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. In general, the dominance of neritic diatoms, such as Thalassionema nitzschioides var. nitzschioides, resting spores of Chaetoceros and Cyclotella litoralis, reflected the continuous offshore influence of coastal upwelling at the Cape Blanc trap site, with stronger intensity in spring/summer. In contrast, the occurrence of pelagic diatoms (e.g. Nitzschia bicapitata, N. interruptestriata, T. nitzschioides var. parva and Fragilariopsis doliolus), and high silicoflagellate fluxes (mainly Dictyocha messanensis) were linked to inshore transport of oceanic waters, generally in winter. With the exception of some fragile, pelagic diatoms, dominant species found in the settled material also occurred in the underlying sediments, suggesting that diatom thanatocoenosis downcore (Organisms preserved from the top to the bottom in sediment core) can be used as a reliable indicator of the intensity and persistence of the offshore spreading of coastal upwelling.本研究呈现了1988年至1991年共四年间，在非洲西北部布兰科角（Cape Blanc）外海约北纬20°、西经20°处的沉降捕集器站位（trap site）所获取的硅藻（diatoms）与硅鞭藻（silicoflagellates）沉降通量观测数据。观测发现通量与物种组成存在显著变化，同时1988年至1989年生物成因蛋白石（biogenic opal，含硅藻）的输出通量出现明显下降，此后通量持续处于低位。我们推测该通量衰减可能与沿岸上升流（coastal upwelling）强度减弱、典型叶绿素丝（chlorophyll filament）向海扩散范围缩减，以及布兰科角外海上升流水体硅酸盐含量降低有关。此外，该沉降捕集器的锚定位置更偏向外海，可能也对通量产生了影响。在整个观测周期内，硅藻始终是生物成因蛋白石通量的最主要贡献组分，且硅藻通量与总质量通量的波动趋势高度一致。尽管物种组成存在季节性变化，但各年份间未出现显著的质的差异。总体而言，近岸硅藻（neritic diatoms）的占比优势——如菱形海线藻原变种（Thalassionema nitzschioides var. nitzschioides）、角毛藻属（Chaetoceros）休眠孢子以及近岸小环藻（Cyclotella litoralis）——反映出布兰科角沉降捕集器站位持续受到沿岸上升流的向海影响，且该影响在春夏季强度更强。与之相反，浮游硅藻（pelagic diatoms，如双头菱形藻（Nitzschia bicapitata）、间纹菱形藻（N. interruptestriata）、菱形海线藻小变种（T. nitzschioides var. parva）以及小双尾藻（Fragilariopsis doliolus））的出现，以及高通量的硅鞭藻（主要为墨西拿网柄藻（Dictyocha messanensis）），通常与冬季海洋水体向近岸输送有关。除部分脆弱的浮游硅藻外，沉降样品中的优势物种同样出现于下伏沉积物中，这表明岩芯深部的硅藻尸积群（diatom thanatocoenosis，即沉积物岩芯自顶至底保存的生物遗体）可作为可靠指标，用以反映沿岸上升流向海扩散的强度与持续性。