Biomass, functional diversity and community composition during seasonal heatwaves
In our modelling study, we observed that heatwaves induce changes in community biomass, diversity, and dominant functional groups of plankton (Figures. 3-6, SM4- SM6). Summer and autumn heatwaves (Figure 4, SM4) cause the highest anomalies in terms of biomass concentration and recovery times followed by spring and winter (Figures SM5-SM6). The strongest copepod biomass decline occurs during summer heatwaves, persisting for two years and eventually recovering to pre-heatwave concentrations after six years (Figure 4). Protists also experience strong biomass declines during and after the summer heatwaves with a stronger periodic signal than in copepods. The biomass anomalies for the autumn heatwave (September- November) are similar to the summer heatwave (Figure SM4). The only exception is that the biomass of active feeders increases when the autumn heatwave occurs. The winter heatwave (Dec-Feb) positively impacts plankton biomass, particularly for active and passive feeders, while total protist biomass declines for the remainder of the year (Figure SM5). The biomass anomalies are less profound a year after the winter heatwave and the biomass returns to pre-heatwave conditions after three years. The spring heatwave (Mar-May) exhibits similar biomass anomaly patterns to the winter heatwave (Figure SM6). The total biomass of both active and passive copepod feeders increases, while the total protist biomass stays the same despite the fluctuations in the size bins.
Looking at the Shannon Diversity Index (Figure 5, Supplementary Material section: “Shannon Index”), heatwaves also affect plankton functional diversity with the anomaly signal being extended into subsequent seasons. For both protists and copepods, functional diversity stays the same or increases during winter, spring, and summer heatwaves, while it decreases during the autumn heatwave. The autumn heatwave causes the strongest anomaly, followed by summer, winter, and spring. After the heatwave, the time-traveling anomaly signals show that heatwaves affect plankton functional groups differently.
Examining community composition, heatwaves alter the order of dominant groups based on their relative contribution to the total biomass at the time (Figure 3). These changes persist for up to six years before returning to pre-heatwave conditions. Like biomass and diversity, the alterations in dominant groups are more profound for the summer and autumn heatwaves (Figure 5). All four heatwave scenarios cause more changes in protists than in copepods. We speculate that this is due to protists shorter live-cycles, higher community diversity, and stronger ecosystem dynamics (resource competition, predation). Still, two temperature norms (20 ˚C and 24 ˚C) dominate the community before, during, and after all heatwaves. Our findings indicate that despite temperature impacts on individuals’ physiology, populations and communities remain resilient during seasonal heatwaves. Smaller-size groups may benefit, but larger groups also show increased biomass, suggesting the influence of resource competition and predator-prey dynamics alongside temperature.