With rising demands and capability to exploit the deep sea, the deep ocean has become a frontier for economic development; however, such exploitation remains highly controversial because the associated risks and environmental impacts are not well understood and the baseline data is usually lacking. There is also a mismatch between the scale of our current knowledge and the scale of the potential impacts, because the large-scale and long-term manipulative impact experiments are practically impossible. To bridge the gap between the mesocosm experiments and the real-world problem, it is important to understand the cumulative impacts and recovery processes of benthic communities undergo large-scale natural disturbance or submarine geohazards.
Gaoping Submarine Canyon (GPSC) off the SW Taiwan (NW Pacific) has been an instructional example of the source-to-sink sediment pathway from the high mountain (head water of Gaoping River at ~4000 m) to the deep South China Sea (> 5000 m to the Malina Trench) and thus an ideal natural laboratory to study the impacts of land-sea interactions on deep-sea benthos. A total of 5 research cruises were conducted during 2014 and 2015 to sample the upper GPSC and the adjacent slope between 200 to 1000 m. The densities of larger macrobenthos (>300 um) were consistently lower in the GPSC than that on the slope, while the densities of smaller meiofauna (>40 um) appeared very similar between the slope and canyon in August 2015 but significantly lower in the GPSC than on the slope in November 2015. The compositions of both size classes were distinctively different between the slope and canyon with more co-occurring taxa on the slope than that in the canyon. Molluscs and paracarid crustaceans did not appear to survive in the canyon, but polychaetes, harpacticoids, nematodes and kinorhyncha seemed to flourish in both the canyon and slope environments.
Our preliminary results suggested that the benthos were likely stressed by the dynamic environments in the GPSC. The effects were more pronounced and consistent for the larger macrobenthos than for the smaller meiobenthos. In contrast to the elevated benthic standing stocks reported in most of the submarine canyons worldwide, the GPSC consistently had lower macrofauna densities than that on the slope throughout the year, suggesting that the benthic communities cannot recover from large-scale and frequent disturbances in the GPSC and the communities remain in early successional stages. Our results also imply that the hyper sedimentation associated with deep-sea mining or mine tailing placements likely have negative impacts on the surrounding benthic communities throughout the length of their operations.