Deep Sea Research Part II: Topical Studies in Oceanography
Re-establishment of an abyssal megabenthic community after experimental physical disturbance of the seafloor
Introduction
Human industrial activities in the deep sea, e.g. dumping of waste, storage of carbon dioxide, or mining of mineral resources, will have a large impact on the abyssal benthos (Jumars, 1981; Thiel, 1991; Thiel and Schriever, 1994). The deep sea is inhabited by a highly diverse fauna. Our knowledge of the taxonomy and ecology of the abyssal benthos is still poor. There is an urgent need for baseline and special environmental studies related to impact assessments of such activities on abyssal communities.
Industrial technologies for manganese nodule mining have already been developed although some technical problems still need to be solved. Knowledge of the impacts created by these devices is largely unknown. At the very least sediment surface structures will be physically changed influencing the structure of benthic communities.
The aim of the DISCOL experiment (‘DISturbance and reCOLonization experiment in a manganese nodule area of the deep South Pacific’; Foell et al (1990), Foell et al (1992a), Foell et al (1992b); Schriever, 1990; Schriever and Thiel, 1992; Thiel and Schriever (1989), Thiel and Schriever (1990)) was to observe the long-term re-establishment of the benthic fauna following a large scale, experimental physical disturbance of the seafloor. The experiment was conducted as part of German national deep-sea environmental protection activities (TUSCH, Thiel, 1995; Thiel and Forschungsverbund Tiefsee-Umweltschutz, 1995) and started in 1989 in the Peru Basin, in the tropical south-eastern Pacific Ocean.
A large area of seabed (11 km2) was ploughed with a specially designed ‘plough-harrow’ (Thiel and Schriever, 1989). Post-impact studies were conducted immediately after impact, half a year later, and again after three years. Dominant taxa of meio-, macro-, and megafauna were investigated to identify indicator species that characterised the recovery process. Seven years after ploughing, the DISCOL Experimental Area (DEA) was revisited during the ECOBENT project and a similar sampling programme was again conducted (Schriever et al., 1996).
The megabenthos comprises a major fraction of the deep-sea benthic biomass. It plays a key role in abyssal ecosystems (Smith and Hamilton, 1983), and therefore has to be included in environmental studies. Megabenthic organisms are defined as species large enough to be determined on photographs (Grassle et al., 1975; Rex, 1981). They are difficult to study using conventional sampling methods because of their low density, and in areas with manganese nodules the animals are typically badly damaged when collected with trawls (Bluhm et al., 1995).
A great deal of data already exist on the density and diversity of megafauna from manganese nodule sites in the Pacific (Hecker and Paul, 1977; Foell et al., 1986; Pawson, 1988; Foell, 1992; Tilot, 1992; Morgan et al., 1993; Bluhm et al., 1995) and Indian Oceans (Sharma and Rao, 1992). A comparison of these data was presented by Bluhm (1994). Data on megafauna recolonisation within the DISCOL experiment were published in Bluhm et al. (1995). Results of the ECOBENT study, regarding the continuing re-establishment of the community, are presented herein.
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Material and methods
The DISCOL Experimental Area (DEA) is located close to a German manganese nodule claim. The DEA is a circular area of 10.8 km2, with its central position at 07°04.4′S and 88°27.6′W. The water depth ranges between 4140 and 4160 m. The abundance of manganese nodules in the DEA before the experiment was about 5–10 kg/m2. The size of individual nodules varied between 8 and 15 cm in diameter (Thiel and Schriever, 1989).
The plough-harrow was towed on diammetric courses through the centre of the DEA a
Qualitative observation
Details of the plough operations, their impact on the seafloor, the photographic observations, the development of surface structures, and the re-establishment of the epifaunal community during DISCOL are described in detail in Bluhm et al. (1995). Additional information concerning the fourth post-impact study (ECOBENT) is given here.
During the third post-impact study we observed that the plough-harrow tracks were less sharp than previously seen owing to near-bottom currents and faunal
Physical disturbance and resedimentation
Physical disturbance of the seafloor is not an uncommon phenomenon. For instance, the carcasses of large animals fall onto the seafloor (Stockton and DeLaca, 1982; Smith (1985), Smith (1986); Priede et al., 1991) and the activities of scavenging animals change the physical and chemical structure of the sediment surface. On a larger scale, turbidity currents, debris flows, and benthic storms modify benthic communities (Nardin et al., 1979; Hollister and Nowell, 1991). Commercial mining will
Conclusion
The results of the DISCOL and ECOBENT surveys show that photo/video image systems are useful in detecting changes of epibenthic communities resulting from physical disturbance impacts. The megafauna play an important role in the deep-sea ecosystem and, therefore, should be monitored in future environmental studies.
During the experiment, natural variations in the taxonomic and abundance compositions of the megafauna communities were observed. Clear relationships to the flux of organic matter
Acknowledgements
I would like to thank all who helped to identify the animals in our image material. The statistical analyses accompanying this study were performed by Ingo Langner. Thanks are also due to Ellen and Christopher Scholl for streamlining the English of the manuscript. The project was funded by the German Bundesministerium für Bildung und Forschung through contracts 03-R-389, 03-R-392, 03-R-396, 03-R-411, 03-F-0010F, and 03-G-0106A. The author is responsible for the content of this paper. DISCOL
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