Coral reefs are in the midst of global decline, resulting from anthropogenic perturbations of climate, nutrient cycles, and fisheries. Future projections of increasing sea surface temperatures alone yield dire predictions for coral reefs and the ecosystem services they provide. This issue is very relevant to the fringing and barrier reefs of the Indo-Pacific region that are home to ~600 species of scleratinian (hard) corals. Recent studies have shown that mean sea temperature is rising by ~0.5 °C every decade resulting in an alarming increase in rainfall events. The impact of this on marine biodiversity is stark and is being further accelerated by ever-increasing human impacts – for instance, reef tourism is currently estimated at ~US$36 billion every year with annual visitor numbers being equivalent to 70 million tourist trips. This has resulted in a disproportionate increase in untreated wastewater from hotels, and restaurants that is discharged into the ocean. Both nutrients and pathogens in wastewater fuel harmful algal blooms and exacerbate the prevalence and severity of disease, respectively. Sedimentation smothers and abrades benthic species, re-suspends nutrients and pathogens, and blocks recruitment. Aquaculture waste poses similar threats, adding nutrients, pathogens, parasites, and sediments. Human activities can also create disease vectors. For instance, plastic debris serves as a vector for pathogens and spreads coral disease.
This issue is especially relevant to one of the most densely populated cities on Earth, Hong Kong, where nearly 1000 Olympic-sized swimming pools equivalent to sewage get dumped into the ocean on a daily basis. Human impacts in Hong Kong date back to the Tang dynasty when the slaked lime industry thrived. Although this died following WWII, several coral communities were lost. The presence of abundant corals in the past to support a slaked lime industry suggests that the coastal waters were extremely hospitable to coral communities, with low sediment load and high water quality. In the 1980s and 1990s, rapid urbanisation through reclamation and dredging facilitated explosive economic development but with trailing wastewater treatment infrastructure. This resulted in a tragic loss of water quality and associated foundational species such as coral reefs and seagrass beds. Moreover, we now see a distinct gradient in water quality (total inorganic nitrogen concentrations) with low biodiversity in the west to high biodiversity in the east.
Yet, Hong Kong is home to more hard coral species than the Caribbean! In fact, the marine environment is home to some especially old coral colonies (~200 years old). So, what makes Hong Kong coral communities special? In the little time, I have spent in this city with a long-standing eutrophication problem, I have observed that Hong Kong’s seawaters are one of the most human-impacted environments on this planet and the coral communities have been naturally selected for only resilient genotypes owing to multiple environmental stressors. In the last few years, the coral biogeochemistry lab at the University of Hong Kong has been studying water quality impacts on marine biodiversity. They now believe that the marine environment has improved considerably for coral survivorship following several Government initiatives. In fact, Hong Kong is a recent signatory to the Convention on Biological Diversity (CBD) and is now implementing a biodiversity strategy and action plan (BSAP). However, coral recruitment continues to be negligible implying that without assisted management and restoration, … coral recruitment continues to be negligible implying that without assisted management and restoration, corals are unlikely to persist in this part of the world. corals are unlikely to persist in this part of the world. Is Hong Kong receiving coral larvae dispersed from its neighbours? Or are Hong Kong corals reproductively isolated? The former will require intervention that could be more aligned with the viewpoints of the Chinese government. The latter will require local protection and assisted restoration.
It is an important time for China and its neighbours to elevate public awareness and support for marine conservation specifically focusing on human activities such as reclamation and pollution. In fact, the region has seen positive shifts in mindset for the shark-fin trade and sustainable seafood campaign. After years of development, even the most pristine reefs around coastal cities are likely to transition into heavily turbid, patchy, and poor-recruitment driven, yet naturally selected and genotypic-resilient corals. With ~200 years of development, Hong Kong is a crystal ball through which we can witness the impacts of coastal development on coral reefs. Hence their name, urban reefs of the future.