Spartina Salt Marshes in Southern England: V. Tidal Submergence and Chlorinity in Poole Harbour

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Abstract
Simultaneous measurements of tidal submergence and soil chlorinity were made in Spartina X townsendii marsh at different points on the main estuarine gradient. The equinoctial tidal curve is steeper and submergence duration at a particular level less in the upper than the lower reaches of the estuary. Spartina is limited to levels where submergence depth is 1 m or less, and duration 6 hr or less in the growth season and 9 hr or less when plants are dormant. Tidal conditions would result in a big change in submergence duration with only small change in sea level at the Spartina limit. This could account for extensive marsh recession observed in marsh levels lying close to mudflat level. The chlorinity gradient decreases gradually in lower and middle reaches and more steeply in upper estuarine reaches. Seasonal chlorinity changes in the marsh were correlated with rainfall, but not with tidal submergence; the water table level remained relatively constant throughout the year. Distribution of marsh plants is discussed in relation to these factors - Phragmites seedlings germinated, but did not establish at 1% chlorinity and mature plants were confined to areas of 1.2% chlorinity or less.