Mortality of marine organisms in cold upwelling events. Image source: Nature Climate Change
A study published in Nature Climate Change on the 15th revealed that the frequency and intensity of extreme cold upwelling events, which lead to large-scale mortality of various marine organisms, are increasing. This may make migrating species more vulnerable to adverse effects of such climate events.
While the impact of heatwaves on marine species has been extensively studied, research on the effects of extreme cold events is severely lacking. Climate change-induced alterations in ocean currents and pressure systems have been linked to the strengthening of upwelling (the process by which deep cold water is brought to the surface). However, it has long been unclear how such events affect the distribution, movement, and survival of marine species.
In this study, a team from James Cook University in Australia analyzed mortality events of over 260 species of marine organisms from 81 different taxa along the South African coast in 2021. Additionally, they examined the frequency and intensity of cold "lethal events" nearshore of the Agulhas Current in the Indian Ocean and the East Australian Current over the past 30 years, using 41 years of sea surface temperature data and 33 years of wind records. Subsequently, using bull sharks as a case study, they linked these events to species migration and survival.
The research team associated the marine mortality events of 2021 with a particularly strong cold upwelling, highlighting how such events can impact a plethora of species occupying warm ecological niches. Further analysis revealed that from 1981 to 2022, the frequency and intensity of such events in these regions have been on the rise. Tagged bull sharks only appeared in colder fringe zones during the warm season, consistent with their avoidance of waters below 19°C. However, these sharks also employ behavioral strategies to minimize the risk of temperature drops, such as swimming to the surface near upwelling regions and increasing habitat use in bays or estuaries. The researchers suggest that the increased risk of upwelling events could lead to "induced shifts," where species ranges expand with warming oceans but are simultaneously exposed to sudden extreme cold events in these new areas.
The team believes that migrating species or those nearing their long-term thermal limits are particularly sensitive to rapid temperature changes caused by increasingly frequent extreme climate events.
