Eutrophication results

Eutrophication may trigger toxic algal blooms such as red tides, brown tides, and the growth of Pfiesteria. The dinoflagellate, Pfiesteria, can release a very powerful toxin into the water, causing bleeding sores on fish, and even death.

Figure 1: Pfiesteria

Figure 2: Algal bloom

Algal blooms are caused by a sudden increase in nutrients. Algae produce oxygen, initially the ecosystem will be well oxygenated and the fish will thrive. However, the algae will grow exponentially and out-compete other plants for light. Algae also grow at a faster rate than herbivores can consume them, so eventually the algae will die and sink to the seabed uneaten. All this dead algae and plant detritus will cause decomposing bacteria to thrive.

Decomposing bacteria are mainly aerobic so, in such large amounts, they will consume oxygen faster than photosynthesis and diffusion can replace it. Eventually, this series of events will lead to hypoxia, as decomposition of plant matter uses up all the available oxygen in the surrounding habitat.

Hypoxia, or oxygen depletion, is an event that occurs in aquatic ecosystems as dissolved oxygen (DO) becomes reduced to a point that is detrimental to the aquatic organisms living in the system (normally below 2 ppm). When the dissolved oxygen is completely depleted in the water it is called anoxia.