Data from: Evidence for stratigraphy in molluscan death assemblages preserved in seagrass beds: St. Croix, US Virgin Islands

Death assemblages that occupy the upper tens of centimeters of sediment in shallow marine settings are often subject to extensive mixing, thereby limiting their acuity for assessing environmentally-mediated compositional changes through time in the local biota. Here, we provide evidence that dense, Thalassia-rich seagrass beds preserve a stratigraphic record of biotic variation because their dense root-rhizome mats inhibit mixing. We sampled benthic mollusk assemblages at seven localities in Thalassia-rich beds around St. Croix, USVI, collecting three separate sediment intervals of ~13 cm each to a total depth of ~40 cm below the sediment-water interface, and found evidence that sedimentary intervals preserved compositional stratigraphy. Further, some localities displayed systematic, directional changes down-core. An examination of interval-to-interval changes in composition revealed that compositional variation was unique from locality to locality rather than reflecting coordinated, island-wide transitions. In general, however, relative abundances of epifaunal gastropods and small lucinid bivalves tended to decrease with depth below the sediment-water interface. Quantitative comparisons of life to death assemblages from each successive sedimentary interval demonstrated that the shallowest death assemblages were typically more similar to the life assemblages than were deeper assemblages, suggesting that deeper intervals provide records of earlier community states.

栖息于浅海环境沉积物表层数十厘米的死亡组合（death assemblages）常遭受强烈混合作用，因此其评估环境介导的本地生物群随时间组成变化的分辨能力受到限制。本研究提供证据表明，茂密的泰来草（Thalassia）富集型海草床能够保存生物变异的地层记录，因其致密的根状茎垫可抑制混合作用。我们于美属维尔京群岛（USVI）圣克罗伊岛（St. Croix）周边的泰来草富集海草床的7个采样点采集底栖软体动物组合（benthic mollusk assemblages）：在沉积物-水界面以下采集3个独立沉积层段，每层段厚度约13 cm，总采样深度约40 cm，结果发现各沉积层段均保存了组成地层学特征。进一步分析显示，部分采样点沿岩芯向下呈现出系统性的定向变化。对层段间组成变化的考察表明，群落组成的变异仅存在于各采样点之间，而非反映全岛统一的群落演替过程。总体而言，表栖腹足类（epifaunal gastropods）与小型满月蛤科双壳类（lucinid bivalves）的相对丰度往往随沉积物-水界面以下深度的增加而降低。对各连续沉积层段的现生组合（life assemblages）与死亡组合开展定量对比后发现，最浅层的死亡组合通常较深层死亡组合更接近现生组合，这表明深层沉积层段记录了更早的群落状态。