| Summary: | Sediment anoxia and associated low redox potential (Eh) are important influences on the distribution of saltmarsh plants. Nevertheless, the detailed variation of Eh in space and time is poorly understood, especially on managed realignment (MR) saltmarshes where anoxia may be a significant constraint on the establishment of characteristic perennial halophytes. This thesis examines the distribution of Eh in natural and MR saltmarshes of North Norfolk and Lincolnshire, UK and its implications for Armeria maritima, Limonium vulgare, Plantago maritima and Triglochin maritima, species important in general saltmarsh (GSM) communities. Detailed measurements of Eh were made using arrays of fixed electrodes and a field datalogger, at different sites/seasons/sediment depths, over tidal cycles, and in relation to microtopography (including on an existing experiment with manipulated microtopography). Laboratory apparatus was devised to examine responses of halophytes to controlled Eh in water culture. Relationships between vegetation and Eh proved complex. Prolonged low-Eh episodes occurred both at MR and natural GSM sites, particularly in summer. Atriplex portulacoides and Puccinellia maritima tended to dominate where low-Eh episodes were more transient, generally near drainage channels or at higher elevations. Atriplex, Armeria and Plantago previously planted in experimental plots survived poorly at low Eh, while Limonium and Triglochin showed higher survival. In cultivation, Atriplex tolerated anoxia for only two weeks, and low Eh did not impose any additional stress. In contrast, Triglochin survived the ten-week duration of the experiment even at low Eh. Thus laboratory results suggested mechanistic bases to explain those in the field. Although MR and GSM sediments can exhibit similar Eh measurements, they may differ with respect to redox capacity; MR sediments sites with high reduction capacities may exert additional stresses that could limit colonisation. Conversely, consistently high Eh could also limit colonisation on MR sites by promoting vigorous competitive growth of Atriplex and Puccinellia.
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