Marin Van Regteren

Importance of salt marshes

Positive effects of salt marshes on biodiversity, nature-based coastal protection and other ecosystem services are unequivocal. The future of salt marshes is threatened by climate change and sea level rise, which could result in salt marsh degradation and erosion. In addition, the presence of dykes constrains the retreat of salt marshes inland. Salt marshes can keep pace with sea level rise through vertical accretion if sufficient sediment is available and vegetation establishment at the salt marsh edge is an essential requirement to keep marshes vital and resilient. The aim of this thesis was to better understand the ecological and physical processes that determine successful initial establishment of pioneer vegetation beyond the marsh edge.

Initial pioneer vegetation establishment

The focus of this thesis lies at the interface of the salt marsh and the intertidal flats. This transition zone has the highest potential for lateral marsh expansion. Expansion requires successful vegetation establishment, which is influenced by many factors including soil conditions, environmental variables, such as inundation regime and sediment dynamics, bioturbation and seed availability. In this thesis, these factors were studied using a combination of descriptive research, laboratory and field experiments.

Based on vegetation cover we found a gradual change from densely vegetated salt marsh to bare intertidal flat (Figure S1). However, we found a distinct border between the pioneer zone and the sparsely vegetated transition zone, based on distinct benthic communities and environmental characteristics (Ch. 2). A strong increase in soil oxygenation and a much higher abundance of seeds characterised the border between the transition zone and the pioneer (Ch. 2). Although the transition zone has the appropriate elevation for vegetation establishment, it much more resembled the bare intertidal flat than the salt marsh pioneer zone.

If seeds are available in an appropriate area the following spring, a window of opportunity is crucial for seed germination and successful establishment (Ch. 3 and 5). Successful germination of the annual pioneer Salicornia procumbens was mainly driven by increased temperature and precipitation with a low sedimentation rate (Ch. 5). Experimental seed-additions in the transition zone demonstrated that boundary conditions for successful pioneer vegetation establishment were met for S. procumbens and Spartina anglica but not for Aster tripolium (Ch. 3 and 5). Sudden sedimentation events were detrimental as they buried small S. procumbens seedlings entirely (Ch. 5).

The seed bank and seed retention

Seeds of multiple pioneer and low marsh plant species were present in the soil of the vegetated marsh (Ch. 2). In the transition zone, the availability of natural viable seeds was low (Ch. 2, 3, and 5). Although seed dispersal of marsh vegetation is mostly local, seeds of the perennial pioneer S. anglica were practically absent from the transition zone (Ch. 3 and 5). Seeds of the other main pioneer, the annual S. procumbens, were present in the seed bank but the peak abundance of seeds occurred too deep for successful germination.

Dispersal timing affected seed retention. Seeds manually dispersed before winter eroded away but seeds dispersed after winter were retained in the transition zone (Ch. 3). Storm conditions during winter, high tides, wind speed and waves, caused the loss of seeds (Ch. 3). Natural seed availability in spring may also be low because freshly dispersed seeds suffer from secondary dispersal during winter (Fig. S2).

Figure S1. Processes that affect successful initial vegetation establishment and thus lateral salt marsh expansion onto the intertidal flat. Underlined are the most crucial mechanisms in dynamic transition zones.

The role of benthos

Numerous, small bioturbators occurred from the pioneer zone to the intertidal flat, mostly oligochaetes, harpacticoid copepods and collembolans (springtails) (Ch. 2). Benthic bioturbators were capable of transporting seeds downwards into the soil, i.e. seed burial, in the field (Ch. 3) and in the lab (Ch. 4). Seeds from different sizes (1-20 mm) and species, S. procumbens, Spartina anglica and A. tripolium, were buried up to 3 cm in a two-month period (Ch. 3). Bioturbation of oligochaetes also hindered pioneer vegetation establishment, toppling and burying small S. procumbens seedlings (Ch. 4).

Oligochaetes are generally understudied due to their size, although they can have a significant impact on the soil by reworking the sediment. Our studies revealed that oligochaetes can cause a ten-fold increase in soil oxygenation depth (Ch. 4). It is improbable that meiobenthic bioturbation will inhibit vegetation establishment on a marsh scale, because benthic bioturbators, mainly oligochaetes, were most abundant in the pioneer zone, the area with ample establishment of annual pioneer vegetation.

Implications and conclusions

The main conclusion of this thesis is that, even with a seed source in proximity (the pioneer zone), seed availability can still form a threshold for initial pioneer vegetation establishment in the transition zone. In spring, the natural availability of viable seeds was low beyond the marsh edge (Ch. 2, 3, and 5). The dynamic nature of the bed level (Intermezzo) in combination with winter storms induced a loss of seeds (Ch. 3). Bioturbating benthos, small but numerous, may indirectly influence vegetation establishment success, either through seed retention by burial or intermediated by altering soil conditions (Ch. 2 and 4). When aiming to promote resilient and healthy salt marshes, the appropriate boundary conditions regarding seed availability and sediment dynamics must be ensured (Figure S1). To account for imminent climate change, coastal management should implement the possibility that seed availability may be further reduced, caused by soil erosion due to storms. This is particularly important since increased storm frequencies are expected. Together with sea level rise, these effects will alter hydrodynamic regimes and can thus constrain lateral expansion or even cause salt marshes erosion.