When oil is discharged to the marine environment a number of weathering processes occur:

Volatile compounds with low boiling points (e.g. saturates up to nC₁₁, mono- and some diaromatic hydrocarbons) are rapidly evaporated after surface spills. These are compounds that are normally rapidly biodegradable in the water column, but evaporation is normally more rapid than biodegradation after a surface spill. Evaporation is slower in cold than in temperate seawater (Brandvik et al. 2005), and this may result in temporarily higher concentrations of volatile toxic compounds (e.g. BTEX) in the seawater. At high concentrations these compounds may prolong microbial lag-phases and delay the onset of biodegradation (Atlas and Bartha 1972; Hokstad et al. 1999), although it is not known if this will have an impact on biodegradation under field conditions. In subsurface releases these volatiles are rapidly dissolved from dispersed oil and we suspect may be biodegraded rather than evaporated. Evaporation also results in increased viscosity of the residual oil (Faksness 2008), which will negatively affect the ability of oil to disperse, thereby slowing biodegradation.

Components dissolved from the oil phase are available for biodegrading microbes in the water column. In cold seawater the dissolution of oil compounds is decreased compared to temperate water (Faksness, 2008). The typical oil compounds in a water-soluble fraction (WSF) from fresh oils include phenols, naphthalenes and 2-3 ring PAHs. In addition, the WSF contains considerable amounts of highly polar compounds with nitrogen, sulphur, and oxygen atoms in their structures (so-called NSO compounds), often present as a chromatographic "hump", termed the "unresolved complex mixture" (UCM). In a study of WSF from an in-reservoir biodegraded oil (Troll) approximately 70 % of the WSF was separated by preparative high-pressure liquid chromatography, into a polar fraction (Melbye et al. 2009).  The non-polar compounds of the WSF are often considered to be rapidly biodegraded in the marine environment (Brakstad and Faksness 2000), and biodegradation of these compounds may result in a significant increase in the UCM  concentration relative to other crude oil components (e.g. Meredith et al. 2000), highlighting their persistence (Han et al. 2008).

Photooxidation is an important process in degrading and transforming crude oil compounds after release to the environment. The polar region exhibits vast seasonal differences in light conditions, and as a result photooxidation varies significantly between the polar summer and winter. UV-irradiation of crude oils has shown that aliphatic compounds are mainly resistant to photodegradation, while aromatic compounds appear particularly sensitive to this process (Maki et al. 2001). In contrast to biodegradation, increased size and alkyl substitution result in increased sensitivity of aromatic hydrocarbons to photochemical oxidation. As photooxidation leads to the inclusion of oxygen atoms in the structures of these compounds, photooxidized products appear mainly in the polar resin fraction of the oil (Maki et al. 2001; Garrett et al. 1998; Prince et al. 2003). Additionally, the average molecular weight of oil compounds is reduced and the oxygen content increased.  Studies have also shown that the photooxidized compounds subsequently exhibit increased susceptibility to biodegradation (Dutta and Harayama 2000; Maki et al. 2001; Ni'matuzahroh et al. 1999). Consistent with the physico-chemical properties of the photooxidized compounds, both the dissolved organic carbon concentration and acute toxicity of the water-soluble fraction of oil increased during the irradiation period (Maki et al. 2001). Thus, photooxidation results in a greater proportion of oxidized compounds that exhibit increased water-solubility and subsequently more significant impacts on toxicity and biodegradation. However, investigation of the relationship between photooxidation, biodegradation and toxicity would be of interest as part of the fate-determination of different oil compounds during the Arctic summer.

In shallow seawater and at higher levels of suspended sediments (e.g. after a storm), sediment particles may adhere to the oil and sink to the subtidal seabed sediments. Oil spills may also drift to shore and be mixed into the intertidal sediments. Oils mixed into the sediments will be subject to microbial processes in the sediment (Refer to Section 3). If seawater replenishment is poor, aerobic processes may consume most of the oxygen, resulting in anoxic conditions. Anaerobic biodegradation of hydrocarbons, by several alternative mechanisms, will occur in the absence of oxygen, as reviewed by Heider (2007).  Genes associated with anaerobic hydrocarbon degradation (e.g. benzyl- and alkylsuccinate synthase genes) have been detected in hydrocarbon-contaminated sediments (Callaghan et al. 2010).  

Water uptake into the spilled oil may cause the formation of viscous and often stable water-in-oil emulsions. Emulsions have been shown to be poorly biodegradable (Brakstad et al. 2011; Cook et al., 2011). Water taken up in the emulsions may contain oil-degrading microbes, but if water is trapped in the emulsions this will not promote biodegradation on the bulk oil as the emulsions may be depleted in essential nutrients. 

With sufficient energy from wave action the oil may break up into droplets in the water column. If oils are easily dispersed, small droplets are generated. The rising or settling rate of the droplets is related to the size and specific gravity of particles. As an example droplets 100 µm in diameter and a lower specific gravity that the surrounding seawater have been observed to rise with a velocity of approximately 1.5 m/h. Thus, larger droplets will resurface rapidly and thin oil films (sheens) may be formed. Oil dispersion is important for biodegradation. Dispersible oils will generate relatively small droplets, resulting in large surface areas for bacterial attachment. For instance, fresh Louisiana Sweet Crude oil can be made to generate dispersions with a median droplet size of 50-150 µm under continuous breaking wave conditions in an oil-on-seawater flume experiment (Brakstad et al. 2011). Several biodegradation studies of dispersed oil in cold seawater (5-8°C) have shown bacterial colonization of oil-droplets and biodegradation of dispersible oils (e.g. Lindstrom and Braddock 2002; MacNaughton et al. 2003; Venosa and Holder 2007; Prince et al. 2012). This colonization may tend to generate flocs of oil and biomass (MacNaughton et al. 2003; Bælum et al. 2012). However, for waxy oils with high pour points, evaporation, dilution and dispersion may be reduced in cold seawater, since precipitated wax may form a matrix which limits internal mixing and acts as a diffusion barrier between the oil and the water (Faksness 2008).

As described above, surface and resurfaced oil may generate thin films on the sea surface. In a series of studies with thin oil films immobilized on hydrophobic adsorbents, n-alkanes in these films were rapidly biodegraded in temperate and cold seawater (0-13°C), while aromatic compounds were subject to mixed dissolution and biodegradation (e.g. Brakstad and Bonaunet 2006; Brakstad et al. 2004). In experiments over a period of 112 days with different oil thicknesses of a wax-rich oil it was apparent that a thickness limit for measurable biodegradation (nC₁₇/Pristane and nC₁₈/Phytane) was between 0.1 and 1.0 mm in cold (6-10°C) seawater (Brandvik et al. 2006).