Vegetation succession of low estuarine marshes is affected by distance to navigation channel and changes in water level

Abstract

Climate change and engineering activities have modified the hydrology and morphology of estuaries. However, the potential effects of these modifications on vegetation succession in estuarine marshes are still poorly understood. Therefore, we studied temporal changes in tidal habitats of the Elbe estuary over a period of 30 years. We compared vegetation maps from 1980 to 2010 and calculated the change in area of habitats with respect to three salinity and three elevational zones. To analyze the direction of the temporal change, we differentiated between progressive and regressive succession. By using regression tree models (conditional inference trees), we identified the most influential factors determining progressive or regressive succession of low marshes. The total area of the estuarine tidal marshes at the Elbe increased by 2 % from 1980 to 2010, but changes were unequal among the salinity zones. In the salt and brackish zones, the area covered by high marshes increased substantially but decreased in the tidal freshwater zone, while that covered by low marshes decreased in all the salinity zones. Additionally, we determined high persistence of tidal flats and high marshes, whereas only 19 to 28 % of the low marshes found in 1980 remained in 2010. In salt and brackish marshes, more than two-thirds of the area that had been identified as low marshes in 1980 had progressively developed into high marshes. In contrast, 44 % of the area of low marshes in tidal freshwater marshes showed regressive succession back into tidal flats. The distance to the navigation channel was the main factor determining successional direction in salt and brackish marshes. Here, greater proximity to the channel was correlated with higher risk of regressive succession. In tidal freshwater marshes, we identified both the distance to the navigation channel and the situation on the river shore (i.e. inner bank, outer bank or straight bank) as the main factors for marsh succession. Here, considerable engineering activities in the channel had simultaneously decreased the mean low water level and increased the mean high water level between 1980 and 2010, which led to an increase in tidal amplitude. It is quite likely that these changes negatively modified marsh distribution, increased regressive succession and, thus, lowered the quality of tidal freshwater marshes.

Detecting the Morphology of Prograding and Retreating Marsh Margins—Example of a Mega-Tidal Bay

“…Rather, the drivers of marsh edge evolution are external forcings such as tidal creek meandering that force retreat processes. Consequently, retreating platforms may prograde again as tidal creek thalwegs move away from them, as suggested by [36]. By extension, we infer that Moricambe Bay has sufficient sediment supply to support rapid infilling and conversion of the bay to marshes were it not for the action of meandering creeks.…”

“…2020, 12, 13 2 of 26 constriction of salt marsh habitat [29][30][31], as well as the mutual interaction between wave impact, retreat processes and the morphology of retreating marsh margins [32][33][34]. While marsh retreat is demonstrably linked to nearby channel deepening in a macro-tidal setting [35,36], the action of tidal currents on marsh margins remains poorly understood relative to wave action.Likewise, remote observation of salt marsh margins are scarce in the literature, in contrast with the wealth of documentation on the use of light detection and ranging (lidar) and hyperspectral data to characterise marsh platform elevation and vegetation [37][38][39][40]. This knowledge gap hampers our understanding of present coastal mobility in general but also our predictions of the future retreat or advance (which we refer to as progradation) of salt marshes.…”

“…2020, 12, 13 2 of 26 constriction of salt marsh habitat [29][30][31], as well as the mutual interaction between wave impact, retreat processes and the morphology of retreating marsh margins [32][33][34]. While marsh retreat is demonstrably linked to nearby channel deepening in a macro-tidal setting [35,36], the action of tidal currents on marsh margins remains poorly understood relative to wave action.…”

Population of the Paraná AntwrenFormicivora acutirostrisreassessed as being 64% smaller: Revision of its conservation status and assessment of its Green Status, with management proposals

“…This may have occurred due to increased tidal height, which could have raised the height and duration of flooding and increased the salinity of the environment, killing trees and increasing light penetration into the lower stratum. This phenomenon has occurred in other parts of the world, with formations adjacent to mangroves and other estuarine formations experiencing vegetation mortality and regressive succession (Butzeck et al . 2016).…”

Plant Community Stability over 40 Years in a Fraser River Estuary Tidal Freshwater Marsh

“…In a time of rapid global change, temporal shifts in plant community composition can indicate ecosystem stress response and inform conservation management interventions. Shifts in community-dominant species may be indicative of interspeci c interactions such as facilitation (Bruno, 2000), succession (Butzeck et al, 2016), or cycles of population dynamics (Holling, 1973). Alternatively, changes in community-dominant species paired with loss of native species diversity and increasing abundance of non-native species may indicate loss of stability through loss of functional redundancy (Donohue et al, 2016;Tilman, 1999;Palmer et al, 1997).…”