Response of foundation macrophytes to near‐natural simulated marine heatwaves

Abstract

Marine heatwaves have been observed worldwide and are expected to increase in both frequency and intensity due to climate change. Such events may cause ecosystem reconfigurations arising from species range contraction or redistribution, with ecological, economic and social implications. Macrophytes such as the brown seaweed Fucus vesiculosus and the seagrass Zostera marina are foundation species in many coastal ecosystems of the temperate northern hemisphere. Hence, their response to extreme events can potentially determine the fate of associated ecosystems. Macrophyte functioning is intimately linked to the maintenance of photosynthesis, growth and reproduction, and resistance against pathogens, epibionts and grazers. We investigated morphological, physiological, pathological and chemical defence responses of western Baltic Sea F. vesiculosus and Z. marina populations to simulated near‐natural marine heatwaves. Along with (a) the control, which constituted no heatwave but natural stochastic temperature variability (0HW), two treatments were applied: (b) two late‐spring heatwaves (June, July) followed by a summer heatwave (August; 3HW) and (c) a summer heatwave only (1HW). The 3HW treatment was applied to test whether preconditioning events can modulate the potential sensitivity to the summer heatwave. Despite the variety of responses measured in both species, only Z. marina growth was impaired by the accumulative heat stress imposed by the 3HW treatment. Photosynthetic rate, however, remained high after the last heatwave indicating potential for recovery. Only epibacterial abundance was significantly affected in F. vesiculosus. Hence both macrophytes, and in particular F. vesiculosus, seem to be fairly tolerant to short‐term marine heatwaves at least at the intensities applied in this experiment (up to 5°C above mean temperature over a period of 9 days). This may partly be due to the fact that F. vesiculosus grows in a highly variable environment, and may have a high phenotypic plasticity.

Simulated Heatwaves Lead to Upregulated Chemical Defense of a Marine Foundation Macrophyte Against Microbial Colonizers

“…Control (0HW), one summer heatwave (1HW) and two late spring/early summer heatwaves followed by a summer heatwave (3HW). Figure has been adapted from Saha et al (2019). The dates on X-axis are given as dd-mm-yyyy.…”

“…In addition, various benthic invertebrates and macroalgae that are common in the Kiel Fjord (e.g., Idotea baltica, Littorina littorea, Gammarus spp., Mytilus edulis, Balanus improvisus, Fucus vesiculosus, Agarophyton vermiculophyllum) were inoculated into all benthocosm subunits at identical densities and natural abundances (Werner et al, 2016), in order to create a diverse benthic ecosystem. The performance of benthic and free living animals and macrophytes has been presented in Pansch et al (2018) and Saha et al (2019). Briefly, the 3HW treatments resulted in a reduction of longitudinal growth by 40% and increased formation of side branches in Z. marina, whereas photosynthesis and respiration rates were not significantly affected by both treatments.…”

Surviving heatwaves: thermal experience predicts life and death in a Southern Ocean diatom

“…Marine heatwaves (MHWs) are one such example of temperature fluctuations, and are defined as "discrete prolonged anomalous warm water events" (Hobday et al, 2016) that can result in rapid population declines and reduced ecosystem functioning (Frölicher & Laufkötter, 2018, Smale et al, 2019. Recent work has uncovered a broad range of organismal response to MHWs, from negative to positive (Bartosiewicz et al, 2019, Britton et al, 2020, Pansch et al, 2018, Saha et al, 2019, Stuhr et al, 2017. Furthermore, varied responses among species to MHWs can lead to significant food web alterations in marine habitats (Jones et al, 2018, Peña et al, 2019, Piatt et al, 2020, Ryan et al, 2017, von Biela et al, 2019, illustrating that the responses of individual species will influence the resilience of entire marine ecosystems under global change.…”

“…In the context of marine heatwaves, the impact of elevated temperatures will be dependent upon the thermal niche of the organisms present, which is subject to both interspecific and intraspecific variation (Boyd et al, 2013). Furthermore, the state of cellular condition before heatwaves has the potential to affect population resilience to heatwaves when they do occur (Ainsworth et al, 2016, Saha et al, 2019, Short et al, 2015, Siegle et al, 2018, Stuhr et al, 2017.…”

“…Several studies in marine organisms show that resilience to MHWs depend upon several factors, including temperatures experienced prior to the thermal extreme (Siegle et al, 2018) and temperature variability (Lugo et al, 2020, Saha et al, 2019, Stuhr et al, 2017, and that resilience varies between species (Lugo et al, 2020, Magozzi & Calosi, 2015, Saha et al, 2019. Given these factors, it is not surprising that studies have a range of findings.…”