ReviewSea surface microlayers: A unified physicochemical and biological perspective of the air–ocean interface
Highlights
► The sea surface microlayer is a biogenic film layer at the air-ocean interface. ► Distinct microbial assemblages have defining roles in microlayer functions. ► The sea surface microlayer is fundamentally involved in air-ocean transfer. ► The sea surface microlayer is linked to aerosol production. ► The sea surface microlayer is reservoir of pollutants.
Introduction
The sea surface microlayer (SML) constitutes the uppermost tens to hundreds of μm of the surface of the ocean which is in direct contact with the atmosphere, and has physicochemical and biological properties that are measurably distinct from underlying waters (Liss and Duce, 2005, Zhang et al., 2003). Evidence for the existence of the SML has a long history. Natural slicks on the sea surface in areas of high biological activity, such as in coastal waters, are well documented. Work in the Sargasso Sea established an early link between visible slicks and phytoplankton blooms, and showed that slicks can have fundamental affects on the physical properties of the sea surface. Evidence for a biological origin and activity was revealed in carbohydrate enrichments in slicks relative to underlying waters (Sieburth and Conover, 1965). What is perhaps less well known, however, is that the SML is almost ubiquitous, pervading most of the ocean surface, even under conditions of high surface turbulence, although it is largely indiscernible with the naked eye. The unique position of the SML at the air–sea interface, thus linking the hydrosphere with the atmosphere, means that the SML is central to a range of global biogeochemical and climate-related processes. Consequently, an improved understanding of the SML is a key objective of the International SOLAS (Surface Ocean–Lower Atmosphere Study) Project (www.solas-int.org) and realising this objective requires integrating several scientific disciplines with interests both in SML structure and function.
Key early SML observations (e.g. Sieburth and Conover, 1965) were subsequently refined by researchers employing diverse scientific approaches operating in parallel, yet largely independent of each other. Important new findings include the enrichment of microgels in the SML and compelling molecular evidence for complex SML microbial communities.
Understanding important marine ecosystem processes and their interactions with the wider Earth system requires a holistic view of the ocean–atmosphere interface (Fig. 1). What is clear is that SML structure and functions are intimately bound together, and that a unified physicochemical and biological perspective must be embraced and developed. In this paper we integrate key scientific disciplines to review and discuss the current status of SML research. In particular we highlight recent important discoveries that have improved current understanding, present previously unpublished data on the rate of SML reestablishment following disruption and identify current knowledge gaps in this fundamentally important ecosystem that demand future scrutiny. We initially discuss the thickness of the SML and outline the critical importance of sampling in understanding the SML. The physiochemical structure of the SML is then discussed as visible slicks and as complex films. The stability and persistence of the SML are then examined, particularly in the context of physiochemical properties. We discuss the central role of microorganisms in sustaining SML physiochemical properties and maintaining key biogeochemical processes. The final sections focus on three specific processes, air–sea gas transfer, aerosol production and pollution.
Section snippets
Sea surface microlayer thickness, sampling and the need for scientific best practice
Notwithstanding recent advances that have broadened our overall understanding of the SML, there remains a need to develop and adopt agreed scientific best practice for SML research and data reporting, following the examples set in other areas of marine research, such as ocean CO2 measurement (Dickson et al., 2007) and ocean acidification (Riebesell et al., 2010).
One fundamentally important area where agreed best practice is essential is SML sampling and how this relates to true SML thickness.
Sea surface microlayers as visible slicks
It is important to differentiate between visible and invisible surface films, based on differences in their physiochemical properties and compositions. Visible films, hereinafter referred to as “slicks”, are monomolecular films (monolayers) with a typical thickness of 2–3 nm (Hühnerfuss, 2006a), and are thus beyond the sampling resolution of the various SML samplers discussed in Section 2. Slicks spontaneously form when the concentration of insoluble surfactants exceeds some unknown threshold
Sea surface microlayers as physicochemically complex gelatinous films
Historically, lipids were thought to comprise a significant fraction of SML material, forming a continuous layer above a protein–polysaccharide layer; this was the basis of an early “wet–dry” stratified model of the SML (Hardy, 1982, Hermansson, 1990, Norkrans, 1980). It is now evident that although lipid material is less abundant in the SML than previously thought, it may still be important in affecting SML physicochemical properties (Frka et al., 2009, Mazurek et al., 2008).
Lipids are
Stability of the sea surface microlayer
Wurl et al. (2011b) showed that the SML is stable enough to exist above the global average wind speed of 6.6 m s−1, with no observed depletion of organic matter in the SML above a wind speed of 5 m s−1. Other studies support this conclusion, with organic matter enrichments observed at wind speeds up to ∼10 m s−1 (Carlson, 1983, Kuznetsova et al., 2004, Reinthaler et al., 2008). Such stability fundamentally distinguishes the invisible SML from visible slicks in the open ocean, the latter typically
Sea surface microlayers as active biofilms
The study of microbial life in aquatic surface films is not new. The Swedish botanist Einar Naumann in 1917 introduced the term neuston to identify organisms associated with the air–water interface that were ecologically distinct from plankton in underlying water (Naumann, 1917). Subsequently studies have now shown that the diversity of microorganisms in the SML can differ significantly from that in the underlying water, even a few centimetres below the SML, with bacterial diversity having
Sea surface microlayers and air–sea gas exchange
Air–sea gas exchange has a fundamental role in global biogeochemistry and is driven by molecular and turbulent diffusion. Turbulent diffusion is defined by the length and velocity scales of turbulent eddies in bulk water and in air close to the air–water interface, while molecular diffusion operates at the scale of the SML where these eddies are suppressed (Upstill-Goddard, 2006). A “resistance” to gas exchange is thus manifested in gas concentration gradients either side of the interface, in
The sea surface microlayer and sea spray aerosol
Sea spray aerosol (SSA) are defined as a suspension in air of particles produced directly at the sea surface due to the bursting of bubbles generated by breaking waves or rainfall. Due to the size spectrum and hygroscopic nature of its component particles SSA is a medium for the uptake and reaction of trace gases and is suggested to be a major source of cloud condensation nuclei (CCN). As well as affecting the microphysical and radiative properties of marine clouds, SSA is a major source of
Sea surface microlayers and marine pollution
A range of chemical pollutants are found in the SML, including hydrocarbons, organochlorine compounds and trace metals, which may typically be ten to one hundred times more concentrated than in underlying waters. SML pollutants therefore have a range of potential environmental and ecotoxicological concerns for the functioning of the SML ecosystem (Wurl and Obbard, 2004). Notably, higher accumulations are found in the SML of coastal water close to anthropogenic sources of pollution (Cincinelli
Future research direction and perspectives
To date, studies on the SML have been largely dominated by opportunistic sampling programmes that do not make multiple return visits to sample the same location. Greater understanding of the SML could be achieved by establishing a network of time-series study sites. Time-series observations of both pelagic and coastal systems, such as the Bermuda Atlantic Time Series (www.bats.bios.edu), the Hawaiian Ocean Time Series (www.hahana.soest.hawaii.edu/hot) and the Western Channel Observatory (//www.westernchannelobservatory.org.uk
Acknowledgments
This review was born out of a multi-disciplinary meeting held at the Marine Biological Association of the United Kingdom (MBA), Plymouth, UK (January 2011) and entitled ‘What is the sea surface microlayer? Towards a unified physical, chemical and biological definition of the air–ocean interface’. We thank the International Management Committee of the Cooperation in Science and Technology (COST) Action 735 for supporting this meeting from funds made available through the European Union. We also
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