Sea spray

Connection between sea foam and sea spray formation. The dark orange line indicates processes common to the formation of both sea spray and sea foam.

When wind, whitecaps, and breaking waves mix air into the sea surface, the air regroups to form bubbles, floats to the surface, and bursts at the air-sea interface.[10] When they burst, they release up to a thousand particles of sea spray,[10][11] which range in size from nanometers to micrometers and can be expelled up to 20 cm from the sea surface.[10] Film droplets make up the majority of the smaller particles created by the initial burst, while jet droplets are generated by a collapse of the bubble cavity and are ejected from the sea surface in the form of a vertical jet.[12][11] In windy conditions, water droplets are mechanically torn off from crests of breaking waves. Sea spray droplets generated via such a mechanism are called spume droplets [11] and are typically larger in size and have less residence time in air. Impingement of plunging waves on sea surface also generates sea spray in the form of splash droplets [11][13]. The composition of the sea spray depends primarily on the composition of the water from which it is produced, but broadly speaking is a mixture of salts and organic matter. Several factors determine the production flux of sea spray, especially wind speed, swell height, swell period, humidity, and temperature differential between the atmosphere and the surface water.[14] Production and size distribution rate of SSAs are thus sensitive to the mixing state.[15] A lesser studied area of sea spray generation is the formation of sea spray as a result of rain drop impact on the sea surface .[11]

In addition to the local conditions that influence sea spray formation, there are also consistent spatial patterns in sea spray production and composition. Because sea spray is generated when air is mixed into the ocean, formation gradients are established by turbulence of the surface water.[14] Wave action along coastal shorelines is generally where turbulence is greatest, so this is where sea spray production is the highest. Particles generated in turbulent coastal areas can travel horizontally up to 25 km within the planetary boundary layer.[14] As distance from shore decreases, sea spray production declines to a level sustained almost exclusively by whitecaps.[14] The proportion of the ocean surface area that is turbulent enough to produce significant sea spray is called the whitecap fraction.[10] The only other production mechanism of sea spray in the open ocean is through direct wind action, where strong winds actually break the surface tension of the water and lift particles into the air.[10] However, particles of seawater generated in this way are often too heavy to remain suspended in the atmosphere and usually are deposited back to the sea within a few dozen meters of transport.[10]

During winter months, the ocean typically experiences stormy, windy conditions that generate more air inundation into the sea and therefore more sea spray.[16] Calmer summer months result in lower overall production of sea spray.[16] During peak primary productivity in the summer, increased organic matter in the surface ocean drives subsequent increases in sea spray. Given that sea spray retains the properties of the water from which it was produced, the composition of sea spray experiences extreme seasonal variation. During the summer, dissolved organic carbon (DOC) can constitute 60-90% of sea spray mass.[16] Even though much more sea spray is produced during the stormy winter season, the composition is nearly all salt because of the low primary production.[16]

The organic matter in sea spray consists of dissolved organic carbon[17] (DOC) and even microbes themselves, like bacteria and viruses.[18] The amount of organic matter in sea spray depends on microbiological processes,[19] though the total effect of these processes is still unknown.[20][21] Chlorophyll-a is often used as a proxy for primary production and organic matter content in sea spray, but its reliability for estimating dissolved organic carbon concentrations is controversial.[21] Biomass often enters sea spray through the death and lysis of algal cells, often caused by viral infections.[20] Cells are broken apart into the dissolved organic carbon that is propelled into the atmosphere when surface bubbles pop. When primary productivity peaks during the summer, algal blooms can generate an enormous amount of organic matter that is eventually incorporated into sea spray.[16][20] In the right conditions, aggregation of the dissolved organic carbon can also form surfactant or sea foam.

The influence of sea spray on the surface heat and moisture exchange peaks during times of greatest difference between air and sea temperatures.[22] When air temperature is low, sea spray sensible heat flux can be nearly as great as the spray latent heat flux at high latitudes.[6] In addition, sea spray enhances the air/sea enthalpy flux during high winds as a result of temperature and humidity redistribution in the marine boundary layer.[7] Sea spray droplets injected into the air thermally equilibrate ~1% of their mass. This leads to the addition of sensible heat prior to ocean reentry, enhancing their potential for significant enthalpy input.[7]

The effects of sea spray transport in the atmospheric boundary layer is not yet completely understood.[11] Sea spray droplets alter the air-sea momentum fluxes by being accelerated and decelerated by the winds.[11] In hurricane-force winds, it is observed that there is some reduction in the air/sea momentum flux.[10] This reduction in momentum flux manifests as saturation of air/sea drag coefficient. Some studies have identified spray effects as one of the potential reasons for the air/sea drag coefficient saturation.[24][25][26] It has been shown through several numerical and theoretical studies that sea spray, if present in significant amounts in the atmospheric boundary layer, leads to saturation of air-sea drag coefficients.[27][28]

Salt deposition from sea spray is the primary factor influencing distribution of plant communities in coastal ecosystems.[29] Ion concentrations of sea spray deposited on land generally mirror their concentrations in the ocean, except that potassium is often higher in sea spray.[8] Deposition of salts on land generally decreases with distance from the ocean but increases with increasing wind speed.[8] Salt deposition from sea spray is correlated with a decrease in plant height and significant scarring, shoot reduction, stem height decrease, and tissue death on the windward side of shrubs and trees.[30][31] Variation in salt deposition also influences competition between plants and establishes gradients of salt tolerance.[30]

While the salts within sea spray can severely inhibit plant growth in coastal ecosystems, selecting for salt-tolerant species, sea spray can also bring vital nutrients to these habitats. For example, one study showed that sea spray in Wales, UK delivers roughly 32 kg of potassium per hectare to coastal sand dunes each year.[10] Because dune soils leach nutrients very quickly, sea spray fertilization could be very influential to dune ecosystems, especially for plants that are less competitive in nutrient-limited environments.

Sea spray containing marine microorganisms can be swept high into the atmosphere where they become aeroplankton. These airborne microorganisms may travel the globe before falling back to earth.

Viruses, bacteria, and plankton are ubiquitous in sea water, and this biodiversity is reflected in the composition of sea spray.[14] Generally speaking, sea spray has slightly lower concentrations of microbes than the water it is produced from. However, the microbial community in sea spray is often distinct from nearby water and sandy beaches, suggesting that some species are more biased towards SSA transportation than others. Sea spray from one beach can contain thousands of operational taxonomic units (OTUs).[14] Nearly 10,000 different OTUs have been discovered in sea spray just between San Francisco, CA and Monterey, CA, with only 11% of them found ubiquitously.[14] This suggests that sea spray in every coastal region likely has its own unique assemblage of microbial diversity, with thousands of new OTUs yet to be discovered. Many of the more common OTUs have been identified to the following taxa: Cryptophyta (order), Stramenopiles (order) and OM60 (family).[14] Many have even been identified to genus: Persicirhabdus, Fluviicola, Synecococcus, Vibrio, and Enterococcus.[14]

Scientists have conjectured a stream of airborne microorganisms circles the planet above weather systems but below commercial air lanes.[32] Some of these peripatetic microorganisms are swept up from terrestrial dust storms, but most originate from the marine microorganisms in sea spray. In 2018 a team of scientists reported that hundreds of millions of viruses and tens of millions of bacteria are deposited daily on every square meter around the planet.[33][34]