Kelp populations at equatorward-range edges are particularly vulnerable to climate change as these locations are undergoing warming at or beyond thermal tolerance thresholds, the study published in the journal Nature January 23, 2023 found.
Due to this, the unique adaptive or evolutionary genetic diversity that the rear-edge populations (populations in warm, low-latitudes) may contain is also under threat due to rapid warming, according to Climate change threatens unique evolutionary diversity in Australian kelp refugia.
Kelp forests provide underwater habitats to hundreds of species of invertebrates, fishes, and other algae and have great ecological and economic value. Loss of kelp forests will also lead to a decline of the unique biodiversity that they support, the researchers wrote in the paper.
Ecklonia radiata, the dominant and most widely distributed Laminarian kelp in the southern hemisphere, rapidly succumb to warmer temperatures in spring and summer when temperatures exceed 27 degrees Celsius, they observed.
New populations were found in shallower and cooler winter months with temperatures around 20°C, the findings indicated.
“Contemporary climate change is threatening the high and unique genetic diversity found among eastern Australian low-latitude range-edge populations, with warming causing declines of E. radiata along this coastline,” read the study. “Projections suggest further declines and range contractions of kelp under future climate scenarios as has occurred in range edge E. radiata populations globally.”
The optimal mean annual temperature estimated during the last glacial maximum is 23°C, located between Rockhampton and Moreton Island.
E. radiata is putatively extinct in the Lord Howe where the temperatures, reaching up to 26°C, of the warm east Australian current exceed thermal tolerances in summer. The kelp species exists in isolated deep-water habitats in Norfolk Island where the temperature is slightly cooler (maximum 25°C).
Kelp can sometimes persist at lower latitudes, aided by cool water upwelling or in deep-water refugia where they are protected by thermocline (the transition layer between the warmer mixed water at the surface and the cooler deep water below), the study showed. “Identification of these refuge areas (a location which supports an isolated or relict population of a once more widespread species) is vital to ensure important genetic diversity is protected and to understand how extant and past climates shape species distribution and evolutionary diversity.”
There is high evolutionary diversity in these low-latitudes as many marine organisms were only able to persist within ice-free refuge areas at lower latitudes during the Ice Age, according to the report.
The authors of the study examined freshly collected samples of the kelp across the south-west Pacific and accessed specimens in herbariums of rare and extinct populations.
“While it is unlikely that in situ protection could halt declines of rear edge kelp populations under scenarios of warming, their unique genetic diversity could be protected and studied ex situ in culture banks for use in restoration, hybridisation or assisted adaptation strategies,” added the report.