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.

Reviews and syntheses: parameter identification in marine planktonic ecosystem modelling

“…Wirtz and Pahlow, 2010;Dutkiewicz et al, 2009;Smith et al, 2015), or by accounting for allometric relationships in growth and plankton interaction (e.g. Banas, 2011;Acevedo-Trejos et al, 2015), or by using microbial traits in a functional gene approach (Reed et al, 2014). Recent studies have begun to simulate ecosystem complexity and allow the model to "self-organize" according to a relatively simple set of ecological and physiological rules or "trade-offs" (Bruggeman and Kooijman, 2007;Follows et al, 2007).…”

PhytoSFDM version 1.0.0: Phytoplankton Size and Functional Diversity Model

“…Most of these models describe the phytoplankton community by a discrete representation of many species or functional groups (Baird and Suthers, 2007;Follows et al, 2007;Bruggeman and Kooijman, 2007;Barton et al, 2010;Banas, 2011;Ward et al, 2012;Smith et al, 2015). Alternatively, models have been developed that treat the whole phytoplankton species assemblage as a single entity (Wirtz and Eckhardt, 1996;Norberg et al, 2001;Merico et al, 2009;Bruggeman, 2009;Wirtz, 2013;Wirtz and Sommer, 2013;Terseleer et al, 2014;Acevedo-Trejos et al, 2015). These models use aggregate community properties such as total biomass, mean trait, and trait variance to describe changes in phytoplankton community composition.…”

“…discrete and aggregate) relies on the use of a key trait, for which relationships with other traits can be formulated. Cell size is recognized as one of the most important traits for characterizing phytoplankton communities (Litchman and Klausmeier, 2008;Litchman et al, 2010;Finkel et al, 2010;Marañón, 2015), and it has been commonly used in plankton ecosystem models (Baird and Suthers, 2007;Banas, 2011;Ward et al, 2012;Wirtz, 2013;Wirtz and Sommer, 2013;Terseleer et al, 2014;Acevedo-Trejos et al, 2015;Smith et al, 2015). This morphological trait affects trophic organization of foodwebs and the sequestration of CO 2 into the ocean interior (Chisholm, 1992).…”

Manipulation of Non-random Species Loss in Natural Phytoplankton: Qualitative and Quantitative Evaluation of Different Approaches

“…Though the underlying reasons are not completely understood yet, current research suggests that it is partly driven by decreased nutrient availability in the euphotic layer of the oceans due to stronger stratification under warmer conditions (Hofmann et al, 2011;Winder and Sommer, 2012;Acevedo-Trejos et al, 2014;Lewandowska et al, 2014). This situation benefits smaller-sized cells.…”