HKU Bulletin November 2020 (Vol. 22 No.1)

HKU researchers have identified areas where local actions may alleviate some of the impact on marine life and shorelines that is typically associated with global climate change. REASONS TO FEEL LESS HELPLESS We found that in the Pearl River Delta, the threat to corals originates fromour inadequate treatment of wastewater. This is interesting because global warming is usually singled out as the cause of coral decline worldwide. Dr David Baker Corals and mangroves are like canaries in the climate change coalmine. Research has shown that these ecosystems are already experiencing harm that may get much worse. But recent work by scholars in HKU’s Swire Institute of Marine Science (SWIMS) and their collaborators has found the situation is a little less hopeless than feared. The coral study showed that water quality may be a more important stressor for coral communities in the Pearl River Delta (PRD) than global warming, while the mangroves study showed it was possible to rehabilitate these ecosystems. Dr David Baker of SWIMS and the School of Biological Sciences was part of a team of scientists from HKU, Princeton University and the Max Planck Institute for Chemistry that conducted the coral study. “We found that in the Pearl River Delta, the threat to corals originates from our inadequate treatment of wastewater. This is interesting because global warming is usually singled out as the cause of coral decline worldwide. We often feel helpless in that scenario because a solution would require everybody on the planet to drastically change their lifestyles. But wastewater treatment is something we can fix more easily,”he said. The study assessed the growth of living coral in the Pearl River Delta over the past two centuries. Corals form bands that are similar to tree bands – during cooler weather they are smaller due to a decrease in calcification and in warmer weather they are larger. This growth is fuelled by resources drawn from seawater, particularly nitrogen which is a key component of protein scaffolding. Nitrogen is prevalent in human sewage and during the period under study, the Pearl River Delta’s population increased from a few thousand to more than 100 million people. Water quality declined in the latter part of the 20 th century and the study was able to link that to coral growth through X-rays of the corals and material extracted from their bands. It also showed that Hong Kong’s improved wastewater treatment from the year 2000 had had a positive impact on corals. “Paradoxically, our study is good news because it implies that the solution to coral decline is in our hands. The improvements in Hong Kong indicate that these efforts must be continued if we want corals to come back here,” Dr Baker said. Turning of the tide Another area of cautious hope is mangroves. Experts warned more than a decade ago that they were being lost faster than almost any other ecosystem, including coral reefs and tropical rainforests. Dr Stefano Cannicci of SWIMS and the School of Biological Sciences was among the team that sounded the alarm. Now he is singing a different tune. The rate of mangrove loss has improved dramatically, from about 1–3 per cent per year in the late 1990s to 0.3–0.6 per cent more recently. This is attributed to successful conservation efforts, such as better monitoring, changing industrial practices, rehabilitation, and expanded management and protection, and was reported in a study with researchers from 24 institutes around the world, including Dr Cannicci. “I am very proud to have been among the academics warning about mangrove loss more than a decade ago because I think it helped turn the tide on mangrove degradation. The perception about their importance to humankind and the planet has changed, although there are still dangers. We need to keep up the effort to manage and preserve mangroves worldwide,”he said. Areas of concern include the uneven distribution in mangrove conservation gains across the world and the risk of mangroves being sandwiched between rising sea levels and developed hinterlands that constrict their growth (see also page 4). Reducing greenhouse gas emissions is not just good for the atmosphere and sea levels – it could also improve water quality. A study by SWIMS and the Department of Earth Sciences of the South China, East China, Yellow and Bohai Seas found nitrogen oxides from air pollution affect water quality when they fall into the water because they enhance the production of algae. When the algae die and sink to the bottom, their decomposition decreases dissolved oxygen in the water, which is bad for marine life. However, their model shows that reducing the emission of anthropogenic nitrogen in the atmosphere could lead to an improvement of water quality, notably in the South China Sea. “Our study shows the potential benefit of reducing fossil fuel burning not only on humans and the ecosystem, but also local activities such as fisheries,”said PhD student Miss Yau Yu-yan, who led the research under the supervision of Dr Benoit Thibodeau. BETTER AIR, BETTER WATER Nevertheless, places like Hong Kong have seen improvements. Dr Cannicci recently recorded about 40 mangrove forests here covering about 350 hectares, which is the largest mangrove patch within the PRD. “Although small and limited in size, Hong Kong mangroves contain a magnificent diversity of plant and animal species: eight species of trees, 53 species of crabs and 42 species of snails. This is more than is known for the mangrove forests of the entire African continent,”he said. A crab resting on a mangrove in Ting Kok, Tai Po, Hong Kong. Researchers drilling coral under water. Anthropogenic pollutants can be observed in the atmosphere of many Chinese coastal cities. A team of international mangrove forest experts now find cause for optimism for global mangrove conservation. COVER STORY 14 The University of Hong Kong Bulletin | November 2020 15

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