Deep-sea nematode assemblage has not recovered 26 years after experimental mining of polymetallic nodules (Clarion-Clipperton Fracture Zone, Tropical Eastern Pacific)

We investigated nematode assemblages inhabiting the 26-year-old track created by experimental deep-sea mining of polymetallic nodules, and two adjacent, undisturbed sites, one with nodules and one without nodules. The aim was to compare density, assemblage structure, and diversity indices in order to assess the process of recovery of the nematode assemblage inhabiting the disturbed site. This experimental dredging was conducted in 1978 by the Ocean Minerals Company (USA) in the area of a French mining claim in the Clarion-Clipperton Fracture Zone (Tropical Eastern Pacific) at a depth of about 5000 m. The nematode assemblage had not returned its initial state 26 years after the experimental dredging: the total nematode density and biomass within the dredging track were significantly lower than outside the track; the biodiversity indices showed significantly lower nematode diversity within the track; and the structure of the nematode assemblage within the track differed significantly from those in the two undisturbed sites outside the track. However, there were no significant differences in the mean body volumes of adult nematodes and adult-juvenile ratios between the track and reference sites. Parameters such as the rate of sediment restoration (which depends on local hydrological conditions) and the degree and character of the disturbance appeared to be of considerable importance for the recovery rate of the deep-sea nematode assemblages and their ability to recolonize disturbed areas. The rates of recolonization and recovery may vary widely in different deep-sea regions.     

A field experimental study on recolonization and succession of subtidal macrobenthic community in sediment contaminated with industrial wastes

A field experiment was carried out in Hong Kong to study the patterns of recolonization and succession of subtidal macrobenthos in defaunated sediment contaminated with industrial wastes and to determine the time required for benthic recovery in the industrial-contaminated sediment. A total of 50 species was found with an average of 172 animals/tray and 24 species/tray recorded one month after deployment. Initial colonizers were predominantly polychaetes (96 animals/tray, accounting for 55.7%) and gastropods (47 animals/tray, accounting for 27.2%). Abundance of macrobenthos increased quickly to a peak (505 animals/tray) after four months, declined afterwards, and increased again till the end of the experiment. Species number peaked (57 species/tray) in the same month as abundance did, and gradually declined thereafter. Abundance, species number and diversity were significantly lower in the industrial-contaminated sediment as compared to the controls during the early successional stages, indicating the harmful effects of industrial wastes on recolonization and succession of macrobenthos. Although no significant differences in community parameters between the industrial-contaminated and the control sediments were found after eleven months, significant difference in species composition still existed after fourteen months, showing a relatively long-term impact of industrial wastes on macrobenthic community structure.     

Benthic changes at McMurdo Station, Antarctica following local sewage treatment and regional iceberg-mediated productivity decline

McMurdo Station, the largest research station in Antarctica, ceased on-site garbage dumping in 1988 and initiated sewage treatment in 2003. In 2003-2004 its sea-ice regime was altered by the massive B-15A and C-19 iceberg groundings in the Ross Sea, approximately 100 km distant. Here we follow macrofaunal response to these changes relative to a baseline sampled since 1988. In the submarine garbage dump, surface contaminants levels have declined but associated macrofaunal recolonization is not yet evident. Although sewage-associated macrofauna were still abundant around the outfall nearly 2 yr after initiation of treatment, small changes downcurrent as far as 434 m from the outfall suggest some community recovery. Widespread community changes in 2003-2004, not seen in the decade previously, suggests that the benthos collectively responded to major changes in sea-ice regime and phytoplankton production caused by the iceberg groundings.     

Self-dynamics as a driver for restoration success in a lowland stream reach

Multiple stressors like alterations of water quality, hydrology and hydromorphology impact riverine ecosystems. To counteract its consequences, restoration measures are required, e.g. demanded by the EU Water Framework Directive. However, small-scale hydromorphological restoration measures often show little success. Besides overriding stressors, recolonization potential and insufficient time for development are often discussed as reasons for this lack of success. Over a period of 17 years a hydromorphological restoration measure in a German lowland stream reach was evaluated, excluding most of these confounding factors. The restoration measure was left to its self-dynamic development, i.e. there was no further intervention by water management, apart from some large wood installations after 8 years. In the accompanying studies, changes in hydromorphology and the organism groups macroinvertebrates (including both structural and functional diversities), fish and macrophytes were investigated.We proved a rapid and stable enhancement of ecological functions as indicated by a diverse and resilient macroinvertebrate biocoenosis. Both taxonomic and functional richness of macroinvertebrates significantly increased, but varied over time. Since the restoration measure allowed self-dynamic development, a taxonomic and functional endpoint was not achieved even after 17 years, even though near-reference conditions were attained after only 4 years. Deficits in fish communities were most probably due to their low recolonization potential.Our results underscore that small scale hydromorphological restoration can be successful if overriding stressors are absent. Furthermore, we proved the importance of self-dynamics in restoration measures, allowing hydromorphological and biological development.     

Late glacial impacts on dispersal and colonization of Atlantic Canada and Maine by freshwater fishes

Late glacial scenarios of ice retreat and biogeography databases constrain the dispersal routes of obligate freshwater fishes into Atlantic Canada and Maine. Evidence indicates glacial ice covered the present-day mainland and offshore islands at 18,000 ¹⁴C yr before present. Possible refugia for extirpated freshwater fishes were the exposed outer edge of the Grand Banks (east), exposed Georges Bank (south-Atlantic Refugium), and the Mississippi Refugium in the west. It is improbable that the region was recolonized from the offshore refugia. Rather, fishes recolonized from the east via the upper St. Lawrence River valley into the upper Saint John River, Maine (Lake Madawaska) from 11,000 to 12,000 ¹⁴C yr BP. The short period of entry resulted in the low diversity of obligate freshwater species in the region. Lake Madawaska was breached and dispersal continued into the remainder of the region after 8000 ¹⁴C yr BP. By 6000 ¹⁴C yr BP, access routes to the east along low-lying coastal zones were blocked by rising sea levels, which isolated Prince Edward Island, Cape Breton Island, and most probably Nova Scotia. Natural dispersal across the region appeared complete by this time.