When conditions are right, algae in marine or freshwater environments can rapidly grow causing a "bloom". A red tide is called a harmful algal bloom. The water will have a murky appearance and depending on the type of algae, the color can range from green, to red, to purple or to pink. Sometimes, oxygen is depleted due to the fact that when there is such a great amount of algae, as it dies off, the bacteria utilize the oxygen in its decomposition. These makes the environment oxygen poor, which can cause fish kills. This in turn affects birds and mammals depending on these fish for food. Red tide can produce toxins that paralyze the central nervous system of fish. This makes them unable to breathe and they die, making the water quality smelly. This red algae if eaten by humans can affect their respiratory system and eyes. Some species of red tide can affect filter feeding shellfish, rendering them poisonous for human consumption--it can lead to death due to respiratory failure and is found in some clams and mussels. Red tides are due to--algae blooms which are a natural seasonal event, coastal upwelling, increased nutrients--like nitrates and phosphorus in the water due to erosion from neighboring farms, coastal runoff, wind erosion carrying dust which is iron rich, increase in ocean temperature and even El Nino events.