Remote sensing analysis of mangrove distribution in Beibu Gulf

(Guangzhou Marine Geological Survey Guangzhou 5 10760)

Brief introduction of the first author: Dr. Li, a professor-level senior engineer, has been engaged in marine geology for a long time, e-MIL: xuejieli @ yeah.net.

Mangrove is a unique plant community growing in tropical and subtropical intertidal zone, which not only plays an extremely important role in protecting the coastal zone, but also is one of the ecosystems with the richest biodiversity, so it has been widely concerned. In this paper, the distribution of mangroves in Beibu Gulf was analyzed by remote sensing method and field route investigation and verification. The results show that there are abundant mangrove resources along the Beibu Gulf (1), which are distributed from Beilun Estuary in the west to Anpu Port in the east. The total distribution area of mangroves in the study area is 7 172 hm2, of which 633 1 hm2 is located in China and grows well. (2) The Beibu Gulf coast is divided into 10 regions. There are five districts with an area over 700 hm2: Beilun Hekou District, Pearl Harbor District, Maoweihai-Qinzhou Port Area, tieshangang district and Dandouhai-Yingluo Port Area. The smallest distribution area is Yintan-Bailonggang District in Beihai, with an area of only 2 17 hm2. (3) In terms of distribution density and fragmentation, Dandouhai-Yingluo Port Area, Pearl Harbor Area and Beilun Estuary Area with the best distribution have good fragmentation, high density and good growth.

Remote sensing; Mangroves; Coastal wetlands; Beibu Gulf

1 overview

Mangrove is a woody plant community composed of evergreen shrubs and trees, which grows in tropical and subtropical low-energy coastal intertidal zones and is submerged by periodic tides [1]. Mangroves mainly grow on muddy beaches in harbors, estuaries and tidal passages, and the environment is between land and sea. They are one of the most abundant, productive and valuable ecosystems on the earth, and play an irreplaceable role in the global ecological balance [2-4]. At the same time, mangroves can alleviate the scouring of the coast by waves and tides and play a very good role in protecting the coastal zone [5], so coastal mangroves have received extensive attention.

Mangroves in China are mainly distributed in Hainan, Guangxi, Guangdong, Fujian, Taiwan Province and the southern coastal areas of Zhejiang. According to the data of Lin Peng et al. (1995), the total mangrove area in China in 1990s was 1.5× 104 hm2. The total mangrove area in the three provinces is 2 1389 ha, accounting for 94.48% of the total mangrove area in China [6].

Mangroves are not only widely distributed in Beibu Gulf [7-8], but also deeply studied [9], involving mangrove community and its ecological environment [10- 13], the relationship between mangrove growth and soil [14- 15], and the protective effect of mangrove on coastal zone [/] However, because mangroves are distributed in intertidal zone, it is difficult to investigate their distribution range. Remote sensing can provide a reliable and convenient method for coastline identification [19-20], mangrove distribution [2 1-23] and coastal environmental management [24-25].

2 data and methods

2. 1 data

We use the multi-spectral data of ALOS satellite remote sensing for interpretation and analysis, and the whole area needs 9 scenes for stitching (Table 1). ALOS data includes four bands with a resolution of 10 meter.

Due to the weather conditions in the first half of the study area, it is difficult to find suitable images, so the images in the second half of 2008 are mainly used here. For a scene, use the image from 2007, because no suitable image was found in 2008 (table 1).

Table 1 remote sensing image data information table

2.2 methods

The methods of wetland remote sensing interpretation include: geometric correction, image stitching and cropping, supervised classification, post-processing of classification, interactive interpretation of visual estimation and field investigation and verification (Figure 1).

2.2. 1 geometric correction

Due to many factors, the position of pixels in remote sensing images is different from that in map coordinates, resulting in geometric distortion. Before application, geometric correction is needed to eliminate or reduce this geometric distortion as much as possible.

The geometric correction of this image is based on the topographic map of 1:50000, and it is corrected by quadratic polynomial. The root mean square error method requires less than 1 pixel (Table 2).

Table 2 Statistical table of geometric correction errors of remote sensing images

Figure 1 technical concept diagram

In addition,1:50,000 topographic map is used for remote sensing image correction. Topographic map is Beijing 1954 coordinate system, Gaussian Kruger projection and 1985 national elevation datum.

2.2.2 Image stitching and cropping

Nine ALOS images in the study area were obtained at four time points (table 1). Due to the influence of weather, season and vegetation, the DN values of images obtained at different times are obviously different, and the mosaic classification effect is not good. Therefore, images obtained at the same time are used and classified after stitching. Images at different times are classified separately, and then all of them are spliced.

2.2.3 Supervision classification

According to the wetland classification system determined in this paper, remote sensing images are supervised and classified by ERDAS Imagine software. Compared with unsupervised classification, supervised classification clarifies all kinds of contents before classification, and the classification effect can be improved by adjusting the sampling of training set.

2.2.4 Post-processing of classified images

The results of supervised classification are mainly re-calibrated by manually interpreting the extracted coastline. For example, mangroves are distributed outside the coastline, and woodland and cultivated land are distributed inside the coastline. Rivers, lakes and ponds are distributed within the coastline, while oceans and beaches are distributed outside the coastline.

2.2.5 Field investigation and verification

In order to verify the remote sensing interpretation results, a field wetland survey and verification was conducted for a period of 1 1 day. From Beilun Estuary in the west to Anpu Port in the east, a field survey was conducted in the coastal zone of the whole region, and the field route survey was completed at 1763 km, and 526 stations were observed, including 36 mangrove stations.

3 Distribution characteristics of mangrove forests in Beibu Gulf

According to the interpretation results of ALOS images in 2008, mangroves developed along the Beibu Gulf, with different degrees of distribution from Beilun Estuary in the westernmost part to Anpu Port in the easternmost part (Figure 2), with an area of 7 172 hm2, of which 633 1 hm2 is located in China. In order to understand its distribution characteristics and changes, Beibu Gulf is divided into 10 regions for description.

Fig. 2 Distribution Map of Mangroves in Beibu Gulf

3. 1 overall distribution

Generally speaking, Beibu Gulf is rich in mangrove resources, which is called one of the three mangrove distribution areas in China [26]. Among them, Dandouhai-Yingluo Port Area (Area 9) in the east is the location of Shankou Mangrove Reserve, with the most developed mangroves, and the existing mangrove distribution area is 1286 hm2. Followed by Beilun Hekou District (1 District) in the west, Pearl Harbor District (District 2) and tieshangang district City (District 8) in the east, with distribution areas of 9 1 1 hm2, 864 hm2 and 836 hm2 respectively (Figure 3).

3.2 Regional distribution

3.2. 1 beilun estuary (1 area)

There are abundant mangroves in Beilun Estuary, mainly distributed in the south bank of Beilun River (Figure 4), with wide beaches and patches of mangroves, mainly belonging to the border of Vietnam. It is also distributed in the north bank of Beilun River, but the beach is narrow and the mangroves are scattered, mainly in the west of Zhushan Street and the north of Rongshutou in the southeast of Jiebian Village, that is, south of Chen Wu. The mangrove area interpreted by remote sensing in this area is 9 1 1 hm2, of which 74 1 hm2 is under the jurisdiction of Vietnam, which is one of the most densely distributed mangrove areas in Beibu Gulf.

3.2.2 Pearl Harbor Area (Area 2)

This area is also one of the most densely distributed mangrove areas, mainly distributed in the north of Pearl Harbor, with a width of 400 ~ 1700 m, and basically distributed continuously from Xiajiabang in the west to Kexi in the east (Figure 5). On the east and west sides of Pearl Harbor, there are few mangroves, only scattered. The mangrove area interpreted by remote sensing is 864 hm2.

Fig. 3 Mangrove area in each district of Beibu Gulf (hm2)

Fig. 4 Distribution of Mangroves in Beilun Estuary

Figure 5 Distribution of Mangroves in Pearl Harbor Area

Fig. 6 Distribution of Mangroves in Fangchenggang Area

Fig. 7 Distribution Map of Mangroves in Maoweihai-Qinzhou Port Area

3.2.3 Fangchenggang District (Area 3)

Mangroves in this area are much less than those in Beilun Estuary and Pearl Harbor (Figure 6). There are mainly two areas with patchy distribution: (1) south of needlefish, with a distribution width of 200 ~ 500 m; ; (2) In the west of the port area, the distribution width is 200 ~1000 m. According to the field investigation, this mangrove forest grows well. The rest are scattered, with a total area of 559 hm2.

3.2.4 Maoweihai-Qinzhou Port Area (Area 4)

The area is large, but there are relatively few mangroves, and most of them are difficult to distribute in a large area (Figure 8). There are patches of mangroves in Qinjiangkou, Zhu Chong Kou and Dushikou at the top of Mao Wei Bay, and there are patches of mangroves among the islands of Longmen Port in the west of Qinzhou Port Economic Development Zone. In addition, there are scattered mangroves in the west of Jingujiang River, Maoweihai and Longmen Port, with a total area of 796 hm2.

3.2.5 Dafeng Jiangkou District (Area 5)

There are also some mangroves near Dafeng Estuary, which are distributed in the west of Xintian, south of Shajiao, southeast of Shipyard Street and west of Dawangshan (Figure 9). In addition, there are many mangroves in the estuary, such as Laokedun, Binglangdun and Laoyadun. The total area is 535 hm2.

Fig. 8 Distribution of Mangroves in Dafeng River Area

3.2.6 Nanliu Jiangkou District (Area 6)

The Nanliujiang River estuary delta area is suitable for mangrove growth because of its small terrain slope and wide beach distribution. Mangroves in this area are mainly distributed in the south of Datouping-Yudun, the southeast of Xintian and the south of Gaodi Box. In addition, it is to the west of Caotou Village, to the west of Tianliao and to the south of Sigutian (Figure 10), with a total area of 44 1 hm2.

3.2.7 Beihai Yintan-Bailonggang District (Area 7)

There are relatively few mangroves in this area, with a total area of 2 17 hm2. The main distribution areas include Xiacun-south of Daguansha, north of Xicun Port and south of Zhu Lin Yantian (Figure 10).

3.2.8 tieshangang district (Area 8)

Tieshangang district is one of the more developed mangrove areas (figure 1 1), mainly distributed in Longyan Cave-Hongshitang-Crab Farm, Bay Corner-Seamount, Goose Neck Wharf in the east, Egg Pond and Xincun in the north of Tieshan Port. , and basically distributed in patches, with less distribution in the west of Tieshan Port. The total area is 836 hm2.

3.2.9 Dandouhai-Yingluo Port Area (Area 9)

Mangroves are well developed in this area, including Shankou National Mangrove Ecological Nature Reserve (Figure 12).

Fig. 9 Distribution of Mangroves in Nanliujiang Estuary

Figure 10 Distribution of Mangroves in Yintan Port-Bailonggang District of Beihai

Yamaguchi National Mangrove Ecological Nature Reserve is one of the first (five) national marine reserves in China approved by the State Council in September 1990. In June, 438+0993, it joined China Man and Biosphere, and in June, 438+0994, it was listed as an important wetland in China. In May, 1997 established a friendly reserve relationship with Lockley Bay National Estuary Research Reserve in Florida, USA. 56686.6886868866 1

Mangroves in this area are typical representatives of mangroves on the land and sea coasts of China. They are natural mangroves with good development, unique structure, large contiguous area and relatively complete preservation.

In terms of distribution, there are large areas in the north and east of Dandou Sea and the north and west of Yingluo Port, with a total area of 1286 hm2.

Figure 1 1 Mangrove Distribution Map of tieshangang district

3.2. 10 Anpu Port Area

Anpu Port is located at the eastern end of the investigation area, and mangroves are mainly distributed in the estuary between Yingzai River and Kyushu River, south of Kyushu Estuary and near Xiadoucang, with good patches, especially in Lianghekou District (Figure 13). The rest areas are rarely distributed, with a total area of 549 hm2.

Figure 12 Mangrove Distribution Map of Dantuhai-Yingluo Port Area

Figure 13 Mangrove Distribution Map in Anpu Port Area

4 conclusion

Using remote sensing method, combined with field route investigation and verification, the distribution of mangroves in Beibu Gulf was analyzed, and the following conclusions were obtained:

(1) There are abundant mangroves along the Beibu Gulf, starting from Beilun Estuary in the west and Anpu Port in the east. The total distribution area of mangroves in the study area is 7 172 hm2, of which 633 1 hm2 is located in China and grows well.

(2) The Beibu Gulf coast is divided into 10 regions. There are five districts with an area over 700 hm2, namely Beilun Hekou District, Pearl Harbor District, Maoweihai-Qinzhou Port Area, tieshangang district and Dandouhai-Yingluo Port Area, which are 9 1 1 hm2, 864 hm2, 796 hm2, 836 hm2 and 65438 respectively.

(3) In terms of distribution density and fragmentation, Dandouhai-Yingluo Port Area, Pearl Harbor Area and Beilun Estuary Area with the best distribution have good fragmentation, high density and good growth.

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Remote sensing study on mangrove distribution in Beibu Gulf

Li and Wan Rongsheng

(Guangzhou Marine Geological Survey, Guangzhou, 5 10760)

Abstract: Mangroves grow in tropical-subtropical intertidal zone, which plays an important role in coastal protection and is one of the ecosystems with the highest biological activity. Based on satellite remote sensing and field investigation, the distribution of Man Garoff in Beibu Gulf was analyzed. The results show that there are a large number of mangroves along the coast of Beibu Gulf, starting from Beilun Estuary in the west and ending at Anpu Port in the east, with a total area of 7172m2, of which 633 1 hm2 is located in China. Divided into 10 partitions for statistical analysis. In Beilun Estuary, Zhugang, Weihai-Qinzhou Port, Tieshan Port and Dandouhai-Luo Yin Port, the distribution area of mangroves is over 700hm2, with 9 1 1 hm2, 864 hm2, 796 hm2, 836 hm2 and 1286 hm2 respectively. From Dandong to Luo Yin Port, Zhugang and Beilun Estuary grow best, with high density and good continuity.

Keywords: remote sensing; Mangroves; Coastal wetlands; Beibu Gulf