autotrophicautotrophic components of the plankton community and a key factor of oceans, seas and freshwater basins ecosystems.planktonecosystems (Phyton)meaning "plant", and (planktos), meaning "wanderer" or "drifter".plant Most phytoplankton are too small to be individually seen with the unaided eye.unaided eye However, when present in high enough numbers, they may appear as a green discoloration of the water due to the presence of chlorophyll within their cells (although the actual color may vary with the species of phytoplankton present due to varying levels of chlorophyll or the presence of accessory pigments such as phycobiliproteins, xanthophylls, etc.).chlorophyllphycobiliproteins xanthophylls

photosynthesizingphotosynthesizing microscopic organisms that inhabit the upper sunlit layer of almost all oceans and bodies of fresh water. They are agents for "primary production," the creation of organic compounds from carbon dioxide dissolved in the water, a process that sustains the aquatic food web.primary production organic compoundscarbon dioxide Phytoplankton obtain energy through the process of photosynthesis and must therefore live in the well-lit surface layer (termed the euphotic zone) of an ocean, sea, lake, or other body of water.energyprocess photosynthesiseuphotic zoneoceansealake Phytoplankton account for half of all photosynthetic activity on Earth.photosynthetic activity Phytoplankton are also crucially dependent on minerals.minerals Phytoplankton depend on other substances to survive as well. Phytoplankton serve as the base of the aquatic food web, providing an essential ecological function for all aquatic life.

A 2010 study published in Nature reported that marine phytoplankton have declined substantially in the world's oceans over the past century. Phytoplankton concentrations in surface waters were estimated to have decreased by about 40% since 1950 alone, at a rate of around 1% per year, possibly in response to ocean warming.ocean warming

Phytoplankton are a key food item in both aquaculture and mariculture.aquaculture mariculture Both utilize phytoplankton as food for the animals being farmed. In mariculture, the phytoplankton is naturally occurring and is introduced into enclosures with the normal circulation of seawater. In aquaculture, phytoplankton must be obtained and introduced directly. The plankton can either be collected from a body of water or cultured, though the former method is seldom used. Phytoplankton is used as a foodstock for the production of rotifers, [27] which are in turn used to feed other organisms. Phytoplankton is also used to feed many varieties of aquacultured molluscs, including pearl oysters and giant clams. rotifers [27]molluscspearloystersgiant clams

a rapid increase or accumulation in the population of algae (typically microscopic) in an aquatic system.algae CyanobacteriaCyanobacteria blooms are often called blue-green algae. Algal blooms may occur in freshwater as well as marine environments.freshwatermarine environments Typically, only one or a small number of phytoplankton species are involved, and some blooms may be recognized by discoloration of the water resulting from the high density of pigmented cells.phytoplankton

Although there is no officially recognized threshold level, algae can be considered to be blooming at concentrations of hundreds to thousands of cells per milliliter, depending on the severity. Algal bloom concentrations may reach millions of cells per milliliter. Algal blooms are often green, but they can also be other colors such as yellow-brown or red, depending on the species of algae. Bright green blooms are a result of cyanobacteria (colloquially known as blue-green algae) such as Microcystis. Blooms may also consist of macroalgal (non-phytoplanktonic) species. These blooms are recognizable by large blades of algae that may wash up onto the shoreline.cyanobacteriaMicrocystismacroalgalphytoplanktonic

Freshwater algal blooms are the result of an excess of nutrients, particularly some phosphates.excess of nutrientsphosphates The excess of nutrients may originate from fertilizers that are applied to land for agricultural or recreational purposes. They may also originate from household cleaning products containing phosphorus.phosphorus These nutrients can then enter watersheds through water runoff. Excess carbon and nitrogen have also been suspected as causes. Presence of residual sodium carbonate acts as catalyst for the algae to bloom by providing dissolved carbon dioxide for enhanced photo synthesis in the presence of nutrients.watershedscarbonnitrogenresidual sodium carbonate

A harmful algal bloom (HAB) is an algal bloom that causes negative impacts to other organisms via production of natural toxins, mechanical damage to other organisms, or by other means. For example the toxicity of microcystin produced by cyanobacteria, can harm water quality or human health. HABs are often associated with large-scale marine mortality events and have been associated with various types of shellfish poisonings.microcystin cyanobacteriashellfish poisonings

Immune system responses have been affected by brevetoxin exposure in another critically endangered species, the Loggerhead sea turtle. Brevetoxin exposure, via inhalation of aerosolized toxins and ingestion of contaminated prey, can have clinical signs of increased lethargy and muscle weakness in loggerhead sea turtles causing these animals to wash ashore in a decreased metabolic state with increases of immune system responses upon blood analysis. Examples of common harmful effects of HABs include:Loggerhead sea turtle 1. the production of neurotoxins which cause mass mortalities in fish, seabirds, sea turtles, and marine mammals 2. human illness or death via consumption of seafood contaminated by toxic algae 3. mechanical damage to other organisms, such as disruption of epithelial gill tissues in fish, resulting in asphyxiation 4. oxygen depletion of the water column (hypoxia or anoxia) from cellular respiration and bacterial degradationanoxia Due to their negative economic and health impacts, HABs are often carefully monitored.

is a term often used to describe HABs in marine coastal areas, as the dinoflagellate species involved in HABs are often red or brown, and tint the sea water to a reddish color. Red tides can also be caused by bioluminescent dinoflagellates such as Noctiluca scintillans.Noctiluca scintillans

singular alga Latin for "seaweed” are a very large and diverse group of eukaryotic organisms, ranging from unicellular genera such as Chlorella and the diatoms to multicellular forms such as the giant kelp, a large brown alga that may grow up to 50 meters in length.LatineukaryoticunicellularChlorelladiatomsmulticellulargiant kelpbrown alga Most are autotrophic and lack many of the distinct cell and tissue types found in land plants such as stomata, xylem and phloem.autotrophicland plantsstomataxylem phloem The largest and most complex marine algae are called seaweeds, while the most complex freshwater forms are the Charophyta, a division of algae that includes Spirogyra and the stoneworts.seaweedsCharophyta divisionSpirogyrastoneworts One definition is that algae "have chlorophyll as their primary photosynthetic pigment and lack a sterile covering of cells around their reproductive cells"