Distinct patterns in the size-scaling of abundance and metabolism in coastal and open-ocean phytoplankton communities

This study was aimed at identifying macroecological patterns in the relationship between phytoplankton cell size, abundance and metabolism in 2 marine ecosystems characterised by marked differences in resource availability and water-column stability. Several patterns emerged: (1) nearly isometric size-scaling of phytoplankton carbon fixation rate was described for both open-ocean and coastal ecosystems (mean slope: 1.17 and 0.90, respectively), supporting the idea that biomass-specific photosynthesis rates are largely independent of cell size; (2) less steep values for the size-scaling of abundance (mean slope: -0.73) were found in the coastal ecosystem compared to the open ocean (mean slope: -1.15); (3) large phytoplankton used more photosynthetic energy than smaller cells in the coastal ecosystem, but a constant flow of energy along the size spectrum was found in the open ocean; and (4) phytoplankton biomass turnover rates were 1 order of magnitude higher in the coastal ecosystem than in the open ocean, implying physiological limitation of phytoplankton growth in the oligotrophic ocean. Bottom-up and top-down mechanisms and their interaction with nutrient supply dynamics were suggested as major factors determining the contrasting phytoplankton size abundance distributions observed in coastal and open-ocean waters.