Red tide in Rio
From November, inumerable microscopic phytoplankton accumulated along the coast, tinting the clear blue waters a dark reddish brown. Flowering, known as red tide or harmful algae bloom event (HAVE, for its acronym in English), was unusually widespread and long-lasting.
Phytoplankton blooms are common at this time of year in Rio, but usually contain species that are beneficial to the ecosystem. On the contrary, the blooms of Harmful algae can appear at any time of the year, generally caused by sewage effluents and heat waves; they tend to be small and last no more than a few days. This red tide event covered more than 200 kilometers of coastline and lasted more than eight weeks. “It is very worrying”Said Priscila Lange, from the Department of Meteorology of the Federal University of Rio de Janeiro.
Some species in a red tide can produce toxins but, so far, those species have not been observed in the Rio bloom. Instead, Lange called the bloom “concerning “due to its likely impact on the marine food web.
From September to January of most years (spring and summer in South America), fresh, nutrient-rich water gushes from the depths of the ocean off Arraial do Cabo, replacing surface waters that have been pushed out to sea by the winds and the Coriolis effect. The abundance of nutrients and sunlight on the ocean surface triggers the proliferation of diatoms and other phytoplankton, which are soon consumed by zooplankton and fish larvae. Ocean currents push outcropping water masses westward toward the city of Rio de Janeiro, and Rio’s warm blue water generally turns cold and dark green in color.
Spring 2021 was different than most years. Lange and his colleagues believe that six weeks of cloudiness and rain hampered the regular growth of diatoms and small flagellates, leaving the river waters clear and full of nutrients. When the skies finally cleared in early November, abundant sunlight and low turbulence set the stage for the red tide. “Once there was light, the red ones (dinoflagellates, Mesodinium rubrum, etc.) bloomed like crazy”. Lange said.
The change happened fast. The first visual observations of the red tide were made on November 3 and later confirmed with water samples taken from a beach in Rio on November 16. The water on Rio’s beaches quickly became very dark and red foam built up.
At the beginning of December, the red tide reached Arraial do Cabo y “ocleared the waters of Rio’s most pristine diving paradiseLange said. Satellite images from December 5 show reddish-brown water running along the coast between the two cities.
By late December, the bloom was fading, but was still visible to the MODIS sensor on NASA’s Aqua satellite, which acquired these images on December 26, 2021. The bloom appears in the natural-color image (left) as a faint, dark eddy of water spreading from the shoreline. An even fainter patch is visible to the left of the eddy. Bloom is clearer in false-color image (right). In this view, shades of green represent concentrations of chlorophyll-a, the main pigment used by phytoplankton to capture sunlight. The darkest shades of green show the areas with the highest concentrations of chlorophyll.
Lange and his colleagues will continue to watch the flowering progress. But even after a massive bloom fades, the effects can be long-lasting. After phytoplankton die, the decomposition process by bacteria can deplete oxygen from the water (hypoxia) and cause the death of fish. Additionally, red tide species can replace other species of phytoplankton that generally support a region’s fish and marine food webs.
Images of NASA Earth Observatory by Lauren Dauphin, using MODIS data from NASA EOSDIS LANCE and GIBS / Worldview. Testo by Kathryn Hansen.
NASA Earth Observatory