Most harmful algal blooms seem to appear from nowhere-it appears that they are suddenly there! In some ways this is true, but in other ways it isn't. First some basic biology about how algae grow.
What causes phytoplankton to grow? It is thought that when the environmental conditions are ideal for the particular organism, cells will begin to grow or divide. Phytoplankton are photosynthetic autotrophs. They only need light and inorganic nutrients such as phosphate (PO4), nitrate (NO3), ammonium (NH4), carbon dioxide or carbonate (CO2 or CO4) to grow. They do this through their "chloroplasts", an internal structure that takes the energy of light and powers the synthesis of carbohydrates, proteins, and fats-all the building blocks of life. In addition, they also need very small amounts of certain trace metals such as iron (Fe), zinc (Zn), and perhaps a few others, such as silicate in the diatoms.
Algae, like other microscopic single celled organisms, grow by
asexual reproduction (although there are instances where algae
can also engage in sexual reproduction-this can have profound
impacts on the ultimate survivability of the species). When these
organisms divide, a duplicate copy of DNA from the mother cell
is present in the daughter cell. Each resulting cell can then
go on to divide again, and again, and again, and so on. This is
exponential growth. Starting with only one cell, if the cell population
from each generation increases by a factor of 2n (where
n is the number of generations), it is clear that after a relatively
small number of generations, the number of cells will be very
In the oceans a generation (doubling time) can range from hours to a few days. Most noticeable algal blooms in the aquatic environment range from 100,000 -1,000,000 cells per liter.
Within a confined area of sea water, there are only a finite amount of nutrients available for the phytoplankton. As they take in nutrients and grow, there is eventually a reduced amount of nutrients available to the resulting cells. As nutrients are used up and assimilated into cell tissue, the growth of the cells begins to slow in response to declining nutrients.
Another factor that may also inhibit cell growth is the presence of toxic components in the water. Some of these compounds can be man-made, for example, herbicides from land run-off, or they might be naturally derived compounds from other organisms (bacteria, fungi, other algae) in the water. These other organism may produce some of these compounds naturally (for example some fungi produce "antibiotics" to ward off other competing microorganisms).
The production of these control chemical compounds may confer a survival benefit to an organism, allowing it to have a small niche the overall scheme of the water column. As on land, it's a "jungle out there." Organisms are all competing for nutrients and survival. Every now and then, one organism is able to outcompete its neighbors and become the "top dog", which in the case of phytoplankton is what we call a "bloom".
Phytoplankton also have another ability to respond to changes in their environment. When times get tough (nutrient levels are low or detrimental temperature change) they can form cysts. The cyst stage is similar to a "hibernation" or dormant state. When circumstances change, some trigger mechanism can cause the algae to come out of the cyst stage and return to a "vegetative" state, to begin the life cycle over again.
Sometimes when algae forms cysts, they can be easily transported by both surface and deep currents over long distances. It is thought that this is how noxious phytoplankton have been spread from one location to another. In a special case, the cyst stage can cause worldwide distribution of HABs. For example, harmful algal cysts have been recovered from the bottom of ballast holds in tanks of oceanic freighter ships and shown to produce viable vegetative cells.
Currents in the oceans can arise from different causes. On the surface, winds can move water layers. Slight differences in temperature can cause sea levels to rise or sink, carrying with these currents the phytoplankton. In some instances, phytoplankton in the water can be swept into areas where nutrients are high, for example near coastal upwelling areas, where growth can be stimulated. Most phytoplankton, at least the non-flagellated kind, spend most of their existence on or near the surface and basically drift with currents and tides. The diatom Pseudo-nitzschia is this kind of organism. However, some phytoplankton have flagella which allows them to move about in the water column. The phytoplankton Alexandrium catenella (the organism responsible for Paralytic Shellfish Poisoning) is such an organism. Many phytoplankton who have this ability also tend to respond to day/night cycles. Usually at night they tend to move down lower in the water column and then during the day they rise up near the surface.
Eventually many phtyoplankton run out of nutrients, lose their buoyancy, and become part of oceanic "snow" that slowly falls into the benthic environment. In some cases, it is thought that this might be a way that marine biotoxins (produced perhaps during stationary phase) become introduced into the benthic environment. On the bottom, creatures can then consume this toxic "snow" and accumulate toxins. Either through the active uptake of live, vegetative cells or perhaps withered dead cells, biotoxins can enter the food web.
It is generally accepted that bloom initiation is caused by the right set of environmental conditions, i.e., nutrients or sunlight or temperature or a combination of these. These conditions can be provided on a local basis by natural run-off from the land or by human (anthropogenic) inputs (e.g., treated or untreated sewage, farming or urban gardening practices). These initiators appear possible for blooms in local estuarine areas but what drives oceanic blooms? We are now aware of large scale or "global" processes, such as El Nino-Southern Oscillation (ENSO) and Decadal Oscillations. These global processes can drive and cause huge weather and climatic occurrences such as higher than average rainfall (thus increasing runoff) and higher air and hence surface temperatures, all impacting surface and deep currents. These events may all impact the frequency and magnitude of oceanic HABs.