Red tide off Rio – “It’s very worrying”

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26 December 2021. Natural color image from NASA’s Aqua satellite.

A dark, rainy spring gave way to a huge, long-lived phytoplankton bloom off the coast of Brazil.

Beachgoers in the Brazilian state of Rio de Janeiro struggled with unwanted sea creatures in late 2021. From November onwards, countless microscopic phytoplankton accumulated along the coast and turned the clear, blue water a dark reddish brown. Known as the Red Tide or HAB (Harmful Algal Bloom), the bloom was unusually widespread and long-lived.

Phytoplankton blooms are common in Rio at this time of year, but they usually contain ecosystem-beneficial species. In contrast, harmful algal blooms, usually triggered by runoff and heat waves, can occur at any time of the year. They are usually small and don’t last more than a few days. This red tide event spanned more than 200 kilometers of coastline and lasted more than eight weeks. “It’s very worrying,” said Priscila Lange of the Department of Meteorology at the Federal University of Rio de Janeiro.

Red Tide Rio 2021 Chlorophyll commented

26 December 2021. False color image from NASA’s Aqua satellite.

Some species in a red tide can produce toxins, but so far these species have not been observed in the Rio bloom. Instead, Lange called the bloom “of concern” because of its likely impact on the marine food web.

From September to January of most years (spring and summer in South America), cool, nutrient-rich waters well up from the ocean depths off Arraial do Cabo, replacing surface water pushed offshore by winds and the Coriolis effect. The abundance of nutrients and sunlight at the ocean’s surface triggers blooms of diatoms and other phytoplankton, which are soon consumed by zooplankton and fish larvae. Ocean currents push the rising waters west toward the city of Rio de Janeiro, and the warm, blue waters off Rio usually turn cold and dark green.

Spring 2021 was different than most years. Lange and colleagues believe six weeks of cloud and rain stunted the usual growth of diatoms and small flagellates, leaving the water off Rio transparent and full of nutrients. When skies finally cleared in early November, abundant sunlight and little turbulence set the stage for the red tide. “Once there was light, the red ones – dinoflagellates, Mesodinium rubrum, etc. – thrived like crazy!” said Lange.

The change was quick. The first visual observations of the red tide were made on November 3rd and then confirmed by water samples taken on November 16th from a Rio beach. The water off the beaches of Rio quickly turned very dark and red sea foam formed.

In early December, the red tide hit Arraial do Cabo and “darkened the waters of Rio’s pristine diving paradise,” Lange said. satellite imagery from December 5 show the red-brown water stretching along the shoreline between the two cities.

In late December, the bloom faded but remained visible to the Moderate Resolution Imaging Spectroradiometer (MODIS). ">NASA‘s Aqua satellite, which captured these images on December 26, 2021. The bloom appears in natural color in the image (above) as a faint, dark swirl of water extending away from the shore. An even fainter spot is visible to the left of the vortex. In the false color image (second image), the bloom is clearer. In this view, shades of green represent concentrations of chlorophyll-a, the primary pigment used by phytoplankton to capture sunlight. The darkest shades of green show areas with the highest concentrations of chlorophyll.

Lange and colleagues will continue to monitor the flowering process. But even after a massive bloom has faded, the effects can be permanent. After the phytoplankton die, the decomposition process by bacteria can deprive the water of oxygen (hypoxia) and cause fish kills. Also, red tide species can replace other phytoplankton species that normally support a region’s fish and marine food webs.

NASA Earth Observatory images by Lauren Dauphin using MODIS data from NASA EOSDIS LANCE and GIBS/Worldview.

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