AbstractWe present the first comprehensive analysis of the Pacific Northwest estuaries (PNWE) zooplankton time series, which encompasses 38 estuaries distributed across more than 1000 km of the North American Pacific Coast. With observations spanning more than 20 yr, we here examine biogeographic trends among zooplankton communities, patterns of biological invasion across the region, and environmental correlates with dominant native and invasive taxa. Our results show that some estuaries across the region are invaded by multiple zooplankton species and that the geographic extent of invasion is far greater than previously reported for at least five species of copepods: Pseudodiaptomus inopinus, Pseudodiaptomus forbesi, Oithona davisae, Limnoithona sinensis, Sinocalanus doerrii, and the cladoceran Bosmina coregoni. Some of these species appear to be rapidly spreading across the region, while others have occupied a relatively static geographic range for decades. The copepod, P. inopinus, is by far the most abundant and geographically widespread of these invaders, comprising more than 90% of all zooplankton abundance at some sites. We propose that the geographic distribution of these invaders is strongly constrained by geomorphic characteristics that define the salinity and mixing regimes in these estuaries, reflecting the strong role that physical forces play in structuring estuarine zooplankton communities.