Harmful Algae Blooms (HABs) can result in a wide variety of sea life mortality from fish and plants to Manatees and Sea Lions and are shocking even to observe.
The exact mechanism of mortality may be a combination of oxygen depletion in water, production of deadly toxins and in some cases irritation to skin and gills which produce excess mucus and interferes with normal respiration.
Toxic algal growth increase can be caused by natural, regular upwelling in the ocean brining nutrient rich bottom water to the surface. However, the development of toxic algae seem to be on the rise both in severity and in the areas affected.
The author remembers shrimp mortality due to observed toxic algae blooms in limited water exchange experimental ponds in Alabama during the late 90s. This memory was prompted by the current overwhelming amount of recent information on the decimation of $800 million worth of salmon in pens due to a toxic algae bloom off the coast of Chile, where 37 out of 415 farms from the second biggest salmon producer in the world were affected. This amount is estimated to be 20% of the total production.
According to many sources, higher temps of 2-4oC above normal due to a strong el Niño, lack of rainfall and water movement, and lots of sunlight caused the toxic algae explosion.
Pseudochattonella verruculosa is the microalgae responsible for the Chile fish kill. It has been identified around Chile salmon farms as far back as 2004. According to article in the journal of Harmful Algae, this algae is "heterokont flagellate", found as far away as Wellington Harbor in New Zealand.
Changes in conditions and patterns in a marine ecosystems such as slower water circulation, with increase in nutrients such as nitrogen and phosphorous due to a variety of point and non point sources of pollution can cause intense algae blooms.
Excess phosphorous release can also occur due to anaerobic sediments amplified by stratification in standing water bodies. In contrast to this, excess rainfall from an el Niño (la Niña) in other areas can cause HAB. The latest information shows that heavy rains from the el Niño have caused a massive fish kill from pollution washing in to local waterways.
Along with higher temps (>25oC) both variable light conditions and stable, stagnant, conditions can contribute to an increase in toxic blue green algae. Increases in available nutrients such as silica, iron and carbon in lesser amounts have been known possible contributors increasing densities of toxic algae. There are about 40 some species of well known harmful algae producing toxins which are created to ward off consumption by ubiquitous, microscopic zooplankton. Harmful Cholera bacteria thrive in zooplankton blooms caused by increase in available algae and as 80% of disease is known to be water born, toxic algae have far reaching effects.
When phosphorous levels become low, some common algae such as Karenia brevis which results in the red tide in the gulf of Mexico, actually increase the toxins they produce. As soon as the algae run out of nitrogen and phosphorous they weaken and are often destroyed by none other than viruses along with herbivorous plankton.
As more algae decay, this may result in a huge oxygen deficit causing death to most aerobic organisms that can't get away from the anaerobic zones created. Lower pH in some waters actually inhibit the toxicity of some freshwater algae while toxins from algal blooms are eventually broken down by bacteria.
