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Hartmut Blahm I, Gerd Schriever 2 & Hjalmar Tbiel '
' Alfred-Wegener-Institut fur Polar- und Meeresforschung, ColumbusstraBe, 27568 Bremerhaven, Germany
2 Biolab Research Institute, Kieler StraBe 51, 24594 Hohenwestedt, Germany.
Megabenthic organisms are defined as species large enough to be identified on photographs. Due to their low density, the megabenthos is difficult to catch by usual sampling methods. However, megabenthic organisms are widely distributed in manganese nodule areas and are suitable as indicator organisms for monitoring environmental changes. The poster presents the recolonization of the megabenthos after the mechanical impact created within the DISCOL / ECOBENT project (ref. poster Thiel et al.).
For monitoring the megabenthos a photo-/videosystem (OFOS) was used, which was equipped with a low-level black and white video camera and a 35mm still camera with photo-on-command capabilities. The system was towed by the German Research Vessel Sonne at a distance of approx. 3m above the seafloor. In the laboratory the image material was carefully examined and all observations transfered into video and photograph databases. Diversiq and densiq calculations were made with a specially created abase IV program, based on the video recordings. Photographic information was used to correct the densiq values to more realistic levels.
Although the sharp edges of the experimentally created disturbance tracks were smoothed due to the influence of bottom currents, the plow-harrow tracks are still visible in the image material seven years after their creation. The depopulated plow-harrow tracks were recolonized by highly motile animals such as the hermit crab Probeebei m~rabilis, Ophiuroidea and Holothuroidea, especially between the second and third post-impact study (half a year and three years after plowing). Total densiq reached baseline values. After seven years hemisessile organisms (e.g. Lophenteropneusta, Anthozoa) were also observed in the impacted area. Although only few data sets of the fourth post impact study have been analyzed yet, megafaunal densiq in the plow tracks had again declined seven years after plowing.
In the areas not directly impacted by the plow-harrow device, densities exceeded the baseline values three years after plowing due to possible external effects, but declined again to baseline levels during the fourth post-impact study. Although the megafauna studies demonstrated the reestablishment of the fauna in the disturbed area seven years later, the impact was still to be recognized.
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