Abstract
Here we review, synthesize, and analyse the size-based approach to model unicellular plankton cells and communities. We first review how cell size influences processes of the individual the cell: uptake of dissolved nutrients and dissolved organic carbon, phototrophy, phagotrophy, and metabolism. We parameterise processes primarily from first principles, using a synthesis of existing data only when needed, and show how these processes determine minimum and maximum cell size and limiting resource concentrations. The cell level processes scale directly up to the structure and function of the entire unicellular plankton ecosystem, from heterotrophic bacteria to zooplankton. The structure is described by the Sheldon size spectrum and by the emergent trophic strategies. We develop an analytical approximate solution of the biomass size spectrum and show how the trophic strategies of osmotrophy, light- and nutrient-limited phototrophy, mixotrophy, phagotrophy depend on the resource environment. We further develop expressions to quantify the functions of the plankton community: production, respiration and losses, and carbon available to production of higher trophic levels, and show how the plankton community responds to changes in temperature and grazing from higher trophic levels. We finally discuss strengths and limitations of size-based representations and models of plankton communities and which additional trait axes will improve the representation of plankton functional diversity.
Competing Interest Statement
The authors have declared no competing interest.
Footnotes
1)Emphasize the "review" aspects of the manuscript in the introduction, in each section 3-5, and in the discussion. 2)Added review of cell shape in section 4.1 (new figure 6) and in discussion of limiting size. 3)Added a paragraph about the functional response (second paragraph in 3.1) 4)Added a paragraph about bottom-up vs. top-down controls and the size spectrum slope (2nd paragraph in discussion) 5)Added a paragraph about trait-based modelling (2nd paragraph in section 6.2). 6)New table 3 and a paragraph in the discussion about sensitivity of community structure to parameters (last paragraph in section 6.1). 7)Reduced the value of rD. This change facilitates that picoplankton are the dominant group at low mixing rates, as is commonly observed.