|Position:||West African coast|
9 - 18 July, 2009
|Gas & Oil Company||Total E&P Nigeria|
|Vessel Name:||Bourbon Diamond|
|SERPENT Representative:||Dr Daniel Jones|
At present there is little information on the benthic communities of the offshore waters of the Gulf of Guinea region and Nigeria is particularly poorly studied. A summary of deep-water biological expeditions to the entire West Africa region reveals only 14 expeditions since 1872. Only 4 expeditions visited the Gulf of Guinea and from these only a few samples were obtained in Nigerian waters.
It has never been a more important time for baseline scientific study, the Gulf of Guinea region is heavily exploited for natural resources, primarily hydrocarbon resources and fish, and is subject to high levels of anthropogenic impact from pollution. The limited environmental information is mostly collected by oil and gas companies and not yet available in the scientific literature.
The deep-waters off Nigeria offer many interesting and valuable opportunities for novel science. SERPENT carried out a visit to the Usan site in deep-water offshore Nigeria. This work was carried out in collaboration with industry, through Total E&P Nigeria (led by Charles Mrabure). Working with Prof. Alex Ugwumba from the University of Ibadan and Dr Adesina Adegbie from the Institute for Oceanography and Marine Research in Lagos SERPENT collected high-resolution images using the industrial remotely operated vehicles (ROVs) on the vessel.
(Left to Right) Charles Mrabure (Total E&P Nigeria), Prof. Alex Ugwumba (University of Ibadan), Dr Adesina Adegbie (Institute for Oceanography and Marine Research, Lagos) and Dr Danel Jones (SERPENT)
The first phase of the work was to characterise the deep-water megabenthic assemblages found in the Usan region for the first time using fully quantitative ROV imaging. Because the Usan field contains a diverse series of seabed habitats we were also able to assess in more detail the effect of the variable seabed slope on the seafloor animal assemblage particularly with regard to the number and diversity of organisms.
Characterisation of the seabed in terms of terrain parameters, such as slope, is a valuable tool for delineating regions of seafloor that may support particular fauna and therefore provide a distinct habitat. These techniques have been trialled with success in shallow waters but have received little attention in deep water. Slope is a vital parameter in these analyses, however the direct effects of slope on the animals of the deep-sea floor have not been assessed. Certainly flat areas tend to exhibit different seabed types and support communities that are different to steeply sloping areas. Slope, while probably not often directly important, is thought to be an important factor in as it has indirect effects on local near-bottom water movement and sedimentation. These factors, in turn, affect important biological processes such as food supply and colonisation.
In addition, we were also able to carry out detailed hydrographic measurements (salinity, depth and temperature), collect some sediment and rock samples as well as and making some collections of the plankton present in the Usan area to drive further research.
A total of 24 invertebrate species were observed at Usan from six animal groups (phyla). The sea urchin, Phormosoma placenta, was the most common animal which accounting for over half of the animals observed . Eight cnidarian species, particularly anemones, were identified but they only represented 4% of the total density. Crustaceans, such as crabs, were numerically important (31 % total fauna) but only represented by three taxa and only one of these, a squat lobster, accounted for the vast majority of the faunal numbers. Although not considered further in this analysis, 10 species of fish were observed at Usan. As a result of limited knowledge of West African megafauna it is difficult to assess how similar the animals at Usan are to other locations. In terms of species, the common species are probably known to science and, the limited evidence suggests, abundant in many areas.
Slope had a major impact on faunal density with a clear and significant linear negative relationship found between total density and slope. When total density is split into more ecologically meaningful units, based on feeding mode, the trends are even clearer. Deposit feeders, the animals that feed directly on the sediment, make up the majority of the animals observed, however, when analysed alone, densities of deposit feeders display an even stronger negative linear relationship with slope. Densities of suspension feeders, the animals that feed from particles in the water, are very low except in the highest slope site, even here mean densities are less than half that of deposit feeders. Slope also has a significant effect on the number of species found.
A total of 46 hydrographic deployments were carried out at Usan and 9600 readings were taken with the instrument. Maximum observed water temperature was 27.6°C and minimum 4.8°C. Salinities ranged between 26.58 and 36.63 (average 34.8) with the lowest salinities only found in the immediate surface waters during rainy periods. The deepest reading was taken at 815 m water depth (5.20° C temperature; 34.76 salinity). The depth of the thermocline (a layer of water with a strong temperature gradient; in the Gulf of Guinea this usually includes water of 20°C) was surprisingly variable. The thermocline depth varied from 41 to 92 m water depth during the survey. The average thermocline depth observed here is greater than suggested in the literature by about 20m. Seasonal fluctuations in temperature are large in the Gulf of Guinea and the period of survey is a transitional time for water mass properties in the area, where climatic data are variable.
A total of 23 species of phytoplankton (drifting plants – usually microscopic) were captured. Of these most (14 species) were diatoms, 8 species were blue-green algae and 1 species was a desmid single-celled alga. In addition, one species of zooplankton (drifting animals), a copepod, was obtained. These species are all thought to be common in the area.
Large brisingid asteroids. Seen throughout rocky area. Arm length is approximately 300-400 mm.
This squat lobster is probably in the genus Munidopsis. It is very common. Up to 100 mm in length.
A Grenadier. Up to 600 mm in total length.
Large lithodid crab seen in the rocky areas only. Up to 500 mm in total width. Has the body shape and gait you would expect from local species Paralomis
cristulata and africana.
Large pennatulid resembling Pennatulea phosphorea. Up to 500mm in height.
A fish, probably Urophycis