Algal blooms

Water is colourless (at least when in small amounts) but in larger volumes, it is typically seen as a deep blue colour. When we think of the ocean, we immediately picture a blue body of water. And yet, due to a combination of environmental factors, natural bodies of water can unexpectedly become colourful; one reason for such changes can be the high concentration of aquatic microorganisms. These can vary rapidly, leading to an increase in the population of algae, great enough to turn the water completely green or sometimes even red! Such population explosions are commonly known as algal blooms.

Algal blooms occur when excessive amounts of nutrients and organic matter, such as nitrogen - and phosphorous-rich compounds, enter slow-moving bodies of water. These nutrients act as fertilizers and promote the rapid growth of algae.

The formation of blooms can have dramatic effects on the chemistry of water, especially on its pH and dissolved oxygen (DO):

- During photosynthesis, algae remove carbon dioxide (CO₂) from the water, which increases the hydroxide (OH−) levels, resulting in an increase in pH; they also produce large amounts of oxygen (O₂), which may result in supersaturated levels of DO;
- During respiration, carbon dioxide is produced and hydroxide levels decrease, which results in a subsequent lowering of the pH. During this process, algal blooms remove DO resulting in, little or no oxygen, remaining in the water.

Such changes to water chemistry make algal blooms quite dangerous for many organisms: when dissolved oxygen levels fall beneath a certain level, fish and other aquatic creatures can no longer survive.
Some algal bloom events can be caused by the proliferation of other groups of organisms, for example cyanobacteria, also termed blue-green algae.
Beyond changes in pH and other chemical properties of the water, algal blooms can also affect its taste and odor, or can create large fibrous mats that obstruct waterways, making them unusable for fishing, swimming, and even for navigation.
Some species of algae can produce very harmful toxins, and the presence of algal blooms in reservoirs of drinking water can be of great concern. In fact, some species are known to produce toxins that have been linked to the deaths of livestock and pets. Many toxic species of phytoplankton display a red or brown hue when they bloom, and are often called “red tides”. They are associated with large-scale marine mortality events: in the Gulf of Mexico, for example, there is a microscopic marine algae (Karenia brevis) that naturally occurs at lower concentrations but can be very dangerous at high concentrations, as it produces a toxin that affects the central nervous system of fish, paralyzing them and inhibiting their respiration.
Red tides occur on coastlines worldwide, and while not all red tides are the result of organisms that produce toxins, some can be potentially harmful to humans: there have been cases of people showing severe respiratory conditions after eating contaminated shellfish.