An understanding of the modern relationship between diatom species and elevation is a prerequisite for using fossil diatoms to reconstruct relative sea level (RSL). We described modern diatom distributions from seven transects covering unvegetated subtidal environments to forested uplands from four tidal wetland sites (Smith Creek, Bone River, Niawiakum River, and Naselle River) of Willapa Bay, Washington, USA. We compared our diatom dataset (320 species from 104 samples) to a series of environmental variables (elevation, grain-size, total organic carbon (TOC_SOM), and porewater salinity) using hierarchical clustering and ordination. While no single variable consistently explains variations in diatom assemblages at every site, elevation, salinity, and substrate (mud fraction and TOC_SOM) are variables affecting diatom distribution along our transects. Elevation was the major environmental control of diatom variability (explained 27–39% variance) at four transects (Bone River Transect 1, Niawiakum River Transect 2, Naselle River Transect 1 and 2). Salinity and substrate were the major environmental controls (explained 12–32% variance) of diatom variability at three transects (Niawiakum River Transect 1: salinity; Smith Creek Transect and Bone River Transect 2: TOC_SOM). Analyses of a combined regional dataset of all transects suggest that elevation is the driver of regional diatom variability in Willapa Bay, with salinity and substrate co-varying along an elevation gradient. We identify species with narrow elevation tolerances that can serve as indicator species of specific environments. Despite the site-specific variability of our modern diatom distribution, the regional dataset provides an important dataset that can be used to reconstruct RSL in Willapa Bay.

了解硅藻物种与海拔之间的现代关系是使用化石硅藻重建相对海平面 (RSL) 的先决条件。我们描述了来自美国华盛顿州威拉帕湾的四个潮汐湿地地点（史密斯溪、博恩河、尼亚维阿库姆河和纳赛尔河）的七个断面的现代硅藻分布，这些断面覆盖了无植被的潮下环境到森林高地。我们使用层次聚类和排序将我们的硅藻数据集（来自 104 个样本的 320 个物种）与一系列环境变量（海拔、粒度、总有机碳（TOC _SOM）和孔隙水盐度）进行了比较。虽然没有一个变量能够始终如一地解释每个地点、海拔、盐度和基质（泥质分数和 TOC _SOM) 是影响沿我们横断面的硅藻分布的变量。海拔是四个断面（骨河断面 1、尼亚维阿库姆河断面 2、纳塞尔河断面 1 和 2）的硅藻变异性（解释为 27-39% 的方差）的主要环境控制。盐度和底物是三个断面（尼亚维阿库姆河断面 1：盐度；史密斯溪断面和骨河断面 2：TOC _SOM）硅藻变异的主要环境控制因素（解释为 12-32% 方差））。对所有横断面的组合区域数据集的分析表明，海拔是威拉帕湾区域硅藻变异的驱动因素，盐度和基质沿海拔梯度共同变化。我们识别具有窄海拔公差的物种，可以作为特定环境的指示物种。尽管我们现代硅藻分布的地点特定可变性，区域数据集提供了一个重要的数据集，可用于重建威拉帕湾的 RSL。