Reefs of tomorrow: nutrients drive coral biodiversity in an urbanized seascape

Session - Human ImpactsMarine ecosystems have experienced dramatic changes since the 1850s in response to human activities. This epoch, often referred to as the Anthropocene, has witnessed the punctuated loss of reef-building corals worldwide. Coral reef degradation may induce subtle changes which remain un-witnessed; this is particularly true when the water quality is altered. Deteriorated water quality hampers coral larvae recruitment, amplifies bio-erosion, favors coral diseases and reduces the threshold of thermal …published_or_final_versio

Reefs of Future Present: Hong Kong Corals from the Qing Dynasty to the Information Age

Oral presentation - Session #:02 : Abstract ID: 30014My geologist friends tell me that Hong Kong has no 'proper' coral reefs. This is because there is no evidence of carbonate formations upon which living corals are still found. Instead, corals grow on exposed bedrock in a patchy distribution and any exposed carbonates are quickly eroded away. Were there proper coral reefs in the past? Consider that during the Qing Dynasty the industrial production of slaked lime was extensive throughout Hong Kong, yielding 40 tons per year from mining living bivalve and coral communities. Indeed, historical records and archaeological evidence describe a very different environment in the past; where apex predators, megafauna, and diverse reef invertebrates were common. Through archeological digs, we show that the diversity and abundance of corals and other invertebrates declined markedly over the last century, including the extirpation of several coral species. We analyzed subtidal push cores to show that coral abundance has decreased by 50%, and coral fragment identification from sediments to delineate a 40% contraction in the range of important staghorn corals. Today, coral decline is closely linked to coastal development and pollution, which limits the recovery of coral communities. However, thanks to government efforts at improving water quality, we demonstrate that active restoration of coral communities is possible. Through targeted propagation and transplantation we show that staghorn corals have high rates of growth and survivorship in areas where they were previously eliminated giving hope to the future of these marginal reefs

New insights on the nitrogen footprint of a coastal megalopolis from coral-hosted Symbiodinium δ15N

The development of megalopolises in coastal areas is often linked with severe eutrophication, requiring mitigation of anthropogenic dissolved inorganic nitrogen (DIN) pollution. Yet, identifying the DIN-sources responsible for eutrophication is challenging, hampering mitigation efforts. Here, we utilize the stable nitrogen isotope ratio of endosymbiotic dinoflagellate Symbiodinium spp. (δ15Nsym) associated with the hard coral Porites to trace DIN sources in one of the most urbanized areas of the planet: the Pearl River Delta (PRD). The mean δ15Nsym value found in the coastal waters of Hong Kong (HK), located on the eastern edge of the PRD, (7.4‰ ± 1.2‰) was +2.7‰ higher than at Dongsha Atoll, a reference site unaffected by anthropogenic-DIN (4.7‰ ± 0.4‰). The isotopic enrichment suggested a consistent dominance of DIN deriving from local and regional sewage discharges on the eastern edge of HK. Furthermore, the strong depletion of the summer δ15Nsym value (−1.6‰) observed in southern HK revealed that the Pearl River plume strongly modulates the coastal DIN pool. Our results revealed the value of benthic marine organisms’ δ15N for deciphering the complex dynamics of coastal eutrophication and highlighted the pivotal role of transboundary coordination in DIN-pollution mitigation

Coral isoscapes reveal varied nitrogen sources and dynamics in Hong Kong’s coastal environment

Poster Presentation: Session #:09 - Abstract ID: 29155 ; Poster ID: 92Hong Kong's marine ecosystem health is under threat due to large inputs of dissolved inorganic nitrogen (DIN) from various sources. We used stable isotope analysis to trace major DIN source(s) affecting corals and examined their spatial and temporal variations in coastal environments. We collected corals along coastlines in a wet and dry season and analyzed the δ15N signals of their Symbiodinium (δ15Ns). Spatially, the mean δ15Ns value was significantly lower in the Southern zone–SHK (5.6±1.5‰) than Northeastern zones–PS (8.0±1.1‰) and MB (8.8±1.3‰) in the wet season. In the dry season, there was no significant difference in the mean δ15Ns values among zones. When compared between seasons, the mean δ15Ns value in SHK in the dry season (6.9±0.7‰) was significantly higher than the wet season. The spatial and seasonal differences in the δ15Ns values highlighted the influence of the Pearl River (PR), which supplies Hong Kong with a high concentration of 14N compared to N derived from oceanic water. The dominance of N from PR could contribute to the low δ15Ns values in SHK in the wet season as it is in close proximity to PR. The seasonal difference, i.e. higher δ15Ns values in SHK in the dry than wet season, can be explained by the decreasing influence of PR as the dry season accounts for ~20% of the annual discharge. These data suggested PR has a major influence to the DIN dynamics and its significance is subject to spatial and seasonal changes

The Old man and the Giant clam: climate variability and prehistoric human migration in the Pacific

The Lapita migration, which occurred 3,000 years ago, is one of the most remarkable prehistoric human migration: the Lapita seafarers went across 4,000 km of uncharted seas to settle in the islands of the southwest Pacific. The factors that drove this event are still unknown. It has been hypothesized that an increase in the El Niño Southern Oscillation –ENSO – may have triggered this migration. To explore the possible influence of ENSO forcing the Lapita migration, bulk oxygen stable isotopes records were obtained from fossil giant clams, unearthed from Lapita archaeological sites of SW Pacific and a modern baseline was obtained from modern conspecifics. Fossil giant clams showed that climate oscillated between 1) present day conditions and 2) warmer / fresher conditions at the inter-annual time-scale. This suggests that Lapita migration occurred concomitantly with a strong ENSO variability. The potential dramatic environmental degradations caused by the increased ENSO variability (droughts, malaria, wildfires…) may have acted as a push factor for the Lapita migration. Frequent shifts in prevailing wind regimes associated with ENSO may have also facilitated the discovery of new islands

Learning from the past: Investigating the anthropogenic impact on A-P reefs since WWII using coral δ15N records

Conference Theme: Challenges of Asia-Pacific Coral Reefs under the Changing OceanAsia-Pacific (AP) coastlines are facing intense coastal development which exerts a strong pressure on coral reefs ecosystems, in particular, by releasing considerable amounts of nutrients (primarily sewage) into coastal waters. Excess nutrients, like nitrogen, increase coral disease, bioerosion, depress fecundity and thus change community structure and function. The decline of coral reefs in the AP region is predicted to have dramatic ecological and socio-economical consequences considering that 76% of all coral species are found in the AP region and that millions of people are reliant on reef resources and services. Although eutrophication is well documented on modern reefs, very little is known about the historical exposure of coral reefs to nitrogen pollution during the last century and how it has shaped modern coral reef ecosystems. World War II and the post-war era provide interesting coastal development scenarios: Japanese military occupation and subsequent withdrawal, triggered punctuated population changes in the AP region which modulated the severity of anthropogenic nutrient release on coral reefs. Given that human sewage is characterized by an anomalously high nitrogen stable isotope ratio (δ15N), and corals use this nitrogen for growth, we can document the increase and decrease of sewage input to the reef over time through the analysis of coral skeletal bands. This talk will present the preliminary results of this retrospective study

Terrestrial pollution versus global warming: what drives coral annual extension in the Anthropocene era? - A case study in Chuuk lagoon, Micronesia

Poster presentation - Session #:38: Abstract ID: 29228 ; Poster ID: 442Terrestrial pollution and CO2-driven global warming are main threats to coral reefs ecosystems. Previous studies revealed the contrasting responses of reef building coral species to human-led changes. Increased sea surface temperature (SST) reduced coral annual extension in the Red Sea, whereas, terrestrial pollution increased extension in Indonesia. These two concurrent threats raise some questions about coral annual extension in the Anthropocene. Coral reefs in Chuuk lagoon have suffered from high anthropogenic impacts through the WWII and subsequent population growth, additionally, the SST increased by about 0.5 °C over the same period. To assess the relative importance of these two factors in coral annual extension, we collected two coral cores of Porites sp. at Newman and Nepukos which are located at Weno island in Chuuk lagoon, Federated States of Micronesia. Age estimation based on band counts of x-radiograph indicated the first core encompasses the time-period 1938-2013 and the second core is estimated to include the entire 20th century. According to correlation analyses, coral annual extension showed no correlation with SST and population density, however, significantly correlated with precipitation on the most recent half of the record. While the second core is still being analyzed, the preliminary results indicate increased human population and concomitant deforestation caused an influx of terrestrial pollutants and subsequently affect coral annual extension rather than global warming nearshore reef

Reefs of the future: Water quality constrains coral biodiversity in an urbanized seascape

abstract no. COS 127-9Background/Question/Methods As foundational species, corals provide a complex habitat that hosts an immense diversity of marine life. Yet, globally corals are in a perilous decline leading to loss of biodiversity and ecosystem services. To date, the relative importance of global (e.g. climate change, acidification) versus local (e.g. overfishing, pollution) are still hotly debated. This debate is rife with shifting baselines, because reef degradation often preceded science and management efforts. Increasingly, studies are focusing on natural (e.g. CO2 seeps) or anthropogenic (e.g. power plant effluent) gradients to test the hypothesis that global change processes are decreasing coral diversity. Yet, few studies have examined how water quality affects coral species richness. Hong Kong is host to more than 80 hard coral species, which cling to the edges of strong water quality gradients across a limited spatial scale. Owing to more than 20 years of water quality and coral diversity monitoring, Hong Kong provides an interesting scenario to test the hypothesis that poor water quality reduces coral species richness. This GIS and field-based study investigates the effect of five water quality parameters (total inorganic nitrogen & phosphate, particulate suspended matter, salinity and dissolved oxygen) on hard coral species richness. Results/Conclusions Coral species richness was negatively correlated with surface productivity (chla). Both nitrogen and phosphorus were negatively correlated with coral diversity and had twice the relative importance as particulate suspended matter. These negative effects were ground-truthed in an explantation experiment along a water quality gradient emanating from a heavily urbanized watershed. Corals growing near sewage effluents suffered high mortality, whereas corals further away showed increasing survivorship and growth. Taken together, we can use these data to identify biodiversity hot-spots and water quality thresholds for hard coral diversity to better inform future MPA and restoration site selection

Not all corals dine in: variation in niche partitioning between corals and their Symbiodinium indicates a range of symbioses

Oral presentation - Session #:11: Abstract ID: 29304The symbiosis between corals and Symbiodinium has allowed coral reefs to achieve high levels of productivity and diversity in oligotrophic habitats. By sharing resources, corals and Symbiodinium access novel niche space. Some have argued that the coral-algal interaction ranges from mutualism to parasitism. Trophic niche theory predicts that mutualist symbionts would occupy the same niche space as their host (shared resources) whereas commensal symbionts would occupy a different niche (fewer shared resources). To test this hypothesis, we measured the δ13C and δ15N of separated coral tissue and symbiont cells from 6 coral genera commonly encountered in Hong Kong. Using Stable Isotope Bayesian Ellipses in R (SIBER) analysis, we compared the isotopic niche placement and area of each host and symbiont pair. Our results supported the hypothesis: in some genera (Acropora and Goniopora) the host and symbiont had nearly 100% overlap of their isotopic niches implying shared nutritional resources, while in others (Platygyra and Favites) there was no overlap. Finally, some genera (Pavona and Porites) have partial overlap suggesting a flexible symbiosis. These patterns were driven by nitrogen, not carbon. Indeed, the difference between the δ15N values of host and symbiont was positively correlated with calyx volume, suggesting that smaller polyps evolved to support obligate symbioses while larger polyps enable corals to meet their nutritional requirements through alternative means