Diatom oospore population characteristics can be used to indicate past variations in salinity from cores taken in deep waters only. In shallow waters, water currents affect the distribution of diatom oospores, such that interpretation of past variations in salinity is affected.

Lamprothamnium papulosum mainly distributed in shallow water (10-40cm depth) around Langton Ferry and Herbury (eastern part of the West Fleet) on fine sand with relatively little water disturbance. Some adult plants also found between Top Ferry and Rodden Hive Point) during present study (full spatial study of distribution in Fleet not carried out - investigation based on distribution data from Holmes 1985 and >95).

Investigation of Lamprothamnium papulosum oospore distribution with depth and with distance from parent population centre & salinity (2.0 to 5.5 gl^-1).

No clear change in diatom population assemblages with longitudinal (E-W) salinity gradient observed. But gradient with depth observed - more fresh and fresh-brackish species found in shallow waters, more brackish and brackish-marine species found in deeper waters. Deeper channels in Fleet very organic-rich compared to shallower bays - accumulate fossil diatoms from both shallow and deeper waters, therefore diatoms will represent >average= salinities for whole Fleet over time, not local variations. Most diatom species found in Fleet were euryhaline - therefore not much use for indicating salinity fluctuations.

Lamprothamnium papulosum is confined to four sites on the south coast of England in Dorset, Hampshire and the Isle of Wight, and one site in Scotland, on North Uist [Loch Maddy]. A substantial population occurs in the Fleet, but populations are small at the other sites (Church, 1992). It grows in natural and artificial brackish lagoons with salinities in the range 10-30 gl^-1, usually on sand, gravel or pebbles in <2m water depth, and is intolerant of strong water currents or wave action. It usually occurs with tasselled pond weeds (Ruppia spp.), but does not compete well with dense vascular plant growth. Often found where there is some disturbance from birds or animals, or in shallow water where fluctuations of water level result in more open vegetation (Round 1981).

Salinity gradient along Fleet under different precipitation conditions are described. Fleet is divided into two regions - East and West, with transition area between. East Fleet (approx.6.5km from Abbotsbury to Smallmouth) salinities are close to marine values, about 30-35 gl^-1 . The transition zone is from approx. 4.5 - 6.5 km from Abbotsbury, where salinity drops dramatically from 30-15 gl^-1 within 2km distance in average runoff conditions. West Fleet, from Abbotsbury to 4.5 km east, the salinity is quite stable from 13-15 gl^-1 under average runoff, and 5-12 gl^-1 under high runoff conditions (Robinson, 1992). Salinities recorded for this project varied along a transect in main part of lagoon from 2.0 gl^-1 at Berry Knap, to 5.5 gl^-1 at the west end of Herbury headland (east end of Langton Ferry bay).

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