Megacity development and the demise of coastal coral communities: Evidence from coral skeleton δ¹⁵N records in the Pearl River estuary

Abstract

Historical coral skeleton (CS) δ18O and δ15N records were produced from samples recovered from sedimentary deposits, held in natural history museum collections, and cored into modern coral heads. These records were used to assess the influence of global warming and regional eutrophication, respectively, on the decline of coastal coral communities following the development of the Pearl River Delta (PRD) megacity, China. We find that, until 2007, ocean warming was not a major threat to coral communities in the Pearl River estuary; instead, nitrogen (N) inputs dominated impacts. The high but stable CS‐δ15N values (9‰–12‰ vs. air) observed from the mid‐Holocene until 1980 indicate that soil and stream denitrification reduced and modulated the hydrologic inputs of N, blunting the rise in coastal N sources during the early phase of the Pearl River estuary urbanization. However, an unprecedented CS‐δ15N peak was observed from 1987 to 1993 (>13‰ vs. air), concomitant to an increase of NH4+ concentration, consistent with the rapid Pearl River estuary urbanization as the main cause for this eutrophication event. We suggest that widespread discharge of domestic sewage entered directly into the estuary, preventing removal by natural denitrification hotspots. We argue that this event caused the dramatic decline of the Pearl River estuary coral communities reported from 1980 to 2000. Subsequently, the coral record shows that the implementation of improved wastewater management policies succeeded in bringing down both CS‐δ15N and NH4+ concentrations in the early 2000s. This study points to the potential importance of eutrophication over ocean warming in coral decline along urbanized coastlines and in particular in the vicinity of megacities.

Coral reef diversity losses in China’s Greater Bay Area were driven by regional stressors

“…6 ). This conclusion corroborates with the findings of Duprey et al ( 42 ), which showed an increase in DIN concentrations that peaked in the 1980s during rapid urbanization. Their conclusions that local eutrophic waters have led to the present coral richness southeast gradient are supported by our subfossil baseline.…”

“…This biodiversity includes ~90 species of scleractinian corals ( 37 , 38 ), which is relatively high for marginal reefs that commonly host less than 50 species ( 39 – 41 ). The co-occurrence of high diversity and extreme urbanization [an increase of >100 million people in the last 40 years; ( 42 )] makes the GBA an appropriate choice for investigating historical trends in coral communities as a result of severe local stressors and more recent global change. Corals are a model system to study impacts to the marine environment as they are fragile animals that can be highly susceptible to human perturbations ( 43 , 44 ).…”

Light Limitation and Depth-Variable Sedimentation Drives Vertical Reef Compression on Turbid Coral Reefs

“…Coral reefs in Southeast (SE) Asia have suffered significant ecological declines (Heery et al, 2018). Widespread reductions in coral cover, species diversity, and reef structural complexity from climate-related disturbances (Guest et al, 2012;Perry and Morgan, 2017;Hughes et al, 2018) are compounded by localized threats to coral reefs, including land-derived siltation and nutrient loading (Kamp-Nielsen et al, 2002;Baum et al, 2015;Duprey et al, 2020). Declining water quality in the region has been attributed to poorly regulated land-use change, rapid human population growth, and the urbanization of coastal watersheds (Kamp-Nielsen et al, 2002;Syvitski et al, 2005).…”

Ontogenetic δ15N Trends and Multidecadal Variability in Shells of the Bivalve Mollusk, Arctica islandica

“…Thus, the isotopic level of all components of the food web will change proportionately to the shift in δ 15 N TDN (e.g., Ohkouchi et al, 2015). Possible reasons for δ 15 N TDN fluctuations include changes in ocean circulation, mixed layer depth, upwelling, pH value, nitrogen cycling processes, influx of freshwater and input of anthropogenic nitrogen by atmospheric deposition or through rivers (e.g., Altabet et al, 1995;Cabana and Rasmussen, 1996;Voss et al, 2005;Sherwood et al, 2011;Ellis, 2012;Wang et al, 2018;Whitney et al, 2019;Duprey et al, 2020;Hopkins et al, 2020;Doherty et al, 2021). In addition, the δ 15 N value of bivalve soft tissues can also be affected by changes in the dietary composition of the bivalve (e.g., Kang et al, 1999;Tue et al, 2012).…”

“…(e.g., Altabet et al, 1995;Voss et al, 2005;Ellis, 2012;Geeraert et al, 2020). With suitable δ 15 N archives it becomes possible to place all such ecological and environmental changes into precise temporal context, determine the pristine state of ecosystems prior to human perturbations and quantify the anthropogenic impact on ecosystems (e.g., Sherwood et al, 2011;Lueders-Dumont et al, 2018;Wang et al, 2018;Duprey et al, 2020), and test respective ecological models (Duce et al, 2008;Serna et al, 2010).…”