Meixia Zhao

Three dimensional shape of the magnetopause: Global MHD results

[1] The numerical results from a physics‐based global magnetohydrodynamic (MHD) model are used to examine the relationship between the shape and size of the magnetopause and the solar wind conditions. The magnetopause location is identified by tracing three‐dimensional streamlines through the simulation domain and is fitted by simple analytical functions. The resulting model is applicable for approximating magnetopause location for dipole tilt angle ∼0° and interplanetary magnetic field (IMF) BX and BY = 0 nT at both low and high magnetospheric latitudes. In both regions the results are compared with available empirical models. It is shown that IMF BZ mainly affects the flaring angle (the magnetopause shape) and has smaller effects on the magnetopause size. In contrast, the solar wind DP mainly affects the magnetopause standoff distance (magnetopause size) and has little effect on the magnetopause shape. Both conclusions are consistent with empirical models.

Two centuries-long records of skeletal calcification in massive Porites colonies from Meiji Reef in the southern South China Sea and its responses to atmospheric CO2 and seawater temperature

Rising atmospheric CO2 and warming of the global climate that have occurred since the industrial revolution are regarded as fatal threats to coral reefs. We analyzed the skeletal calcification rate of 14 massive Porites corals from the Meiji Reef in the southern South China Sea through X-ray photography of coral skeletons. A general pattern of change in coral skeletal calcification was determined. The change pattern of coral calcification on the Meiji Reef over the past two centuries can be divided into five periods: calcification increase in 1770–1830, 1870–1920, and 1980–2000 and calcification decline in 1830–1870 and 1920–1980. Over the past two centuries, the largest increase in calcification was 4.5%, occurring in 1770–1830, whereas the largest decline in calcification was 6.2%, occurring in 1920–1980. Coral calcification slightly increased in the recent 20 years (1980–2000). The response relationship of coral calcification to atmospheric CO2 and sea surface temperature (SST) shows that calcification was not correlated with atmospheric CO2 but responded nonlinearly to SST with maxima at ∼27.2°C in 1900–2000. On the Meiji Reef, increasing atmospheric CO2 had a negligible effect on coral growth in the past century. However, rising SST improved coral growth in the early and middle 20th century, and restricted coral growth in the recent 20 years.

Long-term decline of a fringing coral reef in the Northern South China Sea

Abstract Zhao, M.; Yu, K.; Zhang, Q.; Shi, Q., and Price, G.J., 2012. Long-term decline of a fringing coral reef in the northern South China Sea. The Luhuitou coral reef is a fringing reef at Hainan Island in the northern South China Sea. Since the 1960s, the reef has experienced several significant ecological changes. During that interval, the mean coral cover decreased dramatically from 80–90% in 1962–65 and to just 12% in 2009. In the 1960s, the coral community structure was divided into three well-defined zones: a Goniastrea zone and Montipora zone (both on the reef flat) and an Acropora zone (on the reef slope). However, by 2009, Porites lutea became the dominant species on the reef flat, whereas the predominance of Acropora on the reef slope weakened significantly. There are few long-established Porites lutea colonies present, with approximately 80% being younger than 30 years old. This demographic pattern differs significantly from healthy coral reefs, which are typically dominated by large, well-established (and mature) coral colonies. The long-term decline of the Luhuitou coral reef has most likely been driven as a result of anthropogenic activities, such as overfishing, destructive fishing, reef rock digging, and mariculture and tourism activities. Our study reinforces previous works and highlights the vulnerability of coral reefs to anthropogenic impacts.

Seasonal variations of seawater pCO2 and sea‐air CO2 fluxes in a fringing coral reef, northern South China Sea

Evidence based on four field surveys conducted between July 2009 and April 2011 indicates that both sea surface partial pressures of CO2 (pCO(2)) and sea-air CO2 fluxes at Luhuitou fringing reef in Sanya, Hainan Island, northern South China Sea (SCS) are subject to significant seasonal variations. The diurnal variation of seawater pCO(2) ranges from 264 to 579 mu atm in summer, which is much larger than that in autumn (152-335 mu atm), in winter (84-260 mu atm), and in spring (114-228 mu atm). The sea-air CO2 flux in summer (similar to 9.6 mmol CO2 m(-2) d(-1)) is also larger than that in other seasons (i.e.,similar to 3 mmol CO2 m(-2) d(-1) in spring, similar to 3.5 mmol CO2 m(-2) d(-1) in autumn, and similar to 2.7 mmol CO2 m(-2) d(-1) in winter). The atmospheric pCO(2) in this reef shows small diurnal and seasonal variations. The integration of the time-series pCO(2) data shows that the reef area is a weak source of atmospheric CO2 at similar to 0.54 mol CO2 m(-2) yr(-1). Further analyses indicate that the seasonal variations of the surface seawater pCO(2) in Luhuitou fringing reef are mainly affected by seasonally-dependent biological metabolic processes (organic processe and inorganic process), and that the organic process play a more important role than the inorganic process. Seasonal sea surface temperature (SST) variations and hydrodynamic processes may also have some influence on seawater pCO(2) variation.

Model suggests potential for Porites coral population recovery after removal of anthropogenic disturbance (Luhuitou, Hainan, South China Sea).

Population models are important for resource management and can inform about potential trajectories useful for planning purposes, even with incomplete monitoring data. From size frequency data on Luhuitou fringing reef, Hainan, South China Sea, a matrix population model of massive corals (Porites lutea) was developed and trajectories over 100 years under no disturbance and random disturbances were projected. The model reflects a largely open population of Porites lutea, with low local recruitment and preponderance of imported recruitment. Under no further disturbance, the population of Porites lutea will grow and its size structure will change from predominance of small size classes to large size classes. Therewith, total Porites cover will increase. Even under random disturbances every 10 to 20 years, the Porites population could remain viable, albeit at lower space cover. The models suggest recovery at Luhuitou following the removal of chronic anthropogenic disturbance. Extending the area of coral reef reserves to protect the open coral community and the path of connectivity is advisable and imperative for the conservation of Hainan’s coral reefs.

Source, distribution and influencing factors of sediments on Luhuitou fringing reef, Northern South China Sea

Sediment traps were used for monitoring sedimentary processes on the Luhuitou fringing coral reef. Sedimentation rates, grain sizes and the composition of the collected sediments were then analyzed in this paper. The results indicate that (1) the average sedimentation rate varied from 5.70 mg cm?2 d?1 in spring to 97.41 mg cm?2 d?1 in summer, averaging at 29.9 mg cm?2 d?1; (2) the sediments were composed of organic (~9.5%), authigenic (~44.5%) and terrigenous (~45.9%) materials; (3) the average grain size of the collected particles was dominated by small and fine sediments (>80%), this is because the reef is located at a leeward bay and the waves are relatively weak; and (4) the spatial and temporal patterns of sediments were directly related to the reef’s hydrodynamic condition, for example, the sedimentation rate at a stormy season was about 10 times higher than the stormless period; and the lower reef slope had more fine sands deposited than that of the upper reef slope where the hydrodynamic conditions was relatively stronger. Based on the above data, the calculated total terrigenous input was about 1900 mg cm?2 a?1, which would have a sustainable effect on the coral communities at this reef through suspension/re-suspension.

Comparison of coral diversity between big and small atolls: a case study of Yongle atoll and Lingyang reef, Xisha Islands, central of South China Sea

The South China Sea (SCS) includes large areas of extensive coral reef development but its reefs are still poorly known. Yongle atoll is the biggest typical atoll in the Xisha Islands, central of SCS. Lingyang Reef is an isolated small atoll within the whole big Yongle atoll. A total of 144 and 119 coral species were recorded at big Yongle atoll and small Lingyang Reef, respectively. The real coral richness might be higher because species accumulation curve did not saturate. The coral diversity pattern was similar between big Yongle atoll and small Lingyang Reef. Coral communities fell into three clusters, consistent with their habitats on reef slope, reef flat and lagoon slope. The highest coral diversity was observed on reef slopes and the lowest coral diversity was found on lagoon slope. Genera richness was a better proxy for representing coral species diversity on both the big and small atoll but percent live coral cover was not a robust proxy on the small atoll, which only explained 24% of species diversity. This study demonstrated high coral diversity with consistent pattern along habitat types, as has been shown from many other reefs. While far from exhaustive, the study allows first glimpses on how much biodiversity is contained on SCS coral reefs, and hopes to give an impetus to their conservation. The study also suggests that simplified surveys at a small scale and the use of genera richness as an effective proxy for overall diversity can indeed provide important information to rapidly monitor and evaluate the coral diversity in remote locations.

Nutrient Distribution in Coral Reef Degraded Areas within Sanya Bay, South China Sea

ABSTRACT Guo, J.; Yu, K.; Wang, Y.; Xu, D.; Huang, X.; Zhao, M.; Yang, H., and Zhang, R., 2017. Nutrient distribution in Coral Reef degraded areas within Sanya Bay, South China Sea. An investigation was conducted in August 2014 to explore the spatial distribution of dissolved inorganic nitrogen (DIN) and phosphate ( ) in overlying water, pore water, and surface sediment from Sanya Bay and living coral cover on Luhuitou fringing reef. Generally, nutrient contents gradually decreased from the estuary to the central bay, which confirmed that terrigenous inputs from Sanya River and Sanya Harbor were the primary nutrient sources. The results of K-means cluster analysis suggested the Luhuitou fringing reef is in danger of being affected by the pollution source. The results of nutrient diffusive fluxes at the sediment-seawater interface indicated diffusion from pore water to overlying water. Compared with other coral reef areas around the world, nutrient levels in Luhuitou fringing reef were at the medium level. Significant correlation between temporal variation of nutrients and living coral cover was not found, which suggests that low nutrient enrichment, ranging from 1.89 to 2.89 μmol/L for DIN and from 0.1 to 0.49 μmol/L for , may not be responsible for the coral reef decline on Luhuitou fringing reef from 2002 to 2014.