Abstract
As the ocean warms, the frequency, duration, intensity, and range of marine heatwaves (MHWs) increase. MHWs are becoming a severe challenge for marine ecosystems. However, our understanding in regard to their impacts on algal structure and carbon sequestration is still deficient or fragmentary, particularly when combined with ocean warming and acidification. In this paper, we reviewed the impacts of MHWs individually and combined with ocean warming and acidification on regime shift in algal community and carbon sequestration of both macroalgae and microalgae. Solid evidence shows that MHWs cause the decline of large canopy macroalgae and increase of turf-forming macroalgae in abundance, leading to the regime shift from kelp forests to seaweed turfs. Furthermore, increased grazing pressure on kelps due to tropicalization facilitates the expansion of turfs that prevent the recovery of kelps through plundering light and space. Meanwhile, MHWs could trigger microalgal blooms and the intensity of algal blooms is regulated by the severity of MHWs and nutrient availability. MHWs could lead to the decrease of carbon burial and sequestration by canopy-forming macroalgae due to depressed growth and increased mortality. The effects of MHWs on the productivity of microalgae are latitude-dependent: negative effects at low and mid-latitudes whilst positive effects at high latitudes. Ocean warming and acidification may accelerate the shift from kelps to turfs and thus decrease the carbon sequestration by canopy-forming macroalgae further. We propose that MHWs combined with ocean warming and acidification would reduce the biodiversity and facilitate the thriving of morphologically simple, ephemeral and opportunistic turfs and diatoms in coastal oceans, and phytoplankton with smaller size in open oceans. This structure shift would not be in favor of long-term carbon sequestration. Future studies could be conducted to test this hypothesis and investigate the impacts of MHWs on carbon sequestration under future ocean conditions.
Highlights
The term of marine heatwaves (MHWs) was first proposed by Pearce et al (2011) the variability of sea surface temperature (SST) has been studied for decades in physical oceanography and climate science (Philander, 1983)
It was found that from 1925 to 2016, global average frequency and duration of MHWs increased by 34 and 17%, respectively, resulting in a 54% increase in annual marine heatwave days globally. These trends are largely driven by increases in mean ocean temperatures, suggesting that further increases in marine heatwave days can be expected under the context of global warming (Oliver et al, 2018; Benthuysen et al, 2020)
Through reviewing the published data, we find that MHWs are driving the regime shift from canopy-forming to turf-forming and floating macroalgae and inducing microalgal blooms
Summary
The term of marine heatwaves (MHWs) was first proposed by Pearce et al (2011) the variability of sea surface temperature (SST) has been studied for decades in physical oceanography and climate science (Philander, 1983). MHWs are driven by multiple factors, including air-sea heat fluxes, ocean heat advection, large-scale climate variability, etc. It was found that from 1925 to 2016, global average frequency and duration of MHWs increased by 34 and 17%, respectively, resulting in a 54% increase in annual marine heatwave days globally (from 26 to 40 days). These trends are largely driven by increases in mean ocean temperatures, suggesting that further increases in marine heatwave days can be expected under the context of global warming (Oliver et al, 2018; Benthuysen et al, 2020)
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