Spatial displacement of nearshore vegetation in response to artificial changes in coastal morphology

Abstract

The species composition of nearshore plant communities represents a continuous biological response to environmental gradients perpendicular to a marine shoreline. This response integrates individual adaptations, competition, abiotic variability, and chance events resulting in a definite ‘optimum’ band where a particular species attains highest density. Ultimately, this variation creates distinct vegetation zones where groups of species coincide in their optimum density. This study evaluated possible changes to nearshore plant communities in response to a change in shoreline. Surveys of vegetation assemblages were carried out in six coastal sites. Measurements of soil salinity and exposure to wind were also taken from each site. All six sites outcropped on the same bedrock and had a comparable overall seaward slope. Previous studies indicated that the species sequence was predictable: Arthrocnemum macrostachyum closest to the shoreline, with a Limonium- Crithmum maritimum zone adjacent in the inland direction. This was succeeded by a broader band of Limbarda crithmoides subsp. longifolia. Further inland, the subhalophilous assemblage dominated by Thymbra capitata, was the dominant cover. The perpendicular distance from the shoreline at which peak density of L. crithmoides was recorded, was taken as an indicator of the ‘location’ of the vegetation sequence. The variation in density of L. crithmoides perpendicular to the shoreline was unimodal and was modelled using a Gaussian 3-parameter distribution for each study site. The distance from the shore of the peak L. crithmoides density was positively correlated with the Thomas Exposure Index (adjusted R2 = 0.563). These models were subsequently used to simulate the predicted shift of the vegetation sequence in response to a modified shoreline. The displacement in vegetation distribution following land reclamation is not necessarily subject to linear predictability. The abrupt change in shoreline morphology would modify the effects of environmental stressors, driving a displacement of nearshore plant communities towards the new shoreline. This displacement is unlikely to be either uniform or symmetrical, as the infill may have different sedimentological characteristics from the original substratum and initially will not have a persistent soil seed bank.