Light in Relation to Tidal Immersion and the Growth of Spartina Townsendii (s.l.)

A number of estimates have been made during the past 45 years of the depth and duration of tidal immersion of Spartina marshes. The normal habitat of S. townsendii was said by Oliver (1925) to be soft mud, and extending not further than 3 ft (1 m) below high water mean spring tides (HWMST). Bryce successfully established S. townsendii at levels down to 1 2-2 1 m. below spring tides, with an immersion time of about 4 h (Goodman, Braybrooks & Lambert 1959). Jacquet (1949) was of the opinion that the absolute seaward limit of Spartina marsh must be governed by the total time of immersion. Goodman et al. (1959) found channel 'die-back' of S. townsendii at a level of 1 m below mean high water of ordinary spring tides (MHWOST) at Lymington, Hampshire, where it was subjected to approximately 6 h immersion at spring tides. Detailed surveys of tidal immersion of Spartina marshes were made in 1962-63, in Poole Harbour, Dorset, during a normal spring tide and an equinoctial spring tide (Ranwell et al. 1964). Nomenclature in this paper is in accordance with that of Clapham, Tutin & Warburg (1962) except for species of Spartina where the nomenclature agrees with that of Hubbard (1968), i.e. fertile S. anglica C. E. Hubbard is distinguished from the 'male-sterile' hybrid S. x townsendii H. & J. Groves (sensu stricto). This paper includes a re-assessment of the tidal data collected by Ranwell et al. (1964) which is now used to demonstrate the role of immersion in relation to the availability of light reaching the lowest level of a Spartina marsh. The paper also includes a description of the effects of light intensity and day length on S. anglica and S. x townsendii grown in a controlled environment.