Mechanisms shaping size structure and functional diversity of phytoplankton communities in the ocean

Abstract

The factors regulating phytoplankton community composition play a crucial role in structuring aquatic food webs. However, consensus is still lacking about the mechanisms underlying the observed biogeographical differences in cell size composition of phytoplankton communities. Here we use a trait-based model to disentangle these mechanisms in two contrasting regions of the Atlantic Ocean. In our model, the phytoplankton community can self-assemble based on a trade-off emerging from relationships between cell size and (1) nutrient uptake, (2) zooplankton grazing, and (3) phytoplankton sinking. Grazing 'pushes' the community towards larger cell sizes, whereas nutrient uptake and sinking 'pull' the community towards smaller cell sizes. We find that the stable environmental conditions of the tropics strongly balance these forces leading to persistently small cell sizes and reduced size diversity. In contrast, the seasonality of the temperate region causes the community to regularly reorganize via shifts in species composition and to exhibit, on average, bigger cell sizes and higher size diversity than in the tropics. Our results raise the importance of environmental variability as a key structuring mechanism of plankton communities in the ocean and call for a reassessment of the current understanding of phytoplankton diversity patterns across latitudinal gradients.

Figure 1. Model setups.

The left panel shows the geographical locations of the two considered regions: temperate and tropical. The panels on the right show the temporal changes of the environmental variables, namely mixed-layer depth (MLD), photosynthetically active radiation (PAR), sea surface temperature (SST), and nutrient concentration below the MLD (N₀). The map was generated using R v.3.1.2 (The R Foundation for Statistical Computing, Vienna, Austria).

Figure 2. Mean cell size dynamics in the temperate and tropical regions.

The shaded areas indicate one standard deviation calculated from the size variance (or functional diversity, equation 7). The dashed lines mark the limits of the phytoplankton size classes (PSC), which are picoplankton, nanoplankton, and microplankton. The dots represent PSC observations reconstructed from High Performance Liquid Chromatography (HPLC) data. The green colour scale represents the relative dominance of each PSC in percent. The bottom panels show the influence of the size-scaling processes on mean cell size () in the temperate (C) and in the tropical (D) regions. The orange area represents zooplankton grazing (Equation 12), the blue area represents nutrient uptake (Equation 11), and the green area represents phytoplankton sinking (Equation 13). The positive values of zooplankton grazing indicate that this process drives the community composition towards larger sizes, while nutrient uptake and phytoplankton sinking drive the community towards smaller sizes.

Figure 3. Phase plane of the mean cell size and size variance, the latter reflecting the functional size diversity.

The lines corresponds to a seasonal changes of the two state variables for the tropical and temperate regions. Changes in colour tonalities reflect changes in the relative biomass of each region.

Figure 4. Sensitivity of the diversity ratio between the temperate and tropics to changes in model parameters.

The red line marks the diversity ratio between the two regions obtained with the standard run (≈1.3). The bars show the changes of this ratio for alterations in the model parameters by +50% (grey bars) or −50% (black bars). The names and reference values of the parameters are listed in Table 1.