Since humans are so dependent on the oceans, for example for food and recreation, human health is closely coupled to the health of the oceans. A striking example is the occurrence of Harmful Algal Blooms, or "red tides". These are proliferations of phytoplankton, some, but not all of which are toxic. The toxins produced accumulate in shellfish and can cause severe poisoning to the consumer. Certain forms of harmful algae can cause respiratory discomfort and skin irritation to bathers.