An autonomous benthic lander:: preliminary observations from the UK BENBO thematic programme

A new, multi-purpose autonomous benthic lander is described, and preliminary experimental data are presented relating to deployments in the Atlantic Frontier (eastern north Atlantic) during the recent UK Thematic Programme ‘BENBO’. The autonomous lander was deployed at two contrasting sites — Site A (mouth of Rockall Trough; 3570 m) and Site B (Hatton-Rockall Bank; 1100 m) — before and following the spring-time surface ocean phytoplankton bloom (May & July, 1998, respectively). Diffusive oxygen uptake and nutrient flux data were obtained using two interchangeable modules — a profiling oxygen micro-electrode unit and a benthic chamber unit. Diffusive O2 uptake across the sediment–water interface and the O2 penetration depths within the sediment were determined from the oxygen micro-profiles. The shallower site, which had previously received phyto-detrital input, had a comparatively large diffusive oxygen uptake within the sediment (1.2 mmol m−2 d−1) and a maximum penetration depth of only 21 mm. The deeper site had greater oxygen penetration depths (∼80 mm) but a lower diffusive oxygen uptake of 0.6 mmol m−2 d−1, indicative possibly of little or no phyto-detrital input. Visual observations of retrieved sediment cores support this conclusion, however Site A has also displays generally lower organic content and lower macrobenthic biomass which may contribute to this observation. Nutrient pore water profile data indicated fluxes of nitrate of 0.161 mmol m−2 d−1 and phosphate 0.0008 mmol m−2 d−1 into the overlying water. However, the benthic chamber studies showed virtually no change in nutrient concentrations, due probably to the relatively short deployment time used.