Macrofaunal communities of the Central Indian Basin (CIB) were sampled with a spade before (June 1997), and immediately after (August 1997), and 44 months (April 2001) after a simulated benthic disturbance for polymetallic nodule mining. The average density recorded down to a sediment depth of 40 cm ranged from 89 to 799 ind Á m À2 (mean: 373 AE 221 SD; n ¼ 12) and 178-1066 ind Á m À2 (mean: 507 AE 489 SD; n ¼ 3) in the test and reference area, respectively. Most of the macrobenthic animals (64%) were concentrated in the upper 0 to 2 cm sediment layers, whereas, sizeable fauna (6%) inhabited the 20-40 cm sediment section and the deepest 5 cm section from 35-40 cm contributed only about 2% to the total population density. The fauna, comprised of 12 groups, were dominated by the nematodes, which constituted 54% of the total population. The macrofaunal density in the test site showed a significant increase (x:400 ind Á m À2 ) in the 44 months postdisturbance sampling (x:320 ind Á m À2 ). The population of nematodes and oligochaetes was nearly restored after 44 months, but the polychaetes and crustaceans did not reach the baseline populations measured in June 1997. The top 0-2 cm sediment layer was severely affected by the disturber, and the study suggests that physically disturbed deep-sea macrofauna may require a longer period for restoration and resettlement than normally believed.Exploratory surveys at sea, conducted worldwide have revealed large mineral deposits in the intertidal, shallow subtidal, and deep-sea areas. Among different marine sectors, the deep-sea has attracted considerable commercial interest, as large deposits of polymetallic nodules have been identified in the world oceans (Glasby 1977;Thiel 2001). Nodules occur on the deep-sea floor in all the oceans, including the Indian Ocean where one of the highest nodule concentrations is in the Central Indian Basin (CIB) (Qasim and Nair 1988). Although deep-sea mining may not be economically feasible at present due to low metal prices and expensive mining technology, it could We are grateful to the officers and crew of the research vessel R.V.A.A. Sidorenko for their help in on board sample collection. This study was carried out as a part of ''Environmental Impact Assessment of nodule mining in the Central Indian Basin'' funded by Department of Ocean Development (Govt. of India). Thanks to Dr. Rahul Sharma for useful discussion on disturbance experiment and also for providing the deep-sea photographs. The manuscript was considerably improved due to the helpful comments of an unknown reviewer. This is contribution no. 4064 of NIO, Goa.
The deep-sea is well known for high benthic biodiversity despite being a low-food environment. However, most deep-sea organisms are very small in size as an adaptation to food limitation. Macrofauna are generally considered to be organisms larger than 0.5 mm and smaller than 3 cm. However, the smaller body size in the deep sea has led to the use of mesh sizes ranging between 0.25 and 0.5 mm to collect macrofauna, 0.3 and 0.5 mm being the most commonly used mesh sizes for deep-sea sampling. In this study, we tested the effectiveness of sieves of two different mesh sizes (0.3 and 0.5 mm) in assessing macrofaunal diversity, density and biomass. A total of 66 species were obtained with the smaller mesh, while the larger mesh retained only 40 macrofaunal species. Thus, use of larger mesh resulted in the loss of 39% species over the smaller mesh (p00.0001). However, both sieves yielded high densities of organisms, high species diversity and steep rarefaction curves for nematodes and polychaetes. Using the larger mesh resulted in a significant loss in biomass of 90% and 78% for polychaetes and nematodes, respectively. Vertically in the sediment, faunal density was sampled more effectively with the smaller mesh sieve. Our results show a significant reduction in the number of species, organism density, and biomass of macrofauna with use of a 0.5 mm mesh rather than a 0.3 mm mesh and that a sieve of lower mesh size is more suitable for evaluation of deep-sea macrofauna.
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