Nubi, Olubunmi Ayoola
1, 2
1
Centre for Mathematical Physics - University of Abomey Calavi, Rep. of Benin.
2
Nigerian Institute for Oceanography & Marine Research, 3, Wilmot Point Road, Off Ahmodu
Bello Way, Bar Beach, Victoria Island, PMB 12729, Lagos, Nigeria. oanubi@niomr.org
Bernard Bourlès
3, 4
3 Laboratoire d'études en Géophysique et océanographie spatiales (LEGOS) CNRS : UMR5566 –
IRD – CNES – Observatoire Midi-Pyrénées – INSU – Université Paul Sabatier - Toulouse III
4
LEGOS IRD: UR065
ABSTACT
This research work was carried out as part of the mandate of the Regional Program for Physical
Oceanography in West Africa (PROPAO) to enhance human capacity building in the field of oceanography in West Africa. Nutrient data from the eastern equatorial Atlantic collected during
EGEE/AMMA cruises between 2005 and 2007 were analyzed to illustrate the potential impact of equatorial upwelling on the distribution pattern of nutrients (nitrite, nitrate, phosphate, and silicate).
For the purpose of this report compilation, nutrient data for EGEE 3, 4, 5, and 6 (2006 and 2007) were taken into consideration, and all the plots here reported show the potential influence of equatorial upwelling on the nutrient distribution pattern in the equatorial Atlantic. Further studies shall be dedicated to the analysis of the nutrient distribution and variability in the Gulf of Guinea, and to the processes responsible for this variability, i.e. upwelling, biological, and potential trend due to climate change.
1.0 BACKGROUND
Nutrients are essential for the proper growth and reproduction of individual organisms and, consequently, for the general productivity of marine environments. Nutrients such as nitrate, nitrite, phosphate, and silicate are naturally present in seawater and are essential for growth of phytoplankton and other algae which form the base of the ocean food chain. The availability of inorganic nutrients in the euphotic zone is an important factor controlling primary productivity and consequently the energy available for secondary productivity (Gordon et.al. 1992).
Nutrient distribution has long been used in hydrological studies to show the vertical motions in the surface layer of the ocean (Oudot & Morin 1987). The oceanic distributions of nutrients and patterns of biological production are controlled by the interplay of biogeochemical and physical processes. Biochemical processes lead to the transformation of nutrients between inorganic and organic forms, and also between dissolved and particulate forms. Physical processes redistribute nutrients within the water column through transport and mixing.
The EGEE experiment was the oceanographic component of the AMMA program. It focused on the oceanic circulation, and on air-sea interactions in the Gulf of Guinea. The main scientific objectives are to document: (1) the seasonal and inter-annual variability of the circulation through repeated

trans-equatorial sections with hydrological and current measurements; (2) the processes leading to the strong boreal summer cooling of sea surface temperature along the Equator and at the coast and its link with the African Monsoon.
1.1 AIM OF STUDY
The aim of this study is to describe the seasonal distribution of nitrite, nitrate and phosphate, and silicate in the eastern equatorial Atlantic Ocean, using nutrient data collected during EGEE/AMMA
3, 4, 5, and 6 oceanographic cruises between 2006 and 2007.
1.2 OBJECTIVES
The main objectives are:
1.
To describe the seasonal and interannual variability in nutrient distribution in the eastern equatorial Atlantic;
2.
To study the impact of onset and final phase of upwelling regimes on nutrients distribution in the eastern equatorial Atlantic, relatively to other biological processes.
3.
To study the influence of upwelling on the nutrient distribution at different longitudes in the eastern equatorial Atlantic.
4.
To study possible trends that link with the climate change.
1.3 UPWELLING PHENOMENON – ITS GLOBAL SIGNIFICANCE
Upwelling is an oceanographic phenomenon that involves wind-driven motion of dense, cooler, and usually nutrient-rich water towards the ocean surface, replacing the warmer, usually nutrientdepleted surface water. It is one of several oceanic processes which are thought to have a significant effect on the distribution of pelagic fishes. It apparently influences distribution of fish by modifying environmental conditions to which the fish are sensitive and by increasing the nutrient content of the euphotic zone, which leads to increased productivity throughout the food web
(Ingham, 1970). Upwelling regions therefore result in very high levels of primary production (the amount of carbon fixed by phytoplankton) in comparison to other areas of the ocean. High primary production propagates up the food chain because phytoplankton is at the base of the oceanic food chain.
Upwelling in the eastern equatorial Atlantic, particularly in the Gulf of Guinea has significant impacts on the climate and fisheries of the area, and full understanding of its influence remains an interesting challenge to the science of oceanography and its interplay with meteorology.
1.4 RESULTS AND DISCUSSION
This analysis is preliminary and provided the potential impacts of upwelling on the nutrient distribution in the eastern equatorial Atlantic.

1.5 ONSET AND FINAL PHASE OF THE UPWELLING PERIODS
The outcome of this study revealed the influence of upwelling on the nutrient distribution pattern in the eastern equatorial Atlantic Ocean. These observed distributions reflect the onset of upwelling in
June 2007, and the final phase in September 2007. For the purpose of this extended abstract, only the nitrate plots shall be shown for illustration, having observed that the remaining nutrients followed the same trend. The meridional distribution of nitrate along 10 o
W in June (up) and
September (bottom) 2007 (EGEE5) are as shown in figure 1 below.
Fig 1: Meridional distribution of nitrate (µmol.l
-1 ) along 10 o W in June (top) and September
(bottom), 2007.
These observed distributions, both in June and September 2007 was a reflection of the influence of thermocline, which is believed to be shallow during upwelling periods and also associated with nutrient maximum concentrations. Nutrient maximum was observed closer to the surface in June than in September.

The low levels of all the nutrients at the surface, observed across the entire study stations may be due to high productivity at the surface, where nutrients are being used up for photosynthesis.
1.6 VARIATION BETWEEN UPWELLING AND NON UPWELLING PERIODS
For all the nutrients, higher levels push more to the surface in December than in June 2006. This is an unusual situation, because December is supposed to be a month without upwelling. However, other parameters shall be studied to properly explain the observed trend. Figure 2 below presented the meridional distribution of nitrate along 10 o
W in June 2006 (up / EGEE 3) and December 2006
(bottom / EGEE 4). The plots for other nutrients were of the same trend.
Fig 2: Meridional distribution of nitrate (µmol.l
-1 ) along 10 o W in June ‘06 (top) and Dec ‘06
(bottom).

1.7 VARIABILITY IN NUTRIENT DISTRIBUTION AT DIFFERENT
LONGITUDES – 10 O W AND 2.33
O E
There were variations at different longitude in the nutrient distribution between 10 o
W and 2.33
o
E as upwelling was observed to be at maximum along 10 o W in June 2007 with respect to nitrite, nitrate, and phosphate distribution. The meridional distribution of nitrate (µmol.l
-1
) along 10 o
W (top) and
2.33
o
E (bottom) in June 2007 is as presented in figure 3 below.
Fig 3: Meridional distribution of nitrate (µmol.l
’07
-1 ) along 10 o W (top) and 2.33
o E (bottom) in June
The influence of difference in longitude observed in silicate distribution along 10 o W and 2.33
o E is quite different from those of nitrate and phosphate. Upwelling was at maximum along 2.33
o
E in
June 2007 (figure 4). All the plots presented for the meridional distribution of nutrient along 10 o
W in both June and September, 2007 are in agreement with the qualitative model proposed by
Cromwell (1953), which forecasts an intense divergence, and consequently an upwelling south of the equator in this region.

Fig 4: Meridional distribution of silicate (µmol.l
-1 ) along 10 o W (top) and 2.33
o E (bottom) in
June ’07
However, using nutrient data alone to discuss upwelling and its influence in this region is inconclusive. Part of further studies shall be to discuss upwelling and its impact on nutrient distribution pattern, temperature, dissolved oxygen, and salinity. Ocean colour or chlorophyll data shall also be used vis-à-vis nutrient data, and other parameters to reveal biological productivity in the euphotic zone.
REFERENCES
 Cromwell T., (1953): Circulation in a meridional plane in the central equatorial Pacific, J. Mar.
Res., 12, 196-213.
 Gordon, L.I., Joe C. Jennings, Jr., Andrew A. Ross, and James M. Krest. (1992): A suggested protocol for continuous flow automated analysis of seawater nutrients (Phosphate,
Nitrate, Nitrite and Silicic Acid) in the WOCE Hydrographic Program and the Joint Global
Ocean Fluxes Study. OSU Coll. of Oc. Descr. Chem. Oc. Grp. Tech. Rpt. 92-1.
 Ingham, Merton C. (1970): Coastal upwelling in the northwestern Gulf Of Guinea. Bulletin of
Marine Science, Volume 20, Number 1.
 Oudot Claude and Morin Pascal (1987): The distribution of nutrients in the equatorial
Atlantic : relàtion to physical processes and phytoplankton biomass.
Oceanol. Acta, 1987.