Sentinel-3A has captured a number of images of the variety of species of phytoplankton that bloom in the world’s oceans and lakes.
By Ben Loveday and Hayley Evers-King (PML)
Phytoplankton are microscopic, plant-like, organisms that inhabit the sunlit surface layers of the world’s ocean and lakes. Although tiny, their impact is huge — providing half of the oxygen we breathe, and the basis for nearly all life in the oceans. While essential to the functioning of ocean ecosystems, phytoplankton can also cause harm, if they contain specific toxins, and/or when they form large blooms.
The OLCI sensors on the Copernicus Sentinel-3 satellites, are designed to capture these phenomena in terms of their colour and extent, whenever and wherever they might occur.
This case looks at two years of Sentinel-3A OLCI data on both harmful and non-harmful algal blooms. Species are identified according to regional in situ studies or local news stories on the bloom in question, or as a result of an unambiguous spectral signal (e.g. in the case of coccolithophores).
The images featured below show the satellite view from above the atmosphere (Level 1) and with the effects of the atmosphere removed (Level 2). Through atmospheric correction, here done using the C2RCC complex waters processor, we are able to reveal much more detail about the patterns of plankton in ocean and inland waters.
The combinations and differences in colours across the Level 2 false colour images can give us clues about the plankton present in the water including their distribution, type, and physical and biological characteristics (size, shape, pigments). Subject to other factors (sediment or coccolithophores presence for instance), more green/red features generally indicate higher biomass of plankton.
Beyond being beautiful, this data is also being used for regional algorithms for deriving optical water classes and products for different regions. The ORSECT-UK project will provide optimised optical products from OLCI for the UK government's management of its regional seas.
Coccolithophores are one of the most obvious species of phytoplankton to identify in satellite imagery. They secrete calcium carbonate which forms protective, shield-like, ‘liths’ around their bodies. In its more familiar forms calcium carbonate is what is used to make chalk, and the substance that makes the Cliffs of Dover white. It is this property — its whiteness — that makes blooms of coccolithophores so visible from space.
This example from the Barents Sea on 30 July 2018, shows a bloom of the most common species of coccolithophore — Emiliana huxleyi (EHux). As Ehux is very small (5 µm), it is highly reflective, and produces some of the brightest blooms we can see.
These phytoplankton are harmless and play a vital role in the ocean carbon cycle.
Another species of phytoplankton with an important role in the ocean is Trichodesmium. These species are types of cyanobacteria that fix nitrogen from the atmosphere, converting it to forms that can be used by other organisms.
Individual Trichodesmium join together to form strings and clumps (the light green patches/swirls), which can look quite distinctive, as can possibly be seen in this image from the Red Sea on 16 July 2018.
Other types of cyanobacteria can also form impressively large colonies. These can have significant impacts on the marine environment when they bloom and decay through eutrophication (excessive richness of nutrients in a body of water) and/or the presence of toxins. These impacts affect both marine life and human activities.
Cyanobacteria blooms are particularly common in the Baltic Sea, as shown in this image from 16 July 2018. Multiple cyanobacteria species often occur concurrently, making them hard to conclusively identify from space without in situ data.
Many different species of phytoplankton produce toxins which can impact marine life, and pose a risk to human health. About half of the known Pseudo-nitzschia species are capable of producing domoic acid, a neurotoxin which causes amnesiac shellfish poisoning (ASP) when contaminated shellfish, which feed on phytoplankton, are consumed by humans. As such, these species are of a great concern to the aquaculture industry.
This image shows a bloom of Pseudo-nitzschia that occurred near California, affecting the harvest of crabs, and necessitating the rescue of sea mammals suffering from confusion.
Different types of toxins produced by phytoplankton can have different effects. The species Karenia brevis produces another neurotoxin called a brevetoxin, which is particularly irritating when inhaled and can be very dangerous to those with respiratory conditions such as asthma. The presence of this species can also have impacts on the tourist industry, making beaches unattractive or off-limits to visitors.
A recent, toxic bloom of naturally occurring Karenia Brevis along the Gulf coast of Florida, shown in the image below, resulted in extensive damage to the marine environment, and was severe enough to trigger a state of emergency.
The effect phytoplankton have on ocean colour is the key to detecting it from space. In the most extreme examples, large blooms of phytoplankton can turn the ocean red. A species that is commonly responsible for this phenomenon, known as a ‘red tide', is Linglodinium polydedra.
As well as forming dense blooms, this species produces yessotoxins which have been linked with similar symptoms to those associated with paralytic shellfish poisoning (PSP). As well as creating a striking picture by day, this species creates magnificent displays at night, when they luminesce in response to disturbance by waves.
Another plankton species known for its night time light displays, is Noctiluca scintillans. It is not actually a phytoplankton, as it eats other organisms, rather than creating food using sunlight through photosynthesis. It is commonly known as 'sea-sparkle' and can form large blooms with an orange tint, as shown in the image below of the Arabian Sea.
Mesodinium rubrum/Fibrocapsa Japonica
While some plankton (floating) species use sunlight to photosynthesise and feed, and others consume other organisms — some do both! Mesodinium rubrum* (also known as Myrionecta rubra) is a mixotrophic plankton which eats other species and incorporates their parts in to its cell structure to allow for photosynthesis.
It is a common species that forms red tides, and actually acts as a food source for other mixotrophic plankton species such as Dinophysis, which itself can cause diarrhetic shellfish poisoning (DSP). An example of this bloom can be seen in the inshore bays of the Atlantic coast of Florida.
*Note: While initially thought to be Mesodinium rubrum, later assessment of in situ data reclassified this inshore bloom as Fibrocapsa japonica, but acknowledged that additional species were present. This shows how difficult it is to categorise specific species using only space-based approaches.
Previous cases studies
First images from Sentinel-3B (8 May 2018)
Herring spawn seen from space (9 March 2018)
South African Algal Blooms (May–June 2017)
Using Sentinel-3 to monitor chlorophyll concentrations in the Black Sea (2017)
Blooms in the Baltic Sea (20 July 2016)