Three interesting diatoms from Epping Forest pools
HILARY BELCHER
23 Pepys Way, Girton, Cambridge CB3 OPA
Connaught Water is an artificial lake about 300m long in the southern part of Epping Forest (TQ
404953). A sample of brown scum from a wooden post at the edge of the lake, collected during
August 2000 and examined fresh under the microscope, revealed a mixture of diatom species of
various shapes and sizes. They resembled a multitude of tiny boats, some moored, while others
moved busily about Most of these species occur elsewhere in the county, in rivers, canals or ponds.
However, two species, Gyrosigma macrum and G parkeri (Figs. 1A and B) were found which are
very rare in local inland waters. In Essex, they have previously been found only in Connaught
Water, the Hollow Pond at Whipps Cross (TQ 393887) and the Bulrush Pond near the Rising Sun
Public House (TQ 391897), all collected by Alwyne Wheeler in 1991. Outside the county 1 have
seen G parkeri in a sample from the Parish Waters pool near Wheatfen Broad, Norfolk (TG 329055),
apparently a new county record for fresh water. Krammcr & Lange-Bcrtalot (1986) in their diatom
flora, the current European standard (in German), regard these two species as occurring
characteristically in brackish water and inland saline habitats. It seems surprising that the two
species should occur together in the three Forest waters.
Another diatom which prefers saline conditions (and is termed in the literature halophilic) is
Cylindrotheca gracilis (Figs. 1C and D). This was found in five flooded bomb craters by Scourfield
(1943). We have not seen it in collections from the Forest, but have found it elsewhere occasionally,
in shallow depressions which dry up quickly and which receive chlorides from bird droppings. Its
silica wall, which is spirally twisted (Fig. 1C), is very thin and easily destroyed by acid during the
standard methods of cleaning and mounting diatoms. It is best identified alive, by the several round
chloroplasts (Fig. 1D - instead of the two that the similar species of Nitzschia have) and the way
that it revolves as it travels along. It would be worthwhile looking for this curious diatom again in
places such as drying ditches.
Figure 1: A - Gyrosigma macrum; B - Gyrosigma parkeri; C - Cylindrotheca gracilis, cleaned;
and D - the same species, living.Figure 1: A - Gyrosigma macrum; B - Gyrosigma parkeri; C -
Cylindrotheca gracilis, cleaned; and D - the same species, living.
References
KRAMMER, K. & LANGE-BERTALOT, H. (1986) Susswasserflora von Mittelenropa 2/1
Bacillariophyceae. 1. Naviculaceae. Gustav Fischer, Jena.
SCOURFIELD, D.J. (1943) The rare spiral diatom Cylindrotheca gracilis and its occurrence in Epping
Forest. Essex Naturalist 27: 182-7.
Essex Naturalist (New Series) 19 (2002)
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