"Estuarine Transition Zones As Critical Nursery Areas for the Early Life-History Stages of Fishes"

We review research demonstrating the importance of estuarine transition zones (ETZ) and associated turbidity maxima as nursery areas for young fish. More specifically in the St. Lawrence ETZ, the plankton food web is longitudinally organized across the frontal gradients. Elevated algae biomass occurs at the head of the ETZ, supporting a high zooplankton standing stock which in turn is fed upon by the larvae and juveniles of rainbow smelt and Atlantic tomcod. Estuarine circulation and entrapment processes control the trophic relationships within the ETZ. One important prey item for larval fish in the ETZ is Eurytemora affinis, a cryptic copepod subdivided into morphologically similar but genetically divergent clades. Two of these clades co-occur in the ETZ, one clade is unique in the freshwater part of the ETZ, where it shows similar densities as cladocerans, another important prey item for larval fishes. However, the other clade shows much higher densities in the center of the ETZ, potentially offering better feeding conditions for larval fishes. Smelt is also composed of distinct clades that segregate across the ETZ. The cryptic biodiversity of predators and prey may provide a degree of ecosystem resilience in ETZs faced with major hydrodynamic alterations related to climate change.