ECO PRO The Chemistry of the Sea: What Every Dive Pro Should Know – by Alex Brylske, Ph.D., President, Ocean Education International, LLC
When we think of diving, probably the last thing that comes to mind is chemistry – a subject you probably hated in high school. Yet, beneath all the ocean's beauty lies an invisible framework of chemistry that not only
be no ice and no liquid water on Earth, and probably no life at all! Now that we understand something about the nature of water, the next question is, Why is seawater salty? On aver- age, seawater contains about 35 parts per thousand of dis- solved salts, primarily but not exclusively sodium and chloride (which are the constituents of salt). This salinity re- sults from the weathering of rocks on land, volcanic gases, and hydrothermal vents on the seafloor. Rivers carry these minerals to the ocean, where evaporation leaves them be- hind. For divers, salinity has real implications. It affects your buoyancy – hence why you need more weight in the sea than in freshwater – and it can vary significantly between loca- tions. For example, areas with high evaporation, such as the Red Sea, have higher salinity, whereas places with significant freshwater input, like estuaries, tend to have lower salinity. Salinity also influences the distribution of marine life, with many species adapted to narrow ranges of salinity.
supports life in the sea but also is essential in maintaining Earth’s stability. For example, life as we know it could not exist without the unique structure of the water molecule, and chemistry is one of the factors explaining why ocean cur- rents occur. It also plays a key role in understanding how vital, life-sustaining chemicals cycle between the ocean, at- mosphere, and land. And many of these chemicals form the foundation of all life. Ultimately, the ocean’s chemistry is one of the main factors controlling which plants and animals in- habit its depths. Let’s start with water itself. Water isn’t just the medium we dive through – it’s a molecule with nearly magical properties.
Each H ₂ O molecule has a bent shape, with a slight negative charge near the oxygen atom and positive charges near the hydro- gen atoms. This polarity gives water its “stickiness,” known sci- entifically as hydrogen bonding or polarity. For divers, these hydro- gen bonds explain why surface tension can hold a fog of droplets on the inside of your mask. It’s also why “defog” solutions work – they’re surfactants that break the bonds between water mole- cules, allowing water to form a smooth, transparent film instead of clumping into beads that ran-
One of the most remarkable fea- tures of ocean chemistry is its con- sistency. Despite the variety of sources supplying minerals to the sea, the ocean’s chemical makeup remains surprisingly stable. This stability is maintained by the con- cept of “residence time” – the av- erage duration an element remains dissolved in seawater before it is removed through processes like being absorbed by organisms or precipitating out. Some chemicals, such as chloride, can remain dis- solved in seawater for millions of years, while others, such as the mi- cronutrient iron, remain dissolved
domly bend light waves and obscure your vision. Hydrogen bonding also causes water’s high viscosity and surface tension – factors that affect how organisms move through water. For example, plankton can remain suspended in colder, more vis- cous water with less effort, while swimmers (including divers) must work harder to move through it. But perhaps the most astonishing evidence of the crucial role played by hydrogen bonding is this: Without hydrogen bonding, water would not freeze until it reached -130°F (-90°C) and would then boil away at a chilly -94°F (-70°C). In other words, there would
for less than a century. For divers, this consistency means conditions are predictable, which is part of what makes the ocean such a welcoming environment for life – and a de- pendable place to dive. However, human activities, such as CO ₂ emissions, are beginning to disrupt this chemical bal- ance, particularly through the process of ocean acidification. Regarding CO ₂ emissions, the ocean plays a vital role in controlling Earth’s climate by absorbing atmospheric carbon dioxide. Some of this CO ₂ dissolves directly into seawater,
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