Total Usan Nigeria SERPENT biodiversity survey report

Abstract

The deep-waters off Nigeria offer many interesting and valuable opportunities for novel science. Through Total E&P Nigeria, and with collaboration with the international SERPENT project as well as with local universities and research organisations, industrial remotely operated vehicles (ROVs) were used to carry out high-resolution imaging surveys and make specimen collections in this deep-water region during periods of stand-by time. 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. This was carried out during a visit to Usan in July 2009 on the ROV support vessel Bourbon Diamond. 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. A total of 24 invertebrate species were observed at Usan from six phyla. Echinoderms, such as sea stars and sea cucumbers, were dominant with 10 species found representing 64% of the animals observed. The sea urchin, Phormosoma placenta, was the most common animal which accounted 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. Ten species of fish were observed at Usan. Slope had a major impact on faunal density with a clear and significant linear negative relationship found between total density and slope. Deposit feeders make up the majority of the animals observed display a strong negative linear relationship with slope. Densities of suspension feeders 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. 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 thermocline depth varied from 41 to 92 m 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 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, a copepod, was obtained. These species are all thought to be common in the area.

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