Pollution due to biological organisms

 

Algal Blooms and Harmful Algal Blooms (HABs):

 

o  Blooms of algae occur when conditions (light, temperature, salinity, nutrient availability, low grazing) are favorable for growth.  Most algal blooms are beneficial because algae are the primary food source that supports marine life. However, some types of algae have certain characteristics that sometimes make them harmful to other organisms (by toxins or other means) – these are the species that make up the group we call Harmful Algal Bloom (HAB) species.

 

o  Harmful algal blooms (HABs) occur when algae, which live in the sea and form the base of the food web, produce toxic or harmful effects on people, fishes, shellfish, marine mammals and birds. HABs have been reported in almost every U.S. coastal state.

 

o  The frequency, extent, and severity of HAB events appear to be increasing. HABs are indicators to the health of ecosystems and are of national concern.  Many coastal areas suffer from HAB events each year, threatening coastal ecosystems, impacting local and regional economies, and endangering human health.

 

o  HABs are a natural occurrence but their frequency of occurrence and they are geographic distribution seems to have increased over the last several decades.  The environmental conditions that select for blooms of HAB species are complex (for example, different HAB species have different optimal growth conditions) and not completely understood.

 

o  Some human activities that may have contributed to the increase in HAB occurrence and distribution include:

1) Increases in nutrient loadings (which can change the natural nutrient regime and select for HAB      species, plus more nutrients support more growth),

2) Overfishing (which can decrease the grazing pressure on HAB species), and

3) Ballast water discharge (which can transfer resting stages, or cysts, of HAB species to new areas).

Red tides” and other HABs:

Red tides: Red tide occurs when appropriate ocean conditions allow for massive overgrowth of phytoplankton. These microscopic organisms undergo such rapid growth that the phenomenon, called an algal bloom, sometimes will discolor the waters. Several of these microorganisms produce toxins and store them in their cells, hence the term harmful algal blooms. Shellfish such as clams, mussels, scallops, oysters, and cockles feed upon the phytoplankton, concentrating the poisons within their bodies. The poisons can sicken people who consume the shellfish.

Red tides (fig. 1) are caused by blooms of phytoplankton species that have a reddish pigment.  The main pigment in photosynthetic phytoplankton is chlorophyll, but different phytoplanktons have different “accessory pigments” that give them their unique colors.  For example, many dinoflagellates have the reddish pigment, peridinin, and that is why some dinoflagellate blooms give the water a reddish hue.  Accessory pigments allow phytoplankton to use light at different wavelengths and can even act to protect the cell from sun damage.

 

Not all red tides are caused by the same species and not all red tides are toxic.  The red tides that occur in New England and in Florida are caused by two different organisms.  They are both toxic but produce different toxins. Red tide caused by Karenia brevis, an alga that is brown in color and produces a different type of toxic toxin (fig. 2).

 

Not all HABs are red and some don’t discolor the water at all. Water discoloration depends on the HAB species pigmentation and the concentration of cells in the water.  Also, some toxic algae can be dangerous when they are not very abundant, so the water may not be discolored at all.

How are HABs harmful?

 

Some HAB species produce toxins- When shellfish feed on these algae, the algal toxins accumulate in their tissues and therefore the shellfish becomes poisonous to humans or other animals that consume them.

 

o   Shellfishes: Bioaccumulation of toxic substances occurs in many shellfishesThere are a number of human illnesses caused by eating shellfish contaminated with algal toxins. The different types of poisonings are caused by different algae that produce different toxins. Fortunately, commercially available shellfish are rigorously tested in the US to protect humans from these illnesses.  There are often severe economic impacts, however, from blooms of these algae because the shellfish and tourism industries can suffer dramatic losses.

 

o   Toxins can be transferred through the marine food web as well. Toxic algae have caused the deaths of fishes, whales, manatees, sea lions, dolphins, turtles, and birds. 

 

o   Some HABs cause “hypoxia”—Hypoxia is when oxygen becomes depleted in the water. Some algal blooms occur in such high concentrations, that when they die and bacteria decompose them, the bacteria consume all available oxygen in the water, which can suffocate fish and other organisms (fig. 3).

o   Some HABs block sunlight to other plants—When blooms are so large that you can see them with your unaided eye, they are preventing sunlight from penetrating very far into the water column.  Low light can prevent growth of other plants, such as seagrasses that provide critical nursery habitat for fishes and are also important for minimizing coastal erosion.

 

o   A side effect of high concentrations of some HAB- forming species is that they cause irritation and sometimes completely clog fish gills. This gill irritation occurs because some phytoplankton have spiny structures to help them float (fig. 4) which can lodge in fish gill tissues, causing irritation, overproduction of mucous by the fish, and eventual death.

Jellyfish: Due to the eutrophication Nomura Jellyfish population increased in Japanese waters that negatively affect other species. Human society is impacted as well by eutrophication such as decreases in the resource value of rivers, lakes, and estuaries such that recreation, fishing, hunting, and aesthetic enjoyment are hindered. Health-related problems can occur where eutrophic conditions interfere with drinking water treatment.