The Columbia River Estuary (CRE) on the Pacific coast of North America is a highly modified river-dominated estuary. Land use, human development and tributary input vary along the length of the estuary, which may impact the distribution of zooplankton; however the longitudinal and temporal variability of zooplankton have not been evaluated along the tidally-influenced length of the CRE, which extends 234 river kilometers (rkm) inland. To evaluate zooplankton dynamics along the estuarine gradient, zooplankton and environmental data were collected monthly from five sites distributed along 193 rkm (82% of the length) of the estuary over a 2-year period (2016–2018). We found that zooplankton abundance and assemblage structure exhibited strong seasonal dynamics and that abundance was higher in the year with lower river discharge. At the only saline site sampled near the river mouth, assemblages were comprised of greater proportions of estuarine and marine taxa relative to tidal freshwater sites. Zooplankton assemblages at the river mouth were best explained by salinity, discharge and temperature, while assemblages at tidal freshwater sites were best explained by temperature and season. Non-native taxa, notably the calanoid copepod Pseudodiaptomus forbesi, comprised over 50% of zooplankton abundance at tidal freshwater sites during late summer - early autumn, but were largely absent at the river mouth. Assemblages were highly similar among tidal freshwater sites despite differences in tributary input, land use and development along the estuarine gradient. Our findings suggest that low residence times in river-dominated estuaries, such as the CRE, may contribute to homogenization of zooplankton assemblages, particularly during the high flow period and within the tidal freshwater reach.

北美太平洋海岸的哥伦比亚河口（CRE）是一个高度改造的以河为主的河口。土地利用，人类发展和支流投入沿着河口的长度变化，这可能影响浮游动物的分布；然而，尚未通过潮汐影响的CRE长度对浮游动物的纵向和时间变化进行评估，CRE沿内陆延伸了234公里（rkm）。为了评估沿河口梯度的浮游动物动态，在两年期间（2016-2018年），每月从沿193 rkm（占长度的82％）分布的五个地点每月收集浮游动物和环境数据。我们发现浮游动物的丰度和组合结构表现出较强的季节动态，并且一年中的丰度较高，河流流量较低。在河口附近唯一的盐水采样点，相对于潮汐淡水采样点，河口和海洋生物群的比例更大。最好用盐度，流量和温度来解释河口的浮游动物组合，而用温度和季节来最好地解释潮汐淡水地点的浮游动物组合。非原生生物群，尤其是the足类co足类动物夏末至初秋时分，潮汐淡水站点的浮游动物丰富度超过50％，但在河口基本上不存在。尽管沿河口的支流投入，土地利用和发展存在差异，但在潮汐淡水站点之间的组合高度相似。我们的发现表明，在以河流为主的河口（如CRE）中的低停留时间可能有助于浮游动物组合的均质化，特别是在高流量时期和潮汐淡水河段内。