Decline of Kelp Forests Due to Climate Change

Kelp forests, underwater ecosystems formed by the dense growth of several different species known as kelps, are declining due to climate change, according to a new study published in the journal Nature. The study found that kelp populations at equatorward-range edges are particularly vulnerable to climate change as these locations are undergoing warming at or beyond thermal tolerance thresholds.

Threat to Unique Adaptive or Evolutionary Genetic Diversity

• Due to the vulnerability of equatorward-range edges, 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.
• 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.

Impact on Dominant Kelp Species

• The study found that 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. New populations were found in shallower and cooler winter months with temperatures around 20°C.
• “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.

Future Projections and Range Contractions

• The study also projected further declines and range contractions of kelp under future climate scenarios, similar to what 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).

Identification of Refugia

• 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 that 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.

High Evolutionary Diversity in Low-Latitudes

• 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.