Abstract
Climate change is increasing the frequency and severity of marine heatwaves. A recent extreme warming event (2014-2016) of unprecedented magnitude and duration in the California Current System allowed us to evaluate the response of the kelp forest community near its southern (warm) distribution limit. We obtained sea surface temperatures for the northern Pacific of Baja California, Mexico, and collected kelp forest community data at three islands, before and after the warming event. The warming was the most intense and persistent event observed to date, with low-pass anomalies 1°C warmer than the previous extremes during the 1982-1984 and 1997-1998 El Ninos. The period between 2014-2017 accounted for ~50% of marine heatwaves days in the past 37 years, with the highest maximum temperature intensities peaking at 5.9°C above average temperatures for the period. We found significant declines in the number of Macrocystis pyrifera individuals, except at the northernmost island, and corresponding declines in the number of fronds per kelp individual. We also found significant changes in the community structure associated with the kelp beds: half of the fish and invertebrate species disappeared after the marine heatwaves, species with warmer affinities appeared or increased their abundance, and introduced algae, previously absent, appeared at all islands. Changes in subcanopy and understory algal assemblages were also evident; however, the response varied among islands. These results suggest that the effect of global warming can be more apparent in sensitive species, such as sessile invertebrates, and that warming-related impacts have the potential to facilitate the establishment of tropical and invasive species.
Highlights
Climate change is predicted to increase the frequency and severity of extreme climatic events (Solomon et al, 2007; Perkins et al, 2012; Sydeman et al, 2013; FrĂślicher et al, 2018) such as marine heatwaves (MHWs) (Hobday et al, 2016), which are impacting coastal marine ecosystems worldwide
The 2014â2016 warm-event anomaly registered on the three islands was the largest in extent and most prolonged in duration observed to date, with sea surface temperature anomalies (SSTAs) slightly lower at the northernmost site (ITS) than the southernmost sites (ISM and Isla San Jeronimo (ISJ)), with anomalous temperatures peaking on summer 2015 at 2.15, 2.32, and 2.39âŚC above average, respectively (Supplementary Figure S1)
Among the manifestations of climate change is the intensification of MHWs (Oliver et al, 2018), with corresponding impacts reported globally in different taxonomic groups and geographic areas (Garrabou et al, 2009; Marba and Duarte, 2010; Smale and Wernberg, 2013; Voerman et al, 2013; Short et al, 2015; Cavole et al, 2016; Oliver et al, 2017; Tuckett et al, 2017; Ruthrof et al, 2018; Smale et al, 2019)
Summary
Climate change is predicted to increase the frequency and severity of extreme climatic events (Solomon et al, 2007; Perkins et al, 2012; Sydeman et al, 2013; FrĂślicher et al, 2018) such as marine heatwaves (MHWs) (Hobday et al, 2016), which are impacting coastal marine ecosystems worldwide Temperate foundation (habitat-forming) species, are reported to be sensitive to MHWs (Wernberg et al, 2019), especially to warmer water temperatures and to decreases in nutrient availability due to thermal stratification or to a weakening of coastal upwelling (Carr and Reed, 2015; Schiel and Foster, 2015). They are susceptible to MHWs when located close to their equatorward (warm) range edges (Smale et al, 2019). Changes in the abundance and distribution of these foundation species could have consequences for the structure and function of the entire ecosystem (Harley et al, 2012; Schiel and Foster, 2015; Smale et al, 2019)
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