are normally kept in check due to the same mechanisms that keep all life forms in check. Simply put, if there's not enough food the population dies off until the amount of predators is at a level sustainable by the amount of prey.
In the case of algae, it is not a predator. However, it does require certain nutrients to survive and reproduce. The main ingredients algae require are Nitrogen and Phosphorus. These occur naturally to some extent. Fertilizer used in the many Agricultural Farms of Pine Island as well as Lawn & Garden contain artificially high concentrations of these nutrients. Storm runoff eventually carries some levels of these nutrients below sea level via man-made storm drainage systems and natural streams or underground aquifers.
If too much of these nutrients reach the water the algae will respond by multiplying to the point that the number of nutrients will support them. This is called an Algal Bloom.
Algal Blooms can be deadly to the areas undersea marine life for a number of reasons. One reason is that the smaller organisms produced in the upper waters are short-lived. After dying, they tend to sink into the deeper areas, so that there is a continuous "rain" of dead organic material also known as detritus. Though the algae are a mere single cell, the shear numbers dying and reproducing algea will make the water a cloudy greenish brown and in sever cases reduce visibility to near zero. This of course changes the amount of light that will reach the bottom where other oxygen producing plants lie. The bottom then becomes an aphotic zone meaning it is too dark for photosynthesis (in other words too dark for plants to produce oxygen). If the high nutrient level and algae bloom persists, in time, the lower plants will die and be broken down. The raining detritus and the dying plant life beneath will be feasted on by Aerobic Microbes (aerobic meaning requires oxygen to live). Microbes (aerobic and anaerobic) are always around and will appear on the scene like everything else in life, at the level that their food source will support them. These microbes appear as magically as mold does to bread. The mold is always around us but until it has a suitable food source it's levels are maintained. As the number of anaerobic (oxygen using) microbes goes up, the dissolved oxygen they need to live goes down.
In worst case scenarios the oxygen in levels below the surface is so depleted that only even the anaerobic microbes die off and only Anaerobic microbes (anaerobic means it does not use oxygen to live) can continue to live.
Of course by this time every fish, oyster, crab, and anything else that requires oxygen has suffocated to death due to the lack of oxygen. Their only hope was if they were capable of escaping the area affected which can be quite a large area sometimes many miles in size.
A question that often comes up is why don't the algae die when the oxygen is depleted. The answer is that although algae do continue to die, the algae at the surface layer continue to get a fresh supply of oxygen from where the water surface meets the outside air.
Nature's suggestion: Use less fertilizer
Honorable Mention To:
Biology - Second Edition
Author Neil A. Campbell - University of California, Riverside