Harmful algal bloom

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Definition of Harmful Algal Bloom (HAB):
Harmful algal blooms or HABs are algal blooms composed of phytoplankton that naturally produce biotoxins. Harmful algal blooms (HABs) can occur in marine, estuarine, and fresh waters.
The term 'harmful algal bloom' is sometimes used to designate any phytoplankton bloom event that causes 'negative' impacts on the marine ecosystem, for example oxygen depletion or sunlight shading.
This is the common definition for Harmful Algal Bloom (HAB), other definitions can be discussed in the article

This article deals with toxic algal blooms: effects, environmental conditions, factors that promote HABs and management measures.

Effects of harmful algal blooms

The toxins produced by harmful algal blooms (HABs) have direct negative impacts on human health and on many marine organisms. Marine HABs further impact on other aspects of human wellbeing, including human commercial and recreational uses of the coastal and marine environments, such as fishing, aquaculture and tourism, and non-market, passive uses of the ocean, such as preferences for particular ecological states.

Most algal toxins are neurotoxins, which can affect the nervous, digestive, respiratory, hepatic, dermatological or cardiac systems. Consumption of toxins bio-accumulated in shellfish produces shellfish poisoning (PS) syndromes such as [1] Amnesic SP, Azaspirazid SP, Diarrhetic SP, Neurotoxic SP and Paralytic SP. Toxins in fish can produce Ciguatera Fish Poisoning.

Beach visitors can experience serious health problems when respiring aerosols containing algal biotoxins[2][3]. Toxic HABs have recently emerged as a potential risk for the contamination of drinking water supplied by desalination systems[4][5][6].

Socio-economic costs cannot easily be quantified, but they are considerable[7][1][8]. Estimates are in the order of several billon US$ annually up to about 8 billion US$, related to precautionary closure of mariculture farms, reduced attractiveness of beaches for coastal tourists and economic impacts of marine phycotoxins on human health.

The greatest direct effect of HABs concerns mariculture. Mariculture has experienced tremendous growth in recent decades and has become a food source on which much of the world's population depends. As the growth of mariculture is expected to continue, harmful algal blooms are an increasing threat. Most shellfish species can eliminate phycotoxins within a few weeks, but retention of some toxins (e.g. saxitoxins) in some species, such as sea scallops (Placopecten magellanicus) and Atlantic surfclams (Spisula solidissima), can last up to 5 years[9]. The paradox is that the waste from finfish farms itself promotes conditions for the development of HABs[10][11].

Conditions favouring the development of harmful algal blooms

HABs are natural phenomena, but these events can be favoured by anthropogenic pressures in coastal areas. It is not known exactly how toxin producing algae develop. What is known, however, is that most toxic algae belong to the class of flagellates and cyanobacteria. Environmental conditions favorable for the development of flagellates and cyanobacteria therefore create the greatest risk for the development of HABs. Among the algae of the diatom class (large plankton with silica cell wall) there are also toxic species, but these are rarer than among the flagellates. The following is known about the shift from conditions favorable for the development of diatoms to conditions favorable for the development of flagellates and cyanobacteria, thus promoting the occurrence of harmful algal blooms:

  1. Higher temperatures. The optimal growth of diatoms occurs at relatively low temperatures compared to flagellates and cyanobacteria[12]. This is consistent with experiments that show increasing occurrence of HABs with temperature[13]. Warmer waters are thought to also favour smaller-sized cells (less diatoms and more potentially harmful flagellates and cyanobacteria) as they are more efficient in harvesting light and nutrients and maintaining their position in the euphotic zoneCite error: Closing </ref> missing for <ref> tag.
  2. A high ratio of dissolved nitrogen N versus phosphorus P. This has several causes: (a) Very small cells, such as picocyanobacteria, have a lower requirement for P due to the smaller need for structural components in the cell[14]; (b) Many harmful dinoflagellates are mixotrophic [15][16], which means that they can ingest dissolved and particulate organic material and thus correct an imbalance in the stoichiometric N:P ratio[17][18][19]; (c) Harmful algae can release excess N via toxins [20]. Many cyanobacteria and marine dinoflagellate HABs are more toxic when N is in stoichiometric excess over P. In the dinoflagellate Alexandrium tamarense, saxitoxin production has been shown to increase by three- to fourfold under P deficiency[21]. Increasing N:P ratios in ecosystems therefore shift communities toward systems with trait dominance of higher optimal N:P ratios (higher P uptake affinity and decreasing N retention) which is a typical HAB trait[22].
  3. Increasing proportions of N in the form of ammonium and urea (CO(NH2)2). Causes: (a) Potentially harmful flagellates and cyanobacteria grow better on ammonium (NH4+) whereas diatoms prefer nitrate (NO3-)[23]; (b) Mixotrophic harmful dinoflagellates can use urea as food source[24].
  4. Enhancement of stratified conditions[25]. Causes for favouring potentially harmful flagellates and cyanobacteria over diatoms are: (a) Larger phytoplankton sinks more easily out of the photic zone, thus smaller plankton dominates [26]; (b) Many harmful dinoflagellates are mixotrophs which can swim to the pycnocline to capture organic prey[27].
  5. Reduction of the Si:N ratio. Diatoms require Si for growth; Si limitation favours non-Si species such as flagellates and cyanobacteria[28].

Causes for an increase in HABs

The majority of HABs (dinoflagellates as well as diatoms) rely on vegetative cells to survive inhospitable conditions. Under suboptimal growth conditions, some highly toxic HABs such as Alexandrium spp. can reproduce sexually and form resting cysts. These cysts settle on sediments and then undergo resuspension during storms or coastal upwelling, enabling (re)colonisation of existing and new areas. Advection and dispersion of HABs, increasing turbulent shear forces breaking up cells, and/or nutrient limitation are all understood to contribute to the termination of HABs[8]. Although no quantitative estimates can be given, there is strong evidence that the occurrence of harmful algal blooms has increased during the past decades. The causes for an increase in HABs are related to the furtherance of the above mentioned conditions favorable to their development. Probable causes are[29]:

  1. Increase of nitrogen-rich effluents and atmospheric emissions to the sea. It has been estimated that the atmospheric deposition of nutrients in the ocean is now about 20-fold greater than the Redfield ratio for N:P. The main cause is the increasing use of fertilizers; in the period 1970-2000 the N-fertilizer use has increased much faster than the P-fertilizer use (see Fig. 1) [30]. Only about half of the fertilizer N is taken up by crops; the remainder is partly stored in the soil and partly emitted to the sea via runoff and the atmosphere. Other N-rich sources are the widespread and expanding fish farms, which release N mainly in chemically reduced form (e.g., ammonium, dissolved organic N, DON)[31]. The increase of the N:P ratio is further due to the more efficient reduction of P compared to N in sewage treatment plants and the reduction of P in laundry products.
  2. Fertilizer effluents and emissions produce a shift from nitrate to ammonium and urea, which favours HABs[20][24].
  3. Effect of river dams. Large fractions of the fluvial P load (about 43% of total dissolved P and reactive particulate P) and Si load (about 20% of dissolved Si and reactive particulate Si) are bound to sediments that are retained in upstream reservoirs, whereas about 90% of the total N load is unaffected[28][32]. This results in an increase in the N:P and N:Si ratios of riverine delivery to coastal areas following dam construction.
  4. Effects of climate change: (a) Rising seawater temperatures; (b) Intensification of sea water stratification; (c) Increase in peak river discharges and corresponding increase in nitrogen supply in coastal waters[33]; (d) Increase in nutrient concentrations associated with intensification of upwelling events[34].
  5. Spreading of harmful algae species across the oceans by increased transport of algae with ship ballast water[1].

Fig. 1. N and P content of global fertilizer use. After Glibert and Burford (2017[35]) and FAO (2019[36]).

Measures for reducing the risk of HABs

The factors that promote the occurrence of HABs are expected to become more important in the future. This holds in the first place for global warming and for eutrophication, in particular the nitrogen component of eutrophication. Efforts to combat harmful algal blooms are vital, but simple solutions do not exist. It is widely recognized that action is needed to halt global climate change and to reduce nitrogen emissions from agriculture. To this end, agreements have been made and initiatives have been developed at various administrative levels. Important international frameworks have been set up for climate policy that will eventually reverse the trend of global warming. A comparable encompassing agreement has not yet been established for agricultural emissions, although in Europe the Nitrates Directive has been in force since 1991. This directive has contributed to a reduction in N emissions from European agriculture[37] - however, without special focus on the nitrate: ammonium ratio of the emissions.

Local reduction of nutrient concentrations can be achieved by harvesting marine products that grow on nutrients and provide economic value (benefit from ecosystem goods and services, see Mariculture) [38][39]. Examples are the harvesting of farmed mussels[40] and the harvesting of seaweed[41]. The restoration of critical coastal habitats (seagrass meadows, coral reefs, oyster reefs, mangrove forests and salt-marshes) also contributes to remove nutrients, increases sequestration of organic matter in benthic sediment, and increases rates of denitrification[42].

Certain measures may contribute to mitigate the impact of HABs (for a more detailed and complete overview see e.g. Berdalet et al., 2016[1] and Wells et al., 2020[43]):

  • Development and implementation of new efficient techniques for monitoring HABs and biotoxins and for monitoring marine conditions that are favorable for the development of HABs, in order to improve early warning;
  • Management measures for aquaculture to reduce HAB development, for example by timing the harvest, by enhanced flushing and aeration or by relocation to offshore areas where excess N concentrations are less likely to build up;
  • Furthering understanding of fundamental aspects of HAB species in terms of toxin production, life cycles and interactions with bacteria in order to develop better targeted measures.

Measures to eliminate harmful algae, for example through the use of viruses, grazers or biocides, encounter serious problems due to hazardous side effects[44]. That is why many countries have bans on such measures. Experiments in Korea to remove toxic algae through flocculation using clay particles have reported successful application without harmful side effects[45]. Another, more holistic approach to toxic HAB mitigation experimented in Puget Sound (USA), is the restoration of coastal habitats with seagrass that harbors algicidal bacteria[46][47].

Useful links

Global Harmful Algal Blooms

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The main author of this article is Job Dronkers
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Citation: Job Dronkers (2024): Harmful algal bloom. Available from http://www.coastalwiki.org/wiki/Harmful_algal_bloom [accessed on 15-07-2024]