In many community assemblages, the abundance of organisms is a power-law function of organism size. In phytoplankton communities, changes in size structure associated with increases in resource availability and total biomass have often been interpreted as a release from grazer control. A metapopulation-like approach is used to scale up from the individual physiological responses to environmental conditions to community size structure assuming the community taxonomic composition reflects the species pool. We show that the size scaling of cellular nutrient requirements and growth can cause (1) the power-law relationship between cell size and abundance, (2) dominance of small phytoplankton cells under oligotrophic conditions, and (3) relative increase in abundance of larger phytoplankton cells under eutrophic conditions. If physiological differences associated with the taxonomic composition of different community size fractions are considered, then the model can replicate detailed field observations such as the absence of small, slow-growing Prochlorococcus spp. and the relative dominance of large diatom species in nutrient-rich, upwelling regions of the ocean.

Anthropogenic climate change has shifted the biogeography and phenology of many terrestrial and marine species. Marine phytoplankton communities appear sensitive to climate change, yet understanding of how individual species may respond to anthropogenic climate change remains limited. Here, using historical environmental and phytoplankton observations, we characterize the realized ecological niches for 87 North Atlantic diatom and dinoflagellate taxa and project changes in species biogeography between mean historical (1951–2000) and future (2051–2100) ocean conditions. We find that the central positions of the core range of 74% of taxa shift poleward at a median rate of 12.9 km per decade (km⋅dec⁻¹), and 90% of taxa shift eastward at a median rate of 42.7 km⋅dec⁻¹. The poleward shift is faster than previously reported for marine taxa, and the predominance of longitudinal shifts is driven by dynamic changes in multiple environmental drivers, rather than a strictly poleward, temperature-driven redistribution of ocean habitats. A century of climate change significantly shuffles community composition by a basin-wide median value of 16%, compared with seasonal variations of 46%. The North Atlantic phytoplankton community appears poised for marked shift and shuffle, which may have broad effects on food webs and biogeochemical cycles.

Ocean acidification is changing the nature of inorganic carbon availability in the global oceans. Diatoms account for µ 40% of all marine primary productivity and are major contributors to the export of atmospheric carbon to the deep ocean. Larger diatoms are more likely to be stimulated by future increases in CO₂ availability as a result of their low surface area to volume ratio and lower diffusive flux of CO₂ relative to their carbon demand for growth. Here we quantify the effect of the partial pressure of carbon dioxide (P_CO2), at levels of 190, 380, and 750 µL L⁻¹, on the growth rate, photosystem II electron transport rate (ETR), and elemental composition for five diatom species ranging over five orders of magnitude in cell volume. Growth rates for all species were enhanced under 750 relative to 190 and 380 µL L⁻¹, with little change in ETR or elemental stoichiometries, indicating an enhanced allocation of photochemical energy to growth under elevated P_CO2. P_CO2 enhancement of growth rates was size dependent. Under 750 vs. 190 µL L⁻¹ partial pressures, growth rate was enhanced by µ 5% for the smaller diatom species to µ 30% for the largest species examined. The size dependence of CO₂‐stimulated growth enhancement indicates that ocean acidification may selectively favor an increase in the growth rates of larger vs. smaller phytoplankton species in the sea, with potentially significant consequences for carbon biochemistry.

Significance: Most ecosystem models used to predict changes in community composition with climate change assume species’ responses to environmental conditions are genetically fixed on the century scale, but this hypothesis has not been tested. Using an oceanographic time series with directional environmental changes, we show here that many phytoplankton species are able to track, on average, modest changes in temperature and irradiance, but not decreases in limiting nutrient concentrations, on decadal timescales. This result suggests that models that use genetically fixed traits may not provide reasonable projections for changes in biological communities in response to climate change over decadal to longer timescales.