Abstract
Recent evidence indicates that climate change and intensification of the El Niño Southern Oscillation (ENSO) has increased variation in sea level. Although widespread impacts on intertidal ecosystems are anticipated to arise from the sea level seesaw associated with climate change, none have yet been demonstrated. Intertidal ecosystems, including mangrove forests are among those ecosystems that are highly vulnerable to sea level rise, but they may also be vulnerable to sea level variability and extreme low sea level events. During 16 years of monitoring of a mangrove forest in Mangrove Bay in north Western Australia, we documented two forest dieback events, the most recent one being coincident with the large-scale dieback of mangroves in the Gulf of Carpentaria in northern Australia. Diebacks in Mangrove Bay were coincident with periods of very low sea level, which were associated with increased soil salinization of 20–30% above pre-event levels, leading to canopy loss, reduced Normalized Difference Vegetation Index (NDVI) and reduced recruitment. Our study indicates that an intensification of ENSO will have negative effects on some mangrove forests in parts of the Indo-Pacific that will exacerbate other pressures.
Highlights
Recent evidence indicates that climate change and intensification of the El Niño Southern Oscillation (ENSO) has increased variation in sea level
During El Niño, weak equatorial trade winds cause the thermocline to shoal in the tropical western Pacific and the presence of cool water results in sea levels that can be lower by 20–30 cm, while sea levels are higher in the east[3, 4]
Using unique long-term monitoring of a site at Mangrove Bay in north Western Australia, we provide evidence that extremely low sea levels during recent intense El Niño events[4,5,6,7] have led to mangrove dieback
Summary
The variation in mean sea level over the 16 years of monitoring was negatively correlated with normalized soil porewater salinity (Fig. 3), such that years of high salinity were associated with low mean sea level, indicative of lower levels of tidal inundation. Low sea levels may not cause mortality of mangroves if soil porewater salinity is ameliorated through freshwater inputs from rainfall or river flows, as may have been the case in 2002 in our data set Because both low sea level and low rainfall co-occur during El Niño years in the Indo-Pacific region[26, 27], intensification of ENSO in the coming decades with climate change may be unfavorable for productivity of mangrove forest ecosystems. Mortality of mangrove forests during intense episodic low sea level events may reduce the extent of mangrove ecosystems and lead to enhanced vulnerability to sea level rise as well as the loss of ecosystem functions and services, which include carbon sequestration, nutrient cycling, coastal protection and the provision of habitat for biodiversity
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