Siliceous phytoplankton fluxes off northwest Africa

Four years of observations (1988-1991) of downward fluxes of diatoms and silicoflagellates at a trap site off Cape Blanc (ca. 20°N, 20°W), northwest Africa, are presented. Significant variations in flux and species composition were observed as well as a marked drop in the export of biogenic opal (and diatoms) from 1988 to 1989; fluxes remained low thereafter. We hypothesize that this diminution might be related to decreased coastal upwelling intensity and offshore spreading of the typical chlorophyll filament, and/or a lesser silicate content of upwelling waters off Cape Blanc. In addition, the more seaward positioning of the mooring may have influenced the fluxes. At all times, diatoms were the most prominent contributors to the biogenic opal flux, and diatom fluxes closely paralleled total mass flux fluctuations. Although species composition varied seasonally, no significant qualitative variations were observed from year to year. […]

本报告展示了位于非洲西北部布兰克角（约20°N，20°W）附近海域一处陷阱站点，对硅藻和硅甲藻向下通量进行为期四年（1988-1991年）的观测结果。观测期间，通量及物种组成出现了显著的变化，同时观察到从1988年至1989年，生物成因蛋白石（及硅藻）的输出量明显下降，且此后的通量维持在一个较低水平。我们推测这种减少可能与布兰克角沿海上升流强度的减弱及典型叶绿素丝状体的离岸扩展有关，亦或与布兰克角沿海上升流中硅酸盐含量的降低相关。此外，浮标锚定位置向海面的偏移可能也对通量产生了影响。在所有观测时段内，硅藻始终是生物成因蛋白石通量最显著的贡献者，且硅藻通量与总质量通量波动密切相关。尽管物种组成随季节变化而有所不同，但从年际间来看，并未观察到显著的定性变化。