RV SONNE Fahrtbericht / Cruise Report SO242-1 [SO242/1]: JPI OCEANS Ecological Aspects of Deep-Sea Mining, DISCOL Revisited, Guayaquil - Guayaquil (Equador), 28.07.-25.08.2015

Cruise SO242/1 ran from 28 July to 25 August 2015 starting and ending in Guayaquil, Ecuador. In total, 40 scientists from five European countries took part in this cruise of the JPIO project ‚Ecological Aspects of Deep‐Sea Mining‘ to study the ecological long‐term impact of deep sea disturbances. The working area, the DISCOL area in the Peru Basin, was ploughed in 1989 and thoroughly studied in the years thereafter, the last cruise, SO106, happened in 1996. SO242/2 aimed at mapping the DISCOL Experimental Area (DEA) in great detail using ship‐based and AUV‐based hydroacoustic and optical methods. To see changes between differently disturbed areas and study the possible recovery of the ecosystem, biological sampling occurred with TV‐guided multi‐coring (MUC), box coring (BC) and epi‐benthic sledge tracks (EBS). Additional biological sampling for scavenging animals occurred with baited Amphipod‐Traps within and further outside the DEA. For geochemical sampling, multi‐, box and gravity coring (BC) was used. Two lander systems equipped with physical sensors such as ADCPs and CTDs were used for current measurements and to monitor sediment plume dispersal created by the EBS. Additional visual studies of the fauna distribution occurred with camera tows (OFOS). Five main sampling areas were selected, two within the DEA targeting heavily disturbed (ploughed) locations and three reference areas 3 to 4 nmi outside the DEA. All five areas had been sampled in the past and can be directly compared concerning ecological changes. Despite a four‐day break due to medical reasons the work program could almost be completed. Four of the working areas were at least sampled with five BCs and MUCs each, and one GC. Box coring could not be performed in the western reference area. In total, 5 CTDs, 27 MUCs, 25 BCs, 7 GCs, 8 EBSs, 5 Amphipod‐Traps, 6 lander deployments and 6 OFOS tracks were successfully undertaken and one thermistor mooring was deployed. In addition, 16 AUV dives clearly showed that plough marks are still well visible after 26 years. There is a slight sediment cover next to the plough tracks, but first analyses of the faunal distribution show that the sessile fauna has not yet recolonised the tracks. Stalked sponges, corals and anemones can be found outside the tracks but still within the DEA. Their distribution patterns inside the DEA do not vary clearly from those on reference sites. The Mn‐nodule distribution is not homogenous; there are areas inside the DEA that do not have nodules at the seafloor surface; they are typically linked to depressions that show low backscatter intensity in the AUV side scan sonar data pointing towards less dense sediment infill. In gravity cores, nodules could be recovered even in 9m sediment depth, finding several more or less intact nodules throughout the entire sediment column was common. Water current measurements show slow currents (max. 6cm/s) and a strong tide‐influenced current direction, whereas no general direction could be observed. Two ‘disturbance experiments’ demonstrated that sediment plumes can be monitored using high frequency ADCPs (1200 kHz). The disturbance by the EBS created a sediment plume that stayed close to the seafloor. First analyses of current trajectories showed that the sediment resettled rather quickly. It became clear that plume behaviour during large‐scale mining cannot be extrapolated from these small‐scale and short‐term experiments. In resume, cruise SO242/1 was very successful and research should continue in the DEA area, which is undoubtedly the best studied long‐term deep sea disturbance site in the ocean.