5e. TOPIC 5 – Impact of increasing MHWL on tidal marsh ecosystems
In this topic we look at the impact of increasing MHWL on the tidal marsh ecosystems. Did the increasing high water levels result in increased tidal flooding of the tidal marshes or was vertical sediment accretion in the marshes sufficiently high so that the elevation of the marshes could grow in accordance with the growing high water level? And moreover, if vertical sediment accretion was important, did this result in steepening of the intertidal area and hence in increased landward erosion of the tidal marsh shorelines? The methodology applied for research topic 5 is described in §3.1.6.
Change in marsh platform elevation
For the Scheldt marsh, the mean increase in MHWL over the time period 1931-2010 is 0.53 cm/year (blue line, Figure 68). During this period the marsh evolved from a low elevated marsh in 1931 (platform elevation 0.47 m below MHWL) towards a high elevated marsh in 2010 (platform elevation 0.37 m above MHWL) (brown line, Figure 68). Over a period of 79 year this is an absolute increase in elevation of 1.08 m, which corresponds with a mean rate of 1.4 cm/year. However this rate is not constant in time (orange line, Figure 68), we observe that when the marsh has a low elevation in the tidal frame, the increase in platform elevation is almost 2 cm/year. Once the platform elevation reaches MHWL, the rise in platform elevation decreases towards values of 1-1.2 cm/year. Currently, the platform has an elevation 0.09 m below MHWLS.
For the Elbe marsh, the mean increase in MHWL over the time period 1958-2008 is 0.4 cm/year (blue line, Figure 69). During this period the marsh evolved from a low elevated marsh in 1958 (platform elevation 0.42 m below MHWL) towards a high elevated marsh in 2008 (platform elevation 0.68 m above MHWL) (brown line, Figure 69). Over a period of 50 year this in an absolute increase in platform elevation of 1.57 m, which corresponds with a mean rate of 3.1 cm/year. However this rate is not constant in time, and ranges between 0.75 and 4.5 cm/year (orange line, Figure 69). As for the Scheldt marsh, we don’t observe in time a decrease of the rate in platform elevation change.
The platform elevation change of two brackish marsh sites (one In the Scheldt and one in the Elbe) was evaluated in response to an increase in MHWL. Both marsh sites were able to follow up the increase in MHWL (similar for both sites, 0.5 cm/yr for Scheldt, 0.4 cm/yr for Elbe), and evolved from a low elevated marsh (elevation for both sites from about 0.45 below MHWL) towards a high elevated marsh (for the Scheldt site 0.37 m above MHWL, for the Elbe site 0.68 m above MHWL). However, for the Elbe site, the increase in elevation was established in a shorter period of time (50 versus 79 year), and currently the site has a higher position in the tidal frame than the Scheldt site. For the Scheldt marsh, the mean platform elevation increase is 1.4 cm/yr, for the Elbe marsh this is 3.1 cm/yr.
The observation that both marshes are able to follow up the increase in MHWL, suggests that sediment availability at both marsh sites is sufficient. At the Scheldt site, surface SPM values are about 60 mg/l (see Figure 28 and Figure 9), whereas at the Elbe site this is about 150 mg/l (i.e. close to the turbidity maximum, see Figure 28 and Figure 10). The higher SPM values at the Elbe site may explain why the rate in platform elevation increase is about 2 times larger for the Elbe site than for the Scheldt site (3.1 cm/yr versus 1.4 cm/yr). However, we do not know how SPM values evolved historically. For the Scheldt, we observe that the rate in platform increase decreases in time. Thus, as the platform rises in the tidal frame, the increase in platform elevation decreases (asymptotic curve). This is in accordance with earlier findings on long-term changes of marsh platform elevation (Temmerman et al., 2004). However, for the Elbe we do not observe this asymptotic increase. The increase in platform elevation is here much more variable in time, with a peak increase of about 4.5 cm/yr between 1975 and 1995. This could be linked to the deepening of the Elbe fairway in the 1980’s, leading to an increase in SPM values. Finally, it should be pointed out that the more rapid increase in platform elevation for the Elbe marsh leads to a faster succession in vegetation types and thus differences in organic accumulation rates. Moreover, vegetation characteristics determine to what extent sediment particles can be captured an accrete on the tidal marsh surface. Currently, the dominant plant species for the Scheldt marsh is Elymus athericus, whereas the Elbe marsh is characterized by a higher successive stage dominated by Phragmites australis.
Firstly, we may conclude that despite the current differences in SPM values (60 mg/l versus 150 mg/l), both marshes are able to follow up an increase in MHWL. This observation can be considered as favorable for e.g. coastal protection. Secondly, the differences in SPM values have their implications for the ecological state of tidal marshes. For higher SPM values (natural or anthropogenic cause), the increase in platform elevation is faster. This leads to a faster succession of marsh vegetation types, by which a high succession stage with less plant diversity is reached earlier.
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