10 Harmful Algal Blooms
Algae blooms can be a natural process that occurs in lakes, rivers, and estuaries, when conditions are just right (temperature, nutrients, etc…). But these blooms can become harmful to people when so much algae grows that when it dies, algal decomposition removes all the oxygen from the water column. Another kind of harmful algal bloom can occur when certain species of cyanobacteria, that produce toxins grow rapidly. Human activities (increasing temperatures, excess nutrients) have increased the likelihood and extent of many algae blooms. The video below highlights the key drivers of these blooms.
10.1 Alternative stable states
One important idea that lake ecologists have discovered in the past 30 years is that lakes are susceptible to sudden shifts in ecological status. With regards to harmful algal blooms specifically, this essentially means that nutrients can be added for years to a lake with no visible sign of a transition to a more eutrophic, algal bloom dominated state. Then, all of a sudden the lake can switch into an algal dominated state. The key idea with alternative stable states in lakes is that simply removing the same amount of nutrients from the system that were originally added, will not return the lake to a lower algae state. This idea has been written about 1000s of times, but a great paper from 1993 by Scheffer and others is really well-written and clear.
For a primer on alternative stable states you should visit this page