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Abstract
Detailed research has documented gradual changes to biological communities attributed to increases in global average temperatures. However, localized and abrupt temperature anomalies associated with heatwaves may cause more rapid biological changes. We analysed temperature data from the South Island of New Zealand and investigated whether the hot summer of 2017/18 affected species of bull kelp, Durvillaea antarctica, D. poha and D. willana. Durvillaea spp. are large iconic seaweed that inhabit the low intertidal zone of exposed coastlines, where they underpin biodiversity and ecosystem functioning. Sea surface temperatures during the summer of 2017/18 included the strongest marine heatwaves recorded in 38 years of existing oceanic satellite data for this region. Air temperatures were also high, and, coupled with small wave heights, resulted in strong desiccation stress during daytime low tides. Before-After analysis of drone images of four reef platforms (42, 42, 44 and 45 ˚S) was used to evaluate changes to bull kelp over the hot summer. Bull kelp loss varied among species and reefs, with the greatest (100%) loss of D. poha at Pile Bay in Lyttelton Harbour (44 ˚S). In Pile Bay, sea surface temperature exceeded 23 ˚C and air temperatures exceeded 30 ˚C, while Durvillaea was exposed for up to 3 hours per day during low tide. Follow-up surveys showed that all bull kelps were eliminated from Pile Bay, and from all reefs within and immediately outside of Lyttelton Harbour. Following the localized extinction of bull kelp in Pile Bay, the invasive kelp Undaria pinnatifida recruited in high densities (average of 120 m-2). We conclude that bull kelps are likely to experience additional mortalities in the future because heatwaves are predicted to increase in magnitude and durations. Losses of the endemic D. poha are particularly concerning due to its narrow distributional range.
Highlights
Between 1950 and 2009, mean sea surface temperature (SST) of the Atlantic, Indian, and Pacific Oceans increased by 0.41, 0.65, and 0.31◦C, respectively (Hoegh-Guldberg et al, 2014)
Analysis of Sea surface temperatures (SST) data near Pile Bay within Lyttelton Harbor showed many days with water temperatures exceeding 23◦C during the summer of 2017/18 (Figure 3) and intertidal in situ loggers at Oaro showed several days where atmospheric temperatures on the reef platforms exceeded >45◦C
Prior to the summer of 2017/18, the four intertidal platforms were dominated by D. poha, with smaller patches of D. willana and D. antarctica at the Point and Kaik reefs (Figure 4)
Summary
Between 1950 and 2009, mean sea surface temperature (SST) of the Atlantic, Indian, and Pacific Oceans increased by 0.41, 0.65, and 0.31◦C, respectively (Hoegh-Guldberg et al, 2014). Local Extinction of Bull Kelp gradual temperature increases are discrete heating events covering smaller regions (1–1000 km) and occurring over shorter durations (days–months). These events are referred to as “heatwaves” (Perkins and Alexander, 2013; Hobday et al, 2016; Oliver et al, 2018) and can have more instantaneous and conspicuous effects (Wernberg et al, 2016; Le Nohaïc et al, 2017; Hobday et al, 2018; Smale et al, 2019). In northern Spain, range contractions have been reported for several canopy-forming seaweeds, including Fucus serratus and Himanthalia elongata (Duarte et al, 2013)
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