Bacterial standing stock, meiofauna and sediment–nutrient characteristics: indicators of benthic disturbance in the Central Indian Basin

As part of the environmental impact assessment studies for polymetallic nodule mining, the effect of simulated “benthic disturbance” caused by a benthic hydraulic disturber was studied in the Central Indian Basin (CIB). We have compared the abundance and distribution of meiofauna, macrofauna, total counts of bacteria (TC) and cultivable bacteria in the sediments, retrieved from depths of 5000–5300 m, using box corer, before and after disturbance. Nine box core samples each in pre- and post-disturbance stages, were analyzed for labile organic matter (LOM constituted by carbohydrates, protein and lipids), total organic carbon (TOC), ATP and sediment enzymes, phosphatase and lipase. Immediately after the benthic disturbance, (within 10 days), we observed a decrease in meiofauna, macrofauna and bacterial numbers, accompanied by a decrease in LOM, ATP and lipase activity, indicating importance of quality food for the deep-sea benthos. On the other hand, there was an increase in TOC, phosphatase activity and cultivable bacteria, suggesting beneficial effect of disturbance. The results show that a benthic disturbance caused by a hydraulic device may have mixed effects, such as bringing up nutrients from the subsurface layers on one hand and blanketing the bottom by the discharged sediment plume resulting in a decrease in the number of benthos on the other hand. The distinct change in nutrient characteristics of the bottom sediments caused by the disturbance which is probably comparable to a magnified bioturbation process, can be used as an indicator of benthic disturbance.