Red Tide

A heavy Red Tide season across Southern California coastal waters has tinged the sea a brownish-red color.

The Southern California Bight—the body of water from Point Conception extending into Baja and including the Channel Islands—commonly experiences springtime phytoplankton blooms that result in a “Red Tide,” so named for the brownish-red tint the sea develops. This year, however, the Red Tide, particularly in San Diego and Orange counties, has been especially pronounced. While the upside to the phenomena are spectacular nighttime lightshows as breaking waves and boat wakes activate bioluminescence, the significant downsides are decreased oxygen levels and dying marine life including seabirds, fish and mammals.

A May 12 report from the Southern California Coastal Ocean Observing System noted “a fairly typical… spring phytoplankton bloom” from a robotic microscope deployed on a mooring that sits on the continental shelf offshore of Del Mar, where an upwelling of deeper water delivers nutrients to the well-lit surface layer. But March rains brought a 200 to 400 percent increase to normal precipitation rates for the season.

“In early April,” the report states, “as the rains subsided, the freshwater likely contributed to making the surface waters more hospitable to L. polyedra, an alga that is known for preferring ‘stratified’ conditions when the surface layer is less saline/dense than the bottom layer.” Low wind conditions in early April made good conditions for a “widespread L. polyedra bloom that can be seen in satellite imagery extending from Los Angeles to Baja (with reports as far south as Islas de Cedros),” the report states.

“We now know,” the report’s authors Clarissa Anderson and Megan Hepner-Medina wrote, “that we were seeing the highest cell numbers of L. polyedra ever recorded at Scripps Pier with nine million cells per Liter on April 27 (the previous maximum was just under one million cells/L).” A contributing factor to this year’s intense Red Tide event may be the “prolonged marine heatwave since 2015 that has never really gone away,” the report suggests. “Anomalously warm surface temperatures are common in the region and might be contributing to the spate of ‘Red Tides’ we have seen in the last few years.”

In Ventura and Carpinteria, surfers have reported sinus inflammation after being in the ocean in recent weeks. Although typical Red Tide events are not considered a health hazard, the phenomena causes many ocean users to feel ill. Another contributing factor to the heavy Red Tide may be fertilizers and other nutrients that washed into the sea during the unseasonably heavy rains of March: “We cannot discount the effect nutrients from land could have in this situation,” the authors note. “Nitrate (anthropogenic and natural) and phosphate as well as regenerated forms of nitrogen, such as urea and ammonium, could have helped to keep this bloom cranking, especially as waters warmed and stratification set in.”  

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