Long-term changes in microbial and biochemical parameters in the Central Indian Basin

Natural kilometer-scale spatial variability and seasonal variability in deep-sea sediments at ∼5000 m depth was studied at two reference sites in the Central Indian Basin. Biochemical parameters such as the labile organic matter (LOM) (consisting of carbohydrates, protein and lipids) and total organic matter (TOM) and biological parameters such as total counts of bacteria and adenosine triphosphate (ATP) were estimated after an interval of 54 months. These were compared with nine locations (12–17 km away) where an artificial sediment disturbance was created by a hydraulic benthic disturber. Besides the above-mentioned parameters, extracellular enzymes, alkaline phosphatase, aminopeptidase and lipase were also monitored at these nine locations. Post-disturbance (PoD1) observations were carried out immediately after the benthic disturbance experiment and compared with the pre-disturbance (PreD) pattern. The restoration process in the test site was studied after a period of 44 months (PoD2). There was a drastic reduction in total counts of bacteria, ATP, carbohydrates and lipids accompanied by a two- to four-fold increase in protein content at the undisturbed reference sites after 54 months. A similar reduction in these parameters was observed in the PoD1 compared to the PreD stage at the test site. An initial increase in TOM content and in the activities of three extracellular enzymes in the PoD1 stage was followed by a reduction in TOM, ATP and organic P content and activities of the extracellular enzymes in the PoD2 phase (i.e. after 44 months). Reduction in ATP levels suggests a negative impact of resedimentation on living biomass in the sediments. Although LOM, protein concentrations and the protein/carbohydrate ratio increased in the PoD2 phase relative to the PoD1 phase, the bacterial numbers were below the concentrations in the PreD stage, indicating slow restoration of benthic conditions. Long-term analyses of the indicator parameters thus showed mixed effects of sediment disturbance. A similar pattern at the undisturbed reference location suggests that natural (seasonal) variability outweighed artificial variability caused by the benthic disturbance in the Central Indian Basin and the former is responsible for bringing about changes in deep-sea benthic ecosystem on long-time scales.