How Ocean Water Modifies and Absorbs Solar Radiation

Introduction: Solar Radiation and Ocean Water

There are a multitude of complex factors that influence the absorption of solar radiation by ocean water, including two very specific laws governing the amount of light transmitted through a liquid. The Beer-Lambert law governs the transmission of solar spectra in ocean water, but beyond this foundational principle, other factors also play a role in the way radiation is influenced and absorbed (Stramski and Woz'niak, 2005). The role of suspended particulate matter and the composition of that matter also have much to do with absorption, and can be traced to differing patterns of distribution and diffusion. The seasonal variations and cycles that occur within the ocean are poorly understood, especially when it comes to solar radiation and light diffusion and backscattering. These effects, as far as natural phenomena are concerned, are reasonably well characterized; the factors influencing them are not.

The types of particulate matter are many, and their roles are only now being understood and fitted into the complex picture that constitutes the ocean's solar radiation and light absorption dynamics. Organic particulates such as phytoplankton, as well as man-made pollutants, colloids, and minerals, all have specific effects on ocean water when it comes into contact with solar radiation (Stramski and Woz'niak, 2005). The previously oversimplified understanding of how the size, shape, and composition of these particulates influence their behavior — and alter the optical dynamics of seawater — is starting to be more clearly understood within the scientific community. Ocean water has far more dynamic and complex optical qualities than previously recognized. The purity and general health of the ocean in specific sample regions can also be determined based on the diffusion levels of the water itself (Stramski, Boss, Bogucki, and Voss, 2004). This is not to say that water as an inorganic compound controls ocean health; rather, the presence of certain organisms — namely algae at specific depths and temperatures — helps to estimate the environmental costs of man-made and inorganic pollutants.

Satellite Data and Research

Over the past twenty years, scientists have used satellites to help measure the way that light and solar radiation are diffused in ocean water. By distinguishing how particle size and depth affect this diffusion, scientists have been able to observe and predict diffusion levels using imagery from outer space. According to satellite data, ocean water found primarily in tidal mixed shelf seas and estuaries exhibits relatively uniform and predictable scattering of light based on water color and the particles suspended within it (Stramski, Boss, Bogucki, and Voss, 2004). The levels of particulates as well as their sizes also suggest that a seasonal cycle occurs relative to changes in turbidity and tides. This comes into play with the diffusion of solar spectra because, as particles become smaller during periods of higher turbidity, diffusion occurs at greater depths — a result that has more to do with the dispersion of particulates and their sizes across seasons.

Local knowledge of water turbulence can also help improve satellite estimations of light and radiation diffusion, because as seasonally adjusted turbidity changes, so does the light reflected back to satellites (Lahet and Stramski, 2010). This data demonstrates that solar radiation is reflected by ocean water differently during different seasons and weather patterns. It is also important to note that other specifics — such as the location and tidal activity of the area under study — are significant in measuring this diffusion.

Backscattering and Missing Light Diffusion

Historically, the idea that certain elements in the water can scatter and diffuse light in different ways, under different and often seasonal conditions, has been surrounded by mystery. The concept itself has enjoyed broad scientific acceptance, but the precise mechanisms of light diffusion have recently become a subject of renewed interest now that satellites and advanced instrumentation allow scientists to measure the phenomenon far more accurately. In the past, scientists believed that phytoplankton and other organic compounds were responsible for up to 80% of light backscatter and diffusion in the ocean. Scientific studies seemed to support this view, which was notable given the seasonal nature of phytoplankton blooms. In recent years, however, as the role of man-made particulate matter has come under greater scrutiny, scientists are beginning to recognize that both the seasonality of these blooms and other organic and inorganic compounds contribute more than the seasonal factors alone, and on a larger scale.

Because of the substantial variety of compounds in the water, recent studies have shown that these compounds diffuse and backscatter light in different ways and to different extents than previously understood. It appears that small, non-living particles have a greater backscattering effect on incoming light and solar radiation than organic ones, according to one key study (Stramski, Boss, Bogucki, and Voss, 2004). This means that both the organic and man-made particulates currently present in the ocean are beginning to affect the behavior of the ocean's primary energy source — sunlight and solar radiation — both at the surface and at specific depths. It is worth noting that while the previously adopted scenario of phytoplankton accounting for up to 80% of backscattering was relatively accurate during certain times of year, it is becoming increasingly evident that the effects of particulates depend largely on the depth, consistency, and seasonality of the water and its contents.

The role that phytoplankton play in diffusion is well understood. However, until recently it was not well documented that this role, though substantial, operates only during certain specific periods. For the remainder of the year, other particulates are responsible for backscattering light and diffusing solar radiation (Dera, Sagan, and Stramski, 1993). This reveals a balance within the ocean that is being disrupted. Other conditions — such as air bubbles introduced near shore as waves break, tidal variations, and the presence of shallow seas — help compensate for the effects of non-bloom periods for phytoplankton. Scientists are developing a much clearer and more complex picture of how the seas scatter, diffuse, and absorb light depending on conditions and particulates. As Stramski, Boss, Bogucki, and Voss (2004) conclude, the study of how sunlight and radiation affect the ocean has been grossly oversimplified, and the true processes governing ocean cycles and functions are far more complex than previously believed.

Conclusion

To summarize, ocean water and solar radiation spectra are influenced by myriad factors. This is not to say that scientists will never achieve a firm grasp of the larger picture, but evidence is emerging that the complexities associated with even a single diffusion modifier are quite striking. The particulates suspended in ocean water — whether organic, inorganic, colloidal, or algal — all influence the way in which light interacts with water. The regional and seasonal characteristics of ocean water columns are also significant, since these factors affect how the environment itself interacts with the water surface. Scientists are now better equipped to predict the purity, health, and backscattering capabilities of specific regions of the world's oceans, and will be able to use this knowledge to build more accurate models of how human activity has impacted the ocean and the broader environment.