Intra- and interannual patterns of diatom population occurrence in the Mauritanian coastal upwelling

Qualitative and quantitative analyses of the diatom community were carried out on permanent slides of acid cleaned material (Mountex® mounting medium) at x1000 magnifications by using a Zeiss®Axioscop with phase-contrast illumination (MARUM). Depending on the valve abundance in each sample, several traverses across each permanent slide were examined. The census and definition of counting units for valves follows Schrader and Gersonde (1978). Total amount of counted valves per slide ranged between ca. 350 and 700. Two cover slips per sample were scanned in this way. Census of valves in replicate slides indicate that the analytical error of valve concentration and relative abundance estimates is ≤10 %. The resulting counts yielded absolute and relative abundance of individual diatom taxa as well as daily diatom fluxes (valves m-2 d-1), calculated according to Sancetta and Calvert (1988), as follows: Diatom Flux = [N] x [A/a] x [V] x [Split] [days] x [D] where, [N] number of valves in an known area [a], as a fraction of the total area of a petri dish [A] and the dilution volume [V] in ml. This value is multiplied by the sample split [Split], representing the fraction of total material in the trap, and then divided by the number of [days] of sample deployment and the sediment trap collection area [D].