive bloom. At nighttime, a sudden die-off may occur from depletion of oxygen due to high and competing demand.
Moreover, a placid sea with no strong wind and waves to break its surface exacerbates dissolved oxygen depletion, and contributes to a mass plankton die-off. The next day, if the weather remains sunny and the sea unperturbed, the plankton community continues to bloom and grow wide, and spreads and drifts with the current often towards much shallower waters. A huge bloom may wreak havoc to marine lives from toxins emitted by certain algae and by the mere cover of the bloom that may deplete the DO in reefs and estuaries, and those in fish cages, oyster and even seaweed farms.
The putrid smell of the waters dirtied by decomposing plants and animals may spoil beach resort activities.
Among the organisms in large number in a red tide are the single-celled dinoflagellates. Many dinoflagellates are known to be photosynthetic, but a large fraction of these are in fact mixotrophic, species that combine photosynthesis with ingestion of prey in capturing energy; thus, they can be among the most prolific of the plankton organisms.
Some of these dinoflagellates produce toxin that may accumulate in marine animals and may seriously harm humans if ingested. Individually or as separately found in the ocean, this alga may not produce sufficient amount of toxin to cause harm to marine lives and humans.
In a massive bloom, however, millions of these tiny organisms may together produce an amount of paralyzing neurotoxin that prevents fish and other marine animals from respirating. For instance, in the US summer of 2018, the red tide in the southwestern Gulf Coast of Florida, claimed the lives of thousands of fish, hundreds of sea turtles, and dozens of manatees and dolphins, including a 26-foot whale shark whose body was riddled with neurotoxin produced apparently by a dinoflagellate, known as Karenia bravis.
Meanwhile, the BFAR recently informed public that a toxin-producing dinoflagelate, Pyrodinium bahamense, had been sampled in the coastal waters of northern Leyte and Samar and warned the possible occurrence of red tides in the area. BFAR advised the public to refrain from gathering, selling, and eating all types of shellfishes and acetes sp. locally known as alamang or hipon from said places.
Already, two siblings, aged three and eight years old were reported on September 15, 2020, to have died in Daram, Samar, while four other family members were hospitalized due to alleged paralytic shellfish poisoning (PSP) by eating red tide-infested green sea mussel, or “tahong”.
PSP occurs after ingesting shellfish, such as mussels, oysters, and clams that contain red tide toxins. These toxins can cause severe and life-threatening neurological issues.
No science-based measures have yet been developed and promoted to stop and avoid the occurrence or contain the conflagration of red tides.
Apparently, only an adverse weather, such as a gale or typhoon may break, disperse and drive a red tide away from shore or shallow waters. A dispersal of the drifting bloom may degrade the amount of toxin and may no longer pose a risk to marine lives.
We may yet prevent or lessen the adverse impact of red tides by adopting the ways of nature. Huge blowers may be fabricated and installed in coast guard vessels to blow and disperse red tides away from coastal waters. Water cannons may also be used for the same purpose.
Perhaps, aqua farmers may also be able to protect their crops from the drifting bloom by building protective floating barriers, like the one used in controlling oil slick, some distance away from their oyster farm, fish cages and pens. (MindaViews is the opinion section of MindaNews. William R. Adan, Ph.D., is retired professor and former chancellor of Mindanao State University at Naawan, Misamis Oriental, Philippines.)