Abstract
Knowledge of long-term and multi-scale trends in ecological systems is a vital component in understanding their dynamics. We used Landsat satellite imagery to develop the first long-term (1986-2015) data set describing the cover of dense surface canopies of giant kelp Macrocystis pyrifera around the entire coastline of Tasmania, Australia, and assessed the extent to which potential environmental drivers explain the dynamics of surface canopies at multiple spatial and temporal scales. Broad-scale temporal patterns in canopy cover are correlated with El Niño-Southern Oscillation events, while regional patterns are related to sea surface temperature and nutrient regimes are associated with the East Australian Current. Regression models developed to predict the presence or absence of giant kelp canopy emphasise the importance of sea surface temperature in these systems. Long-term decline in canopy cover is clearly evident in most regions, and in light of increasing thermal stress associated with a changing ocean climate, this raises concern for the future of this species as a major habitat-forming kelp in Australia and some other regions worldwide. Given that Tasmania represents the stronghold of the range of this species in Australia, but is a geographic trap in that there is no suitable habitat for M. pyrifera to the south, our findings support the Federal listing of giant kelp communities in Australia as an endangered marine community type.
Highlights
Many marine ecosystems are under threat as a consequence of global warming (Hughes et al 2003, Wernberg et al 2016)
A state-wide peak in canopy cover was driven predominately by dense surface canopies occurring in the Bruny bioregion (Fig. 2B), which accounted for 274.6 ha (65%) of the maximum cover observed through the time-series
Our results are an example of region-specific signals of global change acting synergistically with local stressors to result in trends that may not be representative of other parts of the world (Krumhansl et al 2016)
Summary
Many marine ecosystems are under threat as a consequence of global warming (Hughes et al 2003, Wernberg et al 2016). Oceans are becoming warmer and more acidic, shifts in ocean circulation are altering temperature and nutrient regimes, and in many locations, anthropogenically associated disturbance events are increasing in frequency and severity, so that marine ecosystems are increasingly subject to multiple environmental stressors. These physical changes to the environment affect biotic communities, including in coastal regions where ecosystems are subject to growing anthropogenic pressure from urbanisation, development and exploitation (Vásquez 2008, Strain et al 2014). Data collection in the marine environment is costly and often labour intensive, so despite recognition of the value of long-term data sets, few studies are able to assess long-term change
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