Detecting artificialization process and corresponding state changes of estuarine ecosystems based on naturalness assessment
Introduction
Natural estuaries with vast intertidal wetlands are deemed as crucial wildlife habitats, water pollutant filter, green barrier, and material circulation channel (Ridgway and Shimmield, 2002, Tian et al., 2008, Engle, 2011), playing an irreplaceable role in conserving coastal biodiversity, reducing offshore contamination and eutrophication, mitigating natural hazards like storm surge, and maintaining normal biogeochemical cycles (Arrigoni et al., 2008, Wild-Allen et al., 2013). However, more and more estuaries worldwide are losing or weakening their capacity for biodiversity conservation and ecosystem services as they are being artificialized increasingly (Borja et al., 2010, Waltham and Sheaves, 2015, Castro et al., 2016). Estuarine artificialization, driven by coastal economic demand and urbanization, significantly alters natural composition, structure and processes of estuarine ecosystems, interfering with their natural succession (Ducrotoy and Dauvin, 2008). Dramatic landscape transformation not only modifies and fragments natural habitats, but also eliminates some natural attributes of ecosystems that are indispensable for maintaining estuarine ecological integrity and ecosystem health (Machado, 2004, Ridder, 2007, Barr and Kliskey, 2014). Unlike natural estuaries, highly artificialized estuaries have poor self-regulation ability and resilience (Dafforn et al., 2015), but suffer considerable human disturbance, such as land reclamation, environmental pollution, overfishing, and introduction of invasive species (Azevedo et al., 2008, Castro et al., 2016), which ultimately leads to estuarine degradation and unsustainability.
In the context of increasing coastal urbanization and industrialization, estuarine artificialization is an unavoidable phenomenon across the world (Perillo et al., 2005, Waltham and Connolly, 2011). Although managers can not protect natural estuaries from being transformed, they are able to spatially plan and control human activities based on changes in ecosystem state to minimize the negative impact of estuarine artificialization (Tagliani et al., 2003, Frashure et al., 2012). There is an urgent need to estimate the artificialized degree of estuarine ecosystems and dynamically monitor their state changes during artificialization process. This provides important decision support for preventing over-artificialization of estuaries as well as preserving and restoring their ecologically critical areas (ECAs) (Korfmacher, 2002, Coughanowr and Whitehead, 2013). Estuarine ECAs mainly refers to those natural wetlands whose habitat conditions and dynamics are associated closely with the level of estuarine ecological security and ecosystem services (Gonçalves et al., 2013, Brisson et al., 2014). Once these ECAs are degraded or lost, they are not ease to recover due to their unique eco-hydrological processes, hydrodynamic conditions, topography, flora and fauna (Laegdsgaard, 2006, Plater and Kirby, 2006, Ritter et al., 2008). Ecosystem state monitoring and evaluation programs for estuarine artificialization can contribute to creating an early warning system for reducing ecological risk caused by estuarine overexploitation (Gonçalves et al., 2013, Park et al., 2015). Also, it offers some baseline information and references for ecological restoration of damaged ECAs (Elliott et al., 2007, Borja et al., 2010).
Naturalness, an ecosystem state parameter that is opposite semantically to the terms artificialness and hemeroby, is defined as the extent to which the original natural attributes of the ecosystem are maintained (Machado, 2004, Schnitzler et al., 2008). Ecosystem naturalness assessment has been widely applied to some fields such as the habitat and biodiversity conservation, nature reserve management, ecological planning and restoration, and regional ecological security prediction (Machado, 2004). An ecosystem with high naturalness means that few socioeconomic factors have been integrated into its natural structure and processes. Therefore, artificialization process monitoring for specific estuaries can also be achieved through the evaluation of ecosystem naturalness. Several researchers presented multiple methods to quantify ecosystem naturalness, like developing naturalness index according to changes in land cover and environmental factors (Jansen et al., 2009, Gimmi and Radeloff, 2013). In recent years, geographic information system (GIS) and remote sensing (RS) technology have further enhanced the convenience of visualization analysis and mapping for ecosystem state diagnosis (Parravicini et al., 2012, Dai et al., 2013).
The Bohai Sea coastal zone in North China is a typical area of the rapid estuarine artificialization. As it is located in the Bohai Economic Circle, one of the regions with the fastest economic growth and highest population density in the world, the rate and intensity of its landscape conversion is much higher than elsewhere in China. Over the past few decades, more than 20 estuaries in this coastal zone have been strongly modified by human development activities. Consequently, the artificialization of estuarine structures and processes not only induces some local eco-environmental problems (Ridgway and Shimmield, 2002, Whitall et al., 2004), but also directly threatens dozens of marine and coastal nature reserves for fish, waterbirds, benthic animals and other endemic habitats (Ma et al., 2009), which are usually established in estuaries and adjacent waters. Previous studies have focused on the landscape pattern dynamics, habitat changes and corresponding environmental effects of estuaries in this area (Zhang et al., 2007, Bi et al., 2014), but rarely involved the topic of estuarine artificialization and attempted to reveal the extent to which existing estuarine ecosystems have deviated from their original natural state. In this paper, we took the Xiaoqinghe estuary in the Bohai Sea coastal zone as a case study and our objectives were to: (1) quantify the naturalness of each ecosystem category and estuarine area of interest for different stages; (2) determine the artificialized degree of specific estuarine areas and corresponding period; (3) illustrate the spatiotemporal changes in state characteristics of estuarine ecosystems and trace their succession process.
Section snippets
Study area
The Xiaoqinghe estuary is located in the southern coast of Laizhou Bay in Bohai Sea, where storm surges occur frequently. Estuarine natural ecosystems include saline wastelands in Zone A, shrub-grass lowlands in Zone B and intertidal wetlands in Zone C (Fig. 1). As a biodiversity-rich site, this estuary is of crucial ecological preservation value. Its intertidal wetlands and adjacent shallow waters are one of the few fish spawning grounds in Bohai Sea and also important waterbird habitats. The
Ecosystem dynamics
The ecosystems in the study area were divided into four categories, i.e. natural ecosystems (NE), near-natural ecosystems (NNE), semi-natural/artificial ecosystems (SNE), and artificial ecosystems (AE). We explored dynamics of these ecosystem categories by analyzing the spatial and attribute data of land use thematic maps in ArcGIS, which were derived from Landsat TM (1984, 1992, 2000 and 2007) and ETM+ (2015) images. According to field survey data from 2011 to 2015, aerial photographs and land
Spatiotemporal patterns of ecosystems
Table 2 presents changes in the quantitative structure of ecosystem categories in the study area. From 1984 to 2015, the NE area decreased by 61.3%, whereas the area of SNE and AE increased by 4.1 and 8.2 times, respectively. As a transition type, NNE changed little in the area proportion, usually fluctuating between 2% and 8%. NE maintained the largest area percentage (over 60%) in 1984–2000, but has been less than SNE since 2007. In general, the area ratio of NE, SNE and AE was sharply
Discussion
In a high-intensity human activity area, original landscapes and ecosystems are difficult to exist and evolve in a natural way (Chapman and Reich, 2007). This was particularly true for the Xiaoqinghe estuary. Our results showed that it has been converted from a natural estuary into a highly artificialized estuary over the past 30 years (Table 4), with the average estuarine naturalness dropping by 43%. Especially in those areas occupied by original salt marshes and shrub-grass lowlands, the
Conclusions
In this study, we explored the changes in ecosystem state of the Xiaoqinghe estuary during its artificialization process. Base on the developed framework and procedures for ecosystem naturalness assessment, the naturalness spatiotemporal maps of this estuary was obtained, and the artificialized degree of different estuarine areas and their corresponding period were further quantitatively identified. We found that both the naturalness of ecosystem categories and regional naturalness in the
Acknowledgments
This study was funded by the Natural Science Foundation of Shandong Province (ZR2013DM004, ZR2015DL005) and National Natural Science Foundation of China (41501508). We would like to thank Xiaobao Sun, Guangjun Wang and Honghai Wang of the Yangkou Wetland and Fisheries Association for their help in field survey and data collection. Also, we are grateful to the editor and anonymous reviewers for their constructive comments and suggestions.
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