Tidal range 0.15m at Abbotsbury, to 2m at Smallmouth (Smith & Laffoley 1992). Charophytes are associated with clean, unpolluted water because most species cannot tolerate high levels of phosphates and nitrates. Levels in excess of 20 µg/l inhibit their growth [doesn=t specify which nutrient or whether concentration expressed as phosphorus] whilst encouraging other plants (Stewart & Church 1992, Red Data Book, Stoneworts, JNCC). Lamprothamnium papulosum can tolerate near marine salinities of up to 35 gl^-1 (Stewart, 1997, Stoneworts - connoisseurs of clean water, British Wildlife supplement). Its optimum salinity is 26 gl^-1, with a range of 8-18 gl^-1 in Denmark (Olsen 1963, Danish Charophyta: Chronological, ecological and biological investigations, Khem: Copenhagen). It is a summer annual, germinating in spring. Oospores are produced between July and September.

Estimations of the historical marine-freshwater regimes of the lagoon are made from diatom species composition and Lamprothamnium papulosum oospore record of core samples with depth, but dating of sediment depths was not performed, so these estimations cannot be allocated to a time period. The work indicates that the Fleet has been brackish, but with marine intrusion similar to the present situation, for many years. Dominance by macrophytes is seen throughout the core, with more subtle changes in the balance of marine, brackish and freshwater diatoms attributed to such events as the construction of Portland Harbour, and the changes in channel morphology of Smallmouth when the new road bridge to Portland was constructed (1984). The overall salinity regime of the Fleet is, however, seen as relatively stable - neither becoming progressively more marine, nor rapidly progressing to a freshwater system.

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