Size and Sexual Reproduction of the Centric Diatom Melosira varians C. A. Ag. in Lough Neagh

Melosira varians C. A. Ag. was the first centric diatom species in which sexual reproduction was observed in a natural population. In L. Neagh, M. varians occurs in the shallow margins, growing in chains of cells, usually on the surface of rocks or plants. Its depth distribution is very restricted, as sufficient light for growth only penetrates about 2-3 m. This zone is frequently affected by wave action and during storms cells are washed offshore into the plankton. Cells have been found regularly in concentrated plankton samples but are usually more abundant after windy weather. The populations considered in this study were collected at a mid-lake site, where the water depth was 14 m, 4 km south from Cranfield (J0485), (which is on the north shore of L. Neagh). Samples from 0, 5 and 10 m depth were concentrated through a 20 mu m mesh net and then observed and measured on an Olympus IMT microscope. Most diatoms go through cycles of size decline, in at least one dimension, and subsequent size regeneration. Essentially, every time a diatom cell divides vegetatively, half of a new cell wall is formed for each of the two new cells. The new walls are formed inside part of the old ones and are, therefore, smaller. In the majority of diatoms size regeneration is accomplished by expansion of the zygote after sexual fertilization.