Modeled pre-heatwave conditions
In the pre-heatwave conditions the Sea Surface Temperature (SST) ranges from 13.5 ˚C (March 1) to 26.2 ˚C (August 4, Figure 1) with winter exhibiting the lowest mean SST, followed by spring, autumn, and summer (Fig 1). The Mixed Layer Depth (MLD) varies from 28 m (September 14) to 93 m (February 23, Figure 1). Winter and spring have the deepest MLD, followed by similar depths in autumn and summer (Figure 1).
The overall daily modeled biomass concentration of protists and copepods declines from early winter (December) until the end of April (Figure 2). Copepod biomass gradually increases from May to its peak in autumn (September/October). The biomass concentration of protists has a strong variation in summer and autumn. Still, overall, it shows an increasing trend from summer to mid-autumn with three biomass peaks in summer (August), early autumn (September) and early winter (December). Autumn holds the highest mean biomass concentration for protists and copepods, followed by summer, winter, and spring (Figure 2).
Protists constitute the most diverse community, followed by active and passive copepod feeders. Depending on the season, ten functional groups contribute 70 % to 86 % of the total protist biomass, while five functional groups represent 82 % to 100% of the total copepod biomass. Protists’ highest diversity is not only due to their big pool of initial groups in the model set-up (112 groups) but mostly due to their short life cycle (asexual, one life stage), their opportunistic ability to grow with different resources (nitrogen, prey, mixotrophy) at any time, and the dynamic losses (grazing, background mortality). In contrast, copepods have longer life cycles (sexual, eight life stages), and their growth relies only on one energy source (prey availability). These trait disparities account for variations in population coexistence, biomass distribution among groups, and temperature norms (Figures 2, 3). Overall, the plankton community is dominated by groups with temperature optima of 20 ˚C and 24 ˚C (Figure 3). Both optima cover temperatures from 10 to 30 ˚C and are within the daily (14 – 30 ˚C) and annual (20 ˚C) temperature range. The temperature norms of dominant groups track the annual mean temperature, not the Sea Surface Temperature seasonality (Figure 3), as life cycles, growth rates, and population dynamics introduce delays between abiotic and biotic seasonalities, allowing populations to persist even when some environmental conditions fall outside their optimal growth range .