A project undertaken at the University of Tasmania, and supervised by Dr Scott Bennett.
The Kelp forests cover 1/3 of the worlds coastlines and provide critical services that benefit ecosystems and people throughout temperate-polar regions worldwide. Australia’s kelp forest ecosystems are global hotspots for marine biodiversity and lie adjacent to around seventy percent of Australia’s population. Australian kelp forests support multi-billion-dollar industries in sectors such as fishing and tourism and are integral to the social and cultural fabric of the nation.
Climate change is undermining these benefits, leading to extensive loss of kelp from many regions, including Australia, where more than 100,000 Ha of kelp forest has been lost to marine heatwaves in the past two decades alone. The dramatic loss of kelp forests in recent decades is threatening Australia’s unique marine biodiversity and eroding the critical services kelp forests provide.
In response, emerging efforts are underway to try and restore kelp forests in several parts of Australia. Until recently there have been fundamental knowledge gaps about how ecological feedbacks within seaweed communities influence the resilience and restoration potential of kelp forests. For example, competitive interactions between established benthic communities and kelps that are planted for restoration, could undermine restoration efforts and waste valuable resources. In contrast, ‘finding the positives’, where established reef communities can facilitate the recruitment and long-term re-establishment of kelp would greatly benefit the efficacy and scaling up of restoration efforts.
The overarching aim of this project was to quantify the role of seaweed-seaweed interactions across natural environmental gradients to identify where reef communities may facilitate or impede the restoration of Australia’s kelp forests.
To address these questions, we conducted a suite of field experiments and surveys throughout south eastern Tasmania, in warm-affiliated kelp forests, giant kelp restoration sites and some of the last remnant giant kelp forests in Australia.
Macrocystis forest Actaeons
We found:
1) Strong competition between warm-affiliated kelp (e.g Ecklonia radiata and Phyllospora comosa) and giant kelp, Macrocystis pyrifera, preventing the recruitment, growth and survival of giant kelp. Targeted removal of Ecklonia and Phyllospora enabled successful recruitment and rapid growth of Macrocystis.
2) Abrasion from competing canopies and light availability were found to be the two most important factors limiting Macrocystis growth within competing kelp canopies. Temperature and nutrient availability, while critical for Macrocystis growth and survival, could not explain strong differences in performance between depths and canopy treatments.
3) Competing kelp species that were removed to enable the establishment of giant kelp, rapidly recruited and recolonized the cleared patches. Within 12 months of removal, cleared patches were indistinguishable from non-cleared patches, with the exception of giant kelp which grew quicker than the warm affiliated species and established themselves alongside Ecklonia and Phyllospora.
4) Biodiversity comparisons between natural remnant giant kelp forests and adjacent warm-affiliated kelp forests, revealed only small differences in fish and large invertebrate communities (Forbes et al. 2024). These findings suggest that warm-affiliated forests are largely fulfilling the function of giant kelp forests in terms of biodiversity maintenance. However chronic loss of giant kelp forests over recent decades, may mean that the reef assemblages that we are observing have already changed from historic baselines, when giant kelp forests were more abundant.
5) Productivity rates of giant kelp forests are over two orders of magnitude greater than warm-affiliated kelp forests. This is due to rapid growth and biomass production of giant kelp, relative to other species. Interestingly, the loss of giant kelp did not result in competitive release and increased growth rates or density of golden kelp.
6) Over the past 40 years, individual rates of giant kelp growth have declined whereas rates of Ecklonia growth and Phyllospora growth have remained relatively stable.
Giant kelp portrait, Port Davey, Tasmania. Stefan Andrews.
The combination of these changes and the massive loss in giant kelp forest cover along Tasmania’s east coast means that there has been a net loss of approximately 800 Tons of kelp production per day from Tasmanian reefs.
Collectively, this work has provided a step-change in understanding of the ecology of kelp forests in Tasmania and the ecological conditions required for restoration. Further, our work has uncovered the biodiversity baseline and composition of kelp forests in Tasmania, providing clear targets for restoration efforts and a novel understanding of what has been lost in recent decades.