Seaweed prospers with kelp from its friends

Adult kelp seaweed engineers its environment to optimise conditions for juvenile species members, Australian researchers have established.

The finding, published in the journal PLOS One, provides valuable insight into the conditions that lead to the collapse of kelp forests, which are recognised as critical structures in many marine ecosystems.

Kelp forests are estimated to provide abundant habitats for a diverse range of species along about 25% of the world’s coastlines, especially in colder waters. Recent research found that threats to their survival are increasing in number and severity, with total forest areas decreasing by roughly 2% each year.

Much of the global research effort aimed at understanding the decline is focussed on external stressors, such as human activity and the effect of invasive species. 

To provide a more detailed picture, a team led by Cayne Layton from the University of Tasmania opted to study the internal dynamics of kelp. In particular, the researchers wanted to investigate how the complex dynamics of patch size and density affected rates of growth and the health of younger members.

To do this, they constructed a series of artificial reefs off the east coast of Tasmania, in a region where endemic kelp forests have been badly hit by both climate change and an explosion of invading sea urchins. They populated the reef structures with the region’s most abundant kelp species, Ecklonia radiata.

Observing the results, Layton and colleagues found that adult kelp engineer their immediate environment by altering water flow, sedimentation and the amount of sunlight in the lower reaches of the patch. The new conditions create optimal conditions not only for the kelp’s own survival and health, but also for the survival of microscopic and macroscopic kelp juveniles.

The researchers found that this ability to sustain and promote robust growth was critically dependent on the overall size of the kelp patch and – to a lesser but still significant extent – its density. Reduction in either value negatively affected the recruitment and survival of juveniles.

This in turn, the scientists reported, triggered a cascade of unfortunate events, “causing breakdown of positive intraspecific feedback and collapse of demographic functions, and overall, leading to reductions in ecosystem stability and resilience well before local extirpation”.

“With fewer juveniles to replace adults and repair damage to the patch, the resilience and stability of kelp forests was further eroded,” explains Layton.

“Even when we introduced healthy juveniles into a degraded patch of kelp, the absence of sufficient adults to provide shelter and a favourable environment resulted in those juveniles dying and led to the collapse of the patch.

“As previous research has identified, this downward spiral caused by the degradation of kelp patches can lead to a shift from these complex and highly diverse kelp forest ecosystems to habitats without kelp that that are simpler, less diverse and less productive.”