Co-existence and survival of diatoms on sand grains

High densities of diatoms were found attached to sand grains down to half a metre below the sediment surface (6.5 g chl a m−2) on an exposed beach (Lough Neagh, N. Ireland). However, seasonal variation of biomass was low (11–21 µg chl a g−1 dry sand). Sand movement was the main controlling factor but silica and nitrate limitation also contributed during summer months. Laboratory assays showed that nutrients were derived mostly from the overlying water. Two species dominated but with very different strategies for coping in this abrasive environment. Cymbellonitzschia diluviana was small, low growing and aggregated in depressions whereas the larger, non-motile and stalked Martyana martyi was dispersed over the most exposed surfaces. Their contrasting distributions are related to their different structure and modes of cell division. However, one adaptation common to both species was the ability to form resting cells. We investigated the environmental factors controlling the formation and recovery of resting cells using both chloroplast shape and fluorescence indicators. Both species showed a gradual change in and out of the resting state, which enabled them to compromise between the need to conserve energy when buried but recover rapidly when brought back to the surface by wave action and water level changes.