基于海平面影響濕地模型的海平面上升影響海岸濕地景觀研究

陳崇賢 楊瀟豪 夏宇

受全球氣候變化的持續(xù)影響，海平面上升已成為沿海地區(qū)未來面臨的重要挑戰(zhàn)[1]。2013年聯合國政府間氣候變化專門委員會（IPCC）發(fā)布的研究報告中指出，如果溫室氣體排放持續(xù)加劇，到21世紀末海平面將上升0.52～0.98 m[2]。許多研究也表明，隨著海平面上升，沿海低地區(qū)域受災風險將會不斷加大，包括海岸侵蝕、低地淹沒、風暴潮加劇等。同時，一系列潛在風險的影響也對未來海岸區(qū)域資源的保護、管理與開發(fā)提出新難題[3]。

表1 不同類型生態(tài)系統服務價值評價方法Tab. 1 Evaluation methods for different types of ecosystem service value

海岸濕地介于海洋和陸地之間，為人類社會提供了豐富的生態(tài)系統服務，在維持海岸帶環(huán)境健康安全、保護生物多樣性、提供生物棲息地等方面發(fā)揮重要作用[4]。自20世紀90年代以來，國內外許多學者針對海平面上升對海岸濕地生態(tài)系統的影響展開研究。美國環(huán)境保護署（EPA）研發(fā)了海平面上升影響濕地模型（Sea Level Affecting Marshes Model，簡稱SLAMM），被廣泛應用于美國海岸濕地資源的風險評估研究中。Craft等[5]通過SLAMM模型評估在海平面加速上升情況下美國佐治亞州海岸潮間帶濕地生態(tài)系統可能受到的影響。Thorne等[6]考慮濕地在水平和垂直維度上對未來海平面上升的自我調節(jié)能力，采用濕地生態(tài)系統恢復吸積速率模型，綜合濕地高程、潮汐、吸積速率、土壤特性及海平面上升情景等因素，動態(tài)模擬美國太平洋沿岸14個河口地區(qū)的濕地在21世紀可能面臨的損失風險。近年來，中國學者[7]開始基于GIS空間疊加技術，應用遙感影像和高程提取模型研究海平面上升對濕地景觀的分析和保護策略。但目前國內尚缺乏針對海平面上升的動態(tài)過程對海岸濕地生態(tài)系統影響的評價研究。綜合考慮海岸地理環(huán)境和濕地自然演變的因素，客觀定量預測評估海平面上升過程對海岸濕地生態(tài)系統的影響，是保障合理保護和開發(fā)海岸濕地景觀資源的重要前提，也是目前國際前沿關注的重要問題。因此，以廣州市為研究區(qū)域，運用SLAMM定量模擬預測近期（2017—2030年）、中期（2017—2050年）和遠期（2017—2100年）海平面上升3個不同階段對海岸濕地景觀的影響，定量評估其生態(tài)系統服務價值的變化，并為海岸濕地景觀資源的開發(fā)利用與保護提供參考。

1 研究區(qū)域概況

廣州市位于廣東省中南部，其海岸帶位于珠江入?？?，緊鄰中國南海，為淤泥質沖積平原，是海、陸、河三者交匯處，岸線長約522 km（圖1）。廣州市域內河網密布，水塘眾多，濕地景觀資源豐富，面積大、分布廣，共有濕地面積86 178.7 hm2，占廣州市土地總面積的11.6%，主要包括近海及海岸濕地、河流濕地、湖泊濕地及庫塘濕地幾大類型，其中近海及海岸濕地面積分布最廣，約39 286.4 hm2[8-9]。

海岸濕地處于海陸交匯區(qū)域，對海岸線的保護和生態(tài)功能的發(fā)揮具有重要作用，根據海岸濕地的組成和成因特點，主要分為潮上帶濕地、潮間帶濕地和潮下帶濕地[10]。選取廣州市為研究地域，以該區(qū)域的海岸濕地為研究對象，分析未來海平面上升對海岸濕地空間分布及生態(tài)系統服務價值的影響。

2 研究方法

2.1 海平面上升情景預測

2013年IPCC發(fā)布的第5次評估報告中預測，到2100年全球海平面將上升0.52～0.98 m。研究選取上限值0.98 m作為計算研究區(qū)域海平面上升情景的依據。全球歷史海平面上升率為0.001 7 m/a，廣州歷史海平面上升率為0.002～0.003 m/a[11]，根據地方海平面上升計算公式求出相應的海平面上升值：

式中，Hlocal為地方海平面上升相對高程，Hglobal為全球海平面上升相對高程，T為預測年份，T0為基準年份，Vglobal為全球歷史海平面上升率，Vlocal為地方歷史海平面上升率。

2.2 海平面上升對濕地類型變化的影響模擬

SLAMM模型主要應用于海平面上升對海岸濕地的影響研究，該模型以地理信息系統（GIS）為基礎，輸入基礎數據，包括地區(qū)海平面上升數值、區(qū)域高程數據、用地類型及影響參數等，通過幾何關系定量分析海平面上升過程中沿海土地利用的變化[12]。在SLAMM6.7模型中，基于RCP8.5情景，結合Landsat8衛(wèi)星遙感圖像和數字高程模型（Digital Elevation Model，簡稱DEM），選擇已開發(fā)用地和未開發(fā)用地進行保護的模式，模擬預測近、中、遠期海平面上升3個不同階段對廣州海岸濕地景觀空間分布和面積變化的影響。由于不同用地類型面積對生態(tài)系統服務價值有直接關聯，參考王軍和李娜等的研究[13-14]，采用不同濕地類型用地面積作為重要估算因子，預測由于海平面上升過程導致不同濕地類型用地面積變化而造成的生態(tài)系統服務價值的變化。

2-1 2017年海平面上升影響濕地空間分布圖Spatial distribution of wetlands affected by sea level rise in 20172-2 2030年海平面上升影響濕地空間分布圖Spatial distribution of wetlands affected by sea level rise in 20302-3 2050年海平面上升影響濕地空間分布圖Spatial distribution of wetlands affected by sea level rise in 20502-4 2100年海平面上升影響濕地空間分布圖Spatial distribution of wetlands affected by sea level rise in 2100

2.3 海平面上升對生態(tài)系統服務的影響評估

研究采用生態(tài)系統服務價值估算方法，以不同濕地類型用地面積和單位面積不同的生態(tài)系統服務價值作為相關變量，兩者相乘得到相應的生態(tài)系統服務類型價值總量。不同濕地面積的變化值依據SLAMM模型的分析結果，不同類型濕地單位面積的生態(tài)系統服務價值采用的方法包括市場價值法、替代成本法、影子工程法、成果參照法、旅行費用法等（表1）。具體計算公式如下：

式中，ESVt為第t年生態(tài)系統服務總值，Vki為第i種濕地、第k類生態(tài)系統服務功能單位面積的服務價值，Ati為第i種濕地、第t年的濕地面積。

3 結果

3.1 近期海平面上升對濕地景觀的影響（2017—2030年）

基于SLAMM分析結果顯示（圖2-1、2-2），近期（即2030年與2017年相比）在海平面上升過程中受影響最大的區(qū)域是廣州南部的南沙和番禺區(qū)域，其總體變化特征是河口水域范圍擴大，并向內陸延伸。灘涂、內陸水域、鹽沼/蘆葦沼澤面積銳減，分布呈破碎化趨勢。濕地景觀受影響最嚴重的區(qū)域是南沙區(qū)的萬頃沙鎮(zhèn)、新墾鎮(zhèn)及龍穴村，海珠及黃埔區(qū)受影響的區(qū)主要是內陸水域的河道。

近期海平面上升造成不同濕地景觀類型面積發(fā)生變化（表2），其中鹽沼/蘆葦沼澤和紅樹林減少趨勢最為顯著，分別減少449.83 hm2和641.21 hm2，減少量占比73.59%和81.54%。過渡帶鹽沼和河口水域出現顯著增長趨勢，其中過渡帶鹽沼是海平面上升中新出現的濕地類型，增長1 589.62 hm2，河口水域面積增加23 492.87 hm2，增加量占比達101.23%。從生態(tài)系統服務價值變化看（表3），近期海平面上升造成生態(tài)系統服務價值總量減少356 819.18萬元，減少量占比28.12%。生態(tài)系統服務價值減少的濕地包括林木沼澤、鹽沼/蘆葦沼澤、紅樹林、陸上開放水域（表4），其中，林木沼澤和紅樹林減少趨勢最顯著，分別減少11 816.16萬元和3 693.87萬元。生態(tài)系統服務價值增加的濕地是過渡帶鹽沼、灘涂和河口水域，分別增長21 668.32、20 826.19和178 354.55萬元。從生態(tài)系統服務價值類型的變化來看（表5），近期海平面上升造成漁業(yè)生產、氣候調節(jié)、調蓄洪水、棲息地、娛樂休閑及教育科研的服務價值減少，其中漁業(yè)生產和教育科研服務價值減少量最顯著，分別減少374 951.94萬元和27 440.07萬元，減少量占比分別為54.66%和50.92%。出現增長的是污染凈化和干擾調節(jié)，分別增長102.22%和29.74%?？梢姡诤Ｆ矫嫔仙?.12 m將對海岸濕地景觀帶來的影響，不僅造成空間分布的改變，而且也帶來濕地類型面積及生態(tài)系統服務功能的變化。其可能是海平面上升帶來的復雜性因素，造成位于低地濕地類型逐漸減少或消失，而在海平面上升過程中又造成海岸環(huán)境變化，促使產生新的濕地類型或原有濕地類型面積增加[15-16]，最終造成生態(tài)系統服務價值總量呈減少趨勢。

3.2 中期海平面上升對濕地景觀的影響（2017—2050年）

中期（即2050年與2017年相比）在海平面上升過程中受影響最大的區(qū)域仍是廣州南部的南沙和番禺區(qū)域，對比近期SLAMM分析結果，河口水域范圍只出現少量擴大，濕地景觀繼續(xù)呈現破碎化分布趨勢（圖2-1、2-3）。

到2050年，中期海平面上升將進一步造成不同濕地景觀類型面積發(fā)生增加或減少（表2），大部分濕地面積減少量加大。從濕地面積減少量占比來看，鹽沼/蘆葦沼澤和紅樹林面積減少仍最大，分別減少462.56 hm2和652.48 hm2，減少量占比分別為75.67%和82.98%，比近期減少分別增加2.08%和1.44%。與近期相比，濕地面積減少趨勢最明顯的是林木沼澤、過渡帶鹽沼和陸上開放水域，林木沼澤和陸上開放水域比近期減少分別增加3.89%和3.61%，而過渡帶鹽沼發(fā)生急劇減少變化，從近期1 589.62 hm2減至26.78 hm2；灘涂和河口水域面積顯著增長，分別增長2 421.43 hm2和25 141.81 hm2，比近期分別增加174.90%和7.10%。從生態(tài)系統服務價值變化看（表3），中期海平面上升造成生態(tài)系統服務價值總量減少337 578.63萬元，減少量占比26.60%，相對近期出現少量增加，增加19 240.55萬元。相比近期，生態(tài)系統服務價值發(fā)生減少的濕地包括林木沼澤、鹽沼/蘆葦沼澤、過渡帶鹽沼、紅樹林、陸上開放水域（表4），其中過渡帶鹽沼顯著減少，從近期新增的21 668.32萬元減至198.78萬元；生態(tài)系統服務價值發(fā)生增加的濕地是灘涂和河口水域，比近期分別增長67 767.91萬元和12 518.55萬元。從生態(tài)系統服務價值類型的變化看（表5），相比近期，漁業(yè)生產、氣候調節(jié)、棲息地、娛樂休閑及教育科研的服務價值進一步減少，其中漁業(yè)生產和教育科研服務價值減少量最顯著，比近期分別減少3.61%和3.43%，同時污染凈化價值也出現少量減少，比近期減少0.87%。相比近期，增加的生態(tài)系統服務價值為調蓄洪水和干擾調節(jié)，分別增加26.35%和109.98%?？梢?，中期海平面上升0.36 m將對海岸濕地景觀進一步帶來復雜變化影響，其中濕地景觀空間分布上未出現較大變化，表明有可能在此情景下該區(qū)域大部分濕地都已受到影響，但該階段海平面上升卻使原有增長的濕地生態(tài)系統服務價值發(fā)生減少，或原有減少的生態(tài)系統服務價值發(fā)生增長，其可能原因是濕地景觀面積發(fā)生過渡性變化，使生態(tài)系統服務價值發(fā)生波動，并最終造成生態(tài)系統服務價值總量雖減少，但比近期卻有增長的趨勢。

表2 3個階段不同濕地面積的變化Tab. 2 Change of different wetlands area in three phases

3.3 遠期海平面上升對濕地景觀的影響（2017—2100年）

對比近期和中期SLAMM分析結果，遠期（即2100年與2017年相比）海平面上升對海岸濕地影響仍是廣州南部的南沙和番禺區(qū)域。造成影響的空間分布特征進一步表明，在此情景下海平面上升對該區(qū)域海岸低地的濕地景觀影響逐漸趨于穩(wěn)定（圖2-1、2-4）。

到2100年，對比近期和中期結果，遠期海平面上升造成不同濕地景觀類型面積發(fā)生變化的趨勢最顯著（表2）。林木沼澤、鹽沼/蘆葦沼澤、紅樹林和陸上開放水域進一步減少，其中林木沼澤和鹽沼/蘆葦沼澤減少量最顯著，比中期分別減少9.24%和10.83%；新出現的過渡帶鹽沼和中期出現增長的灘涂均呈負增長，分別減少8.35 hm2和849.45 hm2。相比中期，河口水域面積顯著增大，面積增加22.08%。從生態(tài)系統服務價值變化看（表3），遠期海平面上升造成生態(tài)系統服務價值總量減少了435 492.59萬元，比中期減少7.72%。相比中期，生態(tài)系統服務價值發(fā)生減少的濕地包括林木沼澤、鹽沼/蘆葦沼澤、過渡帶鹽沼、紅樹林、灘涂和陸上開放水域（表4），其中灘涂生態(tài)系統服務價值急劇減少，從88 594.10萬元減至31 079.16萬元，減少148.44%；生態(tài)系統服務價值增加的是河口水域，從中期的190 873.10萬元增至229 783.73萬元。從生態(tài)系統服務價值類型的變化看（表5），相比中期，生態(tài)系統服務價值減少的有漁業(yè)生產、調蓄洪水、棲息地、干擾調節(jié)、娛樂休閑及教育科研，其中調蓄洪水和干擾調節(jié)減少量最顯著，調蓄洪水比中期減少33.76%，而干擾調節(jié)從中期的增長趨勢變?yōu)闇p少趨勢，比中期減少106.31%；增加的生態(tài)系統服務價值主要為凈化污染，從中期的91 326.16萬元增至110 027.92萬元，增加20.75%?？梢?，遠期海平面上升1 m將對海岸濕地景觀的影響進一步加大，盡管空間分布相比中期和近期未出現明顯變化，但開放水域面積的顯著增加趨勢表明，該情景下不同濕地類型之間的轉變受海平面上升影響較大。遠期階段海平面上升使生態(tài)系統服務價值總量在中期增加的變化上，出現減少的趨勢，該情景下部分生態(tài)系統服務功能較為豐富，或生態(tài)系統服務價值較高的濕地景觀受到影響較大，從而最終形成生態(tài)系統服務價值總量進一步減少的趨勢。

4 討論與結論

海平面上升對海岸生態(tài)環(huán)境的影響已引起國內外學者的廣泛關注，科學客觀地評估海平面上升對海岸生態(tài)帶來的風險是進一步制定合理利用和開發(fā)海岸資源策略的關鍵。研究基于SLAMM6.7模型，綜合考慮海平面上升對海岸濕地景觀影響的動態(tài)變化過程，模擬分析海平面上升3個不同階段對廣州海岸濕地景觀空間分布和面積變化的影響，并對3個階段濕地生態(tài)系統服務價值變化進行定量評估。

表3 3個階段生態(tài)系統服務價值總量的變化Tab. 3 Change of total wetland ecosystem service values in three phases

表4 3個階段不同濕地生態(tài)系統服務價值的變化Tab. 4 Change of different wetland ecosystem service values in three phases

研究結果顯示近期海平面上升使廣州南部的番禺和南沙區(qū)域海岸濕地景觀空間分布呈顯著破碎化趨勢，而中期和遠期海平面上升階段這種影響逐漸趨于穩(wěn)定，其表明研究區(qū)域內有大部分海岸濕地景觀對近期海平面上升的影響較為敏感。同時，海平面上升3個不同階段使不同海岸濕地景觀類型的面積出現增加或減少，大部分濕地面積持續(xù)減少，但部分海岸濕地，如過渡帶鹽沼和灘涂出現近期增長到中遠期逐漸減少的變化，只有河口水域面積不斷擴大，其原因可能是隨著海平面上升，海岸生境發(fā)生變化，一方面新的生境條件促使新的濕地類型出現或擴大已有濕地景觀類型面積，另一方面由于生境條件不利于其他濕地植物的生長，因此造成其減少或消失的變化。同時，當海平面上升進一步不斷加大，淹沒大量低地濕地，最終導致河口水域面積不斷增長。海平面上升還將影響研究區(qū)域內的海岸濕地生態(tài)系統服務價值，總體上生態(tài)系統服務價值呈現“減少（近期）—增加（中期）—減少（遠期）”的波動性變化特征。對比海平面3個不同上升階段，不同濕地生態(tài)系統服務價值顯著減少的是過渡帶鹽沼，顯著增加的是河口水域，波動性顯著的是灘涂濕地，而不同類型生態(tài)系統服務價值顯著減少的是漁業(yè)生產和教育科研價值，顯著增加的是污染凈化價值，波動性顯著的是干擾調節(jié)價值，這也進一步表明海平面上升對海岸生態(tài)環(huán)境影響的復雜性特征。

當前面對城市化不斷擴張，人類在海岸帶活動不斷增強及全球氣候變化帶來海岸風險逐漸加大的多重挑戰(zhàn)，亟需采取有效措施合理利用和保護海岸濕地景觀資源。從景觀生態(tài)規(guī)劃視角看，一方面需要加強科學的風險預測評估研究，根據未來海岸環(huán)境的變化趨勢制定合理的開發(fā)模式，如漸進式的開發(fā)模式注重濕地生境演替的連續(xù)性，能夠適應海岸濕地景觀的自然更新機制。另一方面積極實施海岸濕地景觀生態(tài)恢復和管理措施，由于海岸泥沙開采、圍墾活動及城市水域污染等造成大量海岸濕地退化或消失，因此需要恢復野生動植物群落、設立海岸濕地保護區(qū)、控制海岸污染源及限制旅游人群等一系列人為的有利干預，以緩解在未來復雜海岸環(huán)境變化下濕地進一步退化和損失。

此外，研究基于計算機分析模擬技術，對現實的復雜因子動態(tài)綜合作用考慮存在一定的局限性，且生態(tài)系統服務價值的評估也未有統一的標準和方法。因此，進一步的研究中需要綜合多種情景和影響因子，完善現有生態(tài)系統服務價值的評估方法，構建更加合理科學的海平面上升影響濕地評估模型。

圖表來源：

文中圖表均為作者繪制，其中圖1底圖來源于中華人民共和國自然資源部。

As the impact of global climate change is growing, sea level rise has posed a big challenge for coastal areas in the future. A research report released by Intergovernmental Panel on Climate Change (IPCC) has predicted that the sea level would rise 0.52-0.98 m by the end of 21 century if the greenhouse gases are not prevented effectively.Studies show that low-lying lands in coastal areas will face increasing risks of coastal erosion, coastal flooding and storm surges. This also brings new challenges for future protection, management and development of coastal zone.

Coastal wetlands between the ocean and land provide rich ecosystem service for human society,and play an important role in maintaining the health and safety of coastal ecosystem, protecting biological diversity and providing biological habitat.Since the 1990s, scholars at home and abroad have assessed the impact of sea level rise on coastal wetland ecosystems. Sea Level Affecting Marshes Model (SLAMM), developed by U.S. Environmental Protection Agency (EPA), is widely applied in the vulnerability assessment of coastal wetland in the United States. Craft et al. have assessed the possible effect of the sea level rapid rise by using SLAMM model on the wetland ecosystem of the coastal tide zone in Georgia. Thorne et al. evaluated the selfability of wetland to adjust the sea level rise in the horizontal and vertical dimension with the help of wetland ecology system accretion rate recovering model. GIS spatial overlay analysis technology with remote sensing image and inundation model is also widely used by Chinese researchers to evaluate and develop the protection strategy of wetland system under sea level rise. However, there is a lack of assessment research on the impact of dynamic sea level rise processes on coastal wetland ecosystem services in China. Based on SLAMM, this study takes Guangzhou as a typical case, quantitatively projecting the impacts of sea level rise on coastal wetland by three different phases of sea level rise,including short-term (2017—2030), mid-term(2017—2050) and long-term (2017—2100), and quantitatively evaluate the change of ecosystem service value for providing a reference for future protection and development strategies.

1 Overview of the Research Area

Guangzhou is located in the south-central part of Guangdong Province, and its coastal zone, a muddy alluvial plain across the mouth of the Pearl River, closes to the South China Sea. It is the intersection area of sea, land and river with 522 km coastline (Fig. 1). The wetlands here include the offshore and coastal wetlands,rivers, lakes, ponds wetlands, of which the offshore and coastal wetlands are the most widespread with the area of 39,286.4 hm2. The coastal wetlands can be divided into tide high-zone wetland, tide middlezone wetland and tide low-zone wetland according to the composition and the origin features of wetlands.

2 Research Methods

2.1 Sea Level Rise Projection

According to the 5th evaluation report of global sea level rise projected by IPCC in 2013,globally, the sea level rise will rise 0.52-0.98 m by the end of 2100. The sea level rise rate was 0.002-0.003 m/a in Guangzhou City in the history. The future local sea level is projected by the following formula:

WhereHlocalis the relative elevation of local sea level,Hglobalis the relative elevation of global sea level,Tis the predicted year,T0is the base year,Vglobalis the rate of global historical sea level rise,andVlocalis the rate of local historical sea level rise.

2.2 Impact Simulation of Sea Level Rise on Wetland Types

Based on Geographic Information System(GIS), SLAMM model inputs the data to quantitatively analyze the change of coastal land utilization in the sea level rise by using geometrical relationship. Based on RCP 8.5 scenario, SLAMM 6.7 model combined with digital elevation model (DEM) can be used to make simulated prediction of the effects of sea level rise in three phases on the landscape space distribution and area change in Guangzhou coastal wetlands. According to the researches, land areas in different wetlands, can be used as a key estimating factor for the changes of ecosystem serve value.

表5 3個階段不同生態(tài)系統服務類型價值的變化Tab. 5 Change of different ecosystem service types value in three phases

2.3 Impact Assessment of Sea Level Rise on Ecosystem Service

Based on SLAMM output data, ecosystem service value in different types of wetlands was evaluated through a series of methods including marketing value, replacement cost,shadow engineering, benefit transfer and travel cost (Tab. 1), which could be calculated by the following formula:

Where ESVtis the total value of ecosystem services in yeart,Vkiis the service value of unit area of thek-th ecosystem service type of thei-th wetland type,Atiis the area of thei-th wetland type in thet-th year.

3 Results

3.1 Short-Term Impact of Sea Level Rise on Coastal Wetland (2017—2030)

According to the short-term (compare 2030 with 2017) result of SLAMM (Fig. 2-1, 2-2),Nansha and Panyu districts, located in the southern city of Guangzhou, are the most affected areas during the process of sea level rise. Generally,the estuary area will expand and extends to the original inland. The areas of mud flat, inland waters,salt marshes and reed marshes will be decreased sharply, and the distribution of the wetlands shows a fragmentation trend. The most seriously affected wetlands are in Wanqingsha town, Xinken town and Longxian village in Nansha district.

During the short-term, sea level rise will cause the changes of different wetland types (Tab. 2), of which flood marsh and mangrove show the most significant reduction by 449.83 hm2 and 641.21 hm2, respectively, with the corresponding reduction proportion of 73.59% and 81.54%, respectively.Transitional salt marshes and estuary waters show a significant trend of increase, and the transitional salt marshes are the new wetland landscape types emerging in the process of sea level rise, with the area increased by 1,589.62 hm2. And estuary waters will increase by 23,492.87hm2 with the increase proportion of 101.23%. From the perspective of the ecosystem service value (Tab. 3), sea level rise of short-term will cause the reduction of total ecosystem service value by 3,568.191,8 million yuan, with reduction proportion of 28.12%. It is clearly that when sea level is raised by 0.12 m in the short-term, it will cause not only the changes of spatial distribution of the wetlands, but also the changes both in the areas of wetland type and the ecosystem service function. This mainly because the complex factors caused by the sea level rise lead to the gradual reduction or disappearance of the low-lying wetland types. Meanwhile, it will also cause the changes of coastal habitat, which could promote the emergence of new wetland types or increased area of original wetland types, finally leading to a decreasing trend in the total value of ecosystem service.

3.2 Mid-Term Impact of Sea Level Rise on Coastal Wetland (2017—2050)

Based on the mid-term (compare 2050 with 2017) result of SLAMM, Nansha and Panyu districts, in the southern city of Guangzhou, are also the most affected areas during the process of sea level rise. Compared with the short-term results of SLAMM, the area of estuary waters only shows a small amount of expansion changes,and the wetland landscape continues to show a trend of fragmentation distribution (Fig. 2-1, 2-3).

By 2050, sea level rise will further increase or reduce the area of different wetland landscape types (Tab. 2), and the most of the wetlands shows the further increase in area reduction. In terms of proportion of reduction, salt or reed marshes, and mangroves still show the greatest area reduction by 462.56 hm2 and 652.48 hm2, with the corresponding reduction proportion of 75.67% and 82.98%. And these reductions were increased by 2.08% and 1.44%, respectively compared with the reduction in the short-term. From the change of the ecosystem service value (Tab. 3), sea level rise of mid-term will cause the reduction of total ecosystem service value by 3,375.786,3 million yuan with reduction proportion of 26.60%, however,compared to the short-term, total ecosystem service value in the mid-term shows a little increase by 192.405,5 million yuan. The reduced wetlands in ecosystem service value included forest, salt and reed, transitional marshes and mangrove and the land open waters (Tab. 4),of which the salt marshes in transitional zone was decreased significantly. Compare with the short-term (Tab. 5), the service values in fishery production, climate adjust, habitat, recreation,education and scientific research were reduced significantly by the decrease of 3.61% and 3.43%,respectively. It shows that there is no significant change in the spatial distribution of wetland landscape when sea level was increased by 0.36 m,which may indicate that most of the wetlands in this area have been affected during the short-term.However, sea level rise in this phase makes the originally increased values of wetland ecosystem service decrease or the originally decreased values turn to the increasing trend. The possible reason is that the area of wetland landscape has undergone transitional changes which make the fluctuation in the value of ecosystem service.

3.3 Long-Term Impact of Sea Level Rise on Coastal Wetland (2017—2100)

Compared with the short-term and mid-term results of SLAMM, the coastal wetlands in Nansha district and Panyu district are still effected by the sea level rise in the long-term (compare 2100 with 2017). The spatial distribution further indicates that in this scenario, the impact of sea level rise on the coastal lowland wetland in this region has been gradually stable (Fig. 2-1, 2-4).

By 2100, compared with the results of shortterm and mid-term, long-term sea level rise will causes the most significant change in the area of different wetland types (Tab. 2). The areas of forest swamps, salt or reed marshes, mangroves and land open waters continue to decrease, and forest swamps and salt or reed marshes show the most significant reduction, less than midterm by 9.24% and 10.83%. The emerging transitional zones marshes and the increased mudflats in the mid-term were decreased by 8.35 hm2 and 849.45 hm2, respectively, while the estuary waters area was increased by 22.08%. From the perspective of the changes of ecosystem service value (Tab. 3), sea level rise in the long-term will cause the reduction of total ecosystem service value by 4,354.925,9 million yuan, which was decreased by 7.72% compared with the mid-term. The area of the open waters shows a significant increasing trend, which indicates that in this scenario, the transformation between different wetland types is greatly affected by sea level rise.

4 Discussions and Conclusions

The impacts of sea level rise on coastal environment have attracted broad attention of scholars at home and abroad. Objectively assessing the risks caused by the impacts of sea level rise on coastal ecology is the key to propose strategies for rational utilization and development of coastal zone. Based on SLAMM6.7 model, this study imitates the dynamic process of the impacts of sea level rise on coastal wetlands, projecting the impacts of three phases of sea level rise on the changes of spatial distribution and area of coastal wetland landscape in Guangzhou, and quantitatively assessing the value of wetland ecosystem service in three phases.

The results of the study show that the spatial distribution of coastal wetlands in Panyu district and Nansha district of southern Guangzhou will be significantly fragmented by sea level rise in the short-term, while the impacts will be gradually stable during the sea level rise of mid-term and long-term, indicating that in the study area, most coastal wetlands are sensitive to the impacts of sea level rise in the short-term. At the same time, the most areas of different wetlands affected by the sea level rise in 3 phases have been continually on the decline, while part of the coastal wetlands, such as salt marshes and beaches in transitional zones,showed an increasing trend in short-term and a gradual decline trend in mid-term and long-term.It is because of the continued sea level rise and a large number of drowned low-lying wetlands that the estuary water areas are growing.

This research, based on the computer analysis simulation technology, is still limited in realistic complex factor dynamic synthetic function, and the assessment of ecosystem service value has not reached a united standard, which needs to combine multiple scenarios with influencing factors, to improve the current ecosystem service value assessment method.

Sources of Figures and Tables:

Fig. 1 base map from Ministry of Natural Resources of the People’s Republic of China; Fig. 2 credit by the author;Tab. 1-5 credit by the author.