Share this :

Harmful Algal Blooms

Background :- The Ministry of Earth Science has been actively working with National Oceanic and Atmospheric Administration (NOAA), USA towards strengthening of ocean observations in the Indian Ocean with a view to improve understanding ocean process for better forecasting capabilities including studying the Harmful Algal Blooms (HAB).

The Ministry also has conducted regular surveys along the coastal and oceanic area of India and recorded altogether about 84 algal blooms during the period from 1998 to 2016.

What is a harmful algal bloom?



Satellite view of 2011 massive Lake Erie algal bloom, caused chiefly by phosphate runoff from agricultural fertilizer.

Harmful algal blooms, or HABs, occur when colonies of algae — simple plants that live in the sea and freshwater — grow out of control and produce toxic or harmful effects on people, fish, shellfish, marine mammals and birds. The human illnesses caused by HABs, though rare, can be debilitating or even fatal.
Ranging from microscopic, single-celled organisms to large seaweeds, algae are simple plants that form the base of food webs. Sometimes, however, their roles are more sinister. Under the right conditions, algae may grow out of control — and a few of these “blooms” produce toxins that can kill fish, mammals and birds, and may cause human illness or even death in extreme cases. Other algae are nontoxic, but eat up all of the oxygen in the water as they decay, clog the gills of fish and invertebrates, or smother corals and submerged aquatic vegetation. Still others discolor water, form huge, smelly piles on beaches or contaminate drinking water. Collectively, these events are called harmful algal blooms, or HABs.
Harmful algal blooms, or HABs, are a growing problem in every U.S. coastal and Great Lakes state. While we can't prevent these blooms, we can be better prepared. NOAA leads many research efforts to help coastal communities counter the environmental and health effects associated with these events.

These blooms are a national concern because they affect not only the health of people and marine ecosystems, but also the “health” of our economy — especially coastal communities dependent on the income of jobs generated through fishing and tourism. With climate change and increasing nutrient pollution potentially causing HABs to occur more often and in locations not previously affected, it’s important for us to learn as much as we can about how and why they form and where they are, so that we can reduce their harmful effects.

Why Do HABS Happen ?

While we know of many factors that contribute to HABs, how these factors come together to create a “bloom” of algae is not well understood. HABs occur naturally, but human activities that disturb ecosystems seem to play a role in their more frequent occurrence and intensity. Increased nutrient loadings and pollution, food web alterations, introduced species, water flow modifications and climate change all play a role.

Studies show that many algal species flourish when wind and water currents are favorable. In other cases, HABs may be linked to “overfeeding.” This occurs when nutrients (mainly phosphorus and nitrogen) from sources such as lawns and agriculture flow into bays, rivers, and the sea, and build up at a rate that “overfeeds” the algae that exist normally in the environment. Some HABs appear in the aftermath of natural phenomena like sluggish water circulation, unusually high water temperatures, and extreme weather events like hurricanes, floods, and drought.

A harmful algal bloom offshore of San Diego County, California. Many people use the term “red tide” to refer to harmful algal blooms, but not all HABs turn the water red. Blooms may appear in a variety of colors depending on the species of algae involved – and some HABs have no color at all.

Although all coastal states experience HABs, different organisms live in different places and cause different problems. Other factors, such as the structure of the coast, runoff, oceanography, and other organisms in the water, can also change the scope and severity of HAB impacts.

In the Gulf of Mexico, especially the west coast of Florida and the Texas coast, the most frequent cause of HABs is Karenia brevis. The toxin from this HAB becomes airborne when waves break on the beach, which causes severe respiratory irritation.


Phytoplankton are important constituents of the marine food web and comprise 40 % of the total fixed global primary productivity . Of about 5,000 species of marine hytoplankton that exist in the world, ~ 7 % are responsible for algal blooms (or red tides) which includes diatoms, dinoflagellates, raphidophytes, prymnesiophytes and silicoflagellatesOf this, ~ 2 %of phytoplankton species are harmful or toxic and ~ 75 % are contributed by dinoflagellates.. The occurrence of blooms are spontaneous and remarkable; their growth and persistence are brought about by a combination of physical, chemical and biological factors interacting in ways that are often sudden and unpredictable

Algal blooms that discolour the seawater are commonly referred to as ‘red tide’.

India, being one of the major maritime countries is endowed with a coastline of approximately 7,500 km and embraced by two important seas i.e. the Arabian Sea (AS) on the west and Bay of Bengal (BOB) on the east coast of India. This marine environment embodies diverse habitats such as estuaries, mangrove swamps, brackish water lakes, coral reefs, islands and offshore waters that support a great diversity of flora and fauna. The coastline is dotted with 12 major ports, six each on the west and east coasts of India.In addition, there are 163 minor and intermediate ports along the coastline and sea–islands. These serve as gateways for international and national trade. With increase in shipping traffic, this region is also susceptible to ship–mediated invasion. Bioinvasion is considered is one of the vectors for global expansion of HABs in other parts of the world.

Possible causative factors responsible for algal blooms:-

(a)physical processes such as upwelling, cyclones and eddies

(b) chemical processes such as increased nutrient conditions (eutrophication)

(c) biological processes like competition, grazing and allelopathy

India, Monsoon and Bloom

The seas along the Indian coast – the AS (Arabian Sea)  and BOB (Bay of Bengal), are land–locked in the north and forced by seasonally–reversing monsoon winds, making it unique among the world  oceans. The monsoon winds reverse twice a year, blowing from south–west (SW) during May–September and from the north–east (NE) during November–January with the transition taking place during the months in between.

Differences between regions of AS and BOB arise because of the following reasons- 

Both, the AS and BOB show opposing trends in surface circulation during summer (SW) and winter (NE) monsoons.

The BOB receives larger quantities of fresh water and sediment load from rivers compared to AS.

The AS has salinity usually in the range 35–37 psu due to excess evaporation over rainfall. In contrast, the BOB has much lower salinity (30–33 psu) due to large influx of freshwater from river discharge and high amount of rainfall.

In the BOB, the surface temperatures range between 27 and 29 ⁰C; except for shallow areas near the coast. The fluctuations are much wider (23–29 ⁰C) along the AS coast of India.

Algal bloom reportings:-

In Indian waters, first observations on algal blooms that caused massive fish mortality was  reported by James Hornell in 1908 while cruising along the Malabar coast to Laccadive islands.

A review of algal bloom occurrences in Indian waters from 1908 to 2009 showed a total of 101 bloom incidents

Trichodesmium erythraeum and Noctiluca scintillans were the common blooming species in coastal waters of India.

During the period from 1908 to 1950, the number of blooms was less and it was restricted between the 8 to 12 ⁰N latitudinal margins along the Indian coasts with majority of the blooms recorded from the SW coast of India. From 1950 to 2009, the distribution of bloom incidents has spread from 8 to 20 ⁰N latitude i.e. from southern to northern part along both the coasts of India.

Most of the blooms are generally reported in non-Monsoon times owing to rough weather and low sea vessel movements.Also , the bloom reported till date is higher in AS than BOB.

The positive aspect of diatom blooms is that it may be beneficial to fisheries whereas blooms can negatively affect fisheries and human health.

Notwithstanding the general trend of bloom occurring in non-monsoon times, few condition in AS and BOB trigger bloom in Monsoon times as well.

The west coast of India undergoes periods of strong upwelling during monsoon and delivers cold, nutrient–rich waters from bottom depths. As a result, upwelling coupled with monsoon leads to  high nutrient conditions triggering high primary production.Due to high biological productivity, the intermediate water gets depleted of oxygen creating hypoxic conditions during September–October.Therefore, blooms occurring during monsoon can be the result of increased discharge of nutrients by land run–off, precipitation and upwelling.

The Indian summer monsoon has vigorous intra– seasonal oscillations in the form of active and weak (or broken) spells of rainfall within the monsoon season resulting in sudden changes of salinity and water temperatures This might act as a trigger mechanism to induce the blooming of certain species which prefer a particular range of salinity and temperature in the presence of sufficient nutrients in coastal waters.It is evident that the monsoons can provide favourable conditions to trigger bloom formation of several phytoplankton species. Break in monsoon can also provide a window of opportunity for
certain phytoplankton species to bloom.

Although, the conditions are favourable during monsoon, there are lesser occurrence of blooms during this period.This can be also related to non–efficient utilization of nutrients by phytoplankton under low irradiance due to monsoonal cloud cover.

The BOB is known for its unique characteristic features: large volume of freshwater input from river discharge and rainfall, warmer sea surface temperatures, monsoonal clouds and reversal of currents.

BOB is considered to have lower biological productivity than its western counterpart, the AS. The low biological productivity of BOB has been speculated to be due to various seasons such as narrow shelf, cloud cover during summer monsoon, turbidity resulting from sediment influx and fresh water–induced stratification

Along the east coast of India, upwelling does occur but the strongly stratified surface layer of the BOB restricts the transport of nutrients from deeper layers to the surface. The stratification is especially intense during the SW-monsoon period due to enormous influx of freshwater through precipitation and riverine discharges that leads to the formation of low salinity cap at the surface.

Hence number of bloom in eastern coast of India is less than that of western coast of India.

Impact of algal blooms on human health and sustainable fishery:-

  • Paralytic Shellfish Poisoning (PSP)
  • The Indian fisheries economy depends heavily upon the coastal zone for marine products, so it isespecially sensitive to constraints from red tides and toxic microalgae
  • So far, there are 7 fish–killing species such as Cochlodinium polykrikoides, Karenia brevis, Karenia mikimotoi, Noctiluca scintillans, Trichodesmium erythraeum, Trichodesmium thiebautii and Chattonella marina that form
    algal blooms and are responsible for massive fish mortality in Indian waters

On–going monitoring programmes:-

  1. For monitoring of HABs along the Indian coasts, a national coordinated multi–institutional research program on “HABs in the Indian EEZ” is being funded by Ministry of Earth Sciences (MOES) with Centre for Marine Living Resources and Ecology (CMLRE) as the project coordinator and participation of various institutions.
  2. Ignoring problems caused by ballast water introductions could pose a threat to coastal water bodies.The introduction of harmful aquatic organisms and pathogens to new environments via ships’ ballast water has been considered as an important vector for global expansion of HABs.Ballast Water Management
    Programme–India (BAMPI) which is supported by Directorate General of shipping and Ministry of Shipping, India
  3. A phytoplankton–monitoring programme under the Indian Expendable Bathythermographic (XBT) programme funded by Ministry of Earth Sciences is in existence since early 2000


Print Friendly, PDF & Email
Share this :