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Feed-back effects of increased sedimentation induced by Spartina anglica patches on plant functioning

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Dutch title: Terugkoppelingseffecten van verhoogde sedimentatie veroorzaakt door Spartina anglica
Reference no: OND1262934
Period: 1997

Taxonomic term: Spartina anglica C.E. Hubbard [WoRMS]
Geographical term: ANE, Netherlands, Westerschelde, Platen van Valkenisse [Marine Regions]

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The canopies of marine littoral vegetation tend to reduce the current velocity of water flowing over these systems. As a result the particle carrying capacity of the water decreases, therby allowing increased deposition of particles. Salt marshes, mangroves and seagrass meadows for this reason tipically are sedimentation areas. The effectiveness of the salt marsh halophyte genus Spartina in the trapping of particulate organic matter from the water column is particulary well established, and species belonging to this genus have been used to initiate salt marsh formation. As a result of particle trapping and the retention of litter, the abiotic evironment in Spartina patches will gradually change. The organic matter content of the sediment and the potential nutrient availability will increase over the years as the pioneer vegetation becomes established and the patch size increases. We therefore hypothezie that a sequence of processes occurs; the colonization of an area by Spartina is followed by a change in soil conditions, and this, on its turns has feedback effects on the functioning of the plants. Sedimentation will occur particulary in the central parts of newly established (colonizing) patches, where flow reduction is most pronounced. Shoots developing in the center parts therefore are expected to have access to relatively larger nutrient stocks. At the periphery of the patches, the rhizomes which spread away from the center colonize unvegetated muddy or sandy areas with a presumably lower nutrient content. The different abiotic conditions of the sediment when comparting the central to the peripheral parts of Spartina patches, may give rise to spatial differences in functioning within patches. On the other hand, Spartina spp. are colonial plants, and the interaction which may exist between ramets of colonial species may lead to a rather uniform functioning of the ramets despite differences in the sediment environment Objectives: In the present project, the working hypotheis is colonization of an area by Spartina is followed by a change in soil conditions, and this, on its turn, has feed-back effects on the functioning of the plants. To investigate this hypothesis, the occurrence of spatial differences within patches of Spartina anglica will be studied. The following specific questions will be addressed: (1) Is growth and development of shoots in the center parts of Spartina anglica patches different from that of the peripheral parts? (2) Are any such differences related to spatial differences in sediment conditions caused by the establishment of the vegetation? (3) What is the role of physiological integration between ramets in compensating spatial differences in the sediment environment of patches? Achievements 1997: The samples taken during the campaign of Aug/Sept 1996 were analyzed. the work focused on growth analysis of Spartina anglica in patches found on the tidal flat near Kattendijke. On the relatively higher parts of the tidal flat near the dike, patches of the intertidal seagrass Zostera noltii intermingle with patches of Spartina anglica, indicating that this zone represents the transition phase fron tidalk flat to lower salt marsh. It was found that Spartina biomass in the edge zone of the patches was higher than in the centre zone of the patches, whereas no significant differences in the median particle size of the sediment of edge and centre zones could be established. Forward stepwise regression analysis showed that length of the culms and aboveground biomass of the plant in the center part of the patches was significantly related to phosphorus and nitrogen content of the soil. This was not the case for the edge zone, where no relationships between palnt growth and sedimentparameters could be established. Measurements of the stable carbon isotope composition of the sediment in the center zone suggest that the amount of phosphorus and nitrogen in the sediment in the center zone depends on sedimentation of allochtonous particulate organic matter. We hypothize that limited sedeimentation in the center zone due to the low particle content of the water of the Oosterschelde on the one hand, and to the relatively turblent hydronamic conditions near the dike foot on the other hand, limit nutrient availability after patch establishment when initially present nutrient stocks are depleted. hence, we suggest that positive feed-back effects of canopy-enhanced sedimentation can be important in the early stages of salt marsh development. Planning 1998: The project will be continued in a Spartina anglica vegetation on the tidal flat of Valkenisse in the Westerschelde estuary, to gain further information on the relation between sedimentation and plant performance in pioneer Spartina vegetation. The site is chosen because of the changes in sedimentation/erosion conditions that are expected in relation to the larger dredging programme that will be carried out in the Westerschelde. First sampling at this site has been carried out in October 1997, to establish the 'baseline' conditions at the site, The project will be carried out in close cooperation with the EU-ISLED project (LV 1.891)

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