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:
Dilution-recoupling Hypothesis - an ecosystem-based view Source: 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. These modulations, along with light availability, drive the abundance of phytoplankton in the region. The availability of phytoplankton significantly affects the marine food web and ocean health. The fast-moving currents in the
Gulf Stream meander and pinch-off to create eddies. These eddies retain the physical properties of their parent
water mass (e.g. temperature, density, salinity, and other ocean dynamic properties) when they separate. As the eddies migrate, their physical properties change as they mix with the surrounding water. In the Gulf Stream, migrating eddies are known as anticyclonic or cyclonic eddies based on the direction in which they spin (clockwise vs. counter-clockwise).
Upwelling and
downwelling processes in the eddies create a cold and warm core. Downwelling in the anticyclonic eddy prevents colder water from entering the surface, thus creating a warm-core in the center
. Whereas in the cyclonic eddy, the upwelling entrains deep cold water and forms a cold-core. Previous studies show the deepening effects of MLD under anticyclonic eddies and shoaling of MLD in cyclonic eddies. These phenomena may be due to increased heat loss to the atmosphere in anticyclonic eddies. This loss of heat causes the sinking of dense water, referred to as convective mixing'''''', and the deepening of the MLD. In contrast, in cyclonic eddies the water temperature at the core is less cold than the Anticyclonic eddy. This therefore does not lead to deepening of the MLD. Studies conducted in the region via a network of
Argo floats and model simulations created through satellite data have shown cases of the opposite phenomena. The deepening and shoaling of MLD via eddies is ubiquitous and varies seasonally. The MBL is characterized by the formation of convective cells (or vertical flow of air) above the ocean surface, which perturbs the direction of the mean surface wind and generates texture, roughness, and waves on the sea's surface. Two types of boundary layers exist. One is a stable, convective layer found between the lower 100m of the atmosphere extending up to approximately 3 km in height, and is referred to as the convective boundary layer (CBL). The other boundary layer forms as a result of a surface
atmospheric inversion. This generally occurs closer to the surface in the absence of turbulence and vertical mixing, and is determined through the interpretation of vertical humidity and temperature profiles. The MBL is often a localized and temporally dynamic phenomenon, and therefore its height into the air column can vary considerably from one region to another, or even across the span of a few days. The North Atlantic is a region where diverse and well-formed MBL clouds are commonly formed, and where MBL layer height can be between 2.0-and 0.1 km in height As these particles age or are chemically transformed as a function of time in the air, they may alter microphysical and chemical properties as they react with other airborne particles.
Role of aerosols Aerosols Aerosols are very small, solid particles or liquid droplets suspended in the atmosphere or inside another gas and are formed through natural processes or by human actions. Natural aerosols include volcanic ash, biological particles, and mineral dust, as well as
black carbon from the natural combustion of biomass, such as wildfires. Anthropogenic aerosols are those that have been emitted from human actions, such as fossil fuel burning or industrial emissions. Aerosols are classified as either primary or secondary depending on whether they have been directly emitted into the atmosphere (primary) or whether they have reacted and changed in composition (secondary) after being emitted from their source. Aerosols emitted from the marine environment are one of the largest components of primary natural aerosols. Marine primary aerosols interact with anthropogenic pollution, and through these reactions produce other secondary aerosols. One of the most significant yet uncertain components of predictive climate change models is the impact of aerosols on the climate system. Aerosols affect Earth's radiation balance directly and indirectly. The direct effect occurs when aerosol particles scatter, absorb, or exhibit a combination of these two optical properties when interacting with incoming solar and infrared radiation in the atmosphere. Aerosols that typically scatter light include sulfates, nitrates, and some organic particles, while those that tend to exhibit a net absorption include mineral dust and
black carbon (or soot). The second mechanism by which aerosols alter the planet's temperature is called the indirect effect, which occurs when a cloud's microphysical properties are altered causing either an increase in reflection of incoming solar radiation, or an inhibited ability of clouds to develop precipitation. The first indirect effect is an increase in the amount of water droplets, which leads to an increase in clouds that reflect more solar radiation and therefore cool the planet's surface. The second indirect effect (also called the cloud's lifetime effect) is the increase in droplet numbers, which simultaneously causes an increase in droplet size, and therefore less potential for precipitation. That is, smaller droplets mean clouds live longer and retain higher liquid water content, which is associated with lower precipitation rates and higher cloud
albedo. This highlights the importance of aerosol size as one of the primary determinants of aerosol quantity in the atmosphere, how aerosols are removed from the atmosphere, and the implications of these processes in climate.''' Without a better understanding of aerosol sizes and composition in the North Atlantic Ocean, climate models have limited ability to predict the magnitude of the cooling effect of aerosols in global climate. Note the net cooling effect of sulphates.
Sea-spray Aerosols Although the amount and composition of aerosol particles in the marine atmosphere originate both from continental and oceanic sources and can be transported great distances, freshly emitted
sea-spray aerosols (SSA) constitute one of the major sources of primary aerosols, especially from moderate and strong winds. The estimated global emission of pure sea-salt aerosols are on the order of 2,000-10,000 Tg per year. especially at high latitudes such as the North Atlantic Ocean. Organic matter in these aerosols help nucleation of water droplets at these regions, yet plenty of unknowns remain, such as what fraction contain ice-freezing organic materials, and from what biological sources. Primary marine aerosols created through bubble-bursting emission have been measured in the North Atlantic during spring 2008 by the International Chemistry Experiment in the Arctic Lower Troposphere (ICEALOT). This research cruise measured clean, or background, areas and found them to be mostly composed of primary marine aerosols containing hydroxyl (58% ±13) and alkene (21% ±9) functional groups, indicating the importance of chemical compounds in the air with biological origin. Nonetheless, the small temporal scale of these measurements, plus the inability to determine the exact source of these particles, justifies the scientific need for a better understanding of aerosols over this region. However, recent studies of OA are correlated with
DMS production and to a lesser extent chlorophyll, suggesting that organic material in sea salt aerosols are connected to biological activity in the sea's surface. The mechanisms contributing to marine organic aerosols thus remain unclear, and were a main focus of NAAMES. There is some evidence that marine bioaerosols containing cyanobacteria and microalgae may be harmful to human health. Phytoplankton can absorb and accumulate a variety of toxic substances, such as
methylmercury,
polychlorinated biphenyls (PCBs), and
polycyclic aromatic hydrocarbons. Cyanobacteria are known to produce toxins that can be aerosolized, which when inhaled by humans can affect the nervous and liver systems. For example, Caller et al. (2009) suggested that bioaerosls from cyanobacteria blooms could play a role in high incidences of
amyotrophic lateral sclerosis (ALS). In addition, a group of toxic compounds called
microcystins are produced by some cyanobacteria in the genera
Microcystis, Synechococcus, and
Anabaena. These microcystins have been found in aerosols by a number of investigators, and such aerosols have been implicated as causing isolated cases of
pneumonia,
gastroenteritis, and
non-alcoholic fatty liver disease. with the genus
Ostreopsis causing symptoms such as
dyspnea, fever,
rhinorrhea, and cough. Importantly, marine toxic aerosols have been found as far as 4 km inland, but investigators recommend additional studies that trace the fate of bioaerosols further inland. Of these,
Agaricomycetes constitutes the majority (95%) of fungi classes inside this phylum. Within this group, the
Penicillium genus is most frequently detected in marine fungi aerosols. Fungi bioaerosols can also serve as ice nuclei, and therefore also impact the radiative budget in remote ocean regions, such as the North Atlantic Ocean. The
CLAW hypothesis conceptualizes and tries to quantify the mechanisms by which phytoplankton can alter global cloud cover and provide planetary-scale radiation balance or
homeostasis regulation. As
solar irradiance drives primary production in the upper layers of the ocean, aerosols are released into the
planetary boundary layer. A percentage of these aerosols are assimilated into clouds, which then can generate a negative feedback loop by reflecting solar radiation. The ecosystem-based hypothesis of phytoplankton bloom cycles (explored by NAAMES) suggests that a warming ocean would lead to a decrease in phytoplankton productivity. Decreased phytoplankton would cause a decrease in aerosol availability, which may lead to fewer clouds. This would result in a positive feedback loop, where warmer oceans lead to fewer clouds, which allows for more warming. One of the key components of the CLAW hypothesis is the emission of
dimethylsulfoniopropionate (DMSP) by phytoplankton. Another chemical compound, dimethyl sulfide (DMS), has been identified as a major volatile sulfur compound in most oceans. DMS concentrations in the world's seawater have been estimated to be, on average, on the order of 102.4 nanograms per liter (ng/L). Regional values of the North Atlantic are roughly 66.8 ng/L. These regional values vary seasonally and are influenced by the effects of continental aerosols. Nonetheless, DMS is one of the dominant sources of biogenic volatile sulfur compounds in the marine atmosphere. The SML is considered a "skin" within the top 1 millimeter of water where the exchange of matter and energy occurs between the sea and atmosphere. The biological, chemical, and physical processes occurring here may be some of the most important anywhere on Earth, and this thin layer experiences the first exposure to climatic changes such as heat, trace gases, winds, precipitation, and also wastes such as nanomaterials and plastics. The SML also has important roles in air-sea gas exchange and the production of primary organic aerosols. A study using water samples and ambient conditions from the North Atlantic Ocean found that a polysaccharide-containing
exopolymer and a protein are easily aerosolized in surface ocean waters, and scientists were able to quantify the amount and size resolution of the primary sea to air transport of biogenic material. These materials are small enough (0.2μm) to be largely emitted from phytoplankton and other microorganisms. However, predicting aerosol quantity, size distribution, and composition through water samples are currently problematic. Investigators suggest that future measurements focus on comparing fluorescence detection techniques that are able to detect proteins in aerosols. NAAMES filled this research gap by providing a fluorescent-based instrument (See section on Atmospheric Instruments below), both in the air column and near the sea's surface.
NAAMES Objectives • Identify the different features of the annual cycle of phytoplankton blooms in the North Atlantic and determine the different physical processes affecting those features. To accomplish this objective, a combination of ship-based, airborne, and remote sensing measurements was used. NAAMES conducted multiple campaigns that occurred during the various phases of the cycle in order to capture the important transitory features of the annual bloom for a comprehensive view. • Understand how the different features of the North Atlantic annual phytoplankton cycle interact to "set the stage" for annual blooms. This objective seeks to reconcile the competing resource-based and ecosystem-based hypotheses. NAAMES goal was to provide the mechanistic field studies necessary to understand a more holistic view of the annual bloom cycle. • Determine how the different features of the annual phytoplankton cycle affect marine aerosols and cloud formation. The effects of aerosols on clouds is an understudied topic despite the major implications it could have for predicting future climate change. This objective addressed this gap by using combined measurement methods to understand the contribution of various aerosols to cloud formation produced during each major phase of the annual phytoplankton cycle. == Methodology ==