Cultural and natural eutrophication have both contribu ted to changes in nutrient input to coastal waters, and led to an overall increase in nutrient availability and an alteration in nutrient composition. The first result of these changes is often an increase of total algal biomass and shifts in species composition potentially leading to secondary disturbance such as harmful algal blooms (HABs). HAB species range from marine, brackish to freshwater organisms and cover a broad range of phylo genetic types (dinoflagellates, diatoms, raphidophytes, cyanobacteria). Most HAB species form massive blooms of various colors (red, brown, or green). A few species can produce potent toxins. These toxins can directly kill marine mammals and transfer through the food chain causing harm at different levels from plankton to humans. A potential impact of HABs on human health occurs through the consumption of shellfish that have filtered toxic phytoplankton from the water or planktivorous fish. All poisoning syndromes are serious and can be fatal. They are named paralytic (PSP), diarrhetic (DSP), neu rotoxic (NSP), azapiracid (AZP), and amnesic shellfish poisoning (ASP). All syndromes, except for ASP, are caused by dinoflagellates. ASP is caused by diatoms, a group of phytoplankton usually thought to be nontoxic. In tropical and subtropical zones, another human poison ing syndrome, ciguatera fish poisoning (CFP), is caused by toxic dinoflagellates that grow on substrate in coral reef communities. CFP toxins are transferred from herbi vorous to carnivorous fish that are commercially valuable. Some algal toxins, brevetoxins, are airborne in sea spray, causing respiratory distress in coastal population, for example, in the Gulf of Mexico. Cyanobacteria (blue green algae) naturally bloom in still inland waters, estuaries, and the sea during summer. Some cyanobacteria produce potent cyanotoxins (anatoxins, microcystins, and nodularin), which are dangerous and sometimes fatal to livestock, wildlife, marine animals, and humans. These toxins represent a serious health risk in water bodies used for recreational and/or as freshwater supply reservoirs.
Although references to HABs date back to biblical times, the number of toxic events and subsequent eco nomic losses linked to HABs has increased considerably in recent years around the world. Many reports point out an obvious link between pollution and HABs, but there are also other reasons for the expansion of the HAB problem.
Numerous new bloom events have been discovered because of increased awareness and improved detection methodologies (e.g., molecular probes for cell recognition, PCR probes for rDNA specific to genera or species of HABs, enzyme linked immunosorbent assays (ELISA), remote sensing data from satellites, qualified observers, and efficient monitoring programs). The global increase of aquaculture activities and trade of exotic species has led to improved safety and quality controls that revealed the presence of HAB species and/or toxins in, for exam ple, aquaculture pens, and contaminated seafood. Mortality events and toxicity outbreaks in fish or bivalves resources can no longer go unnoticed. Transport of toxic species in ship ballast water undeniably contributes to the increasingly damaging effect of HABs on fisheries, aqua culture, human health, tourism, and the marine and brackish environment. UNEP has recently ranked HABs among the ten worst threats of invasive species trans ported in ballast water.
Dispersal of HABs is influenced by oceanic and estuar ine circulation, river flow combined with currents, upwelling, salinity, nutrients, and specific life cycles of various HAB species. The apparent increase in toxic diatoms (Pseudo nitzschia spp.) off the US and Canada coast is often coupled to physical forcing (storm, wind, rain, and upwelling) and more rarely to the increase in nitrate N in rivers, for example, from the Mississippi River. Both nutrients and harmful dinoflagellate taxa are introduced from upwelling/downwelling areas to estu aries, coastal bays, or lagoons, for example, the Atlantic coast of France, Spain, and Portugal, Chesapeake Bay, and the Benguela region. Similar processes are observed for cyanobacteria in the Gulf of Finland. Physical con vergence, advection, or accumulation process of oceanic dinoflagellates (Dinophysis, Karenia, and Gymnodinium) in embayments also contribute to the extension of HABs in some areas. Large oceanic current systems transport the N fixing cyanobacterium Trichodesmium from tropical oli gotrophic regions to W. Florida waters, enriched with Saharian iron dust, where it blooms. Some HABs have specific life cycles including resting stages for diatoms (spores), dinoflagellates (cysts), and cyanobacteria (akinetes). These resting stages provide these algae with a competitive advantage over populations that cannot survive in poor conditions.
Climatic and hydrological changes affect nutrient delivery and processing, for example, the input of micro nutrients and freshwater from rainfall and river flow, flooding after hurricanes, and tropical storms also favor HAB's growth and persistence. Certain PSP and CFP producers (dinoflagellates) have increased significantly under large scale changing climatic conditions in tempe rate environments (Kattegat, NW Spain, and SW Portugal) and in the Indo Pacific, respectively. Some of these blooms have been linked to the North Atlantic oscillations (NAO) and to El Nino events that affect local climate in wind driven upwelling systems.
Despite the importance of natural events in algal bloom formation, many examples relate HABs to anthropogenic activities since World War II. Red tides (dinoflagellates) in Asia, for example, the mouth of the Yangtze Estuary in China and the Seto Inland Sea in Japan, are related to the parallel increasing population density and nitrogen (N) and (P) loadings. Nutrient enriched conditions in brackish coastal bays and estuaries have been correlated with high abundance of diatoms (central California, Louisiana in the US), dinoflagellates (off the coast of North Carolina - US, Northern Adriatic, Aegean, and Black seas), and haloto lerant cyanobacteria (Baltic Sea, Brazil, Australia), but the direct cause of this relationship is not fully understood. In tropical regions, eutrophication of reef communities often leads to the overgrowth of macroalgae on corals and high coral mortality that favor the bloom of benthic dinoflagellates (CFP producers). Both elevated N and P concentrations and silicon limitation can favor the dominance of HABs, for example, the haptophyte Phaeocystis in the North Sea. Declining silica input to coastal zones and estuaries is often due to damming of rivers and gives a competitive advantage to marine haptophytes and dinoflagellates over diatoms. The resi dence time of the water in freshwater systems is increased by the construction of the dams. This allows for the development of freshwater diatom blooms. The silicon rich frustules of the diatoms are not remineralized as rapidly as organic matter and so dams effectively retain silicon upstream. The ratio of silicon to nitrogen therefore decreases, and estuarine and coastal diatoms may have insufficient silicon to reproduce. The communities of phytoplankton organisms may therefore shift so that diatoms are replaced by nonsilicon requiring organisms such as dinoflagellates and this may significantly alter the food web.
Many potable water reservoirs are under the pressure of expanding population, and are negatively impacted by both sediment erosion, reduced water flow and elevated N and P loading that will stimulate noxious cyanobacter ial blooms. Similar trends are reported for river systems with weir pools. Many HABs have characteristic modes of nutrition from autotrophy to heterotrophy, that is, can use organic carbon, nitrogen, and phosphorus (mixotrophy and osmotrophy). Recent studies have shown that the increase in organic nutrients could benefit certain HABs (dinoflagellates and prymnesiophytes). The global nitro gen based fertilizer usage has shifted toward urea based products and is expected to continue. Thus, significant amounts of urea are transported to estuarine and coastal waters with the potential for increasing eutrophication of these sensitive areas. Since urea is also one very important nitrogen substrate for some HAB species, the global increase of PSP outbreaks is comparable to the increase of urea use for 1975-2005. Aquaculture sites are also a large source of nutrient from animal excreta, rich in N and P, to coastal sediment. Their contribution to HAB formation will depend on the hydrology of the system, for example, HABs proliferate in calm areas.
Among the natural marine environmental contami nants that are health risks, HABs are most prominent. However, the relative effects of natural versus anthropo genic factors on harmful algal blooms cannot yet be resolved.
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