Climate change affects not only terrestrial life but also the largely uncharted deep-sea ecosystem, which is home to distinct and largely unknown species. Deep-sea creatures, having adapted to their stable and extreme conditions, are especially susceptible to fluctuations in temperature and food supply. This prompts an essential inquiry: which environmental factors are crucial for deep-sea ecosystems, and how could these be altered?
A research project co-directed by Professor Moriaki YASUHARA and Ms. Raine CHONG from the School of Biological Sciences, together with the Swire Institute of Marine Science and the Institute for Climate and Carbon Neutrality at The University of Hong Kong (HKU), alongside Dr. May HUANG from Princeton University’s Department of Geosciences, has unveiled how the deep-sea ecosystem in the Southern Ocean has transformed over the last 500,000 years. The findings, published in Current Biology, indicate that changes in temperature and food input have distinctly influenced the development of deep-sea ecosystems.
The temperature in the deep sea remains relatively constant with only minor variations over extensive periods. Despite this consistency, the organisms that inhabit these depths are exceptionally well-adapted to their stable environments, making even slight temperature changes particularly problematic for them. Unlike surface waters, where sunlight supports primary production, deep-sea regions lack this light, hindering the growth of phytoplankton. Instead, the inhabitants of the deep sea depend on decaying organic matter, referred to as particulate organic material or marine snow, which falls from the surface, including dead plankton that serve as a primary food source for deep-sea dwellers.
The research team conducted a study using data from deep-sea fossils contained in sediment cores that date back 500,000 years. Their findings confirmed that variations in temperature and food availability have significantly impacted deep-sea communities over lengthy timescales, with each influencing different species.
Professor Yasuhara emphasized the importance of advancing fundamental science to comprehend how ecosystems function globally and to address the pressing challenges posed by human-driven climate change.
As awareness grows regarding the ongoing human-induced climate warming and the potential for it to worsen, scientists and engineers are striving to innovate mitigation technologies. These ocean-based climate interventions (OBCI) encompass strategies like marine carbon dioxide removal (mCDR), which seek to combat future warming by depositing and storing carbon or carbon dioxide in deep-sea sediments, where they remain stable due to cold temperatures and high pressure.
A notable instance of mCDR is iron fertilization, where iron is introduced to the ocean surface to boost primary production, leading to an increase in organic carbon sinking to the deep-sea floor. Although mCDR and OBCI technologies are advanced and almost ready for large-scale deployment, they have not yet been implemented extensively, raising concerns about their potential impact on deep-sea ecosystems.
Yasuhara comments, ‘The deep sea covers over 40% of the Earth’s surface and is extremely fragile. It also hosts countless undiscovered species, the majority of which remain a mystery to us. Our research utilizing fossil records from deep-sea sediment core samples over the past 500,000 years indicates that both temperature and food availability—driven by changes in natural iron fertilization from dust input and the consequent increase in surface production—have substantially altered deep-sea ecosystems in various ways. This signifies that we must tread carefully when making decisions about this vital and sensitive ecosystem. Thorough ecosystem impact assessments are essential to determine whether human-induced warming or mCDR, impacting surface productivity, poses a greater threat. Only then can we cautiously and wisely decide on the adoption of mCDR.’
Professor Yasuhara also noted that the Southern Ocean serves as a ‘canary in a coal mine’ due to its crucial role in global ocean circulation and climate systems. ‘Our research draws attention to the sensitivity of its deep-sea ecosystems. It is imperative that we enhance biological monitoring in this region, as it could serve as an early warning system for climate changes. Our findings indicate that the contemporary structure of the Southern Ocean’s deep-sea ecosystem was established approximately 430,000 years ago. It is my hope that this long-standing ecosystem remains intact, particularly considering the uncertainties surrounding the degree of future human-induced warming and its impact on the global climate system.’