Rethinking What Causes Phytoplankton Blooms Michael Behrenfeld Department of Botany & Plant Pathology Oregon State University

Vital Role of the North Atlantic Bloom Fisheries – past, present, future Seabirds Marine Mammals Carbon dioxide & Climate

Traditional View of the North Atlantic Bloom Based on ship observations Focus on spring peak in phytoplankton concentration Relates blooms to spring increases in sunlight energy and shallower surface mixing Mathematically formalized in 1953 by H.U. Sverdrup ‘Critical Depth Hypothesis’ An extension of spring growth by land plants? This view permeates literature and media

Seasonal Environmental Change is Important Environmental controls recognized early-on But, focused on changes in photosynthesis of surface-layer phytoplankton due to changes in sunlight and mixing The upper layer of the ocean is well mixed, but the depth of this mixing changes from 10’s of meters in the summer to 100’s of meters in the winter. Seasonal changes driven by wind (mechanical mixing) and temperature (convective mixing)

Earth System Satellites are Key to New Discoveries Ship measurements are always limited in coverage and often biased toward fair-weather seasons Satellites observe the North Atlantic every day for many years Used 9 years of mixed layer data and SeaWiFS phytoplankton data divided into 12 grid boxes for the North Atlantic Focus on population growth rates, rather than phytoplankton concentration and photosynthesis

Month Population Growth Rate Sunlight January April July October January April July October surface 100 m 200 m Mixing Depth Satellite data tells a very different story about the bloom Traditionally, the bloom is thought to occur here, due to increased sunlight and shallower mixing fueling rapid photosynthesis Traditionally, low light and deep mixing in winter is thought to prevent a bloom Satellite Data show that phytoplankton population growth rates increase in late autumn, reach a peak by early winter, and then remain high until late spring ! In other words, the bloom periods starts earlier than we thought, lasts longer than we thought, and begins when environmental conditions for phytoplankton growth are at their worst

Predator-Prey Heart of Bloom Depth AutumnWinterSpring Mixing changes relationship between predators (zooplankton) & prey (phytoplankton) Deepening dilutes predators & prey, making it harder for predators to find their prey, allowing phytoplankton population growth Shallowing removes phytoplankton and concentrates predators – which is offset by increasing phytoplankton growth rates

Discovery & Importance Bloom-forming ocean ecosystems function differently than we thought The new view of blooms suggests a very different consequence of climate change than the traditional view Deeper mixing can allow stronger blooms Climate warming may reduce winter mixing and favor weaker blooms