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CORAL MORTALITY INDUCED BY THE 2015-2016 El-Niño IN INDONESIA: THE EFFECT OF RAPID SEA LEVEL FALL
Introduction
El Niño-Southern Oscillation (ENSO) is the most important coupled ocean-atmosphere phenomenon impacting climate variability at global and inter annual time scales (McPhaden, 2007). The consequences on coral reefs have been well documented, especially since the 1997-1998 massive coral bleaching event, which reached planetary dimension (Hoegh-Guldberg, 1999). El Niño increases temperature in several coral reef regions and induces zooxanthellae expulsion from the coral polyp, resulting in coral colony looking white, hence “bleaching”. If the situation persists the coral colony eventually dies. Coral bleaching intensity has been related to different temperature thresholds, other environmental factors and stressors, and type of zooxanthellae and corals (Baker et al., 2008). Bleaching episodes due to ocean warming were recorded during the strong 1982-83 El Niño in Australia (Glynn, 2000) and have since been reported worldwide in several instances (Guest et al., 2012; Wouthuyzen et al., 2015). The last bleaching episode has occurred in 2015-2016 during what occurs to be the strongest El Niño event on record (Schiermeier, 2015). Bleaching events were often global in the past, including Indonesia (Suharsono, 1990; Guest et al., 2012; Wouthuyzen et al., 2015). Last reports for Indonesia in 2016 are still under analysis, and survey locations are presented at http://reefcheck.or.id/bleaching-indonesia-peringatan/ (“Reef Check Indonesia,” n.d.). It is thus assumed that coral bleaching induced by ocean warming will be the main culprit if monitoring surveys report post-El Niño coral mortalities.
While in Bunaken National Park in February 23rd – March 5th 2016 for a biodiversity survey, we noticed recent mortalities on the upper part of many massive colonies on several reef flats. This prompted a systematic investigation of the phenomenon spatial distribution. We report here observations on what appears to be the first significant impact of the 2015-2016 El Niño on Indonesia reefs. Unlike what is expected during such a strong event, the mortality was not related to warm water induced-bleaching, but could be tracked to rapid sea level variations. Coral mortality data around Bunaken Island are provided, and we investigate various altimetry and sea level anomaly data sets to explain mortality. The clear link between mortality and sea level fall calls for a refinement of the hierarchy of El Niño impacts and their sequences on coral reefs.
Material and Methods
Bunaken National Park (BNP) is located at the northwest tip of Sulawesi, Indonesia. The location is at the core of the epicenter of marine biodiversity, the so-called Coral Triangle, a vast area spanning Malaysia to Solomon Island, where the number of marine species is maximum (Hoeksema, 2007). BNP includes several islands with Bunaken Island (1.62379°N, 124.76114°E) one of the most studied Indonesian reef site. Bunaken is surrounded by a simple fringing reef system, comprising reef flats, several small enclosed lagoons and forereefs. The tide regime is semi-diurnal, but with marked diurnal inequalities (Ray et al., 2005), with a maximum spring tidal range that can reach 2.52 m.
Two previous BNP surveys for habitat mapping, in May-June 2014 and May-June 2015, did not show any significant signs of widespread mortalities on reef flats. Different species of corals were frequently exposed above water level at low spring tide, yet they were entirely alive (Fig. 11). Microatolls were present. They have not been studied in Bunaken NP, but by similarity with other sites, their growth is likely constrained by a Mean Low Water (MLW), between Mean Low Water Neaps (MLWN) and Mean Low Water Springs (MLWS) (Smithers and Woodroffre, 2000; Goodwin and Harvey, 2008). Several reef flats were characterized by compact communities of massive and semi-massive colonies that could be described as keep-up communities limited in their vertical growth by the MLW (by analogy with the terminology of Holocene reefs provided by Neumann and Macintyre, 1985). In contrast with the 2015 observations, in late February 2016, during a coral biodiversity census survey, we noticed the widespread occurrences of dead massive corals and we performed a systematic investigation on the spatial distribution of the phenomenon. All reef flats around Bunaken Island were visually surveyed and recent mortality was recorded (presence/absence). Geographic coordinates of the presence of mortality were compiled to map its extent. Then, in different locations around the island, mortality was measured on six reef flat locations characterized by high coral cover and different dominant massive coral species, principally Porites lutea and the octocoral Heliopora coerulea, using six 10-meter long Line Intercept Transect (LIT) (English et al., 1997). We recorded the percent cover of live and dead tissue for each coral, the species/genus for each coral, and substrate categories between colonies.
A clear sharp horizontal limit of tissue mortality was present in impacted colonies. The distribution of dead tissue between colonies and among colonies (Fig. 1) suggested that mortality was related to sea level variations, with increased aerial exposure time during the last few months. In order to test this hypothesis, different sea level anomaly products were investigated, based on their temporal coverage and spatial resolution. First, we used gridded altimetry data in terms of Absolute Dynamic Topography (ADT), from the Archiving, Validation and Interpretation of Satellite Oceanographic Data center (AVISO) at the spatial resolution of ¼° and daily temporal resolution. ADT provides the sea level with respect to the geoid. Data is available from 1993 to 2016, allowing a long-term comparison of the sea level trends. The mean ADT over the period were extracted for a small box next to Bunaken Island (1.5-1.7° N; 124.5-124.8° E), a larger box (3 by 3 degrees around the smaller box) centered on Bunaken Island and including the north of Sulawesi and Tomini Bay in the south, and for the entire Indonesia (-14.9-10.0°S, 94.9-140.0°E). The difference between the minimum value (observed in September 2015) and the 2005-2014 mean or the 1993-2016 mean periods were also computed.
Second, to extract geophysical information from higher spatial resolution altimeter data, we used the along-track measurements from SARAL/AltiKa Geophysical Data Records (GDRs) distributed by the AVISO service (http://www.aviso.altimetry.fr/fr/). This data set was chosen because the new Ka-band instrument from SARAL has a finer spatial resolution and enables a better observation of coastal zones (Verron et al., 2015). Data extends from March 2013 (cycle 1 of the satellite mission) to May 2016 (cycle 33), with a repeat period of 35 days. Over this period, we use all altimeter observations located between 10°S-10°N and 105°E-140°E. Two tracks (#535 and #578) intersect the north of Sulawesi island and contain sampling points just off Bunaken Island. The data analysis is done in terms of sea level anomalies (SLA) computed from the 1-Hz altimeter measurements and geophysical corrections provided in GDRs products. The SLA data processing and editing are described in details in Birol and Niño (2015). The 1-Hz SLA data have a spatial resolution of ~7 km along the satellite tracks. In order to quantify the spatial variations of the regional sea level change in March 2013-May 2016, a linear trend model is applied (using a simple linear regression) to the individual SLA time series observed at the different points along the altimeter tracks crossing the area of interest. The trend is the slope of the regression (in cm.y-1). The resulting 3-year sea level trend values can be represented on a map.
Results
Mortality rates per dominant coral genus
For all colonies, dead tissues were found on the top and upper-flank of the colonies, with the lower part of the colonies remaining healthy (Fig. 11). Mortality was not limited to microatoll-shaped colonies only. Round massive colonies were also impacted. On microatolls and other colonies that may have lived close to MLW, the width of dead tissue appeared to be around a maximum of 15 cm. Entirely dead corals were found in the shallowest areas. Dead tissues were systematically covered by turf algae, with cyanobacteria in some cases, suggesting that the stressor responsible for the mortality occurred few months earlier. There were no obvious preferential directions in tissue damage at colony surface as it has been previously reported for intertidal reef flat corals in Thailand (Brown et al., 1994).
Table 10: Mortality levels (mean ± standard deviation, n=6) of all corals for the 6 reef flat sites. The three dominant species were Porites lutea, Heliopora coerulea, and Goniastrea minuta. Several species and genus were found only once. Standard deviation is is not shown when only one measurement per type of coral could be achieved (i.e., one colony per site).
Table of contents :
CHAPTER 1. INTRODUCTION
1.1. The INDESO project
1.2. Coral reefs of Indonesia
1.3. Remote sensing of coral reef habitats and habitat mapping
1.4. Remote sensing of Indonesian coral reefs
1.5. Resilience of coral reefs and mapping resilience
1.6. Change detection of coral reefs using remote sensing
1.7. General research objective& research questions
1.8. Thesis structure
CHAPTER 2. SETTINGS, MATERIAL AND METHODS
2.1. Introduction
2.2. Study site: Bunaken National Park and Bunaken Island
2.3. Field work method for habitat typology
2.4. Training and accuracy assessment points
2.5. Photo-interpretation, digitization, thematic simplification, and final typology of mapped habitats
2.6. Accuracy assessment for habitat mapping
2.7. Multi-sensor image data sets for change detection analysis
CHAPTER 3. REVISITING BUNAKEN NATIONAL PARK (INDONESIA): A HABITAT STAND POINT USING VERY HIGH SPATIAL RESOLUTION REMOTELY SENSED
3.1. Introduction
3.2. Material and Methods
3.2.1. VHR Image
3.2.2. Habitat typology
3.2.3. Habitat mapping for a very high thematic resolution of habitats
3.2.4. Accuracy assessment for a very high thematic resolution of habitats
3.4. Results
3.4.1. Habitat typology
3.4.2. Habitat mapping
3.4.3. Accuracy assessment
3.5. Discussion and conclusion
CHAPTER 4. CORAL MORTALITY INDUCED BY THE 2015-2016 El-Niño IN INDONESIA: THE EFFECT OF RAPID SEA LEVEL FALL
4.1. Introduction
4.2. Material and Methods
4.3. Results
4.3.1. Mortality rates per dominant coral genus
4.3.2. Map of occurrences of mortality
4.3.3. Absolute Dynamic Topography time series
4.3.4. Sea Level Anomaly trends
4.4. Discussion
4.5. Conclusion
CHAPTER 5. ASSESSMENT OF THE RESILIENCE OF BUNAKEN ISLAND CORAL REEFS USING 15 YEARS OF VERY HIGH SPATIAL RESOLUTION SATELLITE IMAGES: A KALEIDOSCOPE OF HABITAT TRAJECTORIES
5.1. Introduction
5.2. Material and methods
5.2.1. Study site
5.2.2. Remote sensing data set
5.2.3. Mining georeferenced historical in situ data
5.2.4. Image and habitat-scenario analyses
5.2.5. Interpretation of changes
5.3. Results
5.3.1. Image time-series quality and correction
5.3.2. Historical data search
5.3.3. Changes for selected polygons
5.4. Discussion
5.4.1. Towards an ecological view of changes using VHR images
5.4.2. A kaleidoscope of habitat trajectories
5.4.3. A kaleidoscope of processes difficult to reconcile
5.4.4. The influence of management and recommendations
5.4.5. Methods for change detection beyond a scenario-based approach
5.4.6. Bunaken, a resilient reef?
5.5. Conclusion
CHAPTER 6. DISCUSSION, PERSPECTIVES AND RECOMMENDATION
