In March 2021, the remains of at least 260 marine animals from 81 species were discovered as a result of a
mysterious phenomenon when giant marine animals
washed ashore in South Africa. Scientists speculated that this event might be linked to a rapid drop in
temperature caused by the upwelling of deep, cold water along the coastline, a process known as
coastal upwelling.
Climate change is causing ocean temperatures to rise, yet at the same time, coastal upwelling is intensifying,
potentially impacting marine megafauna. Recent studies have focused on two rapidly warming boundary currents:
the Agulhas Current off southern Africa and the East Australian Current. Researchers aimed to investigate
whether climate-induced changes in upwelling zones are leading to more frequent and severe cold shock events
that could push sharks beyond their thermal limits.
To assess the impact of upwelling on the 2021 marine die-off, scientists analyzed sea surface temperature data
from satellites and hourly temperature records from temperature loggers. They discovered that in March 2021,
sea surface temperatures along the coast plunged by up to 7°C (12°F) within just 48 hours—from 24°C (75°F)
to below 17°C (63°F). Subsurface temperatures dropped dramatically as well, from 21 °C (70 °F) to 12 °C
(54 °F), a decrease of more than 9 °C (16 °F) in just 24 hours.
Bull sharks begin to experience physiological stress at temperatures below 19°C (66°F), close to their
thermal limits. This led researchers to hypothesize that during the March 2021 cold shock event,
bull sharks were subjected to temperatures several degrees below their tolerance limits—possibly
exceeding 5°C (10°F) below.
To gather more data, researchers deployed acoustic and satellite transmitters on 66 bull sharks:
41 in the Agulhas Current and 25 in the East Australian Current. Acoustic tags recorded the timing of
sharks passing underwater receivers, while satellite tags continuously monitored the water temperature and
depth experienced by the sharks. This allowed scientists to compare the behaviors of sharks in warm,
subtropical waters with those in colder regions.
The research indicated that prolonged low temperatures hindered physiological recovery in sharks and other
marine megafauna. Although some sharks attempted to swim to shallower waters in search of warmth,
the cooling occurred at a pace quicker than their ability to adapt.
Researchers also examined wind records from the South African Weather Service to see if conditions
favorable for upwelling preceded the 2021 event. They documented that preceding the event, strong easterly
winds had pushed cold water to the surface along the coastline. The simultaneous occurrence of cooling,
strong winds, and animal mortality led researchers to conclude that upwelling was the key factor behind
the die-off.
Lastly, by analyzing 41 years of sea surface temperature data and employing mathematical modeling, researchers
determined changes in the frequency and intensity of rapid cooling events. Their findings revealed that these
events can persist much longer than the typical duration of one to three days; the March 2021 event lasted
a full seven days.
The study indicated an increase in both the frequency and intensity of these cold shock events over recent
decades, resulting in more pronounced temperature drops than historically recorded within a 24-hour period.
This trend suggests that the 2021 event might be part of a broader pattern, wherein warming oceans facilitate
species migration, but intensified upwelling suddenly exposes them to lethal cold conditions—a phenomenon
referred to as climate bait and switch.
The researchers concluded that climate change may enhance coastal upwelling, pushing marine megafauna beyond
their thermal thresholds, even in scenarios where initial warming conditions seem advantageous. Their
findings shed light on a crucial yet underexplored facet of climate change regarding temperature fluctuations
and their implications on species survival, ultimately aiding predictions about how marine ecosystems will adapt
to changing ocean conditions in the future.
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Source: sciworthy.com