Abstract
Abstract. Biodiversity of phytoplankton is important for ecosystem stability and marine biogeochemistry. However, the large-scale patterns of diversity are not well understood and are often poorly characterized in terms of statistical relationships with factors such as latitude, temperature and productivity. Here we use ecological theory and a global trait-based ecosystem model to provide mechanistic understanding of patterns of phytoplankton diversity. Our study suggests that phytoplankton diversity across three dimensions of trait space (size, biogeochemical function and thermal tolerance) is controlled by disparate combinations of drivers: the supply rate of the limiting resource, the imbalance in different resource supplies relative to competing phytoplankton demands, size-selective grazing and transport by the moving ocean. Using sensitivity studies we show that each dimension of diversity is controlled by different drivers. Models including only one (or two) of the trait dimensions will have different patterns of diversity than one which incorporates another trait dimension. We use the results of our model exploration to infer the controls on the diversity patterns derived from field observations along meridional transects in the Atlantic and to explain why different taxa and size classes have differing patterns.
Highlights
Phytoplankton are an extremely diverse set of microorganisms spanning more than 7 orders of magnitude in cell volume (Beardall et al, 2008) and an enormous range of cell morphologies, bio(geo)chemical functions, elemental requirements and trophic strategies
The smallest size category (2– 10 μm) has the highest diversity, while there is lower and more regionally varying diversity in the larger size categories, with some regions having none of the largest size class (> 20 μm). This suggests that the controlling mechanism(s) on, for instance, diatom diversity is different to those controlling coccolithophore diversity, which differs to what determines the diversity within different size classes
We find that for the rest of the functional groups, coexistence is strongly controlled by the differences in their resource requirements and the imbalances in the supply rates of multiple resources
Summary
Phytoplankton are an extremely diverse set of microorganisms spanning more than 7 orders of magnitude in cell volume (Beardall et al, 2008) and an enormous range of cell morphologies, bio(geo)chemical functions, elemental requirements and trophic strategies. This range of traits play a key role in regulating the biogeochemistry of the ocean Cermeño et al, 2008; Fuhrman, 2009), including the export of organic matter to the deep ocean (Falkowski et al, 1998; Guidi et al, 2009), which is critical in oceanic carbon sequestration and contributes to modulation of atmospheric CO2 levels and climate. Numerous studies have attempted to understand or predict observed patterns of biodiversity or species richness of marine phytoplankton by correlating with factors such as temperature and latitude
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