Exploring Nature-based Solutions on Refined Waterbird Habitats Restoration in High-density Urban Area: A Case Study of the Futian Mangrove National Important Wetland in Shenzhen, China

Abstract

As a highly urbanized bay area bustling with socio-economic activities, Shenzhen Bay is a pivotal stopover and wintering habitat for migratory birds along the East Asian-Australasian Flyway. The Futian Mangrove National Important Wetland, located in the northeast of Shenzhen bay, is a part of the Guangdong Neilingding–Futian National Nature Reserve. As the smallest national nature reserve in China, the wintering habitat of migratory birds has been significantly impacted by the compacted surrounding built-up environment. It has become an urgent need for refined high-quality ecological restoration for the habitats. This project leveraged Nature-based Solutions to develop a refined model for the ecological restoration of coastal wetland waterbird habitats in compact urban areas. By analyzing waterbird behaviors and habitat requirements, this model outlined six strategies: water surface expansion, water level control, hydrodynamic conditions improvement, shoal transformation, adaptive vegetation management, and disturbance control. To effectively guide the restoration implementation, high-, medium-, and low-adaptive approaches were proposed accordingly. After restoration in 2022, notable increases in target species, such as Platalea minor, were observed. The variety of waterbirds of the reserve in 2022 increased by 33% compared with 2021, while increased by 50.9% compared with 2016, significantly enhancing ecosystem services of the coastal area. As urban renewal in China is shifting towards spatial redevelopment, this model offers valuable insights for ecological restoration aiming at coastal wetland waterbird conservation across the country, and substantially supports establishing the "International Mangrove Center" in Shenzhen.

● Explores Nature-based Solutions on refined coastal wetland restoration in highly urbanized area

● Summarizes the universal habitat requirements for five categories of waterbirds

● Proposes six ecological restoration strategies for waterbird habitats and corresponding high-, medium-, and low-adaptive approaches

Graphical abstract

Keywords

Nature-based Solutions / Coastal Wetland / Ecological Restoration / Waterbird Habitat / Biodiversity

1 Background

Waterbirds are highly sensitive to ecological changes in coastal cities^[1][2]. The area and quality of coastal wetlands significantly influence the biodiversity of local waterbirds and the population of migratory birds using these areas for stopovers^[3]. Urban development in coastal regions, marked by dense construction and intense human activities, has led to a pronounced artificial transformation with decreasing coastal wetlands. This degradation substantially impacts waterbird habitats, resulting in notable declines in both the population and species of migratory birds^[1]. In compact coastal urban regions like the Guangdong–Hong Kong–Macau Greater Bay Area, it is challenging to expand coastal wetlands and waterbird habitats. Thus, refined restoration of existing habitats has become crucial for improving biodiversity and ecological resilience in such regions.

Currently, research and practice on restoring coastal wetlands for waterbirds are increasing. The key technical methods include terrain modification, noise abatement, vegetation planting, food supplementation, and water system construction^[4]~[14]. For instance, David J. Yozzo et al. advocated for using dredged materials to create artificial coral reefs for restoring bird habitats, intertidal marshes, mudflats, and bird islands^[7]. These approaches can significantly enhance local waterbird diversity. However, some focus primarily on the development and management of constructed wetlands with immediate outcomes, rather than on restoring the tidal regulation functions of natural wetlands via natural processes^[15][16]. With various physical traits and living habits, different waterbird species require varied ecological habitat environments: Anatidae prefer open waters with soft sand and mud shoals, part of the sand and gravel substrate with complex structures, and deep waters without vegetation; in contrast, Ardeidae are inclined towards habitats in waters near trees^[17]. Therefore, it is imperative to implement detailed restoration to address the needs of different waterbirds^[5]. Existing habitat restoration methods have limited consideration of target waterbird species with simplistic need analysis, lacking multi-objective solutions that cater to the varied habits of diverse waterbird species.

Nature-based Solution (NbS) has gained prominence as an effective way to address climate change and bolster resilience in recent years. NbS underscores the significance of protecting, managing, or establishing new ecosystems to implement and utilize natural functions, offering cost-effective and adaptable solutions to complex social challenges^[18]~[20]. The adoption of NbS spans a wide array of fields including biodiversity conservation, wetland restoration, and protection of river ecosystems^[8][21][24], showcasing extensive research and practices. However, its integration into refined avian habitat restoration is still nascent. For instance, the coastal wetland restoration project in Yancheng, Jiangsu Province, China created functional zones including bird refuge zone, swimming bird foraging zone, wader bird foraging zone, fish refuge zone, and ecological buffer zone^[8]; the restoration of polders of Jianyang Lake Wetland in Zhejiang Province, China, adopted a "retaining–breaking–integrating" design concept to construct a complex wetland ecosystem composed of forest, pond, farmland, lake, and island to improve habitats for Ardeidae^[24]; the West Pond Living Shoreline Project in Jamaica Bay, New York City, USA, incorporated oyster shell breakwater structures, additional sediment, marsh planting, and erosion control to provide habitat for endangered species and migratory birds^[25]; the Mid-Barataria Sediment Diversion Project Final Restoration Plan in Louisiana, USA proposed to rejuvenate coastal wetland habitats and ecosystem functions by re-establishing the connection between the Mississippi River and the Barataria Basin estuary^[26]; the Greater Niagara Region, Canada applied Ecosystem-based Adaptation to conserve waterbird diversity via living shoreline and beach sand replenishment^[27]. While these cases exemplify the application of NbS, practical implementation of these plans remains at a preliminary stage, lacking refined models or approaches. Employing NbS to develop methods for restoring coastal waterbird habitats has significant potential for further development. Thus, taking the restoration design project of the Futian Mangrove National Important Wetland in Shenzhen as a case study, this paper explores Nature-based Solutions on refined waterbird habitats restoration. It aims at theoretical and technical innovation in coastal wetland ecological restoration, enriching the practical experience of NbS application in bird conservation.

2 Study Site and General Model for Restoration

2.1 Futian Mangrove National Important Wetland in Shenzhen

The Futian Mangrove National Important Wetland in Shenzhen ("Futian Mangrove Wetland" hereafter) is a quintessential example of waterbird habitats in high-density coastal cities in China. Located in the Futian District of Shenzhen City, Guangdong Province, and adjacent to the Mai Po Wetland of Hong Kong, it is an integral part of the Neilingding–Futian National Nature Reserve. It has been selected in the "2020 National Important Wetland List" and designated as a "wetland of international importance" under the Ramsar Convention^[28]. On November 5, 2022, the establishment of the "International Mangrove Center" in Shenzhen was announced on the 14th Meeting of the Conference of the Parties to the Ramsar Convention^[29], with the Futian Mangrove Wetland being a key initiative.

The Futian Mangrove Wetland is a critical overwintering and stopover site on the East Asian–Australasian migration route, which is essential to global bird conservation. According to the Guangdong Neilingding Island–Futian National Nature Reserve Administration ("Reserve Administration" hereafter), every year, hundreds of thousands of waterbirds stop in Shenzhen Bay, including 13 species listed on The International Union for Conservation of Nature (IUCN) Red List of Threatened Species, such as Platalea minor and Limosa limosa.

The core of Futian District in Shenzhen is among the most urbanized areas along China's coastline, characterized by high construction intensity, tall buildings, and vibrant activities of various groups of people, significantly affecting the overwintering of migratory birds. As China's only national nature reserve within core urban area, the wetland lacks a buffer zone from the built-up surroundings, thus confining the range of conservation; and the north of the reserve is adjacent to the Binhai Avenue and the Beijing–Hong Kong–Macao Expressway, where traffic notably impacts bird populations, making protection on the reserve under considerable pressure^[30]. Forming the ecosystem of the Futian Mangrove Wetland together with the periphery mudflats and mangroves, the dike-ponds are pivotal high-tide habitats for migratory birds. Achieving high-quality restoration of these dike-ponds within the limited space of the Futian Mangrove Wetland has become a major challenge. Moreover, as China's urban renewal has shifted towards refined redevelopment, exploring NbS on refined ecological restoration offers substantial support and guidance for better quality and efficiency in these endeavors.

2.2 Site Condition

According to remote sensing data from the Reserve Administration, the total area of fishponds in the Futian Mangrove Wetland is approximately 63.17 hm², consisting of 12 dike-ponds and 1 typhoon shelter pond. Upon years of continuous restoration, 27.10 hm² of the area (Fishponds No. 1 ~ 4 along the west bank of the Fengtang River) have been restored before 2022. The project in this paper focused on the comprehensive restoration of fishponds on the east bank (Tab.1), including Fishponds No. 5 ~ 11, the Northern Freshwater Pond, and the Typhoon Shelter Pond, covering a total area of 36.07 hm². The restoration of primary zones has been completed during March to September 2022, covering Fishponds No. 5 and 6, the Northern Freshwater Pond, and the Typhoon Shelter Pond (Fig.1), effectively integrating Fishponds No. 1 ~ 6 on both banks for ecological management enhancement.

Tab.1 Habitat conditions of fishponds along the east bank of the Fengtang River

Fishpond	Area (hm2)	Number of small ponds	Water depth (m)	Habitat condition
No. 5	6.85	5	1~1.5	· Embankment vegetation dominated by tall trees with numerous invasive plants
				· The northern water surface is encroached by Phragmites australis
No. 6	3.46	4	1.5~2	· Embankment vegetation dominated by trees and shrubs with numerous invasive plants
				· The water body is narrow and elongated, easily disturbed by external factors
No. 7	5.11	4	1.5~2	· Rich embankment vegetation with numerous invasive plants
				· Significantly impacted by urban traffic noise and light
No. 8	3.93	2	2~2.5	· Rich embankment vegetation with numerous invasive plants
				· Islets within the pond
No. 9	4.95	4	2~2.5	· Rich embankment vegetation with numerous invasive plants
				· The water body is divided by cruciform embankments with tall trees
No. 10	3.94	3	1 ~ 2	· Phragmites australis are abundant in the central area, with numerous invasive plants
				· The eastern part is gradually terrestrializing, without tidal channels outwards
No. 11	2.74	0	2~2.5	· Lush embankment vegetation dominated by tall trees
				· Islets with established groves within the pond
Northern Freshwater Pond	4.30	5	0. 3 ~ 0.5 (dry season), 1 (rainy season)	· Lush embankment vegetation enclosed by tall, dense plants, with the water surface encroached by aquatic plants
				· Without tidal channels outwards, rainwater is easily accumulated
Typhoon Shelter Pond	0.79	0	Varies with tidal level	The water surface is covered by Acanthus ilicifolius

Fig.1 Fishpond locations in the reserve.

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2.3 Overall Restoration Model

Drawing on research and practical experience, this project establishes an overall restoration model for waterbird habitats (Fig.2) in the Futian Mangrove Wetland. Initially, combine existing monitoring data with field observations to determine target waterbirds, and analyze the habitat needs of these birds through literature review and bird behavior observations. Then, conduct a site suitability analysis and problem diagnosis on water surface area, water depth, hydrodynamic conditions, shoal and islet morphology, the effects of plants, and the effects of urban and human activities, to propose refined ecological restoration models for target waterbirds. Finally, based on the specific site conditions, develop six strategies that includes high-, medium-, and low-adaptive approaches for implementation.

Fig.2 Refined ecological restoration model for waterbird habitats in high-density urban area.

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3 Site Assessment Based on Habitat Needs of Target Waterbirds

3.1 Bird Resource Analysis

Bird resource analysis was conducted combining monitoring data in past years and field observations. According to the data from the Reserve Administration, a total of 261 bird species from 60 families of 20 orders had been recorded within the Futian Mangrove Wetland by 2021. Among them, 13 species are under national first-class protection, including Platalea minor, Aythya baeri, Tringa guttifer, Egretta eulophotes, Threskiornis melanocephalus, Pelecanus crispus, Ciconia nigra, and Ciconia boyciana, while 46 species under national second-class protection. The Platalea minor is listed as Endangered (EN) on The IUCN Red List of Threatened Species, the population of which in Shenzhen Bay remarkably exceeds 1% of its global population, making it one of star birds in Shenzhen.

According to the Futian Mangrove Reserve Fishponds No. 1 ~ 11 Bird Monitoring Report (2015–2018), a total of 75, 549 birds from 120 species were recorded during June 2015 to April 2018, which is fewer in summer and more in winter. The monitored birds mainly included Scolopacidae (approximately 75%), Anatidae (approximately 13%), Ardeidae, Rallidae, and Phalacrocoracidae, with dominant species including Recurvirostra avosetta, Tringa totanus, Himantopus himantopus, Tringa nebularia, Calidris ferruginea, Tringa stagnatilis, Pluvialis fulva, Anas clypeata, and Anas crecca. As the fishponds on the east bank of the Fengtang River had not been restored then, the waterbirds primarily inhabited the restored Fishponds No. 2 ~ 4 according to winter bird observation data from 2017 to 2018 (8, 319 recorded in Fishpond No. 2, 4, 353 in Fishpond No. 3, and 2, 410 in Fishpond No. 4) (Fig.3).^[31]

Fig.3 Waterbird distribution before restoration.

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To confirm the distribution of waterbirds on-site before the restoration of the east bank fishponds, the project team conducted line transect survey on January 12 and January 19, 2022. Transects were established around the periphery of the fishponds. The team made walking observations along the predetermined route when the tide in Shenzhen Bay was high (above 1.5 m), recording both the water levels of the fishponds and the behaviors of waterbirds. The survey revealed that there were over 50 waterbirds each in Fishponds No. 8, 9, and 11, while the other fishponds had around 10 each, primarily Aythya fuligula, Egretta garzetta, Anas clypeata, and a few Himantopus himantopus and Platalea minor, aligning with the results of historical records.

3.2 Habitat Needs Analysis of Target Waterbirds

Based on the monitoring data and field observations, Scolopacidae, Anatidae, Ardeidae, Rallidae, and Phalacrocoracidae, and one national first-class protected species (Platalea minor) have been identified as target waterbirds within the Futian Mangrove Wetland. Considering factors covering body length, ecological type, and residency type, representative species of the target waterbirds with relatively large population size on-site were selected (Tab.2) to analyze their spatial preferences and human disturbance tolerance, which help determine the essential habitat needs and related indicators.

Tab.2 Overview of representative species of target waterbirds

Target waterbird		Representative species	Body length (cm)	Ecological type	Residency type	Human disturbance tolerance
Scolopacidae	Small	Calidris ferruginea	18.0 ~ 23.0	Wading bird	Transient birds	High
	Medium and large	Limosa limosa	36.0 ~ 44.0	Wading bird	Winter visitors	High
Anatidae	Non-diving	Anas clypeata	44.0 ~ 51.0	Swimming bird	Winter visitors	Medium to high
	Diving	Aythya fuligula	40.0 ~ 47.0	Swimming bird	Winter visitors	High
Ardeidae		Egretta garzetta	55.0 ~ 65.0	Wading bird	Mainly winter visitors, a few summer visitors and residents	Medium
Rallidae		Gallinula chloropus	30.0 ~ 38.0	Wading bird	Winter visitors with some residents	High
Phalacrocoracidae		Phalacrocorax carbo	84.0 ~ 90.0	Swimming bird	Winter visitors	High
National first-class protected species		Platalea minor	60.0 ~ 78.5	Wading bird	Winter visitors	High

NOTEThe information is sourced from Dongniao website, interviews with Reserve Administration, and field observation.

Research indicates that, apart from the special needs of each species, waterbird habitats generally require large, continuous open water surfaces for flocking and safety needs; shallow water for foraging; and a certain proportion of shoals and islets for resting and stopping, with a few aquatic vegetation patches around (without tall trees) and minimal noise disturbance (Tab.3, Fig.4).

Tab.3 Habitat needs of target waterbirds

Target waterbird		Waterbody	Water depth (cm)	Shoal and islet	Vegetation	Other	Source
Scolopacidae	Small	Continuous open water surface, low vegetation coverage, generally larger than 2 hm2, regular shape	3 ~ 5	Soft soil, similar color as Scolopacidae	Low vegetation coverage, area proportion of shoals, water surface, and patches: 40%, 40%, 20%	Noise under 50 dB; flight initiation distance above 40 m	Refs. [10] [17] [32]~[38]
	Medium and large		Under 10
Anatidae	Non-diving	Complex composition of water surface with aquatic vegetation patches	15~30	Soft sand and mud on shoals, part of sandy gravel substrate	Partially covered by aquatic vegetation, creating a safe environment	Slow water flow, foraging area current under 4.8 km/h	Refs. [32] [38]~[41]
	Diving		25 ~ 200
Ardeidae		Open water surface	15~40	Some sandy gravel bases on shoals for standing, not all silt	Vegetation coverage generally under 25%, mainly by trees, better with less closure	Noise under 60 dB	Refs. [14] [17] [42]
Rallidae		Open water surface with aquatic vegetation patches	· Foraging: under 10 · Resting: under 50	—	Vegetation patches on water surface (such as Phragmites australis, marsh grasslands) for activity concealment, rich submersed plants	—	Refs. [17] [39]
Phalacrocoracidae		Continuous open water surface, low vegetation coverage	80 ~ 120	Small islets or logs and rocks in the water for perching	Water surface and aquatic vegetation ratio: 4:6 ~ 6:4	Slow water flow	Refs. [42] [43]
Platalea minor		Continuous open water surface, low vegetation coverage	· Foraging: 5 ~ 10 · Resting: 5 ~ 25	Muddy shoals or substrates	Resting environment with some vegetation coverage around the water for a safe atmosphere	Turbid water, embankments or ridges for concealment are beneficial for foraging	Ref. [38]

Fig.4 Habitat needs for target waterbirds.

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3.3 Habitat Problem Diagnosis

According to the habitat needs of waterbirds above, a suitability analysis was conducted on the dike-ponds of the Futian Mangrove Wetland, assessing water surface area, water depth, hydrodynamic conditions, shoal and islet morphology, the impacts of vegetation, and the disturbance by human activities. It identified six common problems faced by the unrestored fishponds.

(1) Small water surface

The area of open water surface is one of the crucial factors affecting the utilization of waterbird habitats. In terms of total area, most fishponds on the east river bank range from 3 to 5 hm², which theoretically meets the needs of large, continuous open water surfaces for gregarious lifestyle (e.g., 2 ~ 3 hm² for Scolopacidae), but actually failed due to the water surface division into parts of about 1 hm² by embankments.

(2) Deep water level

Water depth is an important factor that influences bird's resting and foraging behaviors, and determines the diversity of waterbird species in wetlands. The water depth of most fishponds was around 2 m, deepening gradually from south to north, and from west to east, where the depth of Fishponds No. 8, 9, and 11 on the east bank were approximately 2.5 or 3 m.

(3) Weak hydrodynamic conditions

The exchange of materials and energy between the fishponds and Shenzhen Bay mainly relies on tidal channels, with water gates built over the channels to regulate water levels. The current water gates, constructed before 2008 with traditional lifting mechanisms, were elevated above the waterbed, leading to insufficient water exchange and regulation with siltation in the waterways. Such weak hydrodynamic conditions resulted in insufficient biological exchange between the fishponds and the external marine area, leading to the shortage of bird food supply.

(4) Lack of shoals and islets

Safety is one of the critical factors for birds to select habitats, provided by a sense of openness on shoals and islets as key resting areas. The existing fishpond embankments and islets were densely covered by tall vegetation, with fast-growing aquatic plants nearly occupied the entire water surface, such as Phragmites australis, Eichhornia crassipes, and Alternanthera philoxeroides, affecting the takeoff and landing of waterbirds and potentially providing concealment for predators. Additionally, there were a few small islets in the middle of the fishponds. With the rich vegetation on the islets had grown into groves, it lacks open space for resting.

(5) Excessive invasive plants

The embankments were overly covered with invasive and exotic plants. The trees were mainly exotic species including Acacia mangium. and Acacia confusa Although Leucaena leucocephala had been repeatedly cleared, there were still many seedlings sprouting. The ground cover mainly consisted of Sphagneticola trilobata with patches of Bidens pilosa. These fast-growing plants had disrupted the native ecological conditions.

(6) Heavy human activity disturbance

The Futian Mangrove Wetland is adjacent to urban built areas, and the northern fishponds were highly disturbed by urban environment and patrolling activities. The Xiasha area of Shenzhen has a high construction density, and was substantially affected by light and curtain walls; in addition, as the Guangshen Expressway closely runs along the boundary of the reserve, despite isolation strips, the noise in the fishponds, especially in the northern area, reaches about 70 dB, exceeding the noise tolerance level of most birds. The northern part of the fishponds is frequented by patrol vehicles, the sound and activities of which disturbed the bird habitat in the northern area.

4 Refined Ecological Restoration of Waterbird Habitats

4.1 Restoration Objectives

For an integral bird conservation area like the Futian Mangrove Wetland, it necessitates the synergy of restoration objectives for all fishponds while setting the specific goals for them each. According to the overall situation, the primary restoration goal was to provide a stable high-tide habitat for migratory waterbirds including Platalea minor, large Scolopacidae, and Anatidae; the secondary goal is to meet the full-life cycle habitat needs of local waterbirds, predominantly Ardeidae. The previously restored Fishponds No. 2 ~ 4 had effectively provided habitats for some Scolopacidae and Anatidae. However, it became a main issue that the habitat suitability for Shenzhen Bay's star bird species (Platalea minor and large Scolopacidae) needs to be improved for the restoration of the east bank of the Fengtang River, while providing composite habitats for various waterbirds reliant on the mangrove wetlands and Shenzhen Bay (Tab.4).

Tab.4 Habitat types and major target birds of fishponds to restore

Fishpond	Habitat type	Major target birds
No. 5	High-tide habitat	Platalea minor, medium and large Scolopacidae, and other large wading birds
No. 6	High-tide habitat	Platalea minor, Ardeidae, and other large wading birds
No. 7	Traditional dike-pond	Various birds reliant on dike-ponds, such as Alcedinidae
No. 8	Traditional dike-pond	Anatidae and Ardeidae
No. 9	Deep-water habitat	Aythya
No. 10	Traditional dike-pond	Various birds reliant on dike-ponds, such as Alcedinidae
No. 11	Deep-water habitat	Local Ardeidae, Phalacrocoracidae, and Aythya
Northern Freshwater Pond	Wetland	Various birds reliant on wetlands
Typhoon Shelter Pond	Tidal flat	Various birds and animals reliant on wetlands

4.2 Ecological Restoration Technical Strategies

In response to the six identified common issues identified and informed by bird habitat requirement research, six ecological restoration technical strategies were proposed. These strategies, along with their adaptive approaches, can be selectively integrated in restoration efforts based on the specific problems and habitat needs of the target waterbirds, allowing for creating a customized strategy package (Fig.5).

Fig.5 Six ecological restoration technical strategies.

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(1) Water surface expansion

To expand continuous open water surfaces preferred by waterbirds, current embankments in the middle of the fishponds should be removed or lowered, and converge segmented ponds. The integrated water surfaces should be larger than 2 hm², ideally maintaining a square shape. The proportion of shoal, water surface, and vegetation patch within a pond should be managed to be approximately 40%, 40%, and 20%, respectively.

(2) Water level control

Finely adjust the fishpond bed topography to create terraced bottoms, and install smart water gates to fulfill the varied water depth requirements of different waterbirds. During the migratory bird stopover season from October to next April, the water level should be controlled to form areas with varied water depths—less than 5 cm, 5 ~ 25 cm, 15 ~ 40 cm, and deeper than 1 m—catering to the resting needs of certain target waterbirds. During the rest time of the year, it should maintain a water depth of over 1 m to suppress the growth of Phragmites australis, thereby keeping the rational proportion of spatial elements within the water area.

(3) Hydrodynamic conditions improvement

This strategy involves dredging silt from the outward tidal channels, constructing internal circular deep channels, and modifying the elevation of water gate bottoms. The dredging degree of tidal channels is determined by the siltation situation, normally removing 30 ~ 50 cm thickness of silt. Along the pond perimeters, excavate deep channels normally requiring a bottom width of 5 m, a side slope at about 45°, and a depth over 2 m. The dimensions of these channels can be adjusted according to specific site conditions, for instance, progressively increasing the depth from the inlet towards the opposite end to optimize hydrodynamic conditions and foster suitable habitats for fish and shrimp. Moreover, aligning the bottom of the water gate with the fishpond bottom ensures complete drainage and can better facilitate the material and energy exchange with the surrounding environment, as well as improving inter-fishpond connectivity.

(4) Shoal transformation

On the basis of improved layout of water surface, construct 1 ~ 2 large central shoals and several small islets, with gentle sloping (5° ~ 25°) into the water and winding shoreline. Utilize the central embankment to form shoals, lengthening the foraging shoreline and expanding the resting area. Use the earth from lowering the embankment to construct small islets. In addition, clear the vegetation on existing islets and adjust the shape and elevation of the islets as required. The substrate for Scolopacidae should be soft soil that matches their body color, while the substrate for Anatidae should be soft sand and mud with sandy gravel.

(5) Adaptive vegetation management

Completely remove invasive plants such as Leucaena leucocephala and Sphagneticola trilobata, as well as aquatic plants like Phragmites australis. For islets within the fishponds, prune native trees to provide perching and nesting spaces for birds such as Ardeidae. For central embankment, clear the fast-growing, potentially invasive trees, transplant native tree species and selectively retain some tree islets. In terms of the southern pond embankments, clean up fast-growing and invasive tree species and keep native trees with proper pruning to ensure them not exceed the height of the mangroves, thereby forming a vegetation pattern lower and sparser in the south and higher and denser in the north.

(6) Disturbance control

To foster a secure environment with vegetation around the water and to shield the natural coastal wetland ecosystem from the large-scale urban municipal projects, the project proposed creating vegetative buffers to mitigate the disturbance of noise, lighting, and human activities. The barrier effect can be enhanced by expanding and elevating these buffers through on-site cutting-and-filling, and transplanting or locally densifying native plants. The widened part of the buffers should be raised to 1 m above the original terrain, gradually sloping into the water.

4.3 Refined Ecological Restoration Model

To improve the suitability of ecological restoration strategies for waterbird habitats, this project took into account the importance level of target waterbird conservation, the necessity of ecological restoration, and the resistance to restoration. It proposed three levels—high, medium, and low—of adaptive approaches tailored to the diverse habitat needs of waterbirds. Practically, by addressing the unique challenges and location features, a combination of these restoration strategies could be applied to forge a refined restoration plan for waterbird habitats in coastal wetlands (Tab.5).

Tab.5 Three levels of adaptive approaches for refined ecological restoration strategies

Strategy	High-adaptive approach	Medium-adaptive approach	Low-adaptive approach
Water surface expansion	· Water surface area larger than 3 hm2 with a proportion of no less than 90%	· Water surface area larger than 2.5 hm2 with a proportion of no less than 80%	· Water surface area larger than 2 hm2 with a proportion of no less than 70%
	· Eliminate internal embankments to fully integrate all water areas, creating a large, open water surface	· Lower internal embankments to visually form open water surfaces	· Partially breach embankments for functional connectivity between water areas
Water level control	· Water level lower than 0.15 m	· Water level between 0.15 ~ 0.4 m	· Water level above 0.4 m
	· Adjust terrain of fishpond bottom into terrace, use smart water gates for real-time shallow water level control	· Slightly adjust the terrain of the bottom of fishpond to form areas with varying depths	· Maintain water level
Hydrodynamic conditions improvement	· High water exchange capacity	· Medium water exchange capacity	· Medium water exchange capacity
	· Dredge tidal channels to enhance hydrodynamic conditions for material and energy exchange	· Lower water gate bottom elevation and construct circular deep channels inside the pond	· Lower water gate bottom elevation
Shoal transformation	· Proportion of shoals larger than 40%	· Proportion of shoals between 20% ~ 40%	· Proportion of shoals less than 20%
	· Construct central shoals and islets, increase shoreline complexity, lower slopes, and clear existing islet vegetation	· Transform internal embankments into shoals and islets	· Partially lower embankments and clear vegetation to visually form shoals
Adaptive vegetation management	· Clear invasive plants and non-native plants from the embankments	· Clear invasive plants and non-native plants from the embankments	· Clear invasive plants from the embankments
	· Transplant native trees to create a vegetation pattern of lower and spaeser in the south to higher and denser in the north	· Retain native trees as tree islets and prune the branches of tall trees	· Clear aquatic plants like Phragmites australis within the fishpond
Disturbance control	· Noise lower than 50 dB	· Noise lower than 65 dB	· Noise lower than 85 dB
	· Expand and elevate the vegetation buffer through cutting-and-filling	· Densely plant existing vegetation buffers partially	· Deep channel as buffer zone instead of habitat for target waterbirds

NOTEThe proportion of water area refers to the area ratio of the water body to the fishpond; the proportion of shoals indicates the area ratio of the shoal to the fishpond.

4.4 Recent Restoration Practices

In the recently completed restoration projects, refined ecological restoration plans were determined by combining various restoration strategies for the target waterbirds of each fishpond (Tab.6).

Tab.6 Ecological restoration plans for four fishponds

Strategy	Water Surface Expansion	Water Level Control	Hydrodynamic Conditions Improvement	Shoal Transformation	Adaptive Vegetation Management	Disturbance Control
Fishpond No. 5	High-adaptive approach	High-adaptive approach	Medium-adaptive approach	High-adaptive approach	Medium-adaptive approach	High-adaptive approach
Fishpond No. 6	Medium-adaptive approach	Medium-adaptive approach	Low-adaptive approach	Medium-adaptive approach	Medium-adaptive approach	Low-adaptive approach
Northern Freshwater Pond	Low-adaptive approach	—	—	High-adaptive approach	Low-adaptive approach	—
Typhoon Shelter Pond	High-adaptive approach	—	High-adaptive approach	—	Medium-adaptive approach	—

(1) Restoration plan for Fishpond No. 5

Fishpond No. 5, located adjacent to the Fengtang River estuary and the tidal flats of Shenzhen Bay, spans a total water area of 6.38 hm² in a square shape. It was divided into 5 sections by embankments, where each section's water surface falls short of 2 hm² with a depth of about 1 m. There had been an abundance of tall trees, and the water area had been significantly encroached upon by Phragmites australis, lacking open water surfaces, shoals, and islets. The northern section experienced substantial disturbances from patrolling and urban construction activities. Before restoration, limited wading birds such as Egretta garzetta, Himantopus himantopus, and Platalea minor were observed in southern small ponds during the drainage, indicating significant potential for ecological restoration.

Fishpond No. 5 was designated as a habitat for large wading birds like Platalea minor. The plan integrated all water areas into one continuous open water surface (Fig.6, Fig.7). With smart water gate installation and adjusted terraced pond bottom, the water level in the central area is maintained between 5 ~ 25 cm. Additionally, embankments were transformed into winding central shoals with gentler slopes to provide resting spaces for migratory birds. A deep channel (5-m-wide, 2-m-deep) was constructed along the perimeter to enrich the aquatic living environment and enhance hydrodynamic conditions. Phragmites australis and invasive plants were removed from the water surface and embankments, with native plants selectively conserved as tree islets. In the north, the vegetation buffer was widened and heightened to mitigate urban and human activity disturbances on birds (Fig.6 ~ Fig.9).

Fig.6 Restoration plan for Fishpond No. 5.

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Fig.7 Section of restoration plan for Fishpond No. 5 ("i": slope; "FL": floor level; "BL": bottom level; "WL": water level).

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Fig.8 Fishpond No. 5 before (left) and after (right) restoration.

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Fig.9 Restoration outcome of Fishpond No. 5.

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(2) Restoration plan for Fishpond No. 6

Fishpond No. 6, located close to the boundary of the reserve, presented a narrow, elongated shape, with a total area of 3.46 hm². It had been segmented into four water bodies by central embankments, all with high water levels. The embankments had been densely dominated by tall trees, suffering heavy external disturbances (Fig.10). Ardeidae had been observed occasionally on the south side, suggesting a moderate potential for restoration. The goal was to provide a habitat for large wading birds including Platalea minor and Ardeidae.

Fig.10 Fishpond No. 6 before (left) and after (right) restoration.

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The plan lowered the internal embankment to create a shoal, visually expanding the water surface. It involved localized adjustments to the micro-topography of the fishpond bottom and lowering the water gate elevation to regulate water levels. Tall trees were preserved for tree islets, while existing islets were transformed into shoals and islets. The earth excavated from breaking the embankment was used to construct new small islets in the southern part of the fishpond. The area adjacent to the northern boundary is designated mainly as a buffer, rather than a habitat for birds.

(3) Restoration plan for the Northern Freshwater Pond

The Northern Freshwater Pond, not directly connected to Shenzhen Bay, mainly relies on rainfall for water replenishment, distinguishing it as the only freshwater pond in the reserve. Covering an area of 4.3 hm², it had been segmented into 5 water areas by 4 embankments. Dense vegetation populated by invasive species, overtook the eastern water surface and embankments, making the area unsuitable for massive migratory bird habitation (Fig.11). This pond had occasionally hosted various wetland-dependent bird species, including Rallidae, Alcedinidae, and Ardeidae. Lutra lutra had been observed under the bridge of the Fengtang River corridor, indicating significant restoration potential.

Fig.11 The Northern Freshwater Pond before restoration (left) and after restoration (right).

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Given the unique spatial conditions of the Northern Freshwater Pond, its restoration was aimed at biodiversity enhancement, primarily addressing limited water surface, lack of shoals and embankments, and excessive invasive plants. By partially breaking the embankments, the water system is functionally interconnected, enhancing the cycling capacity of materials and energy. Tall trees on the embankments are preserved, while invasive plants and aquatic vegetation like Phragmites australis were cleared. After removing vegetation, existing islets were enlarged and new ones were constructed, while increasing shoreline complexity and gentling the slope. The restoration provides habitats for various wetland-dependent birds and animals, and gradually recover its ecosystem functions (Fig.11).

(4) Restoration plan of the Typhoon Shelter Pond

The Typhoon Shelter Pond is located about 200 m north of the tidal flats and directly connected to them through a tidal channel. Due to perennial siltation, the water surface had been fully invaded by Acanthus ilicifolius, making it difficult for migratory birds to use (Fig.12). The intersection of the tidal channel and tidal flats of Shenzhen Bay served as a habitat for various waterbirds. It had significant potential for ecological restoration with Rallidae breeding in the northern forests. To effectively tackle with limited water surface, weak hydrodynamic conditions, and vegetation encroachment in the Typhoon Shelter Pond, and to fully restore it, high- and medium-adaptive approaches were adopted, involving dredging the tidal channel and clearing Acanthus ilicifolius within the pond to create an expansive open water surface and improve hydrodynamic conditions, thereby creating a tidal flat habitat for birds (Fig.12).

Fig.12 The Typhoon Shelter Pond before (left) and after (right) restoration.

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5 Restoration Outcomes

The recent restoration project had been completed by the end of September 2022, coinciding with the arrival of migratory birds in Shenzhen Bay. Preliminary observations for the first stopover season indicated a significant increase in the variety and number of waterbirds across the fishponds. According to statistics, the variety of waterbirds in the reserve in 2022 increased by 33% compared with last year; the total number of waterbirds in the reserve increased from 27, 392 to 37, 079 (an increase of 35%) compared with 2016, and the variety increased from 53 to 80 species (an increase of 51%). Before restoration, only about 30 to 40 Platalea minor were observed in the entire reserve area during winter, while over 150 were observed in the winter of 2022, creating a new record for recent years^[44]. Especially in Fishpond No. 5, over 50 individuals of Platalea minor can be observed each day (Fig.13, Fig.14). With small groups of Scolopacidae and Anatidae observed in Fishpond No. 5, the total estimated number of various waterbirds could reach 200 ~ 300, which is tenfold of the number before restoration. Compared with historical monitoring data from the Reserve Administration, national first-class protected species such as Egretta eulophotes, second-class protected species like Nettapus coromandelianus, Philomachus pugnax, and Hydrophasianus chirurgus were returned the site for the first time in nearly 20 years. In December 2022, a flock of 29 Ciconia boyciana (national first-class protected species) were observed for the first time in nearly 30 years.

Fig.13 Platalea minor recorded in Fishpond No. 5 after restoration.

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Fig.14 Waterbirds in the Futian Mangrove Wetland after restoration.

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6 Conclusions and Perspectives

Focusing on the diverse habitat needs of target waterbirds, this project addresses site challenges specifically. It develops a refined waterbird habitat restoration model in high-density urban areas, which is applied and verified in the restoration project of the Futian Mangrove Wetland in Shenzhen. The implementation of the project effectively has increased the species and number of waterbirds and enhanced the function of Shenzhen Bay as a transit point for international migratory birds.

Building on the proposed restoration model, future efforts can delve into developing further refined management measures for coastal wetlands and waterbird habitats. It is recommended to persistently conduct smart monitoring and evaluation of waterbird habitats in the Futian Mangrove National Important Wetland in Shenzhen for real-time updates on habitat status and bird population dynamics. This will facilitate dynamic optimization and adaptive management, supplying scientific references and technological support for high-quality habitat restoration and the establishment of the International Mangrove Center. Additionally, it aims to bolster the Shenzhen Bay's significance for a wintering site and stopover of international migratory birds.

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Acknowledgements

· "Research on the Driving Mechanisms of Coastal Urban Green Space Resilience: A Case Study of the Guangdong–Hong Kong–Macao Greater Bay Area", National Natural Science Foundation of China (No. 52078004) · "Research on the Resilient Disaster Mitigation Effects and Driving Mechanisms of Urban Green Infrastructure in the Guangdong–Hong Kong–Macao Greater Bay Area", Guangdong Basic and Applied Basic Research Foundation (No. 2021A1515012246) · "Research and Application Demonstration for Coastal Zone Pollution Reduction–Carbon Sequestration Increase–Disaster Response Coupling Mechanism and Synergistic Efficiency Technology", Shenzhen Science and Technology Program (No. KCXST20221021111416039) · "Development of River–Bay Habitat Restoration and Ecological Resilience Building Technologies", Shenzhen Science and Technology Program (No. KCXFZ20211020164205009)

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