ANTHROPOGENIC IMPACTS ON THE DEEP SEA
453
by the collector system, resedimentation of the plume
created by the mining occurs in the near-field. Lumps
and aggregates sink back to the seafloor most probably
in the mined area, as shown by modelling the sediment
plume, and some fine-particulate matter will drift for
longer distances (Zielke et al., 1995; Jankowski et al.,
1996; Segschneider and S¨ undermann, 1997; Thiel, ed.,
2001).
The faunal composition of all size classes remained
permanently altered, because the manganese nodules,
constituting a patchy hard substrate, were ploughed
into the sediment. The megafauna, after an initial
strong disturbance in the ploughing tracks, showed
a slow recovery. After seven years even some of
the less mobile species had returned. Outside the
physically disturbed tracks the megafauna survived the
drifting plumes from ploughing and resettlement of the
sediment; after three years a strong increase in numbers
was observed, whereas undisturbed regions outside the
experimental area showed rather constant megafauna
densities throughout this period (Bluhm, 1993; Bluhm
et al., 1995; Schriever et al., 1997).
The macrofauna recovered slowly after a rapid
increase in abundance in the initial post-impact phase
(0.5 years), but the various dominant taxa behaved
differently. While the isopods and tanaids recovered
very rapidly, the polychaete abundance in disturbed
sediments was affected significantly even after three
years. The abundance of the subsurface deposit feeding
polychaetes in disturbed samples were still significantly
reduced, as is typical for recolonization stages (Rhoads
and Boyer, 1982; Probert, 1984), and the distribution
of the macrofauna in the sediment column extended
deeper in the depressions refilled with sediment than
in undisturbed layers (Borowski and Thiel, 1998). The
“species diversity” (calculated from 78 selected polychaete species) in disturbed samples after three years
was significantly different from that in undisturbed
sediments, and reflected the artificially introduced
heterogeneity of the habitat (Borowski, 1996; Borowski
and Thiel, 1998). A similar result could even be
demonstrated at the taxonomic level of polychaete
families (Borowski and Thiel, 1996). While after seven
years the densities of most taxa in the disturbed
samples had increased to the levels in undisturbed
sediments, those of bivalves behaved differently; their
abundance also increased initially, but in the later
phase decreased again, following medium densities
after 0.5 and 3 years (Borowski, unpublished data).
The meiofauna exhibited a slight increase above
baseline values from the post-impact study to half
a year later. The immediate loss from disturbance
amounted to 8% and 16% for nematodes and harpacticoids, respectively. Both these taxa, and also the
Foraminifera, had already increased after three years –
the nematodes and the harpacticoids to above preimpact levels. Whether this is an effect of variation
in faunal density, as has often been observed during
community re-establishment (e.g., Pearson and Rosenberg, 1978), or an indication of higher organic-matter
sedimentation, as the meiofauna had also increased at
an upcurrent reference station, must remain open. After
seven years, figures for nematode abundance are near
the baseline values (Schriever et al., 1997). Changes
in faunal composition were still detectable, but are
expected to normalize in the future.
In general, it is assumed that a balanced community
will develop in heavily disturbed mining regions, but
with the removal of the nodules it will consist purely
of a soft-bottom fauna, so that a reduction in species
diversity is to be expected as a result of the overall
reduction in habitat diversity. The nodule surface and
its meiofauna differ from those of the sediment (Bussau
et al., 1995). Thiel et al. (1993) and Maybury (1996)
described a specific manganese-nodule crevice fauna
inhabiting a mud-filled canal system in the large
nodules of the Peru Basin. Nematodes, polychaetes, a
sipunculid, harpacticoids and tanaids were discovered
in this special habitat, and the nematode species
Acantholaimus maks has never been found elsewhere.
The DISCOL large-scale in situ experiment was
followed by a series of other large-scale in situ
experiments (Table 13.3), studying the reaction of the
benthos to various rates of resedimentation from an
artificially created plume. These four experiments were
conducted with the same disturber, slightly modified
between experiments, designed to suck up sediment
from the seafloor and eject the material several
metres above, thus creating a plume which would
settle downstream, blanketing the sediment surface and
burying the fauna. The experiment simulated plume
resettlement behind a collector system employed in the
mining of polymetallic nodules.
The results of these experiments are so far limited.
Trueblood and Ozturgut (1997) reported on the United
States Benthic Impact Experiment (BIE). Nine months
after the disturbance, nematodes were significantly
reduced in the sediment redeposition area. Macrofauna
diversity remained the same, but two families (one
polychaete and one isopod) were strongly affected
453
by the collector system, resedimentation of the plume
created by the mining occurs in the near-field. Lumps
and aggregates sink back to the seafloor most probably
in the mined area, as shown by modelling the sediment
plume, and some fine-particulate matter will drift for
longer distances (Zielke et al., 1995; Jankowski et al.,
1996; Segschneider and S¨ undermann, 1997; Thiel, ed.,
2001).
The faunal composition of all size classes remained
permanently altered, because the manganese nodules,
constituting a patchy hard substrate, were ploughed
into the sediment. The megafauna, after an initial
strong disturbance in the ploughing tracks, showed
a slow recovery. After seven years even some of
the less mobile species had returned. Outside the
physically disturbed tracks the megafauna survived the
drifting plumes from ploughing and resettlement of the
sediment; after three years a strong increase in numbers
was observed, whereas undisturbed regions outside the
experimental area showed rather constant megafauna
densities throughout this period (Bluhm, 1993; Bluhm
et al., 1995; Schriever et al., 1997).
The macrofauna recovered slowly after a rapid
increase in abundance in the initial post-impact phase
(0.5 years), but the various dominant taxa behaved
differently. While the isopods and tanaids recovered
very rapidly, the polychaete abundance in disturbed
sediments was affected significantly even after three
years. The abundance of the subsurface deposit feeding
polychaetes in disturbed samples were still significantly
reduced, as is typical for recolonization stages (Rhoads
and Boyer, 1982; Probert, 1984), and the distribution
of the macrofauna in the sediment column extended
deeper in the depressions refilled with sediment than
in undisturbed layers (Borowski and Thiel, 1998). The
“species diversity” (calculated from 78 selected polychaete species) in disturbed samples after three years
was significantly different from that in undisturbed
sediments, and reflected the artificially introduced
heterogeneity of the habitat (Borowski, 1996; Borowski
and Thiel, 1998). A similar result could even be
demonstrated at the taxonomic level of polychaete
families (Borowski and Thiel, 1996). While after seven
years the densities of most taxa in the disturbed
samples had increased to the levels in undisturbed
sediments, those of bivalves behaved differently; their
abundance also increased initially, but in the later
phase decreased again, following medium densities
after 0.5 and 3 years (Borowski, unpublished data).
The meiofauna exhibited a slight increase above
baseline values from the post-impact study to half
a year later. The immediate loss from disturbance
amounted to 8% and 16% for nematodes and harpacticoids, respectively. Both these taxa, and also the
Foraminifera, had already increased after three years –
the nematodes and the harpacticoids to above preimpact levels. Whether this is an effect of variation
in faunal density, as has often been observed during
community re-establishment (e.g., Pearson and Rosenberg, 1978), or an indication of higher organic-matter
sedimentation, as the meiofauna had also increased at
an upcurrent reference station, must remain open. After
seven years, figures for nematode abundance are near
the baseline values (Schriever et al., 1997). Changes
in faunal composition were still detectable, but are
expected to normalize in the future.
In general, it is assumed that a balanced community
will develop in heavily disturbed mining regions, but
with the removal of the nodules it will consist purely
of a soft-bottom fauna, so that a reduction in species
diversity is to be expected as a result of the overall
reduction in habitat diversity. The nodule surface and
its meiofauna differ from those of the sediment (Bussau
et al., 1995). Thiel et al. (1993) and Maybury (1996)
described a specific manganese-nodule crevice fauna
inhabiting a mud-filled canal system in the large
nodules of the Peru Basin. Nematodes, polychaetes, a
sipunculid, harpacticoids and tanaids were discovered
in this special habitat, and the nematode species
Acantholaimus maks has never been found elsewhere.
The DISCOL large-scale in situ experiment was
followed by a series of other large-scale in situ
experiments (Table 13.3), studying the reaction of the
benthos to various rates of resedimentation from an
artificially created plume. These four experiments were
conducted with the same disturber, slightly modified
between experiments, designed to suck up sediment
from the seafloor and eject the material several
metres above, thus creating a plume which would
settle downstream, blanketing the sediment surface and
burying the fauna. The experiment simulated plume
resettlement behind a collector system employed in the
mining of polymetallic nodules.
The results of these experiments are so far limited.
Trueblood and Ozturgut (1997) reported on the United
States Benthic Impact Experiment (BIE). Nine months
after the disturbance, nematodes were significantly
reduced in the sediment redeposition area. Macrofauna
diversity remained the same, but two families (one
polychaete and one isopod) were strongly affected