North Atlantic Aerosols and Marine Ecosystems Study

The North Atlantic Aerosols and Marine Ecosystems Study (NAAMES) was a five-year scientific research program that investigated aspects of phytoplankton dynamics in ocean ecosystems, and how such dynamics influence atmospheric aerosols, clouds, and climate. The study focused on the sub-arctic region of the North Atlantic Ocean, which is the site of one of Earth's largest recurring phytoplankton blooms. The long history of research in this location, as well as relative ease of accessibility, made the North Atlantic an ideal location to test prevailing scientific hypotheses in an effort to better understand the role of phytoplankton aerosol emissions on Earth's energy budget. NAAMES was led by scientists from Oregon State University and the National Aeronautics and Space Administration (NASA). They conducted four field campaigns from 2015-2018 that were designed to target specific phases of the annual phytoplankton cycle: minimum, climax, intermediary decreasing biomass, and increasing intermediary biomass. The campaigns were designed to observe each unique phase, in order to resolve the scientific debates on the timing of bloom formations and the patterns driving annual bloom re-creation. The NAAMES project also investigated the quantity, size, and composition of aerosols generated by primary production in order to understand how bloom cycles affect cloud formations and climate. Scientists employed multiple complementary research methods, including intensive field sampling via research ships, airborne aerosol sampling via airplane, and remote sensing via satellites. The findings from NAAMES, while still forthcoming, have shed light on aerosols and cloud condensation nuclei, phytoplankton annual cycles, phytoplankton physiology, and mesoscale biology. Several methodological advances have also been published, including new remote sensing algorithms and advances in satellite remote sensing.

Background

Competing hypotheses of plankton blooms

NAAMES sought to better understand the impact of bioaerosol emissions on cloud dynamics and climate. It also aimed to test two competing hypotheses on plankton blooms:

Critical Depth Hypothesis - a resource-based view Source: The critical depth hypothesis is a resource-based view of the North Atlantic annual phytoplankton blooms. It is the traditional explanation for the cause of spring blooms and has been documented as a foundational concept in oceanography textbooks for over 50 years. It focuses on the environmental conditions necessary to initiate a bloom such as high nutrients, shallower mixing, increased light, and warmer temperatures. The central argument for the critical depth hypothesis is that blooms are a consequence of increased phytoplankton growth rates resulting from shoaling of the mixed layer above the critical depth. The critical depth is a surface mixing depth where phytoplankton biomass growth equals phytoplankton biomass losses. In this hypothesis, losses are both constant and independent of growth. The decline in biomass may be due to grazing, sinking, dilution, vertical mixing, infection, or parasitism. When the surface mixed layer becomes shallower than the critical depth, initiation of the seasonal bloom occurs due to phytoplankton growth exceeding loss. There is a correlation of phytoplankton growth with springtime increases of light, temperature, and shallower stratification depths. Climate warming may increase stratification or decrease mixed layer depth during the winter, which would enhance the vernal bloom or increase phytoplankton biomass if this hypothesis governed spring phytoplankton bloom dynamics. A primary criticism of this resource-based view is that spring blooms occur in the absence of stratification or shoaling of the mixed layer.

Dilution-recoupling Hypothesis - an ecosystem-based view Source: The dilution-recoupling hypothesis is an ecosystem-based view of the North Atlantic annual phytoplankton bloom. This hypothesis focuses on the physical processes that alter the balance between growth and grazing. The spring bloom is considered to be one feature of an annual cycle, and other features during the cycle "set the stage" for this bloom to occur. This ecosystem-based view is based upon a dilution experiment where the addition of seawater dilutes predators but does not change the growth of phytoplankton. Thus, growth rates increase with dilution. Although the dilution effect is transient, predator-prey interactions can be maintained if the rate of the addition of water equals the rate of growth. The deepening of the surface mixed layer dilutes the predator-prey interactions and decouples growth and grazing. When the mixed layer stops deepening, the increase in growth rate becomes apparent, but now growth and grazing become coupled again. The shoaling of the mixed layer concentrates predators, thereby increasing grazing pressure. However, the increase in light availability counters grazing pressure, which allows growth rates to remain high. In late spring, when the mixed layer is even more shallow, nutrient depletion or overgrazing ends the bloom—losses exceed growth at this point in the cycle. Climate warming would increase stratification and suppress winter mixing that occurs with the deepening of the mixed layer. The suppression of winter mixing would decrease phytoplankton biomass under this hypothesis.

Physical oceanographic processes

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