Dinoflagellate cysts of sediment traps from June 1992 to February 1993 at two sites in the Somali Basin (northwestern Arabian Sea)

To date, relatively little information is available about factors influencing organic-walled cyst production of tropical dinoflagellates and processes influencing the final burial of cysts in bottom sediments, such as transport and preservation. To extend this information, cyst fluxes were documented for three sediment traps from June 1992 to February 1993 at two sites in the Somali Basin (northwestern Arabian Sea) as well as the cyst association of underlying sediments. By comparing cyst associations of contemporaneously collected trap samples at different depths at one site, information about transport and processes of decay in the water column was obtained. Neither transport nor decay appears to have any detectable influence on cyst association during cyst settlement through the water column. Comparing the trap associations with the underlying sediments indicates that downslope transport appears to have influenced the cyst association on a local scale only. Species-selective decay, probably related to the presence of oxygen in bottom sediments, has influenced the cyst association most pronouncedly at the most offshore site. Relating variations in the trap associations with environmental conditions of the overlying surface waters indicates that highest production of both filled and empty cysts occurs during the SW Monsoon upwelling. Based on this correlation three groups of species can be distinguished: Species with highest fluxes during (1) the first-half of the SW Monsoon (June-August); Bitectatodinium spongium, Echinidinium granulatum, Echindinium transparantum, Echinidnium spp., cysts of Protoperidinium compressum and cysts of Protoperidinium subinerme, (2) the transition between the SW-Monsoon and inter-Monsoon; Spiniferites mirabilis and Spiniferites spp., (3) no particular season; all other species. Cyst associations of all trap samples are dominated by cyst of Protoperidinium species. Cysts with highest fluxes during the SW-Monsoon form about a third of the associations.

截至目前，关于热带甲藻（dinoflagellates）有机壁孢囊（organic-walled cyst）产生的影响因素，以及孢囊最终埋藏于海底沉积物的相关过程（如搬运与保存作用）的研究资料仍相对匮乏。为补充该领域的相关数据，我们于1992年6月至1993年2月期间，在索马里海盆（Somali Basin，阿拉伯海西北部）的两个采样站点布设了三台沉积物捕获器（sediment traps），对其中的孢囊通量进行了记录，并同步分析了各站位下方沉积物中的孢囊组合特征。通过对比同一站点不同深度处同期采集的捕获器样品的孢囊组合，我们获取了关于水体中孢囊搬运与降解过程的相关信息。研究结果显示，在孢囊通过水体沉降的过程中，搬运与降解作用并未对孢囊组合产生可检测到的显著影响。将捕获器的孢囊组合与下方沉积物中的孢囊组合进行对比后发现，仅在局地尺度上，下坡搬运作用会对孢囊组合产生一定影响。物种选择性降解作用（可能与海底沉积物中的氧气含量相关），是对最外海站点的孢囊组合影响最为显著的因素。将捕获器的孢囊组合与上覆表层海水的环境条件进行关联分析后可知，饱满孢囊与空壳孢囊的最高产量均出现在西南季风（SW Monsoon）上升流（upwelling）时期。基于这一相关性，可将孢囊产生物种划分为三大类群：（1）西南季风前半期（6-8月）通量最高的物种：海绵双沟甲藻孢囊（Bitectatodinium spongium）、颗粒棘孢甲藻孢囊（Echinidinium granulatum）、透明棘孢甲藻孢囊（Echinidinium transparantum）、棘孢甲藻属（Echinidinium spp.）孢囊、压缩原多甲藻（Protoperidinium compressum）孢囊以及亚鞘原多甲藻（Protoperidinium subinerme）孢囊；（2）西南季风与季风间歇期过渡阶段通量最高的物种：奇异刺甲藻孢囊（Spiniferites mirabilis）以及刺甲藻属（Spiniferites spp.）；（3）无明显季节偏好的物种：其余所有物种。所有捕获器样品的孢囊组合均以原多甲藻属（Protoperidinium）物种的孢囊为主。西南季风时期通量最高的孢囊类群，约占总孢囊组合的三分之一。