Remote Sensing of Mangrove Forests in Three Regions in Panama: Bahía de Parita, Bocas del Toro Archipelago, and Bahía de los Muertos
Sean Murphy and Teresa Pilon
Department of Geography, University of California - Los Angeles
Sean Murphy and Teresa Pilon
Department of Geography, University of California - Los Angeles
Introduction
Mangrove forests are an essential environment that performs many ecosystem services. “Mangroves are a valuable ecological and economic resource, being important nursery grounds and breeding sites for birds, fish, crustaceans, shellfish, reptiles and mammals; a renewable source of wood; accumulation sites for sediment, contaminants, carbon and nutrients; and offer protection against coastal erosion” (Alongi 2002). Mangrove forests possess characteristics that, in total, make them structurally and functionally unique. This unique structure allows for efficient nutrient retention mechanisms, and the abilities to cope with salt and to maintain water and carbon balance (Alongi 2002). Despite the crucial and recognized role that mangroves play in the health of ecosystems, they are being depleted at a rapid rate. Mangrove ecosystems worldwide are threatened by natural disasters and climate change, as well as anthropogenic pressures, including demand for wood and forest products, land conversion to aquaculture and agriculture, and urban development (Heumann 2011). “For all continents, present-day mangrove forest area is substantially smaller than the original area, with a world average loss of 35%” (Bowen, Valiela, York 2001). One country in which mangrove loss has been significant is Panama. The rate of mangrove forest destruction has been highest on the Caribbean coast of Panama, with mangrove forest area dropping from 20,400 ha in 1980 to 5,900 ha by the mid 1990s, a loss of approximately 71%, and a significantly higher rate than the worldwide loss of 20% (Ellison and Farnsworth 1996). Our study will examine three regions of Panama by using remote sensing technology to map mangrove forests present in 1986, 1999, 2007, and 2013, with the purpose of showing the rate of mangrove loss in Panama since the start of the 21st century. With knowledge of the reasons for degradation of mangrove ecosystems around the globe, we hope to determine if climate and direct anthropogenic interferences might be causing this loss in mangrove density. We also hope to compare protected areas to non-protected areas to see how effective Marine Protected Areas are in protecting mangrove populations.
We examine two primary research questions in this study. First, we will test the hypothesis that there has been a reduction in area. We will test this hypothesis with imagery from two different time periods per study area to show the extent of mangrove forests. Then, we will evaluate marine protected areas, land use changes over area and time, and change in surrounding vegetation to determine if these direct anthropogenic interferences have a significant effect on the extent of mangrove ecosystems and their recent rapid decline.
Study Area
Our research studies three coastal areas in Panama that support mangrove forests. Each of these areas plays an important role in Panama’s economic and/or ecological history Our first study area is the coast along Bahía de Parita. The Bahía de Parita is part of the Gulf of Panama in the Pacific Ocean. Our study area focuses on approximately 62 kilometers of coastline (measured on Google Earth). The majority of this coast falls into the province of Coclé, with some extension into the province of Herrera and a very small region of Los Santos. We chose this region because of its historic and current ties to the aquaculture industry. Coclé and Herrera have the largest areas of aquaculture in Panama. The aquaculture in this region is marine shrimp culture, primarily raised in earthen ponds (Pretto Malca, R 2005). Large scale aquaculture in this region has its origins in 1972, when the national government researched and implemented plans to introduce more animal protein into the diet of Panamanians, especially those in rural areas (Pretto Malca, R 2005). In 1974, Agromarina de Panama began an extensive shrimp culture project in the Aguadulce district in the Province of Coclé (Pretto Malca, R 2005). This study region contains this coastal region of the Aguadulce district. Since this time period, aquaculture has been an important national undertaking, with multiple programs and projects including the “Agro-Aquaculture Program for Food Production and Community Development in Marginal Communities” in 1986. We chose this region with the intent to determine any reduction in mangrove forests since 1986 in an area that is heavily used in and important to Panama’s aquaculture industry.
Our second study area is the Bocas del Toro Archipelago. These islands are in Caribbean Sea, approximately 16 kilometers off the mainland of Panama, in the northernmost region of Panama bordering Costa Rica. The Bocas Archipelago contains Isla Bastimentos National Marine Park. Our study region is approximately 432 square kilometers of island and ocean area and contains Isla Bastimentos National Marine Park. Isla Bastimentos National Marine Park was established by the national government in 1988 and the Bocas del Toro Archipelago has been a popular tourist destination ever since tourism was declared a national priority by Panamanian presidents in the 1990s (Die 2012). Tourism is currently the second highest sources of revenue for Panama, earning up to $2.27 million in 2010, with the Bocas del Toro Archipelago highly promoted and frequented as a tourism destination (Die 2012). Much of the tourism on the archipelago is centered around the unique marine life and ecosystems and is showcased to tourists via boat tours, water taxis, and snorkeling and diving (Die 2012). These high impact recreational activities can cause direct damage to delicate ecosystems including physically breaking or altering the ecosystems and pollution from boats and tourist trash. We chose the Bocas del Toro Archipelago to determine if this tourism has an effect on mangroves that is visible at this scale and how the marine protected area of Isla Bastimentos affects mangrove cover in the region.
Our third study region is a coastal region along the Pacific in the province of Chiriquí. This area has an especially high density of mangrove forest surrounding the Bahía de los Muertos in the eastern portion of the study area and a smaller inlet of water in the western portion of the study area. Bahía de los Muertos and the accompanying mangrove forests along the coast extend farther inland than our other two study regions. The study region lies directly south and southeast of the major city of David and the port city of Pedregal, but the rest of the region is primarily forest and land that has been converted to agriculture. The westernmost area of mangrove forests in this study area contains Playa la Barqueta Agrícola Wildlife Refuge, 62.39 square kilometers established as a marine protected area in 1994 as a nesting site for marine turtles (http://www.mpaglobal.org/). In all, this study area contains approximately 1,338 square kilometers of land and ocean area. A key reason for choosing this as our third study area is that it is directly south of the Bocas del Toro Archipelago and Isla Bastimentos National Marine Park. Known differences in mangrove species and especially in human activity on the Pacific and Caribbean sides of Panama led us to focus on this coast, which is a critical/endangered ecosystem (Burdette and Tognetti 2013). A visual analysis of Panama’s coastline shows that this region contains the most extensive area of mangroves on the coast of the province of Chiriquí.
Reference map:
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| Our study focuses on three areas along the coast of Panama. Each of these areas supports mangrove ecosystems and plays an important role in Panama's economic and/or ecological history. The region in the top left is the Bocas del Toro Archipelago, the region in the bottom left is the mangrove forest around the city of David and Bahía de los Muertos, and the region in the center is the coast of Bahía de Parita. |
Methods
Our research uses four Landsat images; images of two scenes from two different years each. Our first images investigate the Coclé and Herrera region of Panama in 1986 and then again in 2000. The first image of Coclé/ Herrera is from December 2, 1986 and was captured by the satellite Landsat 5 using the Thematic Mapper sensor. Our “after” image of Coclé is from March 27, 2000 and was also captured by Landsat 5 using the Thematic Mapper sensor. Our second set of images are studying a portion of the Bocas del Toro and Chiriquí regions. The first image is from January 19, 1999 and was captured by the Landsat 5 Thematic Mapper. The second image from October 7, 2013 and was taken by the Landsat 8 using the OLI_TIRS sensor. Within these study sites we specifically looked at a few subsets. For the Coclé/Herrera region we studied a portion of the coast bordering Bahía de Parita. In the Bocas/ Chiriquí region we looked at the Bocas del Toro Archipelago and a region on the Pacific Coast surrounding Bahía de los Muertos.
Because of the tropical climate of mangrove ecosystems and this region’s proximity to the equator, cloud coverage is an inevitable challenge is obtaining useful images for our research. The images from these dates provide relatively cloud free views of our study area. This time period also shows recent and relevant change, which is essential for understanding what is going on in the ecosystem currently.
Because of the tropical climate of mangrove ecosystems and this region’s proximity to the equator, cloud coverage is an inevitable challenge is obtaining useful images for our research. The images from these dates provide relatively cloud free views of our study area. This time period also shows recent and relevant change, which is essential for understanding what is going on in the ecosystem currently.
In order to investigate mangrove populations of our chosen study area we used the following analysis tools in ENVI Classic. For the Coclé/Herrera region, we first viewed our images in normal color with a linear 2% enhancement applied to clearly display the mangroves present. The images were then shown using the band combination of near-IR (Band 4), mid-IR (Band 5), and red (Band 3) with a linear 2% enhancement applied on the image. This combination allows for mangroves to be more easily detected as they are presented in a striking deep orange color. We then ran a Normalized Difference Vegetation Index to study the photosynthetic activity and quality of the mangrove forests. A density slice was applied including all values from 0.00001 to 1 to show how greenness has decreased between 1986 and 2000. Lastly we displayed the images with a density slice including values from 0.50 to 0.75 to just show how mangrove populations have been reduced.
For the Bocas/ Chiriquí region we observed the images in normal color. The images of the Bocas del Toro Archipelago and the Pacific Coast from 1999 had a linear 2% enhancement applied while the 2013 image had an equalization enhancement applied. The images from 1999 are shown in the 4, 5, 3 band combination with linear 2% enhancements applied. The image from 2013 is shown in the 5,6, 4 band combination with a linear 2% enhancement applied. Images of the Bocas del Toro Archipelago have the shapefile of the Isla Bastimentos National Marine Park overlayed to show how effective the park has been in protecting mangrove populations. Within our region of the Pacific Coast is the Playa la Barqueta Agrícola Wildlife Refuge. To show how this marine protected area has preserved mangroves a shapefile of the park is included in our images.
For the Bocas/ Chiriquí region we observed the images in normal color. The images of the Bocas del Toro Archipelago and the Pacific Coast from 1999 had a linear 2% enhancement applied while the 2013 image had an equalization enhancement applied. The images from 1999 are shown in the 4, 5, 3 band combination with linear 2% enhancements applied. The image from 2013 is shown in the 5,6, 4 band combination with a linear 2% enhancement applied. Images of the Bocas del Toro Archipelago have the shapefile of the Isla Bastimentos National Marine Park overlayed to show how effective the park has been in protecting mangrove populations. Within our region of the Pacific Coast is the Playa la Barqueta Agrícola Wildlife Refuge. To show how this marine protected area has preserved mangroves a shapefile of the park is included in our images.
Coclé/Herrera study region
Coclé/Herrera study region in 1986 and 2000, displayed in normal color, linear 2% enhancement:
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Coclé/Herrera study region in 1986 and 2000, displayed in 4-5-3, linear 2% enhancement:
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Coclé/Herrera study region in 1986 and 2000, Panchromatic Normalized Difference Vegetation Index, positive values:
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Coclé/Herrera study region in 1986 and 2000, Panchromatic Normalized Difference Vegetation Index, values 0.5 to 0.75:
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Close up images of these mangroves and aquaculture ponds along Bahía de Parita in 2013 and 2007:
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| Close up normal color image of mangrove forests and aquaculture ponds from the coast of Bahía de Parita, around the center of the study area on April 9, 2013, obtained from Google Earth. Notice that many of the aquaculture ponds are empty as they are only productive for a certain amount of years. These empty/used aquaculture ponds have not been returned to mangrove forest, which indicates how severely this industry changes the natural landscape. |
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| Close up normal color image of mangrove forests and aquaculture ponds from the coast of Bahía de Parita, around the center of the study area on April 9, 2013, obtained from Google Earth. The eye altitude of 11,511 feet and the high oblique angle of the image show a different and important perspective on the distribution of mangrove forests and aquaculture ponds in this region. |
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| Close up normal color image of mangrove forests and aquaculture ponds from the coast of Bahía de Parita, around the center of the study area on February 13, 2007, obtained from Google Earth. Notice how significantly aquaculture ponds interrupt previously continuous swaths of mangrove forest, creating more areas of edge and less areas of deep forest. |
Bocas del Toro study region
Bocas del Toro study region in 1999, displayed in normal color, linear 2% enhancement, and again in 2013, displayed in normal color, equalization enhancement:
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Bocas del Toro study region in 1999, displayed in 4-5-3, linear 2% enhancement, and again in 2013, displayed in 5-6-4, linear 2% enhancement:
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Close up images of these Bocas del Toro mangroves and surrounding water and land in 2013: |
| Close up normal color image of mangrove coasts and islands in our Bocas del Toro study area on April 9, 2013, obtained from Google Earth. This image contains the land and ocean of the Isla Bastimentos Marine Park. |
Coast of Chiriquí study region
Coast of Chiriquí study region in 1999, displayed in normal color, linear 2% enhancement, and again in 2013, displayed in normal color, equalization enhancement:
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Coast of Chiriquí study region in 1999, displayed in 4-5-3 band combination, linear 2% enhancement, and again in 2013, displayed in 5-6-4 band combination, linear 2% enhancement:
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Close up images of mangroves along the Chiriquí coast in 2011 and 2013:
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| The coast of Chiriquí study region on January 24, 2011, obtained from Google Earth. The Playa la Barqueta Agrícola Wildlife Refuge is in the foreground and Bahía de los Muertos is in the background. Notice how the port city of Pedregal comes into the mangrove forest but the mangrove forest around the city is still intact and comes significantly inland despite its proximity to urban development. |
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| The coast of Chiriquí study region on December 9, 2013, obtained from Google Earth. In the foreground is Bahía de los Muertos and the surrounding mangrove forests and in the background is Playa la Barqueta Agrícola Wildlife Refuge and the port city of Pedregal. The mangroves easily stand out from the surrounding land cover in this oblique image. Notice how the land bordering the mangroves in the foreground is not very green, probably indicating deforestation and/or conversion to agriculture land on this side of the study region. |
Results
Our images of the Coclé/ Herrera region show clear evidence that mangrove populations and overall greenness has been greatly reduced between 1986 and 2000. Our images also show that aquaculture has been expanding along the coast. This drastic reduction in greenness is especially clear in the density slices of the NDVI. The first density slice including values from 0 to 1 shows greener areas ranging from 0.25 to 0.50 in Magenta and less green areas ranging from 0.00001 to 0.25 in Cyan. The image from 2000 shows a much larger area in Cyan than the previous 1986 image, indicating a decrease in overall greenness. The NDVI with the overlay of a density slice showing values from 0.50 to 0.75 only displays specifically mangroves (and inland vegetation) in maroon. These images perceptibly display a great reduction in mangrove populations.
Images of the Bocas del Toro and Chiriquí regions are not as conclusive. Especially in the Bocas del Toro Archipelago there does not appear to be much of a reduction in mangrove populations. Our images from the coast of the Chiriquí show a slight decrease in mangrove populations. There is also a noticeable change in the river patterns visible in our images from 1999 to 2013 that could be negatively impacting mangrove populations. However, this study does not investigate this question and we cannot conclusively say if this is a factor affecting mangroves in this area.
Discussion
In conclusion, it can be stated that in our study site of the Coclé/ Herrera region mangrove populations and overall greenness have been reduced. We can attribute these losses to a few main factors. First is the expansion of aquaculture ponds along the coast. There are currently 1.89 ha of marine shrimp cultures in Coclé and it is also a major area of commercial tilapia cultures. As our accompanying Google Earth images show, aquaculture ponds are built right in the midst of mangrove forests and cause great destruction. In addition to degradation of mangrove forests, the aquaculture ponds also fragment sections of mangrove forests. Habitat fragmentation is a significant problem because it isolates populations of vegetation and animals, reducing their ability to come into contact with one another and raising the probability of genetic depression or an external event damaging the populations. Habitat fragmentation such as this also creates an increased edge effect, exposing more areas of mangrove forest to the outside environment which increases the rate of habitat degradation and change. Other causes of mangrove and greenness reduction are urbanization, agricultural conversion, and overgrazing due to cattle ranches.
The Bocas del Toro Archipelago does not appear to be an area where mangrove populations are being depleted. Although it is a major tourist hotspot and the primary tourist region of Panama, this does not appear to be negatively impacting the mangrove forests of the region. This lack of reduction of mangrove forest populations could be attributed partially to the Isla Bastimentos Marine Park. The marine park prevents the main causes of mangrove destruction. In the Chiriquí area we investigated there appears to be a slight decrease in mangrove forests. Also, the Pacific Coast of the Chiriquí region is classified as critical/endangered habitat area by the World Wildlife Fund. Although the area we investigated appeared to only have a small decrease in mangrove coverage, the greater Pacific coast of Panama is experiencing mangrove loss. The main threats to mangroves on the Pacific coast are conversion to salt flats, crops, cattle farming, tidally flooded land, non-vegetated land and agriculture.
In the Coclé/ Herrera study area the NDVI was the most helpful and successful analysis tool. It clearly supported our research hypothesis by showing a great reduction of overall greenness as well as reduction specifically of mangroves. The normal color images with the 2% enhancement applied were also successful in showing greenness reduction and the expansion of aquaculture ponds. The clarity of the image as well as the brightness and enrichment added by the enhancement added visual support to our hypothesis. Studying the image in the 4,5,3 band combination was also supportive of our hypothesis. These images allow mangroves to stand out in a deep orange color. With the application of the linear 2% enhancement the images become even clearer in showing that mangrove populations are declining in our area of study in the Coclé/ Herrera region.
In the Bocas del Toro Archipelago our analysis tools were not helpful in supporting our hypothesis. There was not a clear reduction in mangrove populations displayed by any of our image analyzations. In the Chiriquí region the 4,5,3 and 5,6,4 band combinations with the linear 2% enhancement applied was the most successful analysis tool in proving our hypothesis to be true. This combination pulled the mangroves out by making them more visually striking in a deep orange color. The enhancement then brightened and sharpened the image to allow the slight decrease in mangrove forest populations to be visualized.
As remote sensing technology advances, it is becoming a more effective tool in studying mangrove habitats. Our project referenced several other scientific papers that detailed how remote sensing was used in studies of mangroves. However, our study is the first to use remote sensing to map out mangrove forests in our chosen regions of Panama.
In the future, remote sensing technologies should continue to be used to explore mangrove forest losses, both in Panama and around the world. Mangroves are a vital ecosystem that must be protected. Identifying areas where mangrove forest reduction is greatest can help to find regions that should be protected in the future. Considering human needs is also essential to conserving mangrove ecosystems. Conservation driven research of these specific study areas should focus on ensuring that tourism in Bocas del Toro has minimum impact on the mangrove forests and identifying and implementing sources of protein for the diets of rural Panamanians that are more environmentally sustainable than aquaculture in mangrove ecosystems.
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