Abstract
Mangrove forests are vulnerable to accelerated sea-level rise associated with climate warming because they occupy a relatively narrow zone on the mid-to-upper-intertidal flats. The fate of these ecosystems largely depends on their capacity to accrete sediment at a rate sufficient to maintain their elevation relative to sea level. We investigated the role of biophysical processes and feedbacks controlling surface-elevation dynamics in a fluvial sediment-rich Avicennia marina mangrove forest (New Zealand) at seasonal-to-inter-annual timescales (over 9 years) using the Rod Surface Elevation Table method. We found that sediment accretion in the forest was not measurably enhanced by episodic and short-lived storm discharges from rivers nor by elevated sea levels during storms. Critically, the coupling of frequent onshore winds and resulting resuspension of intertidal muds, with the fortnightly cycle of spring tide inundation, controlled sediment delivery and resulting accretion rates of 13 to 47 mm y−1. In turn, net surface-elevation trends of 0 to 28 mm y−1 were dominated by the physical processes of sediment accretion and shallow subsidence due to seasonal desiccation and resulting compaction of the infrequently inundated forest platform (4 to 16 mm y−1). Our data suggest that monthly and seasonal variation in tidally controlled hydroperiod and sediment delivery rather than episodic storm events are important for the maintenance of mangrove elevation within the intertidal zone.
Highlights
Global changes in marine environments associated with climate warming are already adversely influencing the capacity of ecosystems to maintain themselves and to provide services valued by society (Gattuso and others 2015)
Sediment accretion in the Firth mangrove forest is controlled by the coupling of frequent onshore winds and resulting resuspension of intertidal muds by small estuarine waves, with the fortnightly cycle of spring tide inundation
These coupled processes regularly deliver fine fluvial muds sequestered in unvegetated mudflats to the fringe forest, scrub and relict fringe forests on the upper-intertidal platform and resulting rapid accretion (13 to 47 mm y-1)
Summary
Global changes in marine environments associated with climate warming are already adversely influencing the capacity of ecosystems to maintain themselves and to provide services valued by society (Gattuso and others 2015). In many estuarine and coastal settings, the potential for mangrove forests to adapt to rising sea levels by landward migration (Egler 1952; Cahoon and others 2009) is constrained by human infrastructure (for example, levee, embankments), resulting in coastal squeeze (Woodroffe and others 2016). Under these conditions, the degree of vulnerability of mangrove forest ecosystems to SLR largely depends on their capacity to accrete sediments at a sufficient rate to maintain their elevation relative to sea level (Friess and others 2012; Woodroffe and others 2016). Root growth and decay and net organic matter accumulation can influence shallow subsidence or expansion (McKee 2011; Krauss and others 2014)
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