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Marine Ecology Progress Series

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Variability of size-structured phytoplankton biomass with chlorophyll a (left). Conceptual depiction of the Enhanced Microbial Loop hypothesis at high chla (right). Conceptual depiction: Dennis Mc Thompson

Taylor AG, Landry MR


Phytoplankton biomass and size structure across trophic gradients in the southern California Current and adjacent ocean ecosystems

Phytoplankton size composition is a major determinant of food-web structure and fluxes. Along a trophic gradient from open-ocean oligotrophy to coastal upwelling in the eastern North Pacific, Taylor & Landry found that large phytoplankton cells (macro = 20–200 µm, nano = 2–20 µm) increase monotonically in biomass with system richness, while picophytoplankton groups (Prochlorococcus, Synechococcus, <2 µm picoeukaryotes) decline in richer habitats. This decrease is consistent with an enhanced microbial loop hypothesis in which increased dissolved organic carbon production leads to higher productivity and grazing turnover of heterotrophic bacteria, which indirectly suppresses picophyoplankton. Density-independent grazing may be a strong driver of picophytoplankton selection across trophic gradients, with implications for strategy trade-offs in growth rate and grazing resistance, and mortality constructs in ecosystem models.


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