THE BIOSTRATIGRAPHY AND TAXONOMY OF SMALL STEPHANODISCUS AND CYCLOSTEPHANOS SPECIES (BACILLARIOPHYCEAE) IN A EUTROPHIC LAKE, AND THEIR ECOLOGICAL IMPLICATIONS

Detailed stratigraphic and taxonomic analyses were made of small Stephanodiscus and Cyclostephanos species in the recent sediments of a small eutrophic lake (Lough Augher, Co. Tyrone, N. Ireland). The morphological variety present was assessed by LM and SEM studies of sediment depths with high valve concentrations. The dominant forms found were Stephanodiscus parvus, S. hantzschii, S. cf. medius, Cyclostephanos dubius, C. cf. tholiformis and C. cf. invisitatus. Mean, range and density were calculated for a number of morphological parameters [costae (alveoli), marginal and central strutted processes, labiate processes], and their relationship to diameter was estimated. Total number of costae or fascicles in S. hantzschii, and alveolae in Cyclostephanos dubius were significantly correlated with diameter, but no significant morphological-diameter relationships were found for S. parvus. The analysis of a fast sediment accumulation rate core (c. 1cm yr-1) permitted high temporal resolution biostratigraphy, including the calculation of diatom accumulation rates. Stratigraphic changes in morphology and diameter occurred, possibly reflecting changing environmental conditions. It is unclear whether observed changes in the ratio S. hantzschii: S. hantzschii f. tenuis valves represent ecological change or preservation variability. Maximum accumulation rates for individual species were 16 x 106 cells cm-2 yr 1 for S. parvus (in 1974), 5x 106 cells cm 2 yr 1 for Cyclostephanos dubius (in 1955) and 14 x 106 cells cm 2 yr-1 for S. hantzschii (in 1980). The clear temporal phasing of the maximum occurrence of the different taxa suggests strongly that individual species have different ecological responses and that application of general theories (e.g. Si:P ratios) to diatom plankton succession at the generic level may be misplaced. © 1990 The British Phycological Society.