Multi-decadal decline in cover of giant kelp Macrocystis pyrifera at the southern limit of its Australian range

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.

One of the least disturbed marine coastal ecosystems on Earth: Spatial and temporal persistence of Darwin’s sub‐Antarctic giant kelp forests

“…However, kelp persistence seems to follow a neutral relationship with SST at higher latitudes, i.e., bull kelp Nereocystis luetkeana in Oregon (Hamilton et al, 2020). Furthermore, our SST analysis confirms that the currents near our study areas show no trends of tropicalisation, a major cause of thermal stress and shifts on kelp distributions in other regions, i.e., Western Australia (Wernberg et al, 2013), or Tasmania, with a sharp decline of kelp canopies since the year 2000 (Butler et al, 2020). The low SST ranges in South Georgia, located south of the Antarctic Polar Front (Moore et al, 1999), could mean that this ecosystem may cope well with polar temperatures, which might be a signal for potential colonisation of the Antarctic, but the small sizes of their canopies might also mean they are close to their lower temperature limit.…”

Disturbance structures canopy and understory productivity along an environmental gradient

“…Such annually recurrent disturbances to giant kelp are typical throughout its range in regions exposed to large seasonal swells (Graham et al, 2007 ; Schiel & Foster, 2015 ). The quarterly disturbance treatment was performed in fixed plots once or twice per season (4–8 times per year) to mimic near continual loss of giant kelp, which can result from prolonged ocean warming and other press disturbances (Butler et al, 2020 ; Cavanaugh et al, 2019 ). Climate change may bring about warmer, stormier seas that would increase both intermittent and sustained kelp canopy losses (Ummenhofer & Meehl, 2017 ).…”

Short‐term stress responses and recovery of giant kelp (Macrocystis pyrifera, Laminariales, Phaeophyceae) juvenile sporophytes to a simulated marine heatwave and nitrate scarcity1

“…2016, Butler et al. 2020). Seaweeds may respond differently to MHW stressors based on each species' genetic diversity, ecophysiological versatility, and resilience (Wernberg et al.…”