1. Introduction 1 BIOLOGICAL

GMES-&-Africa-UG /BIOL-BULL/202006/023

Contact: The Director, Regional Marine Centre, University of Ghana P. O. Box LG.99, Legon, Ghana. Email: [email protected] ; Website: www.gmes.ug.edu.gh

Regional Marine Centre

University of Ghana

MONTHLY

OCEANOGRAPHY

BULLETIN Eastern Tropical Atlantic

Contents

1. Introduction 1

2. Chlorophyll-a concentration 1

3. Sea surface temperature trend 3

4. Annex 4

June 2020

Mean chlorophyll-a anomaly for June 2020 (White patches in the

image area covered by clouds which is prevalent in tropical regions).

Highlights

Monthly mean Chl-a concentration reduced in

the Canary region as compared to the previous

month.

SST is expected to continue decreasing in the

Gulf of Guinea region next month which might

increase fish abundance.

SST in the Canary Current region is expected

to continue rising next month which might

decrease fish abundance.

Faits marquants

La concentration moyenne mensuelle de Chl-a a

diminué dans la région des Canaries par rapport

au mois précédent.

La TSM devrait continuer à diminuer dans la

région du Golfe de Guinée le mois prochain, ce

qui pourrait augmenter l'abondance des poissons.

La TSM dans la région du courant des Canaries

devrait continuer à augmenter le mois prochain,

ce qui pourrait faire baisser l'abondance des

poissons.

BIOLOGICAL

mailto:[email protected]

http://www.gmes.ug.edu.gh/

Regional Marine Centre, University of Ghana June 2020


1. Introduction

The monthly bulletin produced by the Regional Marine Centre provides periodic report on the use of

Earth Observation (EO) data in managing fishery resources in the ECOWAS region. This issue focuses

on changes in biological production derived from ocean colour and sea surface temperature (SST)

observations from space in the western coast of Africa. The analyses cover the spatial distribution in

Chl-a concentrations which is a proxy for phytoplankton – the primary food source for herbivorous

fish, as well as long term comparison of sea surface temperature of the major upwelling centres of the

western coast of Africa.

2. Chlorophyll-a concentration

Phytoplankton are microscopic plant-like organisms which contribute immensely to ocean primary

production by utilizing the sun’s energy. Like many other photosynthetic organisms, phytoplankton

have light absorbing pigments (Chlorophyll) that gives the ocean its blue-green colour. Phytoplankton

are ubiquitous in the vast oceans and satellite derived measurements have become the novelty for

assessing their distribution in the ocean. The distribution of phytoplankton is regulated by nutrient

generation processes that fertilize the euphotic region of the ocean to promote phytoplankton growth.

Upwelling, fronts and currents are oceanographic phenomenon which brings nutrients to the ocean

surface to promote phytoplankton growth. Fishes aggregate to feed in regions of the ocean with high

phytoplankton abundance (high Chl-a concentration) which are often associated with cold SST. Hence,

fisheries resource managers use ocean colour and sea surface temperature to monitor fish distribution

in the ocean.

2.1 Average chlorophyll-a

Due to large amounts of cloud in data, there was a lot of missing Chl-a concentration measurements

on almost the entire Canary region and parts

of the Gulf of Guinea region. The shelves area

of the coast of Ghana and Ivory Coast

recorded high levels of Chl-a concentrations

Also, the coastal waters from north of Guinea

Bissau to the south of Liberia had increased in

Chl-a concentrations (Figure 1). There was also

increased Chl-a concentrations in the

equatorial region which seems to have further

increased westwards to the coastal waters of

Gabon. It is anticipated that the entire Gulf of

Guinea and equatorial waters will continue to

have increase phytoplankton biomass since

the seasonal upwelling will persist for some

few months (Figure 4). Fish abundance is

expected to increase in those areas with Chl-a

concentrations.

Figure 1. Chl-a concentration June 2020





2.2 Monthly mean chlorophyll concentration –June 2020

Except for the northern part of the oceanic and coastal regions of Mauritania, Chl-a concentration

reduced and were confined to the coastal waters

in most part of the Canary. There seems to be a

slight increase in Chl-a concentration off the

continental shelves of Ghana and Ivory Coast,

and in the oceanic regions of the equatorial

Atlantic. The general increase in phytoplankton

biomass in the eastern equatorial Atlantic can be

attributed to the onset of the seasonal upwelling

which supply nutrient-rich bottom water to the

surface for phytoplankton growth. During this

period, fish catch is expected to increase and in

the upcoming month as SST is expected to

decrease in the Gulf of Guinea region (Figure 4).

2.3 Chlorophyll anomaly

Due to the large amount of cloud in data,

Chlorophyll-a concentration anomaly could not

be calculated and were mostly missing. A few

patches of positive Chl-a anomaly were seen on

the shelves of Ivory Coast, Western Sahara and

in the equatorial Atlantic (Figure 3).

Figure 3. Chl-a concentration anomalies June 2020.

Figure 2. 18-year average of Chl-a concentration –June

(2002-2020).





Sea surface temperature trend

As expected, the mean SST trend (2020) showed a decline this month. The 18-year climatology and

mean SST trend for 2019 and 2020 were around the same level (Figure 4). The SST trend (2019)

indicates that there will be a continuous decline in SST next month as the Gulf of Guinea region is

experiencing upwelling. The decrease in SST may cause an increase in phytoplankton abundance in

the Gulf of Guinea region.

Figure 4. Weekly SST trend along the coast of Cote d’Ivoire – Ghana.

SST for June 2020 did not deviate and followed the expected trend, and was around the same level as

the previous year and the mean SST trend from the 18-year climatology (Figure 5). SST in the Canary

Current region is expected continue rising next month. The abundance of phytoplankton may

decrease due to higher SST which may lead to the decrease in fish abundance in the Canary region.

Figure 5. Weekly SST trend along the coast of Senegal-Mauritania.





Annex

Datasets

The products in this bulletin were derived from MODIS Chl-a and SST datasets which are available

from the OceanColor web portal: http://oceancolor.gsfc.nasa.gov/

Indicator calculation

aChl = Chlm – Chly

The chlorophyll-a anomaly represents the deviation of the average chlorophyll-a (Chlm) for a particular

month from the 18-year average (2002 - 2020) for that same month (Chly). A positive anomaly means

chlorophyll-a above the average and can be interpreted as good conditions for phytoplankton growth

in marine ecosystems, and vice versa.

Weekly trend and statistics in SST are calculated from 8-day SST composites from 2002 to 2020.

Abbreviations

SST Sea surface temperature

Chl-a Chlorophyll-a concentration

Acknowledgement

This bulletin was compiled within the framework of the Pan-African GMES & Africa Project. Data used

in the analyses were acquired from the OceanColor web portal. The Regional Marine Resources

Management Centre, University of Ghana acknowledges the contributions of all others who assisted in

the issue of the bulletin. This bulletin has been produced with the financial assistance of the African

Union Commission (AUC).

Disclaimer

The University of Ghana Regional Marine Centre assumes no legal liability or responsibility for how this information is used. This

publication has been produced with the assistance of the European Union. The contents of this publication are the sole

responsibility of the Regional Marine Centre and can in no way be taken to reflect the views of the European Union and the African

Union.



http://oceancolor.gsfc.nasa.gov/

1. Introduction 1 BIOLOGICAL

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