Coastal environmental changes under increasing anthropogenic impacts: an introduction

(Asia Oceania Geosciences Society)


INTRODUCTION
Coastal environmental changes under increasing anthropogenic impacts: an introduction 1 The Anthropocene is being considered as a new geological epoch because human activities now rival geological forces to radically alter the natural state and functioning of the Earth system, producing its own distinctive sedimentary records (Waters et al. 2016). These changes of varied magnitude can be traced over different spatiotemporal scales. The best known issue of course is global warming due to rapidly increasing emissions of CO 2 and other greenhouse gases into Earth's atmosphere since the Industrial Revolution, responsible for more frequent extreme weather events (e.g., heavy rainfall on land and super typhoons in the ocean) (Fan and Liu 2008;Sobel et al. 2016). Rapid population growth and technological advancement in the past centuries have enabled humans to alter landscapes, habitats, and ecosystems through urbanization and industrial and agricultural activities (Price et al. 2011;Hooke et al. 2012). It is estimated that >50% of Earth's ice-free land has been directly modified by human activities (Hooke et al. 2012). For a long time, anthropogenic perturbations have also changed sedimentary processes by increasing soil erosion along hill slopes and accelerating sedimentation behind the reservoirs, and have increasingly altered the cycles of carbon, nitrogen, phosphorus, and other elements in the past decades (Waters et al. 2016). The cumulative land-based perturbations have obviously changed the material flux into the sea (Milliman and Farnsworth 2011), exerting great influence on coastal morphology and ecosystems.
The global coastal zone features extraordinary dynamic and complex interactions among the natural processes at interfaces between land, ocean, and atmosphere, and this has recently been further complicated by rapidly growing human perturbations (Crossland et al. 2005). The narrow land-ocean interface zone also provides the highest primary productivity that supports the richest biodiversity and the highest fishery production in the world's oceans. Attracted by the abundance of fertile agricultural lowlands and fishery resources, humans began to settle along the coastal zone in the Mid-Holocene when global sealevel rise slowed down (Fan et al. 2017). Today, the coasts of the world have developed into global socioeconomic hubs to accommodate nearly 50% of global population on merely 12% of Earth's surface (Crossland et al. 2005). Moreover, continuing growth of coastal population is expected in the near future. Because of increasingly intense human interference, coastal morphology and ecosystems have been greatly transformed from their natural states. Over 50% of the world's coastline has been modified by hard engineering, including seawalls, groins, and breakwaters, which are constructed for land reclamation, fishery practice, and coastal protection from erosion, flooding, and storm strikes (Dafforn et al. 2015). A large volume of coastal wetlands has been lost through coastal erosion and urban development. It was estimated that globally 50% of saltmarsh, 35% of mangrove, 30% of coral reef, and 29% of seagrass have already been lost in the past few decades (Barbier et al. 2012). In China, about 58% of coastal wetlands were lost between 1950 and 2014 (Gu et al. 2018), and only 40% of the mainland coastline remains in a relatively natural state.
Our coastal social-ecological systems are increasingly under more pressures (Newton et al. 2016). First of all, coastal urbanization (e.g., infrastructure construction for land reclamation, coastal defense, harbour, tourism, aquaculture, wind power, and oil and gas exploration) will continue and, in some cases, expand offshore (Dafforn et al. 2015). Secondly, land-based human activities tend to cause a reduction in sediment production and discharge, but an increase of riverine nutrient and pollutant discharge into the sea, consequently exacerbating the problems of coastal erosion and expanding algae blooms and coastal hypoxia. Finally, ocean warming and acidification induced by Anthropocene CO 2 release will continue to exert influence on marine biology. Under these pressures, low-lying mega-river delta plains have been evaluated as the world's most vulnerable regions in response to global climate change (Syvitski et al. 2009;Fan et al. 2017). The increasing ecological degradation in large-river delta-front estuaries has attracted great concern from academics and the general public because of drastically spreading eutrophication and hypoxia in the world's oceans during the past decades (Diaz and Rosenberg 2008;Bianchi et al. 2010).
In the 21st century, coastal sustainability has been increasingly highlighted in the agendas of local governments and international organizations. Both LOICZ-II and Future Earth Coast projects focus on themes of sustainable and healthy development of coastal society and ecosystems (Crossland et al. 2005;Ramesh et al. 2015). Numerous meetings have been held to address these hot topics of coastal vulnerability and sustainability, aiming to provide coastal planners effective and innovative management strategies and tools towards sustainability of coastal social-ecological systems (e.g., Cabral et al. 2015;Newton et al. 2016). This special collection, "Coastal Environmental Changes under Increasing Anthropogenic Impacts", was developed from submissions to three well-attended sessions convened during the AOGS (Asia Oceania Geosciences Society) annual meetings, 2017-2019. A total of 11 papers were selected from presentations that cover the following topics: (i) coastal morphodynamic changes and habitat fragmentations; (ii) estuarine hydrodynamic alterations and regime shift; (iii) coastal sediment dynamics, sediment resuspension, and sediment failure by storm waves; (iv) oil spill spreading dynamics and trajectories; and (v) anthropogenic sedimentary records.

Coastal morphodynamic changes and habitat fragmentations
Numerous sandy and muddy coasts have been undergoing erosion of variable severity, mainly because of reduced sediment supply, rising sea level, and intensifying storm strikes. Erosion-deposition patterns can vary significantly over different coastal sectors with various causes. Fan et al. (2019) examined intermediate-term coastal morphodynamic changes in the Red River Delta through time-series analysis of assimilated geomorphological data from satellite images, historical maps, and charts. The movement of 0 m isobaths was found to be mildly influenced by local human perturbations, but highly sensitive to external stresses, such as river discharges and storm waves. It was shown that most muddy coasts (stretching ∼130 km long in total) have undergone continuous accretion since the 1930s, while sandy beaches (∼30 km long) have suffered chronic erosion. Coastal accretion rates are usually highest at the front of distributary mouths. Estuarine accretion has slowed down significantly after a sharp decrease of sediment discharge from the river since the operation of Hoa Binh Dam reservoir in 1988. The chronic erosion at the beaches along the Hai Hau district initially resulted from the cutoff of direct riverine sediment supply when a distributary was cut off due to channel shift at the early 20th century. Then the erosion continued because of negligible sediment supply from the regional longshore current and continuous sediment loss by local wave-generated longshore and offshore currents. The area of intertidal flats shrank significantly due to large-scale land reclamation and slowing coastal accretion. The area of mangrove forests decreased first due to human deforestation in the mid-20th century, but increased gradually in the past two decades due to artificial plantation. Cong et al. (2019) used Landsat imagery and elevation leveling to examine the coastline and cross-shore profile changes in Xiapu County, northern Fujian Province (southeast China). Both rocky and sandy coasts remained quite stable from 1976 to 2017, albeit with high exposure to wave attacks. Land reclamation on many muddy flats in sheltered embayments have pushed the coastlines seaward. The authors argue that the stable sandy coasts in the area are maintained by those pocket beaches that have reached a dynamic equilibrium state. Slight erosion in some sandy beaches was mainly attributed to rogue sand mining activities. The recent increase in human activities may have exceeded the natural processes in shaping sandy and muddy coastal profiles in the rugged coasts of northern Fujian Province.
Biogeographical shift, and habitat fragmentation or loss, due to growing influence of human perturbations and climate change, have increasingly drawn more attention. Based on time-series analysis of Landsat satellite imagery, Zang et al. (2019) studied the landscape and vegetation change near the 45.33 × 10 4 ha Yancheng National Nature Reserve. More and more natural wetlands have been transformed into artificial wetlands to meet the accelerating economic development and other uses, resulting in increased vegetation fragmentation. Landscape and vegetation at the Yancheng National Nature Reserve central core zone was also greatly modified by human activities in the last decade, producing an overall ecological degradation that has taken a heavy toll on protected species in the Yancheng National Nature Reserve. However, vegetation fragmentation is found to be effective in constraining the spread of Spartina alterniflora, an invasive species that has caused severe ecological problems in the area.

Estuarine hydrodynamic alterations and regime shift
Estuarine hydrodynamics is very complex in nature due to intense river-sea interactions, which can vary significantly over a short distance in response to rapid morphological changes. It can be further complicated by harbour construction, land reclamation, and other human disturbances. Vancouver Harbour in western Canada is an inland depression basin connected with the outer English Bay and the upper river estuary through two narrow straits. Tidal waves are obviously deformed when prograding into Vancouver Harbour due to highly rugged coastline and bathymetry. Using a higher resolution model based on FVCOM (finite volume community ocean model) with a flexible triangle mesh system, Wu et al. (2019) investigated tidal processes in the harbor. The modeled tidal amplitude and current matched very well with the observed data. Using the model, they investigated spatial variations in tidal asymmetry and formation mechanism of the tidally induced vortex dipole, which plays an important role in the transport of tidal energy and sediments in the system. Zhang et al. (2019) also employed FVCOM to simulate tidal processes at the Daya Bay in Guangdong Province (south China). The bay is a tide-dominated shallow embayment with an area of 600 km 2 and a mean water depth of 9.7 m. Due to land reclamation and coastal development, 70% of the coastline has been modified with a total water area of ∼30 km 2 lost during the period 1989-2014. The bay-area shrinkage produces a slight decrease in tidal elevation amplitude, tidal current magnitude, and tidal energy flux for K 1 , M 2 , and M 4 tides, as expected; however, the M 6 amplitude has increased by 1.5-3.5 cm from 1989 to 2014, owing to the enhancement of sexta-diurnal tidal resonance in the basin. Consequently, tidal duration asymmetry in the bay shifted from an ebb-dominance regime to a flood-dominance regime, which tends to accelerate water exchange in the inner part of the bay, favoring maintenance of a healthy ecology.
Abrupt regime shift may occur in some highly altered estuaries to produce severe ecological degradation. In the Republic of Korea, to reduce saltwater intrusion and reclaim more land, approximately half of estuaries have been altered by human-made infrastructure, such as estuarine dams, sluice gates, and weirs. Shin et al. (2019) selected four estuaries to evaluate altered states by estuarine dams using the Hansen and Rattray stratification-circulation classification scheme. In the Han River Estuary, the downstream circulation and estuarine hydrography were not heavily influenced by the presence of an underwater weir; however, a high estuarine dam may completely block the river-sea interaction in some small river mouths during the dry season, changing the outer and inner estuarine parts into a high-salinity coastal bay and a freshwater reservoir, respectively. During the wet season, overflow above the dam and control of the flow by gate operation can temporally change the outer bay into an estuary again, but with a completely different stratification-circulation state. These dam operations also exert great influence on the transport pattern of nutrient, organic, and sediment matter, consequently leading to ecosystem degradation and seabed erosion issues.

Coastal sediment dynamics, sediment resuspension, and sediment failure by storm waves
Coastal and estuarine sediments are highly dynamic under intense wave-current interactions. Huge sediment resuspension and redistribution by storm waves potentially produce abrupt morphological changes, pollutant release, and other environmental issues, causing wide concerns. Z.-K.  examined sediment dynamic variations from spring to neap tides in the shallow platform off the Nanliu River delta in Guangxi Province (south China) using two bottom-mounted quadripod instrumented systems. The results show that waves play a more important role in sediment resuspension than tides as mean wave-induced shear stress (τ w = 0.42 N/m 2 ) was two times the mean currentinduced shear stress (τ c = 0.21 N/m 2 ) during the study period. However, τ w is significantly regulated by tidal level change, as shown by continuous decrease from spring to neap tides. Increased shear stress favors floc formation in such a low suspended sediment concentration and high-salinity water column. Dynamic morphological equilibrium is rapidly maintained over a small temporal and spatial scale owing to limited sediment input. The quasisteady seabed change registered by acoustic Doppler velocimeter measurements was not successfully simulated by the one-dimensional Partheniades-Krone model, potentially resulting from a non-constant erosion coefficient, which was estimated at 5 × 10 −5 kg/m 2 s at the study site.
Y.  investigated the source-sink processes of heavy metals in Quanzhou Bay of Fujian Province (southeast China). Parallel surface sediments were sampled immediately after a typhoon landfall and during normal weather conditions for comparison. The results showed that heavy metals can be divided into two categories. Class I metals (Cu, Zn, Pb, and Mn), mainly sourced from natural weathering of bedrock in the upper reach, are discharged into the sea by typhoon-induced river flooding events, so their post-storm concentrations are higher than under normal weather conditions typically found along the river plume course. Class II metals (V, Cr, Co, and Ni) are mostly derived from industrial sewage with non-point source and discharge into the sea, so their concentrations are higher on muddy flats during normal weather conditions, and may significantly decrease after storms because of intense wave resuspension and releasing processes. However, it is noteworthy that heavy metal remobilization from nearshore sediments and offshore redistribution may produce secondary pollution issues. Extreme wave disturbances may not only produce surface erosion, but also induce liquefaction of high water-saturated sediments in the rapidly accretional delta front, potentially triggering sediment failure, submarine landslides, or sediment gravity flows. To explore their development stages and mechanism, Liu et al. (2019b) performed dynamic triaxial experiments using bulk sediment samples from the Yellow River Delta to simulate sediment liquefaction under strong cyclic loading of waves with a 50 year recurrence interval. The build-up of pore-water pressure usually undergoes three stages: rapid growth at the early short stage, followed by a relatively long but slow growth stage, and ending at peak stable stage. Different liquefaction processes are contingent on sediment properties.

Oil spill spreading dynamics and trajectories
Increasing offshore oil exploitation and international oil trade and transportation significantly raises the risk of oil spills, leaks, and explosions. Precise projection of oil spill trajectories is vital for taking measures to mitigate their environmental impacts. Numerous numerical models have been developed to predict oil spill spreading processes and trajectories, but few can predict the precise trajectories after oil spill events. For example, modeled results by different methods tended to give much more serious impacts of the Deepwater Horizon oil rig event in 2010 on the coast of the Gulf of Mexico. The difference between simulated and observed results can be attributed to multiple factors, but Liu et al. (2011) considered that it is important to improve the wind-forcing functions to drive the ocean models.
Oil slicks cover the sea surface and may change drag properties between the ocean and atmosphere. The question is how to quantify the change of the roughness or drag coefficient by oil coverage, and the magnitude of the surface wind stress and the ocean circulation in response to such change. The change of drag coefficient (or surface roughness) in oil-covered areas is difficult to measure directly, so Shen et al. (2019) employed synthetic aperture radar data to give an estimation of these key parameters. By analyzing data from the Deepwater Horizon oil spill event, it was found that effective wind speeds could be reduced by 50%-100% owing to the presence of surface oil coverage, consequently producing 75%-100% overestimation of wind-driven Ekman current. It is therefore very important to include these effects in modelling oil trajectories, typically for large-scale oil spill events.

Anthropogenic sedimentary records
Sediment routes from the river to the sea have been greatly altered by human activities together with climate change. The Yellow River is an optimal testing ground to study human-regulated river runoff and sediment discharge, and their environmental impacts. Because of dam construction and soil protection projects, sediment flux has decreased from a previous estimate of 1080 × 10 6 t/year to the present ∼150 × 10 6 t/year. This is accompanied by increasing grain size of suspended sediment and decreasing suspended sediment concentration at the river mouth (Wang et al. 2017). Recently, a comparison study of magnetic and geochemical compositions of surface sediments in different ages demonstrated the obvious difference between pre-and post-dam periods (Yang et al. 2019). The sediment signal changes may be registered in the offshore depositional systems dominated by Yellow River derived sediments.
Introduction ix Liu et al. (2019a) presented a detailed analysis of grain-size and rare earth element compositions in a gravity core retrieved from the muddy patch in the north Yellow Sea. The bottom of the 125 cm long core B23 was dated back to the 19th century by the excess 210 Pb activities. Provenance discrimination shows that the lower part, deposited between 1855 and 1885, has higher sediment input from the Yalu River because the Yellow River entered the south Yellow Sea. The middle part, deposited in the period 1885-1980, shows the predominant sediment source from the Yellow River after being diverted to enter the Bohai Sea in 1855. However, the upper part deposited after 1975 is noted to have a higher proportion of sediment input from the Yalu River again, and this is attributable to a dramatic decrease of sediment discharge from the Yellow River in the 1970s due to increasing human disturbances. Simultaneously, coarsening grain size and weakening East Asia winter monsoon also point to the declining sediment discharge with increasing coarse component in the highly regulated Yellow River. A time lag of 1-2 decades is evident between the changes of the river discharge and sedimentary record at the distal muddy depositional area.

Summary
Coastal and estuarine environments have been obviously altered by human activities in terms of hydrodynamics, sediment dynamics, and morphodynamics. These changes have created discernible sedimentary records different from the Holocene. Lessons from these research articles also show high spatiotemporal variations of coastal environments in response to natural and anthropogenic stresses. Therefore, they should be monitored and studied using new techniques with higher resolutions. Because most coastal environmental problems are interlinked, an integrated solutions-oriented approach should be developed aiming for long-term sustainable development of coastal society and ecosystems.