
South African Algal Blooms
March to June 2017


Detection of harmful algal blooms in the southern Benguela, as seen by OLCI aboard Sentinel-3.
22 October 2020
28 February 2017
By Hayley Evers-King (PML ), Marie Smith and Stewart Bernard (CSIR )
The southern Benguela current is a highly productive ecosystem, making it a popular site for fisheries and aquaculture activities.
However, this high productivity can also present a threat, when harmful algal blooms occur.
These occur seasonally in the region and, due to their frequently high Chlorophyll a content, can be detected by satellite ocean colour measurements. Sentinel-3's OLCI allows biogeochemical measurements like concentrations of Chlorophyll a — the photosynthetic pigment of phytoplankton (algae) — and suspended sediments.
Figure 1 shows the Chlorophyll a (Chl-a) concentration associated with one of these blooms which persisted throughout the 2017 austral summer upwelling season.
The Chl-a product is produced with a switching algorithm which uses an adapted version of the OC4MEblue-green band-ratio algorithm for low biomass waters, while a red-Near Infrared band-ratio algorithm is applied over high biomass waters.
Using a time series of OLCI images, the bloom can be tracked (Figure 2). A threshold of the Chl-a concentration can be set, and the first time that this is exceeded within a season can be calculated.
This can give insight in to the typical progression of these events in space and time, a useful predictor for aquaculture farms in the region.
On Figure 2 blue patches indicate earlier bloom initiation, while red indicates later bloom initiation. Initiation is determined as the first time the Chlorophyll a concentration, derived from the satellite images, exceeds a threshold during the upwelling season.
The figure shows the progression of a bloom, southward, through the bay during the 2016/2017 season.
The data and tools developed from these observations feed in to the Ocean and Coasts Information Management System (OCIMS). This is part of Operation Phakisa, a South African government initiative to develop the blue economy across several areas, including aquaculture.
OLCI products are used within the OCIMS HAB Decision Support Tool which provides a risk assessment of potentially harmful high biomass blooms.

Related content
National OCIMS Harmful Algal Bloom Viewer
CSIR Ocean Colour Tumblr site
Previous case study
Blooms in the Baltic Sea (20 July 2016)