South African Algal Blooms

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Detection of harmful algal blooms in the southern Benguela, as seen by OLCI aboard Sentinel-3.

Date & Time
March to June 2017
Level 2 Ocean Colour

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.

Figure 2
Figure 1: Chlorophyll a concentration determined from ocean colour images captured by the OLCI sensor aboard Sentinel-3

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.

Figure 2
Figure 2: Date of first bloom in the St Helena Bay region.

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.

Figure 3
Figure 3: Algal blooms (brown patches) off South Africa's coast in the southern Benguela current. Credit: Hayley Evers-King

Previous case study

Blooms in the Baltic Sea (20 July 2016)

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