## Abstract

The life cycle of Aulacoseira subarctica (O. Muller) Haworth in Lough Neagh, Northern Ireland, is described. Cell numbers can reach up to 17 000 per millilitre in spring. Most cells sediment to the bottom after silica limitation and go into a resting state during summer. The inoculum in autumn partly comes from resuspension, with the surviving cells (0.5-5%) continuing to grow through the winter, doubling every one to two weeks. The population goes through a size reduction and regeneration cycle linked to sexual reproduction. Gametes are only produced in narrower cells (3.8-7.4 <latex>$\mu $</latex>m diameter), usually after interruptions in growth caused by low light conditions (surface irradiance 100-150 <latex>$\mu $</latex>E m<latex>$^{-2}$</latex> s<latex>$^{-1}$</latex>), but availability of nutrients, especially silica and nitrogen, is also important. Even the highest densities of auxospores (20 ml<latex>$^{-1}$</latex>) represent only a small proportion of the total cells present (0.16%). Size regeneration results in initial cells with diameters (14.8 <latex>$\pm $</latex> 2 <latex>$\mu $</latex>m) about three times those of the parent. Larger parent cells usually give rise to larger initial cells. Subsequently, cell division leads to a decrease in population diameter, because of the way new valves are laid down below the girdle bands. Reductions are largest in broader cells (0.32 <latex>$\mu $</latex>m per division) and gradually decrease as cells get narrower. Occasionally large reductions, up to 1 <latex>$\mu $</latex>m, follow periods of environmental stress. By combining these results with studies of changes in cell size (width, length and volume) in related individuals along filaments, it was possible to explain why there have been difficulties in applying the MacDonald-Pfitzer hypothesis to natural populations. Theoretically, the life cycle in L. Neagh might extend over 100 divisions or 15 years but, in practice, cells reach a sexually inducible size in 4-6 years. The discrepancy is because environmental factors (e.g. sedimentation, resuspension, parasitism, etc.) are also important in size selectivity. The interaction of these factors, when combined with intermittent sexual reproduction at low frequencies, results in a relatively stable population size distribution, where there are always some cells in the size range in which sexual differentiation can be induced. Overall, the results demonstrate, that for a full understanding of diatom population dynamics, it is important to quantify events over complete life cycles.