Impact of deep-sea mining plumes on benthic ecosystem functioning

• Autor(a) principal: Ramos, Carolina Jorge
• Data de Publicação: 2023
• Tipo de documento: Dissertação
• Idioma: eng
• Título da fonte: Repositório Científico de Acesso Aberto de Portugal (Repositórios Cientìficos)
• Texto Completo: http://hdl.handle.net/10773/40240
• topic: Deep sea; Mining; CCZ; Meiofauna; Macrofauna; Food web

In the deep sea there are different ecosystems with unique characteristics, being the most common, the abyssal plains. They are the largest habitat in the deep sea and despite being remotely located, they are still affected by anthropogenic impacts. Abyssal plains host an enormous and largely unknown biodiversity, but they also provide important mineral resources such as polymetallic nodules that are formed on the seabed. Nodules can be of various sizes and have great potential to be commercially exploited because they are rich in metals such as copper, nickel, manganese, and cobalt that are currently used in electronics and emerging technologies, The Clarion-Clipperton Zone (CCZ), located in the Equatorial North Pacific, is an area of great interest for deep-sea mining, because it is the area with the greatest abundance of polymetallic nodules in all oceans. The main objective of the present work, carried out under the "MiningImpact" project (JPIO), is to assess the impact of sediment plumes originated by deep-sea mining on the benthic food web functioning using an experimental approach. For this, two scientific expeditions were carried out, that allowed to characterize the baseline benthic assemblage in the Belgian area of CCZ (GSR) (density, biomass community structure and feeding strategies of the meiofauna and macrofauna) and to carry out in situ feeding experiments for assessing the transfer of matter and energy in the benthic food webs, using microalgae Chlorella vulgaris, as analogues of POC supply to the abyssal seafloor. The first experiment was carried out during the cruise SO268, before a nodule collector test operation took place and the second experiment was carried out during cruise IP21, after the collector test took place to assess the short-term impact of the sediment plume deposition. To compare the responses to food supply of organisms in different size classes, the experiments included two time point incubations (5 and 20 days), however, during IP21 it was only possible to conduct a 5-days experiment, The baseline community composition (taxonomic and feeding groups), average densities, and average biomass of the meiofauna and macrofauna were described and the data were analysed using the software package PRIMER v.6. Trophic relationships and changes in the food web were investigated using stable isotope analysis in collaboration with the Japan Agency for Marine-Earth Science and Technology (JAMSTEC). The ¹³C values were used to recognize primary food sources and the ¹⁵N values to indicate the trophic level. The results for the baseline benthic characterization showed that in the experiments after 5 and 20 days before mining started, there were no statistically significant differences in all evaluated parameters. However, when comparing experiments before and after the collector test, the differences were noticeable. Density and biomass both of the meiofauna and macrofauna showed strong decreases. After the impact, most macrofaunal taxa of the baseline community were absent and there was a noticeable decrease in the diversity of feeding types after the impact both on meiofauna and macrofauna which affected the structure of the food chain. Nematoda and Copepoda were the first organisms to react to labelled food and to assimilate it, and therefore they will also be the first to respond to possible disturbances and they will likely be the most affected organisms when mining takes place After the collector test, all organisms that absorbed the labelled food were “deposit-feeders”. The results also showed that mining impacts will be different in organisms with different feeding types, with suspension-feeders being considerably more affected by mining plumes. Although most likely the benthic food web will be severely negatively impacted by deep-sea mining operations, mitigation actions may be implemented. Further studies and the establishment of a guideline that contains information regarding population size, reproduction, diversity, and distributions is urgent for a responsible management and an environmental assessment of the impacts on these benthic ecosystems during mining.