Based on MESEDA results, a new long-term programme for the precautionary assessment of environmental impacts originating from the mining of polymetallic nodules was developed in 1988. The DISCOL project (DIS-turbance and re-COL-onization experiment in a manganese nodule area of the deep South Pacific) became the first large-scale impact assessment study applying this programme. During the time of the project a total of four cruises with the German Research Vessel SONNE at different stages of this impact assessment study were carried out (see Tab. 1).

Table 1: The different phases of the DISCOL project







January / February


SO 61

Baseline study


February / March


SO 61





SO 61

1st post-impact study




SO 64

2nd post-impact study


February / March


SO 77

3rd post-impact study


January – March


SO 106

4th post-impact study


Choosing the DISCOL Experimental Area (DEA)

Fig. 1: The location of the DEA in the Southwest Pacific OceanThe DISCOL area is located in the Peru Basin in the Southern Pacific Ocean at 7° south of the equator (Fig. 1; right). For choosing a suitable working area several conditions and requirements for successful experimental work were considered: the work area needed to be relatively flat without any outcrops and have a low density of manganese nodule coverage to avoid failure of the sampling equipment. The general area was already chosen before the first DISCOL cruise based on information gathered by the Arbeitsgemeinschaft meerestechnisch gewinnbare Rohstoffe (AMR) during early exploration activities. Further inspection of this area using SEABEAM and the Ocean Floor Observatory System (OFOS) finally lead to the selection of the DEA.

The DEA is circular with a diameter of 2 nautical miles (nmi) or 3700 metres (m), with the central position at 07°04.4 S and 88°27.6 W. It covers an area of 3.14 nmi2 or 10.8 square kilometers (km2). The water depth according to the data derived from the SEABEAM survey ranges between 4040 m and 4170 m, with most of the area showing water depths between 4140 m and 4160 m.

Disturbing the seafloor

Fig. 2: The "plough-harrow" disturbance system (Photo: Dr. Gerd Schriever)Before the start of the project in 1989 the first step, however, was to design a disturber system, that would be able to create disturbances similar to the assumed mining effects and remove the polymetallic nodules from the sediment surface. The design evolved into the so-called “plough-harrow” (Fig. 2; left, Thiel and Schriever, 1990), an 8 m-wide system with small ploughs (two-sided shares, 35 centimeter) extending outward on both sides of the harrow to ensure a ploughing effect on the seafloor irrespective of which side of the device engaged the sediment. The idea of disturbing a circular area with radial disturbance tracks resulted in an expected difference between heavily disturbed central and less disturbed peripheral regions. To generate the disturbance the “plough-harrow” was towed over the seafloor behind the ship at 1.5 to 2 knots across the DEA.

During the DISCOL 1 cruise (SO 61) between February and March 1989 a total of 78 so-called plow-harrow tracks were completed and successfully created the disturbance of the seafloor, which was confirmed by several OFOS runs performed subsequently after the “ploughing”. Within the tracks the nodules could be observed to be totally buried or plowed under and the fauna was destroyed. The disturbance also created a large sediment plume, which, through re-sedimentation, created a secondary impact on the nearby otherwise unaffected seafloor and fauna.

It was estimated, that approximately 20 % of the seafloor within the DEA were directly impacted with the “plough-harrow”, while another 70 to 75 % were believed to be impacted indirectly by the created sediment plume. The remaining 5 to 10 %, which were believed to not show any signs of disturbance were located in the southern quadrant of the DEA, since there were only a few tracks run across this section. Additionally, this area was expected to be not highly affected by re-sedimentation because of the prevailing north- to northwestbound currents during the time of the initial disturbance.

Sampling the Seafloor

Fig. 3: Bathymetric chart (Hydrosweep) of the DISCOL area with different sectors (source: Cruise report SO077)To investigate the impact of the disturbance on the seafloor sediments and the deep-sea ecosystem in the disturbed area, the DEA was subdivided into eight sectors facing the main geographical directions (Fig. 3; right). Additionally, the area was subdivided by two concentric circles with a radius of 1000 and 1350 metres, respectively, which ultimately led to the characteristic shape of the DEA (Fig. 1). Sampling took place in the heavily disturbed central sectors of the DEA and in the less disturbed peripheral regions. For each sector, one multiple corer sample was collected to study the meiofauna and three box corer samples were gathered to investigate the abundance of macrofauna in this area. Baited traps were also used, especially during DISCOL 1 and 2, but were deployed mostly outside the DEA to avoid any cross-contamination. In addition to that, several OFOS runs were executed during each phase of the project to evaluate the faunal composition (macrofauna / meiofauna) in the DEA as well as to determine the changes in shape and appearance of the plough-harrow-tracks. At later stages (since 1992) sedimentological and geochemical studies were also implemented and involved the use of a gravity corer and a maxicorer. Detailed information on sampling stations and the gear used during the different DISCOL cruises can be found here.

Due to time constraints, not all sectors could be examined equally well during the 1st post-impact study (Tab.1). Therefore, the sample distribution and sequence were randomly selected with equal numbers of samples in five central and five peripheral sectors. Furthermore, an undisturbed area three nautical miles up-current outside the disturbed area was chosen as a reference site and subsequently sampled as well.

This sampling scheme was repeated during each of the post-impact studies to allow scientifically sound conclusions about the recolonization process of the disturbed area. Seasonal and inter-annual production and sedimentation cycles may be of significance during such long-term ecological studies. However, only the 2nd post-impact study was conducted during another seasonal phase (September) compared to the other three cruises (January to March) with the main sampling taking place in February, so that these factors may be considered to only have little impact on the results of these investigations.


Whereas samplinFig. 4: Investigation areas of the TUSCH group within the DISCOL/ATESEPP project (source: Cruise report SO106)g throughout the first three DISCOL cruises concentrated mostly on the DEA and the impact of the initial disturbance, two other cruises (SO 78 and SO 79) were conducted in the same general area. However, these cruises focused more on sedimentological, geochemical mechanical and planktological studies.

The SO 106 cruise between January and March 1996 brought the different research groups together into a cooperative programme (ATESEPP). Research concentrated on the DEA, but some additional sampling was performed in the other former study areas (Fig. 4; left). During the course of the ATESEPP project the individual study areas of the former cruises were numbered to avoid any confusion. The DEA, which was sampled during three DISCOL cruises (SO 61, SO 64, SO 77) was named ‘Area 2’. Other areas included the FEMILIEU areas (‘1a’; ‘1b’; ‘6a’; ‘6b’) and the SEDIPERU areas (‘4’ and ‘5’) which were studied during the SO 78 and SO 79 cruises, respectively. The ATESEPP project was led by the TUSCH research group and involved many different research groups from various German research institutes. A number of subprojects contributed to the ATESEPP project, which had the aim to combine all collected data in one overall model to simulate different scenarios and in the end predict the impacts of potential technical interventions on deep-sea ecosystems. Therefore, the present state of the ecosystem was described (ECOBENT, FEMILIEU, MEPARSED, SEDIPERU) and monitored in consequence of an experimental large-scale disturbance (ECOBENT, GECOMET, MEPARSED, PARTRANS), which was then simulated by mathematical modelling (FEMILIEU, TRANSMOD).

ATESEPP - The Subprojects

ECOBENT – The goal of the ECOBENT subproject was to investigate the benthic fauna in the abyssal ecosystem of the southeast Pacific Ocean. Therefore, the meio-, macro- and megafauna in terms of their abundance and natural distribution were described, also with regard to the question how bioturbation influenced the undisturbed sediment layers in the area. Studies focused especially on the DEA to survey the recolonization in the disturbed areas and to provide information on the structure of the seafloor surface seven years after the initial impact.

FEMILIEU – This subproject involved studies of the behavior of iron (Fe) in the chemical environment of the deep sea and modelling the chemical milieu as a function of environmental steering parameters. For that purpose, the geochemical system of the deep-sea floor was investigated with focus on oxygen consumption and redox zonation within the sediments, with the background that mechanical changes in the sediment stratification cause also a geochemical changes in the environment, for example with the release and / or absorption of potentially toxic (trace) elements.

GECOMET – In the GECOMET subproject the changes in the geochemical system related to exchange reactions between the particulate and the dissolved phase involving heavy metals were investigated. Here, too, the formation of toxic compounds was assumed. Thus, the circulation of heavy metals inside the sediments was examined in-situ on the seafloor and compared to previous laboratory experiments.

MEPARSED – The aim of this subproject was to study the changes in the seafloor sediments caused by mechanical strain. For this reason, the sediment-petrographic and the soil-mechanic properties of the seafloor sediments were determined in on-board laboratory experiments.

PARTRANS – The PARTRANS subproject involved experimental studies of the sediment plume to obtain oceanographic, sedimentological and chemical baseline data points to evaluate the mechanical impacts on the seafloor. Therefore, the creation of the sediment plume as well as the particulate material and the resettlement of the sediment particles were examined on board the ship in an experimental chamber to ‘calibrate’ former results to predict the effects of a mechanical disturbance of the seafloor. This included the volume of sediment that would be brought into the boundary layer, together with the composition of the particulate material, its distribution and its suspension time in the plume and furthermore, the sinking the velocity, the re-settlement of particles and the intensity of the sediment cover.

SEDIPERU – The goal of the SEDIPERU subproject was to describe sediment structures of both the disturbed and undisturbed areas as accurately as possible determining the regionally varying thickness of sediment layers, the intensity of bottom currents and sediment transport by measuring the intensity of bioturbation as well as other sediment-physical parameters.

TRANSMOD – The TRANSMOD subproject was again subdivided into two projects: (I) Meso-scale sediment transport in the Pacific Ocean and (II) Mathematical modelling of wide-ranging and long-lasting effects of sediment transports. Both projects studied the evolution of the sediment plume using numerical modelling and especially investigated how far sediment particles of certain sizes can be transported in a plume and how much time these fractions take to resettle, aiming to predict the intensity of re-sedimentation at given distances from the disturbance and thus the impact of the created sediment plume on the benthic fauna.

For further reading, please see the following publications and the references therein:      

Thiel, H. and G. Schriever (1991): Deep-Sea Mining, Environmental Impact and the DISCOL Project; Ambio 19(5): 245-250.

Thiel, H. (1991): From Meseda to DISCOL: a new approach to deep-sea mining risk assessments; Mar. Mining 10: 369-386.

Thiel, Hjalmar and Forschungsverbund Tiefsee-Umweltschutz (2001): Evaluation of the environmental consequences of polymetallic nodule mining based on the results of the TUSCH Research Association. Deep-Sea Research II 48 (2001) 3433–3452.

Thiel, H., G. Schriever, A. Ahnert, H. Bluhma, C. Borowski, K. Vopel (2001): The large-scale environmental impact experiment DISCOL - reflection and foresight. Deep-Sea Research II 48 (2001) 3869–3882.