Phase shift from kelp bed to turfing algae increases with multiple stressors, and represents alternative stable states

Chair: Heidi Pethybridge

Craig R Johnson(1), Simon Wotherspoon(1), Sean D Connell(3)

1 Institute for Marine and Antarctic Studies, University of Tasmania, Private Bag 129, Hobart, Tasmania 7001, Australia.
2 School of Earth and Environmental Sciences, University of Adelaide, DX650 418, South Australia 5005, Australia.

Background
The transition from kelp-beds to turfing algae on shallow temperate reefs is increasingly reported around the globe, and represents a significant loss of biodiversity and ecosystem services. Effective management of this issue requires knowledge of:

  • Whether anthropogenic stressors influence the likelihood of the shift from a kelp- to turf-dominated state; and …
  • Whether the phase shift is continuous in response to a changing environment or instead represents a discontinuous shift with hysteresis so that either state can persist under identical environmental conditions (i.e. as alternative stable states).

Methods
Validated simulation models can provide useful answers to both questions provided independent parameter estimates are available from experiments and measurements in the field. We modelled kelp (Ecklonia radiata) – turf dynamics on shallow South Australian reefs.

Findings
The models show clearly that the kelp-turf transition in this region represents a discontinuous phase shift with hysteresis and therefore that the kelp and turf states represent alternative stable community configurations. Importantly, the likelihood of a shift to domination by turf algae increases with acidification, warming and nutrification. As multiple stressors arise the increase in risk of transition to turfing algae is additive, and tipping points shift so that the transition to turf is much more likely.

Conclusions
The hysteresis in this system presents a particular challenge for managers once the shift to turf occurs, and efforts to prevent the loss of kelp in the first place will obviate the need for much greater effort to rehabilitate kelps. Efforts by managers to minimise local stressors (e.g. nutrification) will help mitigate the effects of stressors over which they have little control (acidification and warming).