Latitudinal Variation in Plankton Traits and Ecosystem Function

Abstract

Planktonic ecosystems are usually modeled in terms of autotrophic and heterotrophic compartments. However, the trophic strategy of unicellular organisms can take a range of mixotrophic strategies with both autotrophic and heterotrophic contributions. The dominant emerging strategy found in nature depends on the environment (both biotic and abiotic aspects) and the cell size and influences key ecosystem functions like trophic transfer and carbon export. Ecosystem models that faithfully represent this diversity of trophic strategies are lacking. Here we develop a trait‐based model of unicellular plankton with cell size as the master trait and three other traits that determine trophic strategies: investments in photosynthesis, nutrient uptake, and phagotrophy. This unicellular model spans the entire auto‐ mixo‐ hererotrophic strategy continuum and the entire size range of unicellular organisms. The model reproduces observed latitudinal patterns in biomass, primary productivity, vertical carbon export, and energy transfer efficiency; all increase with increasing latitude. The size range of mixotrophic cells is independent of the season at low latitudes. At high latitudes, the dominance of pure phototrophs during early spring restricts mixotrophic behavior to a narrower range of cell sizes and with the occurrence of relatively smaller mixotrophs during summer. The model's ability to adapt to different environmental conditions, combined with its simple conceptual structure and low number of parameters and state variables (10), makes it ideally suited for global simulation studies under changing environmental conditions.

A Threshold Sea‐Surface Temperature at 14°C for Phytoplankton Nonlinear Responses to Ocean Warming

“…This variability in threshold temperature likely reflects differences in phytoplankton community composition, as well as their physiological status. While latitudinal patterns in the thermal traits of phytoplankton have been observed (Chakraborty et al., 2020; Thomas et al., 2012), far less is known about the distribution of Ks ( N ) values within communities and across latitudes. For example, small phytoplankton with lower Ks ( N ) are influenced strongly by nutrient limitation in high temperature conditions, whereas diatoms with higher Ks ( N ) are also limited by nutrients in low temperature conditions.…”

Linking plankton size spectra and community composition to carbon export and its efficiency

“…The model presented here builds upon previous work of size-based, emergent food web models (e.g. Follows et al, 2007; Bruggeman & Kooijman, 2007; Ward et al, 2012; Chakraborty et al, 2020). However, most of these models center on the microbial community.…”

Destabilization, stabilization, and multiple attractors in saturated mixotrophic environments

“…have one positive co-ordinate, each. By (5), there are no other equilibria. A sketch of the phase-portrait (see Figure 1) or a check of the eigenvalues shows that (10) is locally stable.…”

“…Proof. We use (5) to prove the inequality for k. The inequality c < 1 follow more easily and directly from (4) than from (5).…”

“…A large number of plankton taxa, known as mixotrophs, are able to simultaneously exploit both phototrophic and phagotrophic pathway of nutrition. These mixotrophs play important roles in exporting organic matters to higher trophic levels (Ward and Follows (2016)), carbon export to deep water (Mitra et al ( 2014)), nutrient cycling (Stoecker et al (2017)) and primary production (Chakraborty et al (2019)). Mixotrophs are found almost everywhere in the illuminated water column, both in freshwater and marine environments (Hartmann et al (2012); Stoecker et al (2009)), oligotrophic and eutrophic systems (Burkholder et al, (2008)), and from polar to equatorial regions (Zubkov and Tarran (2008); Stoecker et al (2009); Sanders and Gast (2012)).…”