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.