Shelf–offshore exchange processes, and their effects on mesozooplankton biomass and community composition patterns in the northeast Pacific

The local zooplankton communities that inhabit a several-hundred kilometer-wide band along the eastern margin of the North Pacific typically include mixtures of species endemic to both nearshore and open-ocean environments. Many have behavioral and life-history adaptations that enhance their probability of survival and retention in either nearshore or offshore regions. Transects across the continental margin typically show a gradient in community composition from primarily nearshore to primarily oceanic, and this gradient is usually steepest near the continental shelf break. However, the transition is often neither monotonic nor complete. Barriers to exchange of water and organisms across this transition are leaky and variable in their location and effectiveness. In this paper, we review mechanisms and consequences of shelf–offshore exchange of zooplankton. Many of these processes vary in sign and intensity with latitude and also in time (wind events, seasonal cycle, and interannual). In general, surface-layer transport of zooplankton is more likely to be seaward in summer and in lower-latitude “upwelling” regions, more likely to be shoreward in winter and in higher-latitude “downwelling” regions. Because zooplankton life cycles and depth distributions are also strongly seasonal, there are important interactions that could potentially drive large match–mismatch variations in distribution and survivorship of both zooplankton and their predators.