A Methods Manual for the Collection, Preparation and Analysis of ...

Occasional filaments should be recorded as the corresponding number of diatom units. Ifa large number of diatom units are found in filaments, a new preparation technique, usinga more aggressive mix of oxidising agents, should be considered.In order to eliminate the risk of including separate fragments of broken valves orfrustules, a consistent approach must be decided on before starting a project. Valvesshould be counted only if approximately three quarters is present, or alternatively brokenvalves may be excluded from the count altogether. Since the scale of physical damageduring the sampling and preparation stages is unlikely to be significant, the presence ofmany small fragments of diatoms may indicate that dead diatoms are being washed infrom upstream sites.A diatom may not be identifiable for a number of reasons, including the presentation of agirdle view, the presence of overlying material obscuring the view, or the taxon not beingrecognised by the analyst. If many valves are obscured, then new slides should beprepared using more dilute suspensions.Some taxa are identifiable from girdle (side) views, either because the girdle view isparticularly characteristic (e.g. Rhoicosphenia abbreviata) or because the girdle view canbe assigned with confidence to a particular taxon by “matching” it with correspondingvalve views of taxa found in the sample. However, this is not always possible and, incases of doubt, the analyst should record the girdle views at the lowest level to which theycan be assigned with confidence (e.g. “unidentified Gomphonema sp.”, “unidentifiedpennate girdle view”).This convention should also be applied to other individuals found on the slide but notidentifiable by the analyst. A large number of such individuals may indicate a problemeither with the slide preparation or the identification skills of the analyst.As most diatom indices presume that all taxa in a sample are identified, it isrecommended that not more than five per cent of the total count should compriseunidentifiable individuals. If a diatom unit cannot be identified for any reason,photographs, digital images or detailed drawings should be made. Notes should also betaken of the shape and dimensions of the diatom unit, striae density and arrangement (atthe centre and poles), shape and size of the central area, number and position of punctaeand arrangement of raphe endings.For some purposes, especially biodiversity studies, it is useful to continue to scan theslide after the required number of diatom units has been counted, and any taxaencountered that were not included in the count should be identified and recorded as“present”.A further scan using a medium power magnification (e.g. 400 x) may also be appropriatein order to note any larger taxa (e.g. Gyrosigma spp.) which may be missed at highermagnifications.42