Increasing importance of small phytoplankton in a warmer ocean

TLDR

Marine phytoplankton size structure and temperature relationships lack a theoretical explanation, and the smallest members—picophytoplankton—are cyanobacteria and eukaryotic algae under 2 µm. The study aims to explain the consistent increase in picophytoplankton biomass with temperature by combining the temperature–size relationship and allometric size‑scaling of population abundance. The authors use a merged dataset from the eastern and western temperate North Atlantic Ocean, spanning −0.6 to 22 °C, to apply these rules. Temperature alone explains 73 % of the variance in the relative contribution of small cells to total phytoplankton biomass, predicts a gradual shift toward smaller primary producers in a warmer ocean, and suggests future changes in oceanic ecosystem functioning.

Abstract

Abstract The macroecological relationships among marine phytoplankton total cell density, community size structure and temperature have lacked a theoretical explanation. The tiniest members of this planktonic group comprise cyanobacteria and eukaryotic algae smaller than 2 μm in diameter, collectively known as picophytoplankton. We combine here two ecological rules, the temperature–size relationship with the allometric size‐scaling of population abundance to explain a remarkably consistent pattern of increasing picophytoplankton biomass with temperature over the −0.6 to 22 °C range in a merged dataset obtained in the eastern and western temperate North Atlantic Ocean across a diverse range of environmental conditions. Our results show that temperature alone was able to explain 73% of the variance in the relative contribution of small cells to total phytoplankton biomass regardless of differences in trophic status or inorganic nutrient loading. Our analysis predicts a gradual shift toward smaller primary producers in a warmer ocean. Because the fate of photosynthesized organic carbon largely depends on phytoplankton size, we anticipate future alterations in the functioning of oceanic ecosystems.