Our planet is going through severe climatic changes that include the occurrence of extreme events. Heatwaves, which correspond to an abrupt temperature increase above the seasonal mean, are expected to become more frequent and last longer in the future. They have already caused massive mortalities of macrophytes in marine coastal ecosystems, resulting in the decline of carbon storage and biodiversity shift. The objective of this work was to understand how heatwaves influence the functioning of a temperate marine food web supported by seagrasses and macroalgae. To this aim we carried out a near-natural outdoor mesocosm experiment and analyzed carbon flows in food webs exposed to either single (1HW) or three sequential (3HW) summer heatwaves. The food web studied is representative of coastal ecosystems along the German coast of the Baltic Sea. The carbon budget was quantified by measuring biomass, photosynthetic activity and respiration rates of most abundant species. Stable isotopes were applied to define the feeding preferences of consumers. The dataset collected was used to construct energy flow networks modeled with ecological network analysis. Network and statistical analyses enabled quantifying trophic transfer efficiencies, amounts of carbon recycled, main trophic chains responsible for carbon circulation, food web stability and resilience.
Results/Conclusions
Both single and sequential heatwaves jeopardized the biodiversity of the consumers but food web functioning was mostly impacted by 3HW. The single heatwave resulted in higher trophic specialization due to few herbivores increasing their feeding intensity. The three consecutive heatwaves lowered the primary productivity and as in the case of single heatwaves an increase of trophic specialization was observed. Such changes led to a simplified food web structure with most of the carbon transferred through a few and well-defined paths. No signs of ecological memory were detected since food webs exposed to 3HW displayed more dramatic changes compared to those experiencing single heatwaves. The 1HW represented an intermediate state towards the most impacted conditions exhibited by 3HW. Heatwaves increased the severity of herbivory thus exposing the food webs to stronger top-down control. When occurring in a sequence they reduced both biodiversity and productivity, possibly harming food web resilience. Summer heatwaves sharpen the fragility of temperate marine food webs due to biodiversity loss and the prevalence of a few dominant pathways for carbon circulation.