Dynamics of marsh-mangrove ecotone since the mid-Holocene: A palynological study of mangrove encroachment and sea level rise in the Shark River Estuary, Florida

Abstract

Sea level rise and the associated inland shift of the marsh-mangrove ecotone in south Florida have raised many scientific and management concerns in recent years. Holocene paleoecological records can provide an important baseline to shed light on the long-term dynamics of vegetation changes across this ecotone in the past, which is needed to predict the future. In this study, we present palynological, X-ray fluorescence, and loss-on ignition data from four sedimentary cores recovered from a 20-km marine-to-freshwater transect along the Shark River Estuary, southwest Everglades, to document the patterns and processes of coastal vegetation changes in response to sea level rise since the mid-Holocene. Our record indicates that freshwater marsh progressively replaced marl prairies at the Shark River Estuary between 5700 and 4400 cal yr BP. As marine transgression continued, marine influence reached the threshold necessary for mangroves to establish at the current mouth of the Shark River Slough at 3800 cal yr BP. During the next 3000 years, although sea level rise in the Western North Atlantic slowed down to 0.4 mm/yr, a spatial and temporal gradient was evident as the marsh-mangrove ecotone shifted inland by 20 km from 3800 to 800 cal yr BP, accompanied by a gradual landward replacement of freshwater marsh by mangrove forest. If sea level continues to rise at 2.33 mm/yr in the 21st century in south Florida, it is possible that marine influence will reach the threshold for mangroves to establish in the central Everglades, and we could expect a much more aggressive mangrove encroachment toward the northern and interior parts of south Florida in the next few centuries.

Changes in Modern Pollen Assemblages and Soil Geochemistry along Coastal Environmental Gradients in the Everglades of South Florida

“…In addition, our pollen results also revealed an interesting point regarding the abundance of Amaranthaceae. Among the 25 surface samples retrieved from two transects, the average and maximum percentage for Amaranthaceae pollen is ∼15% and ∼30% of the total pollen sum, far less than the Amaranthaceae dominated environments (up to 90%) revealed by paleoecological records prior to 2,000 cal yr BP from the Shark River Slough (Willard and Bernhardt, 2011;Yao et al, 2015;Yao and Liu, 2017) and during the last two millennia from the north and central Everglades (Willard et al, 2001(Willard et al, , 2006Bernhardt and Willard, 2009). This discrepancy raises the possibility that the high Amaranthaceae zones in these pollen records represent no-analog vegetation communities.…”

“…The concentration of charcoal fragments in this group is the lowest among all surface samples (<2,000/cm 3 ). This probably reflects long hydroperiod and the absence of fires at these sites since SRS-5 and SRS-6 are 9.9 and 4.1 miles from the Gulf of Mexico and regularly receive tidal flooding Yao and Liu, 2017).…”

“…Superimposed on these environmental gradients are biotic gradients, as defined by a land-to-sea array of vegetation communities. Pollen analysis and X-ray fluorescence (XRF) analysis have been used in paleoecological studies to reconstruct the Holocene history of vegetation changes in the coastal wetlands of the southeastern U.S. (Willard et al, 2001;Donders et al, 2005;van Soelen et al, 2010van Soelen et al, , 2012Yao et al, 2015;Yao and Liu, 2017). However, with few exceptions (e.g., Willard et al, 2001), these reconstructions are not supported by surface samples documenting the changes in modern pollen assemblages and soil chemistry along various environmental gradients in these coastal wetland sites, particularly along the secondary environmental gradient in estuarine wetlands.…”

“…In order to provide essential baseline data for the restoration of the hydrological conditions of the Everglades, numerous studies are being conducted along the coastal zones in the Everglades National Park (ENP) to understand the long-term ecological processes and developmental history of these natural wetlands. Recent paleoecological studies from the region suggest that the vegetation shifts in wetland communities during the middle and late Holocene are closely associated with the regional and globalscale climatic phenomena such as position of the Intertropical Convergence Zone (ITCZ) (Donders et al, 2005), the state of the North Atlantic Oscillation (NAO) (Willard and Cronin, 2007;Bernhardt and Willard, 2009), and the Holocene sealevel rise (Yao et al, 2015;Yao and Liu, 2017). To produce accurate results using fossil pollen analysis, it is important to understand how the modern vegetation and environmental gradients are represented in the modern pollen rain of the study area.…”

Decadal‐scale dynamics of an Amazonian mangrove caused by climate and sea level changes: Inferences from spatial–temporal analysis and digital elevation models

“…Natural processes, which impact on mangroves, are changes in air/water temperature gradients, sea level change, change in salinity by river discharge, variation in sediment and nutrient fluxes and disturbance events such as hurricanes (Blasco et al, 1996;Amaral et al, 2006;McLeod and Salm, 2006;Krauss et al, 2008;Cohen et al, 2012;Alongi, 2015;Liu et al, 2015;Yao and Liu, 2017). Stresses caused by variation in these environmental processes will probably be exacerbated over the 21st century in the face of the ongoing global changes.…”