3. Harmful Algal Blooms

How remote sensing can help to detect and follow algal blooms

Monitoring and prediction of algal blooms in the marine environment has become more and more important in the recent years as they are increasingly perceived as a potential threat. Especially in the field of natural resource management and in the public health sector, reliable tools to detect and monitor Harmful Algal Bloom (HAB) events are needed so that mitigation actions can be effectively taken.

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Water sampling from a research vessel
Water sampling from a research vessel.
Photo: U.S. Department of Energy, Office of Science

Measurements carried out from a research vessel, so called in-situ (on the spot) data acquisition, is very useful for determining different algal species. However, when it comes to questions concerning the area and movement of an algal bloom, this method is not cost-effective.


Remote sensing thus plays a significant role in the detection, monitoring and prediction of algal blooms in the marine environment.

How does it work?

The main advantages of using remote sensing are a high spatial and temporal resolution, meaning that large areas of algal blooms can be studied and that their movement can be followed by looking at images captured at different times.

By comparing the two images below you can see how the chlorophyll concentration in the sea can change within just one week. The pseudo-coloured images show the relative chlorophyll content of the sea from blue and green (low) to red and yellow (high).

Chlorophyll concentration in the North Sea and Skagerrak Region, 12.03. - 19.03.2009
Chlorophyll concentration in the North Sea and Skagerrak Region derived from MERIS data. 7 days composite of the period 12.03. - 19.03.2009
Source: NERSC / ESA
Chlorophyll concentration in the North Sea and Skagerrak Region, 16.03. - 23.03.2009
Chlorophyll concentration in the North Sea and Skagerrak Region derived from MERIS data. 7 days composite of the period 16.03. - 23.03.2009
Source: NERSC / ESA
Chlorophyll concentration in the North Sea and Skagerrak Region derived from MERIS data. 7 days composite of the periods 16.03. - 23.03.2009 (left) and 22.03. - 29.03.2009 (right).
Source: NERSC / ESA

Remote sensing of Algal Blooms is mainly based on measurements of ocean colour. Optical remote sensing of algal blooms takes advantage of the fact that water is strongly affected by algal pigments.

In the open ocean, variations of the water colour are solely determined by phytoplankton.

In near shore and coastal areas , the optical properties of the sea water are much more complex. Suspended sediments and dissolved organic compounds are also responsible for ocean colour. Riverine inputs and regional wind and ocean current patterns cause turbidity in coastal waters which in turn change the optical properties of the water.

What colour does the ocean have?

Your answer is probably blue and you are quite right with this - for most of the world's oceans. However, one of the most important things that influence the colour of the ocean are phytoplankton which contain chlorophyll.

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Plankton bloom in the Atlantic Ocean

Plankton bloom in the Bay of Biscay.
This Envisat image shows the vast Bay of Biscay on the Atlantic Ocean.
Along the central part of the coastal shores ( Spanish and French ) it is possible to see different shades of colours in the sea, different from the usual deep blue, due to a strong plankton blooming. These colours are caused by massive nutrients arrival.

Source: ESA

The equation is fairly simple:

The more phytoplankton in the water, the greener it is;
the less phytoplankton, the bluer it is.



What do satellites measure?

Satellite sensors detect the light reflected from the sea surface in different wavelengths. Chlorophyll concentrations can then be derived from satellite data by calculating the blue/green ratio of the ocean colour. The more blue colour is absorbed, the more green colour is reflected, indicating a higher concentration of phytoplankton in the sea water and the other way around.

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Percent reflectance of blue and green water
Percent reflectance of clear (blue) and algae-laden (green) water (data from Han, 1997).

In coastal areas larger proportions of inorganic matter are suspended and dissolved in the sea water. Here, another method is used that calculates chlorophyll using the red/near infrared ratio.


Remote sensing of algal blooms: requirements and limitation

Optical remote sensing can provide valuable (and beautiful!) images of algal blooms when a certain set of criteria is met. These are

  • No cloud cover
  • The bloom is near to the surface
  • The bloom causes perceptible change in ocean colour

Due to abundant cloud cover over the sea the image below, for example, is not very helpful in terms of detecting marine algae.

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MERIS image with cloud cover around Ireland
Ireland, uncharacteristically devoid of cloud cover, is shown here in this Medium Resolution Imaging Spectrometer (MERIS) image.
High pressure dominated northern latitudes during August 2003, causing bright sunshine and intense heat over the northern land masses.
On the eastern coast, the emerald colour in the sea corresponds to sediment and perhaps algae welling up in the shallow coastal waters from Greenore point in the south at the mouth of the Barrow River to Dublin Bay in the north at the mouth of the Liffey River. This image was made using bands 7, 5, and 2 of the MERIS instrument, which correspond to visible light and are particularly well adapted to detecting chlorophyll pigments in water.

Source: ESA

Question:

From what you have learned about the nature of algal blooms and the methods to look at them from space, would you think that from a satellite image it is possible to determine whether an algal species is harmful or not?



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