By Brittany Hook / Scripps – UCSD / April 25, 2024
In the spring of 2020, a historic red tide event occurred in waters off Southern California. Driven by a bloom of the dinoflagellate Lingulodinium polyedra, this event brought nighttime displays of bioluminescence to beaches along the coast, from Baja California to Santa Barbara. While the bloom gained international attention for its stunning visual displays, it also had significant negative impacts, including mass mortality among fish and other marine organisms.
Marine scientists from UC San Diego’s Scripps Institution of Oceanography, National Oceanic and Atmospheric Administration (NOAA), and other organizations seized the opportunity to study the unprecedented event and its impacts on marine life, both in the wild and in aquariums. In a recent study, this multi-institutional team of 34 researchers identified deteriorating water quality — notably prolonged low oxygen conditions — as a significant factor in the mass mortality event. Their findings were published in the journal Elementa, and featured in a special issue focused on the 2020 red tide.
“Red tides are notoriously difficult to predict and study, so the 2020 event presented a unique opportunity for the marine science community to collaborate and use existing infrastructure to figure out what was causing these die-offs,” said lead author Zachary Skelton, a marine biologist under contract at NOAA’s Southwest Fisheries Science Center (SWFSC) and recent graduate of Scripps Oceanography. “Our paper provides valuable insights into how low- or no-oxygen conditions can harm certain marine species, particularly those inhabiting estuaries and shallow coastal areas.”
The study was informed by data from existing long-term observation programs, alongside reports from community members, field surveys, and case studies from local aquariums that use local seawater to sustain their systems. The researchers found a correlation between extreme changes in water conditions, notably dissolved oxygen and pH levels, and the dynamics of the red tide bloom.
Sensor data along the coastline revealed extreme hypoxic (low oxygen) and hyperoxic (excess oxygen) conditions, especially in estuaries and near-shore coastal environments. These conditions impacted the health of numerous marine species, leading to the documented deaths of thousands of fishes and invertebrates, as well as several seabirds. Case studies from four local aquariums — including Birch Aquarium at Scripps, as well as two experimental aquariums at Scripps Oceanography and one at NOAA SWFSC — indicated that factors beyond large changes in oxygen and pH levels might have contributed to the observed mortality.
Since the red tide coincided with the early stages of the COVID-19 pandemic, the team had to obtain special permission from UC San Diego and other local institutions and authorities to conduct fieldwork. Once these permissions were secured, Skelton, along with Ben Frable, collection manager of marine vertebrates at Scripps Oceanography, visited multiple beaches and estuaries around San Diego to count, photograph, and measure the dead fish and invertebrates.
Their observations and community reports — shared via social media, email, and the citizen science platform iNaturalist — revealed mass die-offs of fish and invertebrates between late April and mid-May 2020, following sharp drops in oxygen levels. Fish die-offs were disproportionately reported in the La Jolla area (including La Jolla Cove, La Jolla Shores Beach, and Scripps Beach), the Encinitas area, and Agua Hedionda Lagoon in Carlsbad. Many of these locations are enclosed or geographically sheltered areas, which limits mixing and makes it easier for blooms to concentrate.
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